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Mild prolonged schistosomiasis in mice
53 TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE. Vol. 58. No. 1. January, 1964.
MILD
PROLONGED
SCHISTOSOMIASIS
IN MICE:
ALTERATIONS IN HOST RESPONSE WITH TIME AND THE DEVELOPMENT OF PORTAL FIBROSIS BY
Z I L T O N A. ANDRADE AND K E N N E T H S. WARREN
(From Hospital das Clinicas, University of Bahia, Brazil). During studies of schistosomiasis by one of us (K.S.W.) it was observed that mice with low w o r m burdens could accumulate, over a long period of time, large n u m b e r s of eggs in their livers with relatively mild consequences (WARREN, in press). T h e s e animals, killed late in the course of infection, presented small, well-delimited granulomas scattered throughout the liver tissue. M a n y of these lesions were surrounding e m p t y egg shells, and were shrunken and fibrotic ; but some of t h e m were associated with fully mature eggs, and these granulomas consisted of slight fibroblastic proliferation and a few histiocytes. T h i s was in sharp contrast to the large exudative, necrotic reactions developed around mature eggs in the liver of animals killed earlier. I t seemed, therefore, that we m a y have been observing a true i m m u n e reaction to schistosome ova late in the course of a light infection, as compared to the enhanced and destructive inflammation of hypersensitivity seen earlier and frequently observed in heavy infections. F r o m his studies on tuberculosis RICH (1947) has developed the concept that " the i m m u n e state enhances, so greatly, the protective power of the inflammatory mechanism that, far f r o m requiring the exaggerated and accelerated inflammation of hypersensitivity, an astonishingly small a m o u n t of inflammation suffices for the effective protection of the b o d y . " Since it seemed possible that the observed differences in granuloma formation at different ages of infection might indicate morphologically the change f r o m a hypersensitive to an i m m u n e type of inflammatory reaction, we decided to investigate the histological details in the liver of mice with prolonged infections. MATERIALS AND METHODS Eighty-four adult Swiss albino female mice maintained on a conventional pellet diet were exposed to cercariae (about 28 for each mouse) of a Puerto Rican strain of S. mansoni according to the method of OLIVIERand STIREWALT(1952). Seven weeks following infection, adult worms were recovered from the portal and mesenteric veins by a modification of the perfusion method of YOLLES et al. (1947). Perfusions were also performed at 25 weeks after infection. Two mice were killed at 10, 13 and 15 weeks, three mice at 20 weeks and six mice at 25 weeks following infection. Pathophysiological changes in these and in the remaining mice are reported elsewhere (WARREN, in press). Fragments of liver were placed in buffered formalin and the paraffin sections were submitted to the following staining techniques : hemotoxylin-eosin, Mallory's blue anilin for connective tissue, Gomori's silver impregnation for reticulum and McManus-Hotchkiss' PAS method. RESULTS T a b l e I demonstrates the w o r m burdens found in the mice infected with 28 cercariae of S. mamoni. Histological changes referring to the hepatic granulomas around mature eggs of S. mansoni and to non-granulomatous changes about the portal areas are shown in
54
MILD
PROLONGED
SCHISTOSOMIASIS IN MICE
Tables II and III, respectively. Only granulomas around mature eggs were considered. T h e mature egg was identified according to PRATA (1957) as o n e having a thick, linear shell outline and an eosinophilic miracidium. Inside the miracidium there were several basophilic granules, some of them arranged centrally, forming a rosette-like structure. T h e miracidium has a life span of 2.5 to 3 weeks (GONNERT, 1955), and an egg with a fully mature miracidium lasts for 11 to 12 days (PRATA, 1957). By the 10th week after infection there was infiltration by a considerable n u m b e r of eosinophils, both around the granulomas and about, the portal spaces. T h e eosinophilic infiltrate varied in density and caused the periphery of the granulomas to appear ragged. In the central necrotic area near the egg, eosinophils also accumulated and showed signs of disintegration (Table II). TABLE I.
Worms recovered by perfusion at 7 and 25 weeks following exposure to 28 cercariae of S. mansoni
Weeks after infection
No. animals
Average worms
Average worm pairs
Range of pairs
7
10
5.7 ± 0.8
2.2 ± 0.3
1-4
25
5
2.4 ± 0.6
0.8 4- 0.1
0-2
TABLE II.
Histological changes in granulomas around mature eggs of S. mansoni in the liver of mouse.
Weeks after infection
No. animals
Granuloma size
Peripheral delimitation
Predominant cell type
1
Necrosis around ova
10
large
very poor
eosinophils
+q-q-
13
large
poor
eosinophils
+±+
15
medium
good
histiocytes
++
good
histiocytes fibroblasts I
good
fibroblasts histiocytes
20
3
small
25
6
small
÷
1
By the 15th week after exposure to cercariae a marked decrease in numbers of eosinophils and, consequently, a better peripheral delimitation of the granulomas occurred. This also resulted in a considerable decrease in the size of the granulomatous lesions. Necrosis around mature eggs was seen up t o 20 weeks after infection. No necrotic areas were seen in mice examined at 25 weeks. T h e granuloma around mature eggs showed variation in size and appearance in different animals and in the same animal up to the 15th week after infection. F r o m then ,On, the granulomas were remarkably alike, being small, rounded, well-delimited, and composed of histiocytes and fibroblasts concentrically arranged around a mature egg. F r o m the 15th week on, some granulomas surrounding older disintegrating eggs, that were undergoing resorption, showed advanced fibrotic changes. With time these granulomas presented marked regression in size. T h e y were discriminated from those around mature eggs and were not considered for comparative purposes in the present study.
FIG. 1. 10 weeks after infection. Large granulomas showing a necrotic central area and many eosinophils about the peripheri. H.E. x 150.
2. 20 weeks after infection. Granuloma around mature egg is small and composed predominantly of histiocytes and fibroblasts and has a peripheral lymphocytic infiltration. Older granulomas are undergoing scar transformation. H.E. x 120. FIG.
3. 25 weeks after infection. There is only a small cellular accumulation around a mature At left, an old fibrotic egg. granuloma is undergoing resorption. H.E. x 120.
FIG.
FIG. 4. 10 weeks after infection. A predominantly eosinophilic infiltration is present in the portal space. H.E. x 120.
FIG. 5. 20 weeks after infection. Infiltration in the portal space has a focal distribution and is composed mainly of histiocytes and Iymphocytes. Portal -fibrosis is apparent. H.E. x 120.
FIG. 6. 25 weeks after infection. Fibrosis of the portal space, with vascular dilatation, focal cellular accumulation, phlebosclerosis, and focal bile duct proliferation. Note resemblance to human “ pipestem” fibrosis. H.E. x 120.
55
Z I L T O N A. ANDRADE AND K E N N E T H S. WARREN
The changes in the portal areas refer to those aside from the granulomatous reactions around ova (Table III). During the schistosome infection, from the 10th week on, the portal spaces exhibited a marked inflammatory infiltration which changed, cytologically and in its pattern of distribution, with time. Later, a diffuse fibrosis resembling the "clay pipe-stem " t y p e seen in human beings, appeared. By 25 weeks after infection, the portal spaces disclosed diffuse fibrosis, phlebosclerosis, focal bile duct proliferation, vascular dilatation and focal cellular accumulations. TABLE I I I .
Periportal changes (not directly due to granu!omas) in the livers of mice infected with S. mansoni.
Weeks after infection
Cellular infiltration
No,
Fibrosis Oedema
Vessel
animals Distribution
Predominant cell type
10
diffuse
eosinophils
no changes
13
diffuse
eosinophils
no changes
15
diffuse and focal
e0sinophils lymphocytes
no changes
focal accumulations
lymphocytes histiocytes
dilated
focal accumulations
histiocytes lymphocytes plasmocytes
20 25
numerous dilated phlebosclerosis
Lesions in the hepatic parenchyma, consisting of focal liver cell necrosis, were seen up to the 15th week and were absent thereafter. Kupffer cell mobilization was also prominent, but became progressively less evident after the 20th week of infection. A brownish pigment and a PAS positive material was sometimes seen in the cytoplasm of these cells.
COMMENTS
A host harbouring a parasite can be harmed quantitatively in two ways : by the number of pathological stimuli and by the degree of response to each of those stimuli. Studies in experimental schistosomiasis in the past have concentrated on the former by investigating the ability of the animal to limit a challenging infection. Not only have decreased numbers of worms and stunted worms been noticed on repeated infections (NAIMARKet al., 1960 ; OLIVIER and SCHNEIDERMANN,1953 ; STIREWALT,1953) ; or in animals injected with antigens (SADUN and LIN, 1959) ; (WATTS, 1949) ; but marked inflammatory reactions around migrating schistosomules in " immunized " animals have been observed (MACALHAES FILHO and BA~ROS COELHO, 1957), Some abnormal hosts have also been found to have similar types of responses on initial exposure to S. mansoni (CouTINHO-ABATHand JAMPOLSKY, 1957 ; LICHTENBERCet al., 1962).
~b
MILD PROLONGED SCHISTOSOMIASIS IN NIICE
It is possible that the host response to the pathological elements of the parasite might also change with time or with previous infection, but few studies have been performed in this area. GONNERT(1955) has shown in mice that after treatment, when no eggs enter the liver, the granulomatous lesions shrink dramatically with time. WARREN(1962) confirmed these results and found that signs of se~/ere liver damage, such as hepato-splenomegaly and eosophageal varices also disappear as the lesions decrease in size. WARREN(in press) also found that mice harbouring low worm burdens can tolerate much greater numbers of eggs per g. of liver, than can heavily infected mice, without severe consequences ; the eggs enter the liver over a long period of time, and the initial lesions show a marked decrease in size by the time large numbers of eggs accumulate. It has also been said that if a monkey is immunized or its resistance raised it can stand more infection or a higher worm load (EL HALAWANI, 1962). In the present study it has been shown that a host that initially has a severe exudafive hypersensitivity type of reaction, can, with time, develop a far more economical, immune type of response to schistosome eggs. It thus seems that although a host can protect itself by limiting the number of pathological stimuli (fewer worms, fewer eggs) it can also achieve a significant degree of protection by having or developing a simpler and more efficient tissue response to these stimuli. These two aspects of acquired immunity to schistosomiasis, a decrease in the numbers of pathological stimuli, and a diminished host response to those stimuli, now appear to resemble closely the natural immunity as it appears in the group of " poorly susceptible " animals described by LICHTEI~BERGet al. (1962). They found a pattern consisting of few worms, stunted worms, poor egg production and viability, and small size and intensity of granulomatous response in the host. The factor or factors involved in the immune response to schistosomiasis are not clear. Circulating antibodies are said not to be important (SMITHERS, 1962). The morphological resemblance of the changes studied to tuberculosis, in which an exudative destructive reaction in a susceptible, unprotected host changes into a small proliferative lesion after immunity is established, is striking. This would suggest that cellular rather than humoral factors are of prime importance. Changes in the portal areas of these mice with prolonged mild schistosome infections resemble those of early Symmers' clay-pipe-stem fibrosis (SYMMERS,1904). There appears to be a relation between the development of this portal fibrosis and the severe portal hepatitis which is invariably seen, and is independent of the number and the degree of severity of the granulomas in the liver. The fibrosis develops progressively and could be the result of prolonged irritation caused by inflammation, or both the inflammation and fibrosis could represent a response to a diffuse, mild irritant in the portal area. The degree of inflammation has long been known to be out of proportion to the parasitic elements in the liver (FAIRLEY, t920), and it has been suggested that it represents a delayed-type hypersensitivity reaction to schistosomal elements or to liver cell breakdown products or to both (ANDRADE, 1962). It must be noted that the inflammatory changes in the portal areas, though altered, are still prominent at 20 and 25 weeks following infection when the granulomatous response around mature eggs has diminished markedly. Further studies are necessary to determine whether this inflammation and fibrosis are a residuum of the early changes and will disappear, or are a continuing and developing response to the infection. In conclusion, the mouse has been found to develop a syndrome similar to hepatosplenic schistosomiasis in human beings (DE WITT and W A R R E N , 1959). This syndrome will appear in animals with as few as one or two pairs of worms, over a prolonged period
ZILTON A. ANDRADEAND KENNETH S. WARREN
57
of time (WARREN, in press). In such chronic mild infections the host appears t o be able to protect itself by two mechanisms ; a gradual resorption of granulomas around dead eggs and a more effective and economical response to new eggs. SUMMARY
Mice with mild prolonged infection with Schistosoma mansoni showed a considerable variation in the appearance of the hepatic granulomas appearing around mature eggs in the course of infection. An earlier large, necrotic, exudative-proliferative lesion turned later into small, rounded granulomas composed of a few histiocytes and fibroblasts. This was interpreted as morphological expression of hypersensitivity and immunity developing in the course of prolonged infection. Gradual resorption of older granulomas and a more effective and economical response to new eggs were the principal mechanisms by which the host protected itself more efficiently. T h e mild prolonged infection was accompanied by portal inflammation and later a]so by fibrosis, which was similar to the early " pipe-stem " fibrosis as seen in human beings. Whether this was a late sequela of earlier changes or a progressive lesion due to the infection was not determined. REFERENCES ANDRADE, Z. A. (1962). 0 Hospital, 62, 17]. COUTINHO-ABATH, E. & JAMPOLSKY, R. (1957).
Ann. Soc. Biol. Pernambuco, 15, 125. DE WITT, W. B. & WARREN, K. S. (1959). Amer. J. Trop. Med. Hyg., 8, 440. EL HALAWANI, A. E., (1962). Ciba Foundation Symposium, Bilharziasis. London : Churchill, p. 71. FAIRLEY, N. H. (1920). 07. Path. Bact., 23, 289. GONNERT, R. (1955). Z. Tropenmed. u. Parasit., 6, 33. LICHTENBERG,F., SADUN, E. a . & BRUCE, J. I. (1962). Amer. 07. trop. Med. Hyg., 11, 347. MAGALHAESFILHO, A. & BARROSCOELHO,R. (1957). Ann. Soc. Biol. Pernambuco, 18, 269. NAIMARK, D. H., BENESON,A. S., OLIVER-GONZALES,J,, McMULLER, D. B. & RITCHIE,L. S. (1960). Amer. J. trop. Med. Hyg., 9, 430. OLIVlER, L. & SClqNEIDERMANN,M. (1953). Ibid., 2, 298. - & STIREWALT,M. A. (1952). J. Parasit., 38, 19. PRATA, A. (1957). Thesis. 197. Salvador-Bahia. RICH, A. (1947). Harvey Lect., 42, 106. SADUN, E. H. & LINN, S. S. (1959). J. Parasit., 45, 543. SMITHERS, S. R. (1962). Ciba Foundation Symposium, Bilharziasis. London: Churchill, 239. STIREWALT, M. A. (1953). Amer. J. Trop. Med. Hyg., 2, 867. SYMMERS, W. ST. C. (1904). J. Path. Bact., 9, 237. WARREN, K . S . Amer. J. trop. Med. Hyg. (in press). - (1962). Trans. R. Soc. trop. Med. Hyg., 56, 510. WATTS, N. P. (1949). J. Immunol., 62, 183. YOLLES, T. K., MOORE, D. V., DE GIUSTI, D. L., RIPSOM, C. A. (1947). ~J. Parasit., 33, 419.
MILD
PROLONGED
SCHISTOSOMIASIS
IN MICE:
ALTERATIONS IN HOST RESPONSE WITH TIME AND THE DEVELOPMENT OF PORTAL FIBROSIS BY
Z I L T O N A. ANDRADE AND K E N N E T H S. WARREN
(From Hospital das Clinicas, University of Bahia, Brazil). During studies of schistosomiasis by one of us (K.S.W.) it was observed that mice with low w o r m burdens could accumulate, over a long period of time, large n u m b e r s of eggs in their livers with relatively mild consequences (WARREN, in press). T h e s e animals, killed late in the course of infection, presented small, well-delimited granulomas scattered throughout the liver tissue. M a n y of these lesions were surrounding e m p t y egg shells, and were shrunken and fibrotic ; but some of t h e m were associated with fully mature eggs, and these granulomas consisted of slight fibroblastic proliferation and a few histiocytes. T h i s was in sharp contrast to the large exudative, necrotic reactions developed around mature eggs in the liver of animals killed earlier. I t seemed, therefore, that we m a y have been observing a true i m m u n e reaction to schistosome ova late in the course of a light infection, as compared to the enhanced and destructive inflammation of hypersensitivity seen earlier and frequently observed in heavy infections. F r o m his studies on tuberculosis RICH (1947) has developed the concept that " the i m m u n e state enhances, so greatly, the protective power of the inflammatory mechanism that, far f r o m requiring the exaggerated and accelerated inflammation of hypersensitivity, an astonishingly small a m o u n t of inflammation suffices for the effective protection of the b o d y . " Since it seemed possible that the observed differences in granuloma formation at different ages of infection might indicate morphologically the change f r o m a hypersensitive to an i m m u n e type of inflammatory reaction, we decided to investigate the histological details in the liver of mice with prolonged infections. MATERIALS AND METHODS Eighty-four adult Swiss albino female mice maintained on a conventional pellet diet were exposed to cercariae (about 28 for each mouse) of a Puerto Rican strain of S. mansoni according to the method of OLIVIERand STIREWALT(1952). Seven weeks following infection, adult worms were recovered from the portal and mesenteric veins by a modification of the perfusion method of YOLLES et al. (1947). Perfusions were also performed at 25 weeks after infection. Two mice were killed at 10, 13 and 15 weeks, three mice at 20 weeks and six mice at 25 weeks following infection. Pathophysiological changes in these and in the remaining mice are reported elsewhere (WARREN, in press). Fragments of liver were placed in buffered formalin and the paraffin sections were submitted to the following staining techniques : hemotoxylin-eosin, Mallory's blue anilin for connective tissue, Gomori's silver impregnation for reticulum and McManus-Hotchkiss' PAS method. RESULTS T a b l e I demonstrates the w o r m burdens found in the mice infected with 28 cercariae of S. mamoni. Histological changes referring to the hepatic granulomas around mature eggs of S. mansoni and to non-granulomatous changes about the portal areas are shown in
54
MILD
PROLONGED
SCHISTOSOMIASIS IN MICE
Tables II and III, respectively. Only granulomas around mature eggs were considered. T h e mature egg was identified according to PRATA (1957) as o n e having a thick, linear shell outline and an eosinophilic miracidium. Inside the miracidium there were several basophilic granules, some of them arranged centrally, forming a rosette-like structure. T h e miracidium has a life span of 2.5 to 3 weeks (GONNERT, 1955), and an egg with a fully mature miracidium lasts for 11 to 12 days (PRATA, 1957). By the 10th week after infection there was infiltration by a considerable n u m b e r of eosinophils, both around the granulomas and about, the portal spaces. T h e eosinophilic infiltrate varied in density and caused the periphery of the granulomas to appear ragged. In the central necrotic area near the egg, eosinophils also accumulated and showed signs of disintegration (Table II). TABLE I.
Worms recovered by perfusion at 7 and 25 weeks following exposure to 28 cercariae of S. mansoni
Weeks after infection
No. animals
Average worms
Average worm pairs
Range of pairs
7
10
5.7 ± 0.8
2.2 ± 0.3
1-4
25
5
2.4 ± 0.6
0.8 4- 0.1
0-2
TABLE II.
Histological changes in granulomas around mature eggs of S. mansoni in the liver of mouse.
Weeks after infection
No. animals
Granuloma size
Peripheral delimitation
Predominant cell type
1
Necrosis around ova
10
large
very poor
eosinophils
+q-q-
13
large
poor
eosinophils
+±+
15
medium
good
histiocytes
++
good
histiocytes fibroblasts I
good
fibroblasts histiocytes
20
3
small
25
6
small
÷
1
By the 15th week after exposure to cercariae a marked decrease in numbers of eosinophils and, consequently, a better peripheral delimitation of the granulomas occurred. This also resulted in a considerable decrease in the size of the granulomatous lesions. Necrosis around mature eggs was seen up t o 20 weeks after infection. No necrotic areas were seen in mice examined at 25 weeks. T h e granuloma around mature eggs showed variation in size and appearance in different animals and in the same animal up to the 15th week after infection. F r o m then ,On, the granulomas were remarkably alike, being small, rounded, well-delimited, and composed of histiocytes and fibroblasts concentrically arranged around a mature egg. F r o m the 15th week on, some granulomas surrounding older disintegrating eggs, that were undergoing resorption, showed advanced fibrotic changes. With time these granulomas presented marked regression in size. T h e y were discriminated from those around mature eggs and were not considered for comparative purposes in the present study.
FIG. 1. 10 weeks after infection. Large granulomas showing a necrotic central area and many eosinophils about the peripheri. H.E. x 150.
2. 20 weeks after infection. Granuloma around mature egg is small and composed predominantly of histiocytes and fibroblasts and has a peripheral lymphocytic infiltration. Older granulomas are undergoing scar transformation. H.E. x 120. FIG.
3. 25 weeks after infection. There is only a small cellular accumulation around a mature At left, an old fibrotic egg. granuloma is undergoing resorption. H.E. x 120.
FIG.
FIG. 4. 10 weeks after infection. A predominantly eosinophilic infiltration is present in the portal space. H.E. x 120.
FIG. 5. 20 weeks after infection. Infiltration in the portal space has a focal distribution and is composed mainly of histiocytes and Iymphocytes. Portal -fibrosis is apparent. H.E. x 120.
FIG. 6. 25 weeks after infection. Fibrosis of the portal space, with vascular dilatation, focal cellular accumulation, phlebosclerosis, and focal bile duct proliferation. Note resemblance to human “ pipestem” fibrosis. H.E. x 120.
55
Z I L T O N A. ANDRADE AND K E N N E T H S. WARREN
The changes in the portal areas refer to those aside from the granulomatous reactions around ova (Table III). During the schistosome infection, from the 10th week on, the portal spaces exhibited a marked inflammatory infiltration which changed, cytologically and in its pattern of distribution, with time. Later, a diffuse fibrosis resembling the "clay pipe-stem " t y p e seen in human beings, appeared. By 25 weeks after infection, the portal spaces disclosed diffuse fibrosis, phlebosclerosis, focal bile duct proliferation, vascular dilatation and focal cellular accumulations. TABLE I I I .
Periportal changes (not directly due to granu!omas) in the livers of mice infected with S. mansoni.
Weeks after infection
Cellular infiltration
No,
Fibrosis Oedema
Vessel
animals Distribution
Predominant cell type
10
diffuse
eosinophils
no changes
13
diffuse
eosinophils
no changes
15
diffuse and focal
e0sinophils lymphocytes
no changes
focal accumulations
lymphocytes histiocytes
dilated
focal accumulations
histiocytes lymphocytes plasmocytes
20 25
numerous dilated phlebosclerosis
Lesions in the hepatic parenchyma, consisting of focal liver cell necrosis, were seen up to the 15th week and were absent thereafter. Kupffer cell mobilization was also prominent, but became progressively less evident after the 20th week of infection. A brownish pigment and a PAS positive material was sometimes seen in the cytoplasm of these cells.
COMMENTS
A host harbouring a parasite can be harmed quantitatively in two ways : by the number of pathological stimuli and by the degree of response to each of those stimuli. Studies in experimental schistosomiasis in the past have concentrated on the former by investigating the ability of the animal to limit a challenging infection. Not only have decreased numbers of worms and stunted worms been noticed on repeated infections (NAIMARKet al., 1960 ; OLIVIER and SCHNEIDERMANN,1953 ; STIREWALT,1953) ; or in animals injected with antigens (SADUN and LIN, 1959) ; (WATTS, 1949) ; but marked inflammatory reactions around migrating schistosomules in " immunized " animals have been observed (MACALHAES FILHO and BA~ROS COELHO, 1957), Some abnormal hosts have also been found to have similar types of responses on initial exposure to S. mansoni (CouTINHO-ABATHand JAMPOLSKY, 1957 ; LICHTENBERCet al., 1962).
~b
MILD PROLONGED SCHISTOSOMIASIS IN NIICE
It is possible that the host response to the pathological elements of the parasite might also change with time or with previous infection, but few studies have been performed in this area. GONNERT(1955) has shown in mice that after treatment, when no eggs enter the liver, the granulomatous lesions shrink dramatically with time. WARREN(1962) confirmed these results and found that signs of se~/ere liver damage, such as hepato-splenomegaly and eosophageal varices also disappear as the lesions decrease in size. WARREN(in press) also found that mice harbouring low worm burdens can tolerate much greater numbers of eggs per g. of liver, than can heavily infected mice, without severe consequences ; the eggs enter the liver over a long period of time, and the initial lesions show a marked decrease in size by the time large numbers of eggs accumulate. It has also been said that if a monkey is immunized or its resistance raised it can stand more infection or a higher worm load (EL HALAWANI, 1962). In the present study it has been shown that a host that initially has a severe exudafive hypersensitivity type of reaction, can, with time, develop a far more economical, immune type of response to schistosome eggs. It thus seems that although a host can protect itself by limiting the number of pathological stimuli (fewer worms, fewer eggs) it can also achieve a significant degree of protection by having or developing a simpler and more efficient tissue response to these stimuli. These two aspects of acquired immunity to schistosomiasis, a decrease in the numbers of pathological stimuli, and a diminished host response to those stimuli, now appear to resemble closely the natural immunity as it appears in the group of " poorly susceptible " animals described by LICHTEI~BERGet al. (1962). They found a pattern consisting of few worms, stunted worms, poor egg production and viability, and small size and intensity of granulomatous response in the host. The factor or factors involved in the immune response to schistosomiasis are not clear. Circulating antibodies are said not to be important (SMITHERS, 1962). The morphological resemblance of the changes studied to tuberculosis, in which an exudative destructive reaction in a susceptible, unprotected host changes into a small proliferative lesion after immunity is established, is striking. This would suggest that cellular rather than humoral factors are of prime importance. Changes in the portal areas of these mice with prolonged mild schistosome infections resemble those of early Symmers' clay-pipe-stem fibrosis (SYMMERS,1904). There appears to be a relation between the development of this portal fibrosis and the severe portal hepatitis which is invariably seen, and is independent of the number and the degree of severity of the granulomas in the liver. The fibrosis develops progressively and could be the result of prolonged irritation caused by inflammation, or both the inflammation and fibrosis could represent a response to a diffuse, mild irritant in the portal area. The degree of inflammation has long been known to be out of proportion to the parasitic elements in the liver (FAIRLEY, t920), and it has been suggested that it represents a delayed-type hypersensitivity reaction to schistosomal elements or to liver cell breakdown products or to both (ANDRADE, 1962). It must be noted that the inflammatory changes in the portal areas, though altered, are still prominent at 20 and 25 weeks following infection when the granulomatous response around mature eggs has diminished markedly. Further studies are necessary to determine whether this inflammation and fibrosis are a residuum of the early changes and will disappear, or are a continuing and developing response to the infection. In conclusion, the mouse has been found to develop a syndrome similar to hepatosplenic schistosomiasis in human beings (DE WITT and W A R R E N , 1959). This syndrome will appear in animals with as few as one or two pairs of worms, over a prolonged period
ZILTON A. ANDRADEAND KENNETH S. WARREN
57
of time (WARREN, in press). In such chronic mild infections the host appears t o be able to protect itself by two mechanisms ; a gradual resorption of granulomas around dead eggs and a more effective and economical response to new eggs. SUMMARY
Mice with mild prolonged infection with Schistosoma mansoni showed a considerable variation in the appearance of the hepatic granulomas appearing around mature eggs in the course of infection. An earlier large, necrotic, exudative-proliferative lesion turned later into small, rounded granulomas composed of a few histiocytes and fibroblasts. This was interpreted as morphological expression of hypersensitivity and immunity developing in the course of prolonged infection. Gradual resorption of older granulomas and a more effective and economical response to new eggs were the principal mechanisms by which the host protected itself more efficiently. T h e mild prolonged infection was accompanied by portal inflammation and later a]so by fibrosis, which was similar to the early " pipe-stem " fibrosis as seen in human beings. Whether this was a late sequela of earlier changes or a progressive lesion due to the infection was not determined. REFERENCES ANDRADE, Z. A. (1962). 0 Hospital, 62, 17]. COUTINHO-ABATH, E. & JAMPOLSKY, R. (1957).
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