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The Relative Susceptibility of Chickens of Different Ages to Coccidiosis Caused by Eimeria Necatrix
T h e Relative Susceptibility of Chickens of Different Ages to Coccidiosis Caused by Eimeria Necatrix STERLING BRACKETT AND ALEXANDER BLIZNICK Chemotherapy Division, Stamford Research Laboratories, American Cyanamid Company, Stamford, Connecticut (Received for publication May 4, 1951)
HERE seems to be some feeling that young chickens are less susceptible than older chickens to infections with Eimeria necatrix. This evidently stems from the studies of Tyzzer, Theiler and Jones (1932) in which they report an experiment in which 7 of 8 birds 35 days old died of experimental infections while only 1 of 7 birds 8 days old died of the infection. The extended discussion of possible factors involved in this matter is excellent and complete and will not be repeated here. In this experiment the birds of different ages were permitted to eat uniformly infected feed for four hours after fasting overnight. This would seem to regulate the oocyst dosage by the size (volume) of the digestive tract. Obviously, the older or larger chickens ingested a larger number of oocysts than the smaller chickens, which may account for the results obtained. Since, as Tyzzer et al. (1932) point out, there are other possible explanations for their observed results, but especially because the data upon which the speculations were made were extremely limited, we ran several experiments in an attempt to clarify this point. Though our data too are limited, they are presented here since we feel that they show that young birds may be as susceptible to E. necatrix as are older birds, or at least, based on our standards,
they are highly susceptible tq this infection. In a preliminary experiment 20 birds, 5 days old, were each inoculated with 50,000 oocysts of E. necatrix.1 Only 4 older birds2 (3 months old) were available for comparison, and these were given 50,000 oocysts also. None of the four older birds died of the infection, while 7 of the 20 younger birds died with characteristic lesions of E. necatrix on the 6th to the 10th days after inoculation. In a second experiment graded doses of oocysts were given to younger birds (10 days old), while a large dose was given to older birds (about 10 to 12 weeks of age; in the table of data the relative age of the birds is indicated by body weight). Data from this experiment are condensed in Table 1. From these data it is certainly evident that young birds are far from being resist1 This strain of E. necatrix was originally obtained from Dr. E. H. Peterson, University of Washington, in April 1949, to whom we wish to express our appreciation. The strain was since "purified" in respect to E. tenella several times by rectal passage of merozoites from intestinal scrapings of birds 5 days after inoculation to clean birds from the ceca of which oocysts were obtained 7 days after the original oocyst inoculation (i.e. 2 days after the latter received merozoites rectally). 2 All birds were kept in a separate isolation room until they were ready for inoculation, at which time they were removed to another experimental room. These rooms are several hundred feet apart and precautions were taken not to introduce infections into the isolation room.
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T
AGE AND SUSCEPTIBILITY TO E. NECATRIX TABLE 1.—The pathogenic effect of E. necatrix in young and older chickens No. of Oocysts per No. Av. wt. oocysts at Group of outset inoculated gram of birds per body (g.) bird weight
1 2 3 4 5
20 20 20 20 23
70 69 71 71 1,235
100,000 50,000 20,000 10,000 100,000
1,430
715 300 150 80
% mortality
100 75 20 15 26
sites. Some light is shed on this latter point in an experiment with mild infections where oocyst production was determined. The details of the oocyst counting procedures used here have been described in an earlier paper (Brackett and Bliznick, 1949). The results of this test are summarized in Table 2. (Five birds were used in each group.) These data suggest that, if anything, a smaller proportion of the parasites inoculated develop in the older birds than in the younger birds, although the differences are not great and could not be claimed to be significant. DISCUSSION
The conclusions of Tyzzer et al. (1932) that young birds are less susceptible than older birds to E. necatrix is based on three observations. The first is referred to in a general way and no indication is given of the number of birds used, the size of the inoculum other than calling it "massive," or the exact results. In the second test, which is called a preliminary test, five 5day old chicks and three 67-day old chicks were each given oocysts in varying amounts. The lesions were counted, but no correlation was found between the number of oocysts fed and the resulting number of lesions. In a final test, which has already been referred to, 7 young and 14 older birds were used. In view of the
TABLE 2.—The oocyst production of mild infections with E. necatrix in young and older chickens
Group
Av. number oocysts (in millions) per bird
Age of birds at outset
No. oocysts inoculated
9 days
In feces 7-9th days
In ceca on 9th day
Total
200
3.31
2.89
6.20
9 days
200
3.42
0.69
4.11
2 mos.
200
1.56
0.26
1.82
2 mos.
200
1.95
1.26
3.21
Av.
Approximate oocyst production per oocyst inoculated
5.15
26,000
2.51
12,500
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ant to this infection, since some died after receiving as few as 10,000 oocysts and a large proportion died following inoculation with 50,000 oocysts. Under similar conditions, in our experience, E. tenella is not more and may be somewhat less pathogenic. With equal doses, on an absolute basis, in the foregoing experiment (100,000 oocysts), the older birds were less seriously affected than the younger birds. On a relative basis, the older birds might be somewhat less resistant since 80 oocysts per gram of body weight resulted in 26% mortality, while about 300 ocysts per gram of body weight were required to produce about the same degree of mortality in the younger birds. In using mortality as an end point, any differences that occur may be due to differences in resistance to the pathogenic effects of an infection and may not necessarily, indicate the relative resistance to the invasion or multiplication of the para-
147
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least, the older birds may be more resistant to the parasites than younger birds. The results with the more intense infections (Table 1), where it would appear that on an oocyst per gram of body weight basis, the older birds are more susceptible to the infection, are in contrast, and we can offer no explanation. Perhaps with these inoculations with larger numbers of oocysts, one or more of the suggestions of Tyzzer et al. come into play. CONCLUSIONS The following conclusions regarding E. necatrix seem to be in order. (1) Relatively small numbers of oocysts (25-50 thousand) will cause a high degree of mortality in young chickens. (2) Following the ingestion of equal numbers of oocysts, young birds will be more severely affected than older birds. (3) When an inoculation is given on an oocyst per gram of body weight basis, older birds may be more severely affected than younger birds. Under some conditions, small chickens may feed on contaminated material with little or no mortality resulting while older birds feeding on the same material may suffer considerable mortality, presumably because they ingested a good many more oocysts. This, of course, is of practical significance to the poultrymen because, under certain conditions where he transfers previously unexposed chickens to contaminated quarters he may encounter more difficulty with older than with younger chickens. REFERENCES Brackett, S., and A. Bliznick, 1949. The effect of small doses of drugs on oocyst production of infections with Eimeria tenella. N. Y. Acad. Sci. 52: 595-610. Tyzzer, E. E., H. Theiler and E. J. Jones, 1932. Coccidiosis in gallinaceous birds. II. A comparative study of species of Eimeria of the chicken. Am. J. Hyg. 15:319-393.
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small number of birds in the three tests and the irregular results seen. in the second test, we do not feel that it was completely established that younger birds are more resistant. There would seem to be two points to establish, namely (1) whether young birds are susceptible to E. necatrix infections and (2) the relative susceptibility of younger and older birds. Our results (Table 1) indicate that young birds are highly susceptible to this infection. The failure of Tyzzer et al. to produce mortality in younger birds can only be explained by assuming that these birds did not ingest enough viable oocysts to produce fatalities. On the other hand, the larger birds which unquestionably must have ingested much more of the experimentally contaminated feed, because of their larger digestive tract, received enough viable oocysts to produce a high degree of mortality. Tyzzer et al. (1932) attached more significance to the possible influence of the following factors in explaining the higher mortality in the older birds. (1) The greater length of the intestine of larger birds might increase the opportunity for the sporozoites to penetrate. (2) The rate of passage of material through the gut might be slower in larger birds, thus improving the opportunity for sporozoite penetration. (3) There might be differences in the ratio between the area and the volume of the intestine of different aged birds. (4) Differences between the size of the villi which would favor sporozoite penetration in older birds. (5) And finally physiological conditions might be more favorable for sporozoite penetration in older birds. Our studies with mild infections, using oocyst production as the evidence of the degree of parasite development (Table 2), indicate that under these conditions, at
HERE seems to be some feeling that young chickens are less susceptible than older chickens to infections with Eimeria necatrix. This evidently stems from the studies of Tyzzer, Theiler and Jones (1932) in which they report an experiment in which 7 of 8 birds 35 days old died of experimental infections while only 1 of 7 birds 8 days old died of the infection. The extended discussion of possible factors involved in this matter is excellent and complete and will not be repeated here. In this experiment the birds of different ages were permitted to eat uniformly infected feed for four hours after fasting overnight. This would seem to regulate the oocyst dosage by the size (volume) of the digestive tract. Obviously, the older or larger chickens ingested a larger number of oocysts than the smaller chickens, which may account for the results obtained. Since, as Tyzzer et al. (1932) point out, there are other possible explanations for their observed results, but especially because the data upon which the speculations were made were extremely limited, we ran several experiments in an attempt to clarify this point. Though our data too are limited, they are presented here since we feel that they show that young birds may be as susceptible to E. necatrix as are older birds, or at least, based on our standards,
they are highly susceptible tq this infection. In a preliminary experiment 20 birds, 5 days old, were each inoculated with 50,000 oocysts of E. necatrix.1 Only 4 older birds2 (3 months old) were available for comparison, and these were given 50,000 oocysts also. None of the four older birds died of the infection, while 7 of the 20 younger birds died with characteristic lesions of E. necatrix on the 6th to the 10th days after inoculation. In a second experiment graded doses of oocysts were given to younger birds (10 days old), while a large dose was given to older birds (about 10 to 12 weeks of age; in the table of data the relative age of the birds is indicated by body weight). Data from this experiment are condensed in Table 1. From these data it is certainly evident that young birds are far from being resist1 This strain of E. necatrix was originally obtained from Dr. E. H. Peterson, University of Washington, in April 1949, to whom we wish to express our appreciation. The strain was since "purified" in respect to E. tenella several times by rectal passage of merozoites from intestinal scrapings of birds 5 days after inoculation to clean birds from the ceca of which oocysts were obtained 7 days after the original oocyst inoculation (i.e. 2 days after the latter received merozoites rectally). 2 All birds were kept in a separate isolation room until they were ready for inoculation, at which time they were removed to another experimental room. These rooms are several hundred feet apart and precautions were taken not to introduce infections into the isolation room.
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T
AGE AND SUSCEPTIBILITY TO E. NECATRIX TABLE 1.—The pathogenic effect of E. necatrix in young and older chickens No. of Oocysts per No. Av. wt. oocysts at Group of outset inoculated gram of birds per body (g.) bird weight
1 2 3 4 5
20 20 20 20 23
70 69 71 71 1,235
100,000 50,000 20,000 10,000 100,000
1,430
715 300 150 80
% mortality
100 75 20 15 26
sites. Some light is shed on this latter point in an experiment with mild infections where oocyst production was determined. The details of the oocyst counting procedures used here have been described in an earlier paper (Brackett and Bliznick, 1949). The results of this test are summarized in Table 2. (Five birds were used in each group.) These data suggest that, if anything, a smaller proportion of the parasites inoculated develop in the older birds than in the younger birds, although the differences are not great and could not be claimed to be significant. DISCUSSION
The conclusions of Tyzzer et al. (1932) that young birds are less susceptible than older birds to E. necatrix is based on three observations. The first is referred to in a general way and no indication is given of the number of birds used, the size of the inoculum other than calling it "massive," or the exact results. In the second test, which is called a preliminary test, five 5day old chicks and three 67-day old chicks were each given oocysts in varying amounts. The lesions were counted, but no correlation was found between the number of oocysts fed and the resulting number of lesions. In a final test, which has already been referred to, 7 young and 14 older birds were used. In view of the
TABLE 2.—The oocyst production of mild infections with E. necatrix in young and older chickens
Group
Av. number oocysts (in millions) per bird
Age of birds at outset
No. oocysts inoculated
9 days
In feces 7-9th days
In ceca on 9th day
Total
200
3.31
2.89
6.20
9 days
200
3.42
0.69
4.11
2 mos.
200
1.56
0.26
1.82
2 mos.
200
1.95
1.26
3.21
Av.
Approximate oocyst production per oocyst inoculated
5.15
26,000
2.51
12,500
Downloaded from http://ps.oxfordjournals.org/ at UPVA on May 29, 2015
ant to this infection, since some died after receiving as few as 10,000 oocysts and a large proportion died following inoculation with 50,000 oocysts. Under similar conditions, in our experience, E. tenella is not more and may be somewhat less pathogenic. With equal doses, on an absolute basis, in the foregoing experiment (100,000 oocysts), the older birds were less seriously affected than the younger birds. On a relative basis, the older birds might be somewhat less resistant since 80 oocysts per gram of body weight resulted in 26% mortality, while about 300 ocysts per gram of body weight were required to produce about the same degree of mortality in the younger birds. In using mortality as an end point, any differences that occur may be due to differences in resistance to the pathogenic effects of an infection and may not necessarily, indicate the relative resistance to the invasion or multiplication of the para-
147
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least, the older birds may be more resistant to the parasites than younger birds. The results with the more intense infections (Table 1), where it would appear that on an oocyst per gram of body weight basis, the older birds are more susceptible to the infection, are in contrast, and we can offer no explanation. Perhaps with these inoculations with larger numbers of oocysts, one or more of the suggestions of Tyzzer et al. come into play. CONCLUSIONS The following conclusions regarding E. necatrix seem to be in order. (1) Relatively small numbers of oocysts (25-50 thousand) will cause a high degree of mortality in young chickens. (2) Following the ingestion of equal numbers of oocysts, young birds will be more severely affected than older birds. (3) When an inoculation is given on an oocyst per gram of body weight basis, older birds may be more severely affected than younger birds. Under some conditions, small chickens may feed on contaminated material with little or no mortality resulting while older birds feeding on the same material may suffer considerable mortality, presumably because they ingested a good many more oocysts. This, of course, is of practical significance to the poultrymen because, under certain conditions where he transfers previously unexposed chickens to contaminated quarters he may encounter more difficulty with older than with younger chickens. REFERENCES Brackett, S., and A. Bliznick, 1949. The effect of small doses of drugs on oocyst production of infections with Eimeria tenella. N. Y. Acad. Sci. 52: 595-610. Tyzzer, E. E., H. Theiler and E. J. Jones, 1932. Coccidiosis in gallinaceous birds. II. A comparative study of species of Eimeria of the chicken. Am. J. Hyg. 15:319-393.
Downloaded from http://ps.oxfordjournals.org/ at UPVA on May 29, 2015
small number of birds in the three tests and the irregular results seen. in the second test, we do not feel that it was completely established that younger birds are more resistant. There would seem to be two points to establish, namely (1) whether young birds are susceptible to E. necatrix infections and (2) the relative susceptibility of younger and older birds. Our results (Table 1) indicate that young birds are highly susceptible to this infection. The failure of Tyzzer et al. to produce mortality in younger birds can only be explained by assuming that these birds did not ingest enough viable oocysts to produce fatalities. On the other hand, the larger birds which unquestionably must have ingested much more of the experimentally contaminated feed, because of their larger digestive tract, received enough viable oocysts to produce a high degree of mortality. Tyzzer et al. (1932) attached more significance to the possible influence of the following factors in explaining the higher mortality in the older birds. (1) The greater length of the intestine of larger birds might increase the opportunity for the sporozoites to penetrate. (2) The rate of passage of material through the gut might be slower in larger birds, thus improving the opportunity for sporozoite penetration. (3) There might be differences in the ratio between the area and the volume of the intestine of different aged birds. (4) Differences between the size of the villi which would favor sporozoite penetration in older birds. (5) And finally physiological conditions might be more favorable for sporozoite penetration in older birds. Our studies with mild infections, using oocyst production as the evidence of the degree of parasite development (Table 2), indicate that under these conditions, at