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Postwar Aspects of Some Tropical Diseases
CLINICS ON OTHER SUBJECTS POSTWAR ASPECTS OF SOME TROPICAL DISEASES GEORGE G. STILWELL
AT the present time there is considerable discussion throughout the United States concerning the possibility of a great postwar increase in the incidence of tropical diseases both in the returning armed forces and in the civilian population. There is also speculation about whether new diseases not heretofore present in this country will be imported, and in such event, if the proper vectors are present, whether such new diseases will establish footholds and spread throughout areas which are suitable for the propagation of these diseases. No doubt the practicing physician will see many more cases of socalled tropical diseases in the next decade or two than he has encountered in his past experience, and his diagnostic acumen often will be taxed by the bizarre clinical pictures which some of these unfamiliar disease entities will present. In recognition of this probability, there has been considerable acceleration during the last two years in the teaching program of tropical medicine from the standpoint of both the clinician and the laboratory diagnostician. This is evident in the greatly expanded departments of tropical medicine in both the Army and the Navy Medical Corps. The medical schools of North America also have felt the effect of this increased emphasis on thorough training in tropical diseases, and a plan for improvement of the teaching of tropical medicine in the medical schools of the United States and Canada has been developed through co-operation of the Association of American Medical Colleges, the Office of the Surgeon General of the United States Army, the National Research Council, the Army Medical School and Tulane University. This plan is being aided financially by the John and Mary R. Markle Foundation. Various aspects of this problem have been studied by specialized workers in tropical diseases and by leaders in the field of public health. The conclusion drawn from these studies is that the return to this country of many diseased members of the armed forces presents definite potential dangers to the health of the nation. It is also the consensus that the importance of these potential dangers perhaps has been overemphasized in the mind of the average physician. It is evident that the Army, the Navy, and the public health authorities are fully aware of all the possibilities inherent in the forthcoming situation, and that these agents, together with a medical profession better trained in tropical medicine, will be able to cope with any developments which might ensue. The importance of this problem as related to the general population of our country warrants a brief discussion of some of the more 897
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important of these potentially dangerous diseases. Particular emphasis should be placed on an attempt to anticipate what may be the future course in the United States of the diseases which are imported by our returned fighting men. It is plainly evident that our soldiers and sailors are being exposed to more diseases in the present global conflict than they have been at any other time in. history, and it is a foregone conclusion that they will bring some of these diseases home with them. Several important factors concerned in the method of spread of these diseases should be considered before the individual disease entities are discussed. The acute diseases, which have a comparatively short period of incubation and communicability, are less likely to be introduced to this country than those diseases that are of a chronic nature or that possess a long incubation period. Those diseases which require for their propagation a specific intermediate host may be imported into this country but they cannot beccme established as endemic foci unless there is already present this specific intermediate host. It is possible that in these cases the specific host also may be introduced and may thrive in some sections of the United States. There may be a greatly increased number of asymptomatic carriers of certain diseases returning to this country when demobilization occurs. It is possible that new and much more pathogenic strains of organisms may be imported which will produce disease of heightened severity in the civilian population. Finally, some diseases which are at present localized in comparatively small areas in this country may be much more widely distributed by the return of men from these endemic areas to their homes. ACUTE DISEASES
The group of acute diseases by virtue of their short incubation period probably will not present any particularly grave postwar menace to civilian health. The first of such acute diseases is yellow fever, whose etiologic agent is a virus. Yellow fever may be considered to be a historical curiosity here in the United States because it was last seen in 1905 in New Orleans. Without eternal vigilance, however, anotherintroduction of this disease is possible. As far as geographic distances are concerned, we are farther away from yellow fever than ever before because it is now absent from Mexico, the West Indies, and the upper part of Central America. If we use air travel time as a measure of distance, however, we are much nearer vellow fever than ever before because this infection is still found :in Colombia, Venezuela, 'and some parts of Panama. There appears to be little chance that members of the armed forces will introduce the virus to this country by returning from areas where yellow fever is endemic, while the disease is still in the incubation
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period. This is because of the practice of universal vaccination against yellow fever of all personnel entering areas where the disease is endemic. By use of the mouse protection test, it has been found that yellow fever exists in new and vast areas where previously it has escaped detection. These areas are in Africa, where the endemic zone extends eastward more than 3,000 miles to the upper Nile, and in Brazil along the Amazon basin, together with important areas of Colombia and Venezuela. The discovery of yellow fever in the jungles of South America has emphasized the importance of vaccination as a means of prevention because the ordinarily effective measures of mosquito control that have been so successful against the vector of urban yellow fever; namely, the domestic Aedes aegypti, are powerless against yellow fever which occurs in the great expanses of almost uninhabited tropical forest. . In such areas vaccination is the best means available for protection against the disease. The vaccine in present use is a strain of living yellow fever virus which has been modified by prolonged passage through tissue cultures until it has lost much of its neurotropic and viscerotropic virulence. It still, however, retains considerable antigenic effectiveness. Many studies indicate that the immunity produced in most vaccinated individuals persists for at least four years. Effective protection may last even longer than this in view of the lifelong immunity conveyed by the disease itself. The vaccine in its present state is as Slafe as any biologic agent containing virus can be. Before the present form of vaccine was available, small amounts of human serum were used in the manufacture to aid in preserving the virus. A form of hepatitis resembling catarrhal jaundice developed in many persons injected with the vaccine containing this human serum. After exhaustive study it was concluded that this hepatitis probably was due to some icterogenic agent, perhaps a virus, encountered occasionally in human serum and inadvertently introduced into the vaccine during its manufacture. In the newest type of vaccine, chick embryo juice is substituted for the human serum. During the last two years millions of injections of this serum-free vaccine have been administered and there have been no further reports of jaundice or other symptoms except slight fever and occasional malaise occurring about a week after injection. The greatest danger of a possible postwar introduction of yellow fever is through the medium of air travel. Several measures are being u~ed to control this danger, and vigilance must be maintained if we are to avoid this menace. All persons traveling into endemic areas are urged to be vaccinated against yellow fever. Persons returning from endemic regions are given regular inspections by quarantine authorities. Their temperature is recorded. Anyone who sho.ws signs of illness or an elevated temperature is detained pending a definite diagnosis. If a person is well but has not been vaccinated against yel-
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low fever and possibly has been exposed to the disease, he is kept under observation at his destination by health officials until the six day incubation period is past. These latter precautions are not necessary if the destination is north of any areas where Aedes aegypti is found. Airplanes returning from areas where yellow fever occurs are thoroughly sprayed to kill all incoming insects. The impression should not be gained that vaccination is a preventive agent which makes further mosquito control unnecessary. Extermination of Aedes aegypti is still the best method for control of yellow fever in cities and other accessible places in the tropics and subtropics. Because Aedes aegypti is primarily domestic in its habits it is quite accessible and therefore is particularly vulnerable to antimosquito measures. The efficacy of these measures has been proved in Brazil where in most cities and in many comparatively extensive areas Aedes aegypti can no longer be found. Sawyer2 has stated that there are three lines of defense against postwar introduction of yellow fever. The first line is vaccination and the elimination of the mosquito vector from seaports in endemic areas. The second line of defense may be regarded as the various quarantine procedures at our borders. The third line of defense is the eradication of Aedes. aegypti wherever it exists in the United States. If these measures are taken, they are adequate to keep our communities noninfectable by yellow fever without resorting to general vaccination of the civilian population. Dengue fever has the same mosquito vector, Aedes aegypti, as does yellow fever, and thus the problems of mosquito control are common to these two diseases. Dengue fever is already endemic in the American tropical and subtropical regions. It is important from a military standpoint as it may cause epidemics in the armed forces rendering troops temporarily unfit for active fighting. Because it has a short incubation period, the only danger of an extensive new introduction of it into the United States lies in the possible importation of the disease from Latin America. Until Aedes aegypti is eradicated from our southern states the danger of future epidemics remains a definite threat. Among rickettsial diseases, one is to be feared above all during times when great masses of troops are engaged in conflict. That one is louse-borne typhus fever. We need not consider here the comparatively innocuous murine or flea-borne typhus fever, which has been endemic for years in the southern areas of the United States. Louse-borne typhus is most prevalent in late winter and spring, and thus the cold weather present in the United States would not be a deterrent to its spread. The body louse is the vector cOhcerned in this form of typhus. At the present time, except for slum areas, this insect is comparatively scarce in this country. Therefore, we should feel fairly secure against any extensive invasion of this type of typhus
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fever. This security is further bolstered by the fact that modern delousing procedures in returning troops are much more efficient than in World War I. The tremendous efficacy of the newest insecticide, commonly known as DDT (1,1,1 trichlor-,2,2 diparachlorphenyl-ethane), is proving a vital factor in the present low rate of typhus, both in the armed forces and in the civilian populations with which they are mingling. Thus it appears improbable that there is much danger of wholesale introduction of typhus fever into this country by our armed forces. Another factor which will be of great importance in preventing postwar spread of this disease is the universal vaccination against typhus which is being performed on all military personnel who· go to areas where they may possibly be exposed to typhus fever. The efficacy of the type of typhus vaccine being used at present is evident in the practically complete absence of typhus fever in our troops in North Africa and Italy. This freedom from typhus is even more remarkable when we consider that extensive epidemics of abnormally severe typhus have occurred in the natives of the African area within the last two years, and that many cases of typhus have developed in unvaccinated British troops living in the same localities as our vaccinated troops. The future situation in this country with respect to· the danger of introduction of typhus to the civilian population appears quite safe if the occurrence of the body louse is kept controlled. Several other acute diseases may be briefly considered. Plague should not be any more of a problem after the war than it is at present in the western states. A vaccine is available for use in our troops abroad and it appears to be moderately effective. There should be no new outbreak of plague unless there is some breakdown of our present efficient system of ship inspection which might allow new introduction of infected rats. Phlebotomus fever, or sandfly fever, is another disease which has occurred in a large number of troops. It is of short duration, usually three or four days, but it is of military importance because the afflicted person is quite disabled for military action during its course. It should not create any postwar problems because the Phlebotomus fly is found here in only a few small regions. Cholera has not been encountered to any great extent in the fighting men. However, in the event of a more extensive Burman and In~ian campaign causing widespread native epidemics, the disease may appear in our men stationed in these areas. A fairly efficient vaccine is available for prophylactic use in persons who are to enter zones where cholera is endemic. The likelihood is extremely remote that any person could become a carrier of the Vibrio and bring it back to this country. Thus we should not have any problems with this disease after peace comes.
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Among the chronic diseases which might create postwar problems, first and foremost from the standpoint of military importance is malaria. The scope of this discussion does not permit full consideration of the diffuse manifestations of this problem, but a few of the more important aspects will be outlined. It is obvious that large numbers of our troops are being infected with malaria, particularly those fighting in the South Pacific area and in Italy. In spite of proper treatment many relapses have occurred, and the disease may remain latent for many months. Suppressive treatment with atabrine tends to prevent the initial clinical appearance of malaria for periods as long as a year after infection. Thus it appears impossible to prevent the return to this country of large numbers of men infected with malaria. The chief problems arising from the return of these malarious individuals from abroad may be summarized, according to MtCoy,1 as follows: 1. There is a possibility of establishing new endemic foci of disease in regions which are now free from malaria. 2. Importation of new strains of the parasite may occur in areas where the disease is already prevalent, with a resulting increase in the amount of malaria in these areas. 3. There must be prompt recognition and proper treatment of relapses occurring in soldiers returned to this country. Much attention has been directed to the first of these problems; namely, the establishment of new endemic foci. Every section of the United States contains anopheline vectors capable of transmitting malaria. Anopheles quadrimaculatus is fairly numerous during the warm seasons in many areas which at present do not have malaria. This is especially true in such regions as the Upper Mississippi basin and the Hudson Valley. One suggestion whieh has been made is to segregate for a certain period of time all personnel returning from malarious areas, especially if these men are to be sent to parts of the country where malaria is not present. This procedure is obviously impractical because relapses may occur after many months, during which time there has been no clinical or laboratory evidence of infection. The past history of previous outbreaks of malaria in nonendemic areas of the United States would indicate that the potential dangers in allowing dispersion of these infected individuals are not particularly great. These outbreaks of malaria have consisted of only a few cases and have been characterized by a tendency to remain confined to small areas and a propensity for spontaneous subsidence even without added measures of mosquito control. The second problem is the possibility of importation of new strains of malarial parasites. Clinical experience with infected troops in the South Pacific tends to indicate that the strains of malarial organisms being encountered there are quite different from those already en-
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demic in the United States. These conclusions are drawn from the fact that the relapse rate is much higher in individuals acquiring malaria in .the Pacific islands than it is in persons infected in the United States. This greatly increased relapse rate has occurred in spite of intensive treatment. Another bit of clinical evidence to strengthen this conclusion is the high rate of infection among American Negro troops. These colored troops have had the same rate of malarial infection as have the white troops living under the same combat conditions. This is in contrast to the condition existing in the southern part of the United States where Negroes appear to have considerable immunity to the malarial parasites found in this country. There is a definite possibility that these more virulent strains will be imported into communities where the inhabitants have little or no natural immunity to these new parasites. However, the danger of this introduction resulting in any serious epidemic is remote. This is because most of the cases of relapsing malaria in our returning troops are due to Plasmodium vivax, the tertian type. The reason for the preponderance of this species in the relapsing cases is that it is more difficult to produce a permanent cure in this type of infection than in others. Malaria due to Plasmodium vivax, however, is not considered to be especially dangerous as a source of widespread diseilSe in this country because experience in the past shows that serious epidemics of malaria are practically always due to Plasmodium falciparum, the subtertian or estivo-autumnal type. It is this latter type which pred0minates in the tropics. The third problem, that of prompt recognition and treatment of malarial relapses, resolves itself primarily as a function of the private physician. Many physicians of the United States, particularly those in the northern sections, have had practically no experience with malaria from the diagnostic, therapeutic or public health aspects. If the physician fails to recognize cases of relapsing malaria, the:apeutic measures may be delayed until the patient is in serious condition. Delay in therapy is especially hazardous to the patient suffering from estivoautumnal malaria with cerebral involvement. The public health aspect of delayed recognition of these cases is important, as this delay will greatly aid in spreading the disease to other individuals. To counteract this gap in experience and knowledge, an extensive educational program is necessary to familiarize the physicians of this country with the protean manifestations of malaria and to keep them constantly a~are of the possibility of these cases falling into their hands. After the war this situation will be alleviated considerably by the return of thousands of physicians who have had extensive experience in tropical diseases during their military services. These physicians undoubtedly will contribute greatly to increased efficiency in the practice of tropical medicine throughout the country. Filariasis constitutes another chronic disease which is a vexing prob-
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lem to the military medical personnel. The term "filariasis" in a broad sense includes several categories of parasitic nematode infestations. This discussion is limited to that form due to Wuchereria bancrofti. In this disease the adult worms live in the circulatory or lymphatic systems, the connective tissues or the serous cavities. The adults produce certain small larval forms, called microfilariae, which commonly invade the circulating blood or lymph spaces. The seriousness of the filariasis problem lies in the fact that, while it does not cause large numbers of troops to become physically unfit for military duty, it does produce profound mental disturbances in the men affected with this disease. Thus the primary problem in filariasis is one of proper psychotherapy. These important psychiatric problems arise because the men have seen in the native population horrible examples of long-standing and neglected cases of filariasis with extreme degrees of devastating elephantiasis and tremendously deforming scrotal involvement. Thousands of our troops have been living and fighting in hyperendemic areas of filarial disease, and a large number of these men have been infected with micro filariae. When the acute lymphangitis and lymphadenopathy associated with early filariasis develop, these men immediately conclude that their physical state ultimately will be the same as that of the natives they have seen. This naturally produces a profound sense of mental depression. Many write home to break engagements, fearing they will infect their future wives with the same disease. They write to their families saying they can never return home or lead normal lives because they have contracted a loathsome disease. These men tend to become chronic hospital invalids even though their physical condition may be steadily improving. It is self evident then that the first and most important form of therapy is to change the mental attitude of these men. They must be reminded that the natives they have seen have lived under unhygienic conditions, and constantly have been reinfected with micro filariae over a period of many years. They must be reassured emphatically that since they have been removed from endemic areas no further infection will take place. They must be reassured constantly that probably no progressive or disfiguring elephantiasis or scrotal deformities will develop. Many hundreds of men who presumably have filariasis have been returned to this country. It is much better psychologically to keep these men in barracks and to refrain from hospitalizing them unless such a procedure is absolutely necessary. A series of graduated physical exercises is prescribed for them, and in the majority of cases the men are soon able to return to full duty. Many of them have married and the number of pregnancies normally expected has ensued from these marriages. Continued observation of these patients indicates that the physical signs of disease are disappearing. Extensive lymphadenopathy has not been a prominent feature in the majority of cases. Clinical evidence
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would lead to the conclusion that many of the adult worms are disintegrating. The diagnosis in the great majority of cases has been made on the history of exposure in a hyperendemic area with the subsequent development of lymphangitis and lymphadenopathy. Biopsies of acutely involved lymph nodes have been performed in a few cases and the adult worms have been identified. This procedure has been abandoned, however, as it leads to extensive inflammation in the lymphatics. As far as could be ascertained at the time of preparation of this paper, circulating microfilariae have not been found in the blood of any of these patients who have filariasis. It is true that in many of these instances the infection had not been present long enough to allow the demonstration of micro filariae in the blood stream. However, the complete absence of larval forms in the blood of everyone of these patients, in spite of extensive search, adds another bit of evidence to support the conclusion that the disease will regress rather than progress in our returning men. The possibility of filariasis again becoming established in the United States as a result of infection spread from our returning military personnel appears extremely remote. This is in spite of the fact that complete development of the larval forms of Wuchereria bancrofti has been observed in thirty-two different types of mosquitoes, and that Culex fatigans, the most common vector, is found in our country. For many years an endemic focus of filariasis existed at Charleston, South Carolina, and microfilariae could be found in an appreciable number of the inhabitants of this locality. At the present time this region is practically free from this disease. This focus of infection apparently arose from importation of slaves from Africa who were heavily infested with microfilariae. As a suitable mosquito vector was present, the disease continued to be active for many years, but because of improved general hygienic conditions and mosquito control measures the infestation gradually has died out. It does not seem likely then that new endemic areas of filariasis will be set up in the United States in view of the fact that, up to the present time at least, there has been complete absence of micro filariae in the returned cases. If at a later date micro filariae do appear in these men it seems likely that the public health authorities with modern methods of mosquito control available will be able to prevent any spread to the general population. Bacillary dysentery has been of some concern to members of our a~med forces. Several epidemics have occurred in army camps in this country and in Africa. Fortunately, chemotherapy with sulfonamide drugs has proved. extremely effective in curing the acute phases of the disease, and there has not been a great amount of resulting military disability. Past experience with this disease tends to indicate that a certain proportion of patients who have active disease and also some of the asymptomatic carriers will harbor the organism for several years. Therefore, it is highly probable that new and more
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pbtently virulent strains of the bacillus will be imported into the United States with returning troops. There is a possibility that these highly pathogenic strains might give rise to postwar epidemics, especially in institutions such as mental or convalescent hospitals, and in army camps. This potential hazard can be minimized by strict attention to maintaining efficient conditions of general sanitation and persOfial hygiene. Another form of dysentery which is of military and civil importance is that caused by the presence of Endamoeba histolytica. This is by no means solely a tropical disease, as a considerable percentage of all individuals in the United States have been shown to harbor this organism. The fact remains that symptoms of amebiasis are more frequent and usually much more severe in tropical areas than they are in tetnperate zones. The chance of infection in the troops fighting in the most forward war zones is great because the necessity of hurried treatment of the drinking water may allow the cyst forms of the organism to pass through whatever process is used to render the water potable. It is well known that different strains of Endamoeba histolytica vary greatly in their pathogenic capabilities. It is possible that there is a definite potential danger in the importation of new strains of this protozoal organism. This danger is heightened by the fact that the incubation period may be extremely long. Another danger is that the disease may become chronic and almost asymptomatic in undiagnosed or inadequately treated cases. Persons so affected can become a source of infection to their families and to larger groups with which they may come in contact. However, the medical profession has become much more cognizant of this type of dysentery since the Chicago epidemic of 1933 and the danger of amebiasis becoming much more widespread than it is at present is probably not particularly grave. Trypanos01niasis possibly may be acquired by our troops stationed in two different areas. The African form of the disease, which is due to the presence of Trypanosoma gambiense, is found in equatorial Africa where many of our air transport lines have been placed. Men stationed there may acquire the infection without any clinical symptoms of disease until after return to the United States. Any spread of this form of the disease remains a relatively remote possibility, as the necessary vector, the tsetse fly, occurs only in Africa. Rigid vigilance in the spraying of airplanes and in quarantine inspection should prevent the establishment of the tsetse fly in the American tropics. Greatly encouraging results have followed the treatment of early cases of African trypanosomiasis by use of two new arsenical drugs; namely, melarsen oxide and a compound known as "70A." It appears therefore that we have little to fear from this disease. American trypanosomiasis, due to Trypanosoma cruzi, might occur in personnel sent to Central or South America. This disease probably
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will not become a serious problem because this particular form of trypanosomiasis usually is mild in adults. Cone-nosed bugs and some types of wild rodents in the southwestern United States harbor Trypanosoma cruzi, but as yet no naturally occurring cases in human beings have been reported. Leishmaniasis is a chronic protozoal disease to which many of our troops undoubtedly are being exposed. It occurs in a visceral form, kala-azar, in the Mediterranean region, parts of India, northern China and in northeastern Brazil. The cutaneous form may be found in two main regions, one in North Africa and the Near East and the other in South and Central America. Scattered cases of leishmaniasis probably will occur in men returning from any of these parts of the world. The disease should not produce any serious postwar problems as the vector necessary for its propagation does not occur at the present time in our country. Schistosomiasis is the only trematode infestation which is of sufficient potential danger to warrant discussion. Troops situated in areas where schisto.somiasis is endemic may acquire this disease by immersion in water containing the infected snails. The larvae in the water are able to penetrate the skin even though the men so exposed are fully clothed. It is known that there is considerable specificity of the snails which are the intermediate hosts of these parasites. There is no definite proof that a species of snail exists in the .United States which is capable of acting as an intermediate host in this particular disease. We do know that there are schistosomes of lower animals present in our country and the larvae of these forms are capable of producing a dermatitis in human beings, the so-called swimmers' itch. In view of this fact, there may be snails that are capable of acting as intermediate hosts in human schistosomiasis. If so, this infestation might possibly spread to the civilian population. This phase of the problem needs further study. The possibility of importation of a few cases of leprosy is a problem which may assume undue importance in the minds of the civilian population because of the distorted ideas which most people entertain concerning the danger of this disease. A small number of cases of leprosy may develop among those men who have come into intimate contact with leprous natives. Clinical evidence of infection may not develop for years after their return to this country. We need not fear any wholesale spread of the disease because of these individual cases, however, as experience with the Scandinavian cases brought to Minnesota years ago indicates that leprosy has a strong tendency to disappear spontaneously in most parts of the United States. Endemic foci of leprosy still remain in some regions of Texas, Louisiana and Florida, but the disease probably will not extend beyond these limits, except for occasional scattered cases of unexplained epidemiologic etiology. Coccidioidomycosis is a fungous disease which, while not a tropical disease, is worthy of mention as a possible postwar problem. The
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occurrence of this infection is confined practically entirely to the United States, and the endemic foci occur in California, Arizona and Texas. An especially large number of cases apparently originate in the San Joaquin Valley in California where several army camps have been located. Because the disease is apparently acquired by inhalation of dust containing spores, thousands of nonimmune individuals stationed in these areas for military maneuvers have been exposed to the infection. Thus, it is possible that there may be a great increase in the number of cases occurring throughout the country as these men are demobilized or discharged. Coccidioidomycosis gives rise to a symptom complex which may be difficult to diagnose at times, and hence the medical profession must be constantly aware of the possibility of encountering this disease. There is apparently no danger of the condition assuming widespread epidemic proportions as there is, evidently, no mechanism of direct transmission from person to person. CONCLUSIONS
1. Since the men and women of the armed fQrces of the United
States are scattered over more widespread and far-flung areas of the earth than ever before in our history, they are being exposed to, and are being infected with, a greater variety of tropical diseases than ever before. 2. The importation of these diseases in our returning military personnel will present some postwar problems in medical practice; however, the dangers of widespread epidemics of these diseases extending into our civilian population have been for the most part greatly exaggerated. 3. The most important source of potential danger in the importation of these tropical diseases probably lies in the introduction of new and highly pathogenic strains of parasites which already exist in the United States, together with the importation of new insect vectors of diseases which have never before been present in our country. 4. The Army, Navy and United States Public Health Service are fully cognizant of these potential dangers and have organized new and rigid quarantine services to control as far as possible the importation of these diseases by military traffic. S. The general medical profession is becoming better instructed in tropical diseases and the practice of good tropical medicine will be further bolstered by the return to private practice of thousands of young Army and Navy physicians who have had considerable training and experience in dealing with diseases of the tropics. REFERENCES 1. McCoy, O. R.: Public health implications of tropical and imported diseases.
Am. J. Pub. Health. 34:15-19 (Jan.) 1944. 2. Sawyer, W. A.: Public health implications of tropical and imported diseases. Am. J. Pub. Health. 34:7-14 (Jan.) 1944.
AT the present time there is considerable discussion throughout the United States concerning the possibility of a great postwar increase in the incidence of tropical diseases both in the returning armed forces and in the civilian population. There is also speculation about whether new diseases not heretofore present in this country will be imported, and in such event, if the proper vectors are present, whether such new diseases will establish footholds and spread throughout areas which are suitable for the propagation of these diseases. No doubt the practicing physician will see many more cases of socalled tropical diseases in the next decade or two than he has encountered in his past experience, and his diagnostic acumen often will be taxed by the bizarre clinical pictures which some of these unfamiliar disease entities will present. In recognition of this probability, there has been considerable acceleration during the last two years in the teaching program of tropical medicine from the standpoint of both the clinician and the laboratory diagnostician. This is evident in the greatly expanded departments of tropical medicine in both the Army and the Navy Medical Corps. The medical schools of North America also have felt the effect of this increased emphasis on thorough training in tropical diseases, and a plan for improvement of the teaching of tropical medicine in the medical schools of the United States and Canada has been developed through co-operation of the Association of American Medical Colleges, the Office of the Surgeon General of the United States Army, the National Research Council, the Army Medical School and Tulane University. This plan is being aided financially by the John and Mary R. Markle Foundation. Various aspects of this problem have been studied by specialized workers in tropical diseases and by leaders in the field of public health. The conclusion drawn from these studies is that the return to this country of many diseased members of the armed forces presents definite potential dangers to the health of the nation. It is also the consensus that the importance of these potential dangers perhaps has been overemphasized in the mind of the average physician. It is evident that the Army, the Navy, and the public health authorities are fully aware of all the possibilities inherent in the forthcoming situation, and that these agents, together with a medical profession better trained in tropical medicine, will be able to cope with any developments which might ensue. The importance of this problem as related to the general population of our country warrants a brief discussion of some of the more 897
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important of these potentially dangerous diseases. Particular emphasis should be placed on an attempt to anticipate what may be the future course in the United States of the diseases which are imported by our returned fighting men. It is plainly evident that our soldiers and sailors are being exposed to more diseases in the present global conflict than they have been at any other time in. history, and it is a foregone conclusion that they will bring some of these diseases home with them. Several important factors concerned in the method of spread of these diseases should be considered before the individual disease entities are discussed. The acute diseases, which have a comparatively short period of incubation and communicability, are less likely to be introduced to this country than those diseases that are of a chronic nature or that possess a long incubation period. Those diseases which require for their propagation a specific intermediate host may be imported into this country but they cannot beccme established as endemic foci unless there is already present this specific intermediate host. It is possible that in these cases the specific host also may be introduced and may thrive in some sections of the United States. There may be a greatly increased number of asymptomatic carriers of certain diseases returning to this country when demobilization occurs. It is possible that new and much more pathogenic strains of organisms may be imported which will produce disease of heightened severity in the civilian population. Finally, some diseases which are at present localized in comparatively small areas in this country may be much more widely distributed by the return of men from these endemic areas to their homes. ACUTE DISEASES
The group of acute diseases by virtue of their short incubation period probably will not present any particularly grave postwar menace to civilian health. The first of such acute diseases is yellow fever, whose etiologic agent is a virus. Yellow fever may be considered to be a historical curiosity here in the United States because it was last seen in 1905 in New Orleans. Without eternal vigilance, however, anotherintroduction of this disease is possible. As far as geographic distances are concerned, we are farther away from yellow fever than ever before because it is now absent from Mexico, the West Indies, and the upper part of Central America. If we use air travel time as a measure of distance, however, we are much nearer vellow fever than ever before because this infection is still found :in Colombia, Venezuela, 'and some parts of Panama. There appears to be little chance that members of the armed forces will introduce the virus to this country by returning from areas where yellow fever is endemic, while the disease is still in the incubation
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period. This is because of the practice of universal vaccination against yellow fever of all personnel entering areas where the disease is endemic. By use of the mouse protection test, it has been found that yellow fever exists in new and vast areas where previously it has escaped detection. These areas are in Africa, where the endemic zone extends eastward more than 3,000 miles to the upper Nile, and in Brazil along the Amazon basin, together with important areas of Colombia and Venezuela. The discovery of yellow fever in the jungles of South America has emphasized the importance of vaccination as a means of prevention because the ordinarily effective measures of mosquito control that have been so successful against the vector of urban yellow fever; namely, the domestic Aedes aegypti, are powerless against yellow fever which occurs in the great expanses of almost uninhabited tropical forest. . In such areas vaccination is the best means available for protection against the disease. The vaccine in present use is a strain of living yellow fever virus which has been modified by prolonged passage through tissue cultures until it has lost much of its neurotropic and viscerotropic virulence. It still, however, retains considerable antigenic effectiveness. Many studies indicate that the immunity produced in most vaccinated individuals persists for at least four years. Effective protection may last even longer than this in view of the lifelong immunity conveyed by the disease itself. The vaccine in its present state is as Slafe as any biologic agent containing virus can be. Before the present form of vaccine was available, small amounts of human serum were used in the manufacture to aid in preserving the virus. A form of hepatitis resembling catarrhal jaundice developed in many persons injected with the vaccine containing this human serum. After exhaustive study it was concluded that this hepatitis probably was due to some icterogenic agent, perhaps a virus, encountered occasionally in human serum and inadvertently introduced into the vaccine during its manufacture. In the newest type of vaccine, chick embryo juice is substituted for the human serum. During the last two years millions of injections of this serum-free vaccine have been administered and there have been no further reports of jaundice or other symptoms except slight fever and occasional malaise occurring about a week after injection. The greatest danger of a possible postwar introduction of yellow fever is through the medium of air travel. Several measures are being u~ed to control this danger, and vigilance must be maintained if we are to avoid this menace. All persons traveling into endemic areas are urged to be vaccinated against yellow fever. Persons returning from endemic regions are given regular inspections by quarantine authorities. Their temperature is recorded. Anyone who sho.ws signs of illness or an elevated temperature is detained pending a definite diagnosis. If a person is well but has not been vaccinated against yel-
900
GEORGE G. STIL WELL
low fever and possibly has been exposed to the disease, he is kept under observation at his destination by health officials until the six day incubation period is past. These latter precautions are not necessary if the destination is north of any areas where Aedes aegypti is found. Airplanes returning from areas where yellow fever occurs are thoroughly sprayed to kill all incoming insects. The impression should not be gained that vaccination is a preventive agent which makes further mosquito control unnecessary. Extermination of Aedes aegypti is still the best method for control of yellow fever in cities and other accessible places in the tropics and subtropics. Because Aedes aegypti is primarily domestic in its habits it is quite accessible and therefore is particularly vulnerable to antimosquito measures. The efficacy of these measures has been proved in Brazil where in most cities and in many comparatively extensive areas Aedes aegypti can no longer be found. Sawyer2 has stated that there are three lines of defense against postwar introduction of yellow fever. The first line is vaccination and the elimination of the mosquito vector from seaports in endemic areas. The second line of defense may be regarded as the various quarantine procedures at our borders. The third line of defense is the eradication of Aedes. aegypti wherever it exists in the United States. If these measures are taken, they are adequate to keep our communities noninfectable by yellow fever without resorting to general vaccination of the civilian population. Dengue fever has the same mosquito vector, Aedes aegypti, as does yellow fever, and thus the problems of mosquito control are common to these two diseases. Dengue fever is already endemic in the American tropical and subtropical regions. It is important from a military standpoint as it may cause epidemics in the armed forces rendering troops temporarily unfit for active fighting. Because it has a short incubation period, the only danger of an extensive new introduction of it into the United States lies in the possible importation of the disease from Latin America. Until Aedes aegypti is eradicated from our southern states the danger of future epidemics remains a definite threat. Among rickettsial diseases, one is to be feared above all during times when great masses of troops are engaged in conflict. That one is louse-borne typhus fever. We need not consider here the comparatively innocuous murine or flea-borne typhus fever, which has been endemic for years in the southern areas of the United States. Louse-borne typhus is most prevalent in late winter and spring, and thus the cold weather present in the United States would not be a deterrent to its spread. The body louse is the vector cOhcerned in this form of typhus. At the present time, except for slum areas, this insect is comparatively scarce in this country. Therefore, we should feel fairly secure against any extensive invasion of this type of typhus
POSTWAR ASPECTS OF SOME TROPICAL DISEASES
901
fever. This security is further bolstered by the fact that modern delousing procedures in returning troops are much more efficient than in World War I. The tremendous efficacy of the newest insecticide, commonly known as DDT (1,1,1 trichlor-,2,2 diparachlorphenyl-ethane), is proving a vital factor in the present low rate of typhus, both in the armed forces and in the civilian populations with which they are mingling. Thus it appears improbable that there is much danger of wholesale introduction of typhus fever into this country by our armed forces. Another factor which will be of great importance in preventing postwar spread of this disease is the universal vaccination against typhus which is being performed on all military personnel who· go to areas where they may possibly be exposed to typhus fever. The efficacy of the type of typhus vaccine being used at present is evident in the practically complete absence of typhus fever in our troops in North Africa and Italy. This freedom from typhus is even more remarkable when we consider that extensive epidemics of abnormally severe typhus have occurred in the natives of the African area within the last two years, and that many cases of typhus have developed in unvaccinated British troops living in the same localities as our vaccinated troops. The future situation in this country with respect to· the danger of introduction of typhus to the civilian population appears quite safe if the occurrence of the body louse is kept controlled. Several other acute diseases may be briefly considered. Plague should not be any more of a problem after the war than it is at present in the western states. A vaccine is available for use in our troops abroad and it appears to be moderately effective. There should be no new outbreak of plague unless there is some breakdown of our present efficient system of ship inspection which might allow new introduction of infected rats. Phlebotomus fever, or sandfly fever, is another disease which has occurred in a large number of troops. It is of short duration, usually three or four days, but it is of military importance because the afflicted person is quite disabled for military action during its course. It should not create any postwar problems because the Phlebotomus fly is found here in only a few small regions. Cholera has not been encountered to any great extent in the fighting men. However, in the event of a more extensive Burman and In~ian campaign causing widespread native epidemics, the disease may appear in our men stationed in these areas. A fairly efficient vaccine is available for prophylactic use in persons who are to enter zones where cholera is endemic. The likelihood is extremely remote that any person could become a carrier of the Vibrio and bring it back to this country. Thus we should not have any problems with this disease after peace comes.
902
GEORGE G. STIL WELL CHRONIC DISEASES
Among the chronic diseases which might create postwar problems, first and foremost from the standpoint of military importance is malaria. The scope of this discussion does not permit full consideration of the diffuse manifestations of this problem, but a few of the more important aspects will be outlined. It is obvious that large numbers of our troops are being infected with malaria, particularly those fighting in the South Pacific area and in Italy. In spite of proper treatment many relapses have occurred, and the disease may remain latent for many months. Suppressive treatment with atabrine tends to prevent the initial clinical appearance of malaria for periods as long as a year after infection. Thus it appears impossible to prevent the return to this country of large numbers of men infected with malaria. The chief problems arising from the return of these malarious individuals from abroad may be summarized, according to MtCoy,1 as follows: 1. There is a possibility of establishing new endemic foci of disease in regions which are now free from malaria. 2. Importation of new strains of the parasite may occur in areas where the disease is already prevalent, with a resulting increase in the amount of malaria in these areas. 3. There must be prompt recognition and proper treatment of relapses occurring in soldiers returned to this country. Much attention has been directed to the first of these problems; namely, the establishment of new endemic foci. Every section of the United States contains anopheline vectors capable of transmitting malaria. Anopheles quadrimaculatus is fairly numerous during the warm seasons in many areas which at present do not have malaria. This is especially true in such regions as the Upper Mississippi basin and the Hudson Valley. One suggestion whieh has been made is to segregate for a certain period of time all personnel returning from malarious areas, especially if these men are to be sent to parts of the country where malaria is not present. This procedure is obviously impractical because relapses may occur after many months, during which time there has been no clinical or laboratory evidence of infection. The past history of previous outbreaks of malaria in nonendemic areas of the United States would indicate that the potential dangers in allowing dispersion of these infected individuals are not particularly great. These outbreaks of malaria have consisted of only a few cases and have been characterized by a tendency to remain confined to small areas and a propensity for spontaneous subsidence even without added measures of mosquito control. The second problem is the possibility of importation of new strains of malarial parasites. Clinical experience with infected troops in the South Pacific tends to indicate that the strains of malarial organisms being encountered there are quite different from those already en-
POSTWAR ASPECfS OF SOME TROPICAL DISEASES
903
demic in the United States. These conclusions are drawn from the fact that the relapse rate is much higher in individuals acquiring malaria in .the Pacific islands than it is in persons infected in the United States. This greatly increased relapse rate has occurred in spite of intensive treatment. Another bit of clinical evidence to strengthen this conclusion is the high rate of infection among American Negro troops. These colored troops have had the same rate of malarial infection as have the white troops living under the same combat conditions. This is in contrast to the condition existing in the southern part of the United States where Negroes appear to have considerable immunity to the malarial parasites found in this country. There is a definite possibility that these more virulent strains will be imported into communities where the inhabitants have little or no natural immunity to these new parasites. However, the danger of this introduction resulting in any serious epidemic is remote. This is because most of the cases of relapsing malaria in our returning troops are due to Plasmodium vivax, the tertian type. The reason for the preponderance of this species in the relapsing cases is that it is more difficult to produce a permanent cure in this type of infection than in others. Malaria due to Plasmodium vivax, however, is not considered to be especially dangerous as a source of widespread diseilSe in this country because experience in the past shows that serious epidemics of malaria are practically always due to Plasmodium falciparum, the subtertian or estivo-autumnal type. It is this latter type which pred0minates in the tropics. The third problem, that of prompt recognition and treatment of malarial relapses, resolves itself primarily as a function of the private physician. Many physicians of the United States, particularly those in the northern sections, have had practically no experience with malaria from the diagnostic, therapeutic or public health aspects. If the physician fails to recognize cases of relapsing malaria, the:apeutic measures may be delayed until the patient is in serious condition. Delay in therapy is especially hazardous to the patient suffering from estivoautumnal malaria with cerebral involvement. The public health aspect of delayed recognition of these cases is important, as this delay will greatly aid in spreading the disease to other individuals. To counteract this gap in experience and knowledge, an extensive educational program is necessary to familiarize the physicians of this country with the protean manifestations of malaria and to keep them constantly a~are of the possibility of these cases falling into their hands. After the war this situation will be alleviated considerably by the return of thousands of physicians who have had extensive experience in tropical diseases during their military services. These physicians undoubtedly will contribute greatly to increased efficiency in the practice of tropical medicine throughout the country. Filariasis constitutes another chronic disease which is a vexing prob-
904
GEOR(~E
G. STILWELL
lem to the military medical personnel. The term "filariasis" in a broad sense includes several categories of parasitic nematode infestations. This discussion is limited to that form due to Wuchereria bancrofti. In this disease the adult worms live in the circulatory or lymphatic systems, the connective tissues or the serous cavities. The adults produce certain small larval forms, called microfilariae, which commonly invade the circulating blood or lymph spaces. The seriousness of the filariasis problem lies in the fact that, while it does not cause large numbers of troops to become physically unfit for military duty, it does produce profound mental disturbances in the men affected with this disease. Thus the primary problem in filariasis is one of proper psychotherapy. These important psychiatric problems arise because the men have seen in the native population horrible examples of long-standing and neglected cases of filariasis with extreme degrees of devastating elephantiasis and tremendously deforming scrotal involvement. Thousands of our troops have been living and fighting in hyperendemic areas of filarial disease, and a large number of these men have been infected with micro filariae. When the acute lymphangitis and lymphadenopathy associated with early filariasis develop, these men immediately conclude that their physical state ultimately will be the same as that of the natives they have seen. This naturally produces a profound sense of mental depression. Many write home to break engagements, fearing they will infect their future wives with the same disease. They write to their families saying they can never return home or lead normal lives because they have contracted a loathsome disease. These men tend to become chronic hospital invalids even though their physical condition may be steadily improving. It is self evident then that the first and most important form of therapy is to change the mental attitude of these men. They must be reminded that the natives they have seen have lived under unhygienic conditions, and constantly have been reinfected with micro filariae over a period of many years. They must be reassured emphatically that since they have been removed from endemic areas no further infection will take place. They must be reassured constantly that probably no progressive or disfiguring elephantiasis or scrotal deformities will develop. Many hundreds of men who presumably have filariasis have been returned to this country. It is much better psychologically to keep these men in barracks and to refrain from hospitalizing them unless such a procedure is absolutely necessary. A series of graduated physical exercises is prescribed for them, and in the majority of cases the men are soon able to return to full duty. Many of them have married and the number of pregnancies normally expected has ensued from these marriages. Continued observation of these patients indicates that the physical signs of disease are disappearing. Extensive lymphadenopathy has not been a prominent feature in the majority of cases. Clinical evidence
POSTW AR ASPECTS OF SOME TROPICAL DISEASES
905
would lead to the conclusion that many of the adult worms are disintegrating. The diagnosis in the great majority of cases has been made on the history of exposure in a hyperendemic area with the subsequent development of lymphangitis and lymphadenopathy. Biopsies of acutely involved lymph nodes have been performed in a few cases and the adult worms have been identified. This procedure has been abandoned, however, as it leads to extensive inflammation in the lymphatics. As far as could be ascertained at the time of preparation of this paper, circulating microfilariae have not been found in the blood of any of these patients who have filariasis. It is true that in many of these instances the infection had not been present long enough to allow the demonstration of micro filariae in the blood stream. However, the complete absence of larval forms in the blood of everyone of these patients, in spite of extensive search, adds another bit of evidence to support the conclusion that the disease will regress rather than progress in our returning men. The possibility of filariasis again becoming established in the United States as a result of infection spread from our returning military personnel appears extremely remote. This is in spite of the fact that complete development of the larval forms of Wuchereria bancrofti has been observed in thirty-two different types of mosquitoes, and that Culex fatigans, the most common vector, is found in our country. For many years an endemic focus of filariasis existed at Charleston, South Carolina, and microfilariae could be found in an appreciable number of the inhabitants of this locality. At the present time this region is practically free from this disease. This focus of infection apparently arose from importation of slaves from Africa who were heavily infested with microfilariae. As a suitable mosquito vector was present, the disease continued to be active for many years, but because of improved general hygienic conditions and mosquito control measures the infestation gradually has died out. It does not seem likely then that new endemic areas of filariasis will be set up in the United States in view of the fact that, up to the present time at least, there has been complete absence of micro filariae in the returned cases. If at a later date micro filariae do appear in these men it seems likely that the public health authorities with modern methods of mosquito control available will be able to prevent any spread to the general population. Bacillary dysentery has been of some concern to members of our a~med forces. Several epidemics have occurred in army camps in this country and in Africa. Fortunately, chemotherapy with sulfonamide drugs has proved. extremely effective in curing the acute phases of the disease, and there has not been a great amount of resulting military disability. Past experience with this disease tends to indicate that a certain proportion of patients who have active disease and also some of the asymptomatic carriers will harbor the organism for several years. Therefore, it is highly probable that new and more
906
GEORGE G. STIL WELL
pbtently virulent strains of the bacillus will be imported into the United States with returning troops. There is a possibility that these highly pathogenic strains might give rise to postwar epidemics, especially in institutions such as mental or convalescent hospitals, and in army camps. This potential hazard can be minimized by strict attention to maintaining efficient conditions of general sanitation and persOfial hygiene. Another form of dysentery which is of military and civil importance is that caused by the presence of Endamoeba histolytica. This is by no means solely a tropical disease, as a considerable percentage of all individuals in the United States have been shown to harbor this organism. The fact remains that symptoms of amebiasis are more frequent and usually much more severe in tropical areas than they are in tetnperate zones. The chance of infection in the troops fighting in the most forward war zones is great because the necessity of hurried treatment of the drinking water may allow the cyst forms of the organism to pass through whatever process is used to render the water potable. It is well known that different strains of Endamoeba histolytica vary greatly in their pathogenic capabilities. It is possible that there is a definite potential danger in the importation of new strains of this protozoal organism. This danger is heightened by the fact that the incubation period may be extremely long. Another danger is that the disease may become chronic and almost asymptomatic in undiagnosed or inadequately treated cases. Persons so affected can become a source of infection to their families and to larger groups with which they may come in contact. However, the medical profession has become much more cognizant of this type of dysentery since the Chicago epidemic of 1933 and the danger of amebiasis becoming much more widespread than it is at present is probably not particularly grave. Trypanos01niasis possibly may be acquired by our troops stationed in two different areas. The African form of the disease, which is due to the presence of Trypanosoma gambiense, is found in equatorial Africa where many of our air transport lines have been placed. Men stationed there may acquire the infection without any clinical symptoms of disease until after return to the United States. Any spread of this form of the disease remains a relatively remote possibility, as the necessary vector, the tsetse fly, occurs only in Africa. Rigid vigilance in the spraying of airplanes and in quarantine inspection should prevent the establishment of the tsetse fly in the American tropics. Greatly encouraging results have followed the treatment of early cases of African trypanosomiasis by use of two new arsenical drugs; namely, melarsen oxide and a compound known as "70A." It appears therefore that we have little to fear from this disease. American trypanosomiasis, due to Trypanosoma cruzi, might occur in personnel sent to Central or South America. This disease probably
POSTWAR ASPECTS OF SOME TROPICAL DISEASES
907
will not become a serious problem because this particular form of trypanosomiasis usually is mild in adults. Cone-nosed bugs and some types of wild rodents in the southwestern United States harbor Trypanosoma cruzi, but as yet no naturally occurring cases in human beings have been reported. Leishmaniasis is a chronic protozoal disease to which many of our troops undoubtedly are being exposed. It occurs in a visceral form, kala-azar, in the Mediterranean region, parts of India, northern China and in northeastern Brazil. The cutaneous form may be found in two main regions, one in North Africa and the Near East and the other in South and Central America. Scattered cases of leishmaniasis probably will occur in men returning from any of these parts of the world. The disease should not produce any serious postwar problems as the vector necessary for its propagation does not occur at the present time in our country. Schistosomiasis is the only trematode infestation which is of sufficient potential danger to warrant discussion. Troops situated in areas where schisto.somiasis is endemic may acquire this disease by immersion in water containing the infected snails. The larvae in the water are able to penetrate the skin even though the men so exposed are fully clothed. It is known that there is considerable specificity of the snails which are the intermediate hosts of these parasites. There is no definite proof that a species of snail exists in the .United States which is capable of acting as an intermediate host in this particular disease. We do know that there are schistosomes of lower animals present in our country and the larvae of these forms are capable of producing a dermatitis in human beings, the so-called swimmers' itch. In view of this fact, there may be snails that are capable of acting as intermediate hosts in human schistosomiasis. If so, this infestation might possibly spread to the civilian population. This phase of the problem needs further study. The possibility of importation of a few cases of leprosy is a problem which may assume undue importance in the minds of the civilian population because of the distorted ideas which most people entertain concerning the danger of this disease. A small number of cases of leprosy may develop among those men who have come into intimate contact with leprous natives. Clinical evidence of infection may not develop for years after their return to this country. We need not fear any wholesale spread of the disease because of these individual cases, however, as experience with the Scandinavian cases brought to Minnesota years ago indicates that leprosy has a strong tendency to disappear spontaneously in most parts of the United States. Endemic foci of leprosy still remain in some regions of Texas, Louisiana and Florida, but the disease probably will not extend beyond these limits, except for occasional scattered cases of unexplained epidemiologic etiology. Coccidioidomycosis is a fungous disease which, while not a tropical disease, is worthy of mention as a possible postwar problem. The
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GEORGE G. STIL WELL
occurrence of this infection is confined practically entirely to the United States, and the endemic foci occur in California, Arizona and Texas. An especially large number of cases apparently originate in the San Joaquin Valley in California where several army camps have been located. Because the disease is apparently acquired by inhalation of dust containing spores, thousands of nonimmune individuals stationed in these areas for military maneuvers have been exposed to the infection. Thus, it is possible that there may be a great increase in the number of cases occurring throughout the country as these men are demobilized or discharged. Coccidioidomycosis gives rise to a symptom complex which may be difficult to diagnose at times, and hence the medical profession must be constantly aware of the possibility of encountering this disease. There is apparently no danger of the condition assuming widespread epidemic proportions as there is, evidently, no mechanism of direct transmission from person to person. CONCLUSIONS
1. Since the men and women of the armed fQrces of the United
States are scattered over more widespread and far-flung areas of the earth than ever before in our history, they are being exposed to, and are being infected with, a greater variety of tropical diseases than ever before. 2. The importation of these diseases in our returning military personnel will present some postwar problems in medical practice; however, the dangers of widespread epidemics of these diseases extending into our civilian population have been for the most part greatly exaggerated. 3. The most important source of potential danger in the importation of these tropical diseases probably lies in the introduction of new and highly pathogenic strains of parasites which already exist in the United States, together with the importation of new insect vectors of diseases which have never before been present in our country. 4. The Army, Navy and United States Public Health Service are fully cognizant of these potential dangers and have organized new and rigid quarantine services to control as far as possible the importation of these diseases by military traffic. S. The general medical profession is becoming better instructed in tropical diseases and the practice of good tropical medicine will be further bolstered by the return to private practice of thousands of young Army and Navy physicians who have had considerable training and experience in dealing with diseases of the tropics. REFERENCES 1. McCoy, O. R.: Public health implications of tropical and imported diseases.
Am. J. Pub. Health. 34:15-19 (Jan.) 1944. 2. Sawyer, W. A.: Public health implications of tropical and imported diseases. Am. J. Pub. Health. 34:7-14 (Jan.) 1944.