The Prevalence of Serum Antibodies to Toxoplasma gondii in Ontario Mammals 1. R. Tizard, J. Harmeson and C. H. Lai* ABSTRACT The prevalence of seropositive reactions to Toxoplasma gondii was studied in farm animals, companion animals, wild rodents and birds. Of the animals tested, 17% of cattle, 65% of sheep, 45% of pigs, 9%7, of horses, 33% of dogs and 20% of cats were seropositive by the Sabin-Feldman dye test. In addition 11%7o of mice (Mus musculus), 5% of deer mice (Peromyscus maniculatus), 3% of rats (Rattus norvegicus) and less than 2% of sparrows (Passer domesticus) were seropositive. All samples from short-tailed field mice (Microtus pennsylvanicus), squirrels (Sciurus carolinensis), chipmunks (Tamias striatus), meadow jumping mice (Zapus hudsonius) and starlings (Sturnus vulgaris) were seronegative. The significance of these findings in relation to the epizootiology of toxoplasmosis in Ontario is discussed.

RESUMe Les auteurs ont recherche la predominance de reactions serologiques positives, a l'endroit de Toxoplasma gondii, chez des animaux domestiques, des animaux de compagnie, des rongeurs sauvages et des oiseaux. Parmi les animaux qu'ils eprouverent au cours de cette experience, 17% des bovins, 65% des moutons, 45%- des porcs, 9% des chevaux, 33% des chiens et 20% des chats reagirent de fagon positive 'a l'epreuve de Sabin-Feldman. De plus, 11% des souris (Mus musculus), 5% des souris sylvestres (Peromyscus maniculatus), 3% des rats (Rattus norvegicus) et moins de 2% des moineaux (Passer domesticus) donnierent des resultats positifs. Tous

*Toxoplasmosis

Diagnostic Laboratory, Department of

Veterinary Microbiology and Immunology, Ontario Veterinary College, University of Guelph, Guelph, On-

tario, Canada.

Submitted May 18, 1977.

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les echantillons des mulots (Microtus pennsylvanicus), des ecureuils (Sciurus carolinensis), des tamias (Tamias striatus), des souris sauteuses des champs (Zepus hudsonius) et des etourneaux (Sturnus vulgaris) s'avererent negatifs. Les auteurs commentent la signification de leurs resultats par rapport 'a l'epizootiologie de la toxoplasmose, en Ontario.

INTRODUCTION Toxoplasmosis is endemic in Ontario (3, 8, 22, 24). Approximately 40% of the human population possess serum antibodies to Toxoplwsna gondii when measured either by the indirect fluorescent antibody test (3) or by the Sabin-Feldman dye test (8, 24). Humans may acquire infection by at least three routes: by transplacental transfer of organisms from pregnant mothers to their fetus, through the consumption of raw or grossly undercooked meat or milk containing cysts, or by the accidental ingestion of oocysts shed in feces from infected cats (15, 18, 27). The congenital route of infection is probably the least important of these three routes in Ontario (22) but the relative importance of the other two is unclear. It is generally assumed that toxoplasmosis must be enzootic in the small rodent population which serves as a reservoir of infection for cats (27). Cats in turn are considered to act as sources of infection for both humans and domestic herbivores. Herbivores may then provide an alternative source of infection for humans through infected meat. This paper reports the results of a survey of sera taken from each of these groups of animals in order to provide basic data for studying the epizootiology of this condition in Southern Ontario. 177

MATERIALS AND METHODS SAMPLES

TESTING PROCEDURES All serum samples were stored frozen at -20°C until required. Immediately prior to testing they were inactivated by incubation at 56°C for 45 -minutes and then diluted to 1:16 in normal saline. Antibodies to T. gondii were detected by means of the Sabin-Feldman dye test (SFDT) (20). The test was conducted in a standard manner using human accessory factor and the RH strain of T. gondii. In view of published reports that birds although infected with this organism may remain seronegative (12, 26), attempts to isolate organisms were routinely undertaken on all birds. Brain, liver, heart and spleen were removed from each bird, pooled and homogenized in 5 ml normal saline. Each of five mice were inoculated intraperitoneally with 0.5 ml of this homogenate and examined at regular intervals. After two weeks an attempt was made to aspirate toxoplasms from their peritoneal cavities. Each animal was then exsanguinated, its serum was tested for antitoxoplasma antibodies by SFDT and its brain, liver and spleen examined histologically for the presence of Toxoplasma cysts.

Farm animal samples (cattle, horses, pigs and sheep) were obtained from five major sources (Table I). Samples from the Clinical Pathology Laboratory, Ontario Veterinary College (OVC) represented a "normal hospital population" but also included some samples from nonhospitalized animals. These samples had been taken for clinical reasons but not primarily for toxoplasmosis testing. Horse serum samples obtained from the Large Animal Clinic at OVC were from a "normal hospital population" but were taken for the purpose of this survey. Samples obtained through the courtesy of the Animal Pathology Laboratory, Agriculture Canada, Guelph were originally submitted for brucellosis serology and are representative of a population of normal animals. Samples from the Serology Laboratory of the Veterinary Services Branch (VSB), Ontario Ministry of Agriculture and Food, were submitted to that laboratory by veterinarians for routine serology and are probably comparable to the "normal hospital population" samples obtained from OVC. The bovine DATA ANALYSIS samples obtained at an abattoir in Guleph were from feedlot animals from various Data from each animal tested (date of locations in southwestern Ontario. testing, species, result, age, sex, source Companion animal samples were ob- and original location of specimen) were tained from some of the same sources cross tabulated and subjected to statistical (Table I). Blood samples taken from cats analysis using the SPSS 6 program on an and dogs at the Guelph Humane Society IBM 370/155 computer of the Institute of animal shelter are considered to be re- Computer Science at the University of presentative of a healthy but uncontrolled Guelph. animal population. Some cats encountered on routine farm visits were also sampled. Small rodents were livetrapped1 at nine locations within and around the city of Guelph. These locations included barns, RESULTS woodlots, fields, riverbanks, gardens and the city garbage dump. No more than ten rodents were trapped at any one location. Birds were caught with mist nets in a garden in suburban Guelph, at the Large CATLE Animal Clinic, OVC, at the Swine ExperOf the bovine samples tested 17% were imental Station of the Ontario Ministry of Agriculture and Food at Arkell, Ontario positive (Table II). The average age of and on a farm located approximately ten the animals (excluding the abattoir samples) was 4.02 years. In general, the miles southwest of Guelph. source of the samples appeared to have little effect, although 15% of the samples IHavahart traps. Havahart Inc., Ossining, New York. from the abattoir were positive as com-

178

Can. J. comp. Med.

TABLE I. The Sources and Numbers of Serum Samples Used in this Survey Source Bovine Equine Porcine Ovine Canine Feline A ....................... 75 1 4 197 42 B ..218 C ....................... 1120 152 20 2 32 34 D 518 267 E ....................... 564 F 5 63 G 1 13 Total ....................... 238 1759 671 273 235 152 A = Clinical Pathology Laboratory, Ontario Veterinary College, Guelph B = Large Animal Clinic, Ontario Veterinary College, Guelph C = Serology Laboratory, Veterinary Services Branch, Ontario Ministry of Agriculture and Food, Guelph D = Animal Pathology Laboratory, Agriculture Canada, Guelph E = A Guelph abattoir F = Guelph Humane Society G = Various farms TABLE II. The Prevalence of Antibodies to T. gondii in the Samples Tested

Species Meat Animals Sheep ............... Pigs ................ Cattle............... Companion Animals Horses .............. Cats ................ Dogs................

Small Rodents Mouse (Mus musculus) Rat (Ratius norvegicus) Short-tailed field mouse (Microtus pennsylvanicus) .......... Deer mouse (Peromyscus maniculatus)..... Squirrel (Sciurus carolinensis) ......... Chipmunk (Tamias striatus) ............ Meadow jumping mouse (Zapus hudsonius) ............ Birds House sparrow (Passer domesticus) ......... Common starling (Sturnus vulgaris) . . .

No Positive/No % Tested Positive

176/273 300/671 309/1759

65 45 17

21/238 30/152 111/335

9 20 33

4/36 1/35

11 3

0/2

0

1/19 0/16 0/6

5 0 0

0/2

1/71 0/28

bovine samples taken in February were positive while the percentage of seropositive reactions declined over the summer so that only 15 of 118 (12.7%) samples taken in November were positive.

PIGS AND SHEEP Positive reactions were obtained in 45% of the pig samples and 65 % of the sheep samples. There were no significant differences in prevalence between samples obtained from different sources and insufficient information was available to draw reliable conclusions on the influence of sex on these results. The average age of the pig population tested was 1.55 years. Information on the age of sheep was not available. HORSES

Equine samples had a considerably lower rate of infection than the other large domestic animals, only 9% being seropositive. The average age of this population was 5.7 years. DOGS

pared with 18-19 % in samples from the other sources. There were no significant differences in the prevalence of positive reactions between animals of different sex but age differences were marked. In animals whose age was known, the prevalence of positive reactions climbed from 15 % in animals one year of age to 37% in animals eight years of age or older. It was also observed that 30 out of 84 (36%)

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Of the dog sera tested 33% were positive. The average age of the population tested was 4.96 years. No differences in prevalence were observed between sexes nor between samples obtained from different sources. Of 13 one year old dogs, three (8%) were seropositive while 12 of 26 (46%) dogs eight or more years of age were positive.

179

CATS

WILD BIRDS

Of the cats tested 20% were seropositive. With regard to sources, 23 % of samples submitted by veterinarians (i.e. to OVC or VSB) were positive while 19% of samples from the humane society were positive. The average age of the population tested was 1.15 years and the incidence rate appeared to be approximately 20%/year (Fig. 1).

One male house sparrow, trapped on a farm ten miles south of Guelph, was found to be seropositive. Attempts to isolate the organism from this and other birds trap-

ped were uniformly negative.

DISCUSSION

SMALL RODENTS

The prevalence of positive reactions in When the SFDT was first introduced the rodents tested was relatively low as might be anticipated in animals with a (20) there was concern as to its specificity short lifespan. The highest prevalence was and it was eventually concluded that the found in house mice (Mus musculus) with test, as performed in humans, was essenfour of 36 (11%) animals tested positive. tially specific for T. gondii (7). It has These four mice were captured in barns recently been shown however, that the on two farms at which toxoplasmosis ap- closely related coccidian Hammondia hampeared to be highly prevalent. On farm mondi (10) will cross react in the dye A, two of four mice, one of two cats, four test (4). The test is believed not to detect of five calves and the one member of the antibodies to sarcocysts in either humans farm family tested were seropositive. On (2, 14) or sheep (1). It is therefore farm B, two of eight mice, neither of two probable that this test will detect antideer mice and 34 out of 52 cattle tested bodies only to T. gondii or to very closely were positive. Of these four house mice, related organisms (13) although the posone was male and three were females, two sibility of other unrecognized cross-reacwere juvenile while two were adult. The tivity cannot be excluded. The reported prevalence of seropositive one positive rat was a juvenile female trapped close to the Speed River in central reactions in cattle in North America has Guelph. No positive reactions were ob- ranged from zero to 49% (Table III) but the prevalence of 17% reported here is tained in any of the other rodents tested. relatively high for a herbivore. There are a number of possible reasons for this. Probably the most important of these is 3 the exposure of cattle to cats over the 3 . 100 winter months when housed indoors. Barn cats appear to prefer to defecate in granPercentage ular feed (I. R. Tizard, unpublished ob80 Positive servations) and it is likely that cattle can 2 2 3 3 acquire infection by consuming feed con0 * taminated in this way. The possible im60 portance of this route is supported by the extremely high prevalence of positive reactions in samples taken from cattle in 40 February. 10 The very high prevalence of positive re6 20 actions in sheep (65%) is with one exception (7) considerably higher than that reported from elsewhere in North America. This is in contrast to absence of sero0 .1 3 4 positive reactions in 102 sheep in SaskatAge (Years) chewan (17) but is almost identical to the Fig. 1. The relationship between age of cats and the prevalence of positive reactions to T. gondii. Each value represents the number positive over the number 63 % prevalence of positive reactions retested. corded in Ontario goats (23). This may -

-

-

-

2

0

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Can. J. comp. Med.

TABLE III. The Reported Prevalence (%) of Seropositive Reactions to T. gondii in Farm Animals in North America

Location Ontario.....................

Ontario..................... Saskatchewan ............... New York.................. Connecticut................. Kentucky................... Tennessee................... Iowa....................... Illinois ..................... Arizona..................... California...................

Bovine 17

Ovine

0 0-49

0

0

56 24 37

7 0 24-32

suggest that a common mechanism of infection exists although its precise nature is unclear. Both sheep and goats are capable of grazing very close to the ground and so are potentially capable of picking up oocysts but will not normally do so unless no other food is available. Sheep are the one species of domestic animals in Ontario in which clinical toxoplasmosis is known to be a significant problem (11). The prevalence rate in pig sera is very similar to reported figures from the Northeastern United States (43% in New York (7) and 42% in Connecticut (28)), (Table III) and is, in general, comparable with results obtained in other omnivores (Fig. 2). This may be a reflection of the feeding habits of this species. The most likely sources of infection are through the accidental ingestion of oocysts while rooting and the deliberate consumption of dead rodents when available. The percentage of positive reactions in horses is comparable to results obtained by others in North America (Table III) and approximates to the expected infection rate in herbivores. The reasons for this prevalence being lower in horses than in cattle is unknown but may be due to the custom of feeding horses from an elevated manger, so reducing the possibility of oocyst contamination of feed. The level of infection in domestic dogs (33%) is considerably lower than that found in wild canids in this region. For example, coyotes (Canis latrans) are 65% seropositive (21). The difference may be attributable to the inhibition of natural hunting in the dog. This prevalence is within the range of positive reactions reported elsewhere (Table IV). The incidence rate in dogs is probably considerably less than that in cats in view of the relatively

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April, 1978

65

5 28

Species Caprine Porcine 45 63 _40-48 43 42

Equine 9 -

-

6

19-26 7 4 -

27

4

_14

-

27

References This paper 23 17 7 28 7 6 Cited in 25 Cited in 25 7 25

high age of the dog population tested compared with that of cats. The prevalence in cats reported here is relatively low by American standards (4, 6, 7) but considerably higher than that in Saskatchewan (17). Given an approximate incidence rate of 20%/year (Fig. 1), it is possible to calculate an upper limit on the percentage of cats shedding oocysts at any one time. For example, if 20% of cats become infected in their first year and shed oocysts for two weeks (9) then at any one time 0.05% of cats will be shedding oocysts. However, while all cats which acquire infection by eating cysts in muscle will shed oocysts, less than half the animals which acquire infection from oocysts or tachyzoites do so (4). Consequently this figure of 0.05% represents only an upper limit on the prevalence of shedding cats. Certainly, in the Guelph region, the percentage of cats shedding oocysts is somewhat less than this (Dr. J. P. Lautenslager, personal communication). This calculated percentage is also less than the percentage of cats found to be shedding oocysts in other locations in Northeastern TABLE IV. The Reported Prevalence (%) of Seropositive Reactions to T. gondii in Dogs and Cats in North America Location

Ontario............ Saskatchewan ...... Tennessee.......... Northern California. Iowa.............. New York......... Arizona............ Pennsylvania....... Massachusetts...... Middle Atlantic States...........

Dogs Cats Reference 33 25 This paper 3.4 17 15-35 40 6 5 25 16 7 28 31 7 30 7 7 59 34 7 25

16

181

fox

80-

ercentage bsitive 70

N

sheep

goat

6

COYt mink bear

40-

tMAN

30-

skunk

dog

rcoo

cat

20

bovin

mrtin

10rabbit 0

Herbivores

_____ Omnivores Carnivores

Food Habits Fig. 2. The relationship between diet and the prevalence of positive reactions to T. gondii. Data taken from Reference 21 and this paper.

the Microtus population (5). Indeed, if an animal such as a fox consumes more than a hundred Microtus in a year then it will almost certainly become infected even although the prevalence in the prey species is less than 1%. The number of Microtus tested by Quinn et al (19) or by ourselves are quite insufficient to exclude this species as a significant source of infection. The low prevalence of seropositive reactions in wild birds tends to suggest that they too may be epidemiologically relatively unimportant except as prey for carnivores. In this case, the food pyramid effect described above could ensure that infection occurs at a considerably higher rate in the predator species. In general, the results of this survey together with other published data for Ontario confirm the general association between rodent consumption and the prevalence of toxoplasmosis (Fig. 2). Cattle have a higher seropositivity rate than would be anticipated from their diet but this may be due to their close association with cats over the winter months. Dogs have a lower than expected rate possibly

because of the way they are treated in this society. Humans conform to the general trend since although raw meat consump-

America (1%ln Columbus, Ohio, 1.5% in tion is rare it is

Urbana, Illinois) (4).b at Toxoplasma infection appears to be and a relatively low levelin level in wild rodents and birds. Other North American reports indicate that 4% of Mn and 3% of Rattus norvegicus in urban Memphis are seropositive (6) while 8% of Rattus rattus of M. and 3 2% are seropositive in Hawaii (26). In this study only two

Toxoplativelymlow

Microtus

wilarstodben mvscuilss

mutscuus pennsyrvafuncus were found to

were captured

and both be seronegative. Quinn et at (19) tested twenty animals of this species captured in the Guelph region and all were seronegative. In spite of these results we believe that this species may represent a significant source of toxoplasmosis in carnivores for the following reasons. Firstly, Quinn et at (19) found 18% of the short-tailed shrew (Blarind brevicauda) to be seropositive and thls animal is a carnivore whlch commonly preys on Microtuls. Secondly, Tizard et at (21) found 84% of foxes in this region to be seropositive and this animal too preys largely on Microtus. Finally, toxoplasmosis is reported to occur in this species in other locations and may at times have a significant influence on the size of

182

probably compensated for

xouetto oocysts. oy,s byThe exposure most significant unexplained feature of the epidemiology of this condition is the

extraordinarily high prevalence of positive reactions in sheep and goats. As pointed out earlier, this may be due to their particular grazing habits or perhaps to some unknown epidemiological factor. It is also possible that sheep and goats may be infected by an organism which cross-reacts with T. gondii in the SFDT. In conclusion, it must be pointed out that a significant proportion of the meat produced in Ontario is infected with T. gondii and care should be taken not only to ensure that meat is properly cooked but also to ensure that milk is pasteurized and that hands are well washed after handling raw meat. Although only a very low percentage of cats are probably active shedders of oocysts they almost certainly represent a significant source of infection for animals in this province.

ACKNOWLEDGMENTS This study was supported by the Ontario

Can. J. comp. Med.

Ministry of Health. We should like to thank Mr. S. S. Chauhan for technical assistance and the directors of the various laboratories mentioned for the generous provision of serum samples.

REFERENCES 1. AWAD, F. I. and R. LAINSON. A note on the serology of sarcosporidiosis and toxoplasmosis. J. clin. Path. 7: 152-156. 1954. 2. BEVERLEY, J. K. A., C. P. BEATTIE and C. ROSEMAN. Human toxoplasma infection. J. Hyg., Camb. 52: 37-46. 1954. 3. DESAVIGNY, D. H. Canadian Public Health Ass. Meeting, Ottawa. 1974. 4. DUBEY, J. 0. A review of Sarcocystis of domestic animals and other coccidia of cats and dogs. J. Am. vet. med. Ass. 169: 1061-1078. 1976. 5. ELTON, C. Voles, Mice and Lemmings. Oxford University Press. 1942. 6. EYLES, D. E., C. L. GIBSON, N. COLEMAN, C. S. SMITH, J. R. JUMPER and F. E. JONES. The prevalence of toxoplasmosis in wild and domesticated animals in the Memphis region. Am. J. trop. Med. Hyg. 8: 505-510. 1959. 7. FELDMAN, H. A. and L. T. MILLER. Serological study of toxoplasmosis prevalence. Am. J. Hyg. 64: 320-335. 1956. S. FRENCH, C. E. and N. A. FISH. A survey of toxoplasmosis in an Ontario community. J. Can. med. Ass. 84: 757-767. 1961. 9. FRENKEL, J. K. and J. P. DUBEY. Toxoplasmosis and its prevention in cats and man. J. infect. Dis. 126: 664-673. 1972. 10. FRENKEL, J. K. and J. P. DUBEY. Hammondia hammondi, gen. nov. sp. nov., from domestic cats, a new coccidian related to toxoplasma and sarcocystis. Z. ParasitKde 46: 3-12. 1976. 11. HULLAND, T. J. and S. B. TOBE. Toxoplasmosis as a cause of abortion in Ontario sheep. Can. vet. J. 2: 45-51. 1961. 12. JACOBS, L., M. L. MELTON and M. K. COOK. Experimental toxoplasmosis in pigeons. Expl Parasit. 2: 403-416. 1953.

13. MARKUS, M. B. Serology of toxoplasmosis, isosporosis and sarcosporidiosis. New Engl. J. Med. 289: 980. 1973. 14. MARCUS, M. B. Serology of human sarcosporidiosis. Trans. R. Soc. trop. Med. Hyg. 68: 415-416. 1974. 15. MILLER, N. L., J. K. FRENKEL and J. P. DUBEY. Oral infections with toxoplasma cysts and oocysts in felines, other mammals and in birds. J. Parasit. 58: 928-937. 1972. 16. MORRIS, J. A., C. G. AULISIO and J. M. MCCOWN. Serological evidence of toxoplasmosis in animals. J. infect. Dis. 98: 52-54. 1956. 17. NATION, P. N. and J. R. ALLEN. Antibodies to Toxoplasma gondii in Saskatchewan cats, sheep and cattle. Can. vet. J. 17: 308-310. 1976. 18. QUINN, P. J. and B. M. McCRAW. Current status of Toxoplasma and toxoplasmosis. A review. Can. vet. J. 13: 247-262. 1972. 19. QUINN, P. J., R. 0. RAMSDEN and D. H. JOHNSON. Toxoplasmosis: A serological survey in Ontario wildlife. J. Wildl. Dis. 12: 504-505. 1976. 20. SABIN, A. B. and H. A. FELDMAN. Dyes as microchemical indicators of a new immunity phenomenon affecting a protozoan parasite (Toxoplasma). Science 108: 660-663. 1948. 21. TIZARD, I. R., J. B. BILLETT and R. 0. RAMSDEN. The prevalence of antibodies against Toxoplasma gondii in some Ontario mammals. J. Wildl. Dis. 12: 322-325. 1976. 22. TIZARD. I. R., N. A. FISH and P. J. QUINN. Some observations on the epidemiology of toxoplasmosis in Canada. J. Hyg., Camb. 77: 11-21. 1976. 23. TIZARD, I. R., M. CARRINGTON and C. H. LAI. Toxoplasmosis in goats in southern Ontario. A public health hazard? Can. vet. J. 18: 274-277. 1977. 24. TIZARD, I. R., S. S. CHAUHAN and C. H. LAI. The prevalenice and epidemiology of toxoplasmosis in Ontario. J. Hyg., Camb. 78: 275-289. 1977. 25. VANDERWAGEN, L. D., D. E. BEHYMER, H. P. RIEMANN and C. E. FRANTI. A survey for Toxoplasma antibodies in northern California livestock and dogs. J. Am. vet. med. Ass. 164: 1034-1037. 1974. 26. WALLACE, G. D. Intermediate and transport hosts in the natural history of Toxoplasma gondii. Am. J. trop. Med. Hyg. 22: 456-464. 1973. 27. WALLACE, G. D. The role of the cat in the natural history of Toxoplasma gondil. Am. J. trop. Med. Hyg. 22: 313-322. 1973. 28. WEINMAN, D. and A. H. CHANDLER. Toxoplasmosis in man and swine. An investigation into the possible relationship. J. Am. vet. med. Ass. 161: 229-232. 1956.

BOOK REVIEW THE BIOLOGY AND MEDICINE OF RABBITS AND RODENTS. J. E. Harkness and J. E. Wagner. Published by Macmillan of Canada, Toronto. 1977. 150 pages.

This handy little one hundred and fifty page book is advertised as the answer to the needs of the veterinarian, student and biomedical investigator. It is refreshing in this age of the promotion of so many false expectations, that here is a case where an honest pitch has been made. The organization of the information of this wide ranging field into so few pages and yet managing to include all the relevant material is, in my estimation, rather phenomenal. No claims are made for its being exhaustive. But each subject that is covered is done carefully, emphasizing the common

Volume 42 - April, 1978

and yet giving much helpful information. For the veterinarian this hand book allows him to extend his already extensive knowledge and understanding of medicine to handle laboratory type species in his practise. The authors have resisted the urge to overwhelm with the unusual but have provided a bibliography which will introduce one to many subjects and persons in the field. The case report method of self-teaching is another example of the thoughtful organization of this book. As you may have guessed I am impressed with the book. I would recommend it highly to those it was supposedly written for and anyone else who wants to learn. J. D. Schroder.

183

The prevalence of serum antibodies to Toxoplasma gondii in Ontario mammals.

The Prevalence of Serum Antibodies to Toxoplasma gondii in Ontario Mammals 1. R. Tizard, J. Harmeson and C. H. Lai* ABSTRACT The prevalence of seropos...
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