Toxoplasmosis in Lower Mammals*

SYNOPSIS. Sera from 270 domestic and wild mammals were tested for antibodies against Toxoplasma gondii by the indirect hemagglutination method. The specificity of reactions was determined by the hemagglutination-inhibition test. Positive results at a titer of 1:64 were given by 10 of 138 cattle, 3 of 74 swine, 1 of 8 cats and none of 3 horses. Of the wild animals tested, only 1 of 9 woodchucks was positive; 17 raccoons, 14 opossums and 7 skunks were negative.     

Cyst-Induced Toxoplasmosis in Cats*

SYNOPSIS. The life cycle of Toxoplasma gondii is described from cats orally inoculated with Toxoplasma cysts. Five new structural stages of Toxoplasma designated as “types” A-E were found in the epithelial cells of the small and large intestine. Type A is the smallest of all 5 intestinal Toxoplasma types. It occurs as collections of 2-3 organisms in the jejunum 12–18 hr after infection. Type B organisms are characterized by a centrally located nucleus, a prominent nucleolus and dark blue cytoplasm giving rise to the appearance of bipolar staining with Giemsa. Type B occurs 12–54 hr after infection and appears to divide by simple endodyogeny and by multiple endodyogeny (endopolygeny). Type C organisms are elongate with subterminal nuclei and strongly PAS-positive cytoplasm. They occur at 24–54 hr and divide by schizogony. Type D organisms are smaller than type C and contain only a few PAS-positive granules. They occur from 32 hr to 15 days after inoculation and account for over 90% of all parasites in the small intestine during this time. Three subtypes divide by endodyogeny, schizogony and by splitting of their merozoites from the main nucleated mass without leaving a residual body. Type E organisms resemble one of the subtype D which divide by schizogony, but they leave a residual body. They occur from 3–15 days after inoculation. Gametocytes occur thruout the small intestine but more commonly in the ileum 3-15 days after infection. Male gametocytes contain on an average of 12 microgametes and comprise 2–4% of the gametocyte population. The prepatent period after cystinduced infection is 3–5 days with the peak oocyst production between 5–8 days and a patent period varying from 7–20 days. Variable numbers of trophozoites are present in the lamina propria of the small intestine and in the extra-intestinal tissues within a few hr after inoculation. After 9–10 days cysts were seen in the heart and later in the brain. The lesions of toxoplasmosis are compared in newborn and weanling kittens and in adult cats after oral and subcutaneous inoculation with cysts. After the ingestion of cysts, newborn kittens developed enteritis, hepatitis, myocarditis, myositis, pneumonitis and encephalitis and were moribund by the 9th day. Kittens aged 2 weeks and older developed enteritis, myocarditis, encephalitis and myositis but often survived; adult cats usually remained asymptomatic. After subcutaneous inoculation of cysts, newborn and weanling kittens died of acute toxoplasmosis with severe pneumonia, myocarditis, encephalitis and hepatitis.     

Toxoplasmosis in Sheep

The epidemiology of toxoplasmosis in sheep was studied during a period of 31/2 to 6 years in 4 flocks in which abortions from the infection previously had occurred. The epidemiological pattern indicated that a heavy contamination of the environment may persist for about 2 years. Even in flocks with an apparently normal reproductive performance, toxoplasmosis may cause barrenness, abortion or delivery of dead lambs in 1–2 % of the breeding ewes. The prevalence of infected sheep increased with the age. The incidence of infection was significantly lower in 6–12 months old lambs than in mature ewes. The incidence of infection was higher on lowland pastures than on mountain pastures in not populated areas. On lowland pastures the incidence of infection was lower during the summer than during the winter. kw|Keywords|k]toxoplasma infection; k]epidemiology; k]sheep {fn1|This work was supported by grants from the Norwegian Research Council for Science and for the Humanities.}     

Feline Toxoplasmosis from Acutely Infected Mice and the Development of Toxoplasma Cysts*

SYNOPSIS. The development of Toxoplasma cysts was studied in mice inoculated with tachyzoites by several routes. After 1–30 days of infection, murine tissues were examined microscopically, and portions or whole carcasses were fed to mice and cats. The feces of the cats were examined for oocyst shedding. Cyst-like structures containing distinct PAS-positive granules were first seen after 3 days of infection with tachyzoites, and became numerous by 6 days. Argyrophilic walls were first seen after 6 days, and became numerous by 16 days of infection with tachyzoites. Prepatent periods to oocyst shedding (PPO) were either “short” (3–10 days) or “long” (19–48 days). The “short” PPO was found only in cats that had ingested mice infected for 3 days or longer, and was related to the development of PAS-positive granules in T. gondii, and to high, 60–100%, oral infectivity rates for cats. The “long” PPO followed the ingestion of mice infected for only 1–2 days, and was related to tachyzoites without distinct PAS-positive granules and low, 32% or less, infectivity for cats. The “long” PPO followed also the ingestion of oocysts and the parenteral inoculation of tachyzoites, bradyzoites, or sporozoites. Using the “short” PPO as a criterion for detecting cysts in tissues, it was shown that (a) numerous cysts developed in mice 5 days after inoculation with tachyzoites, 7–9 days after inoculation with cysts, and 9–10 days after inoculation with oocysts, and (b) cysts developed faster and more frequently in the brain and muscle than in lungs, liver, spleen, and kidneys of mice inoculated with tachyzoites.     

Toxoplasmosis in Sheep

The presence of Toxoplasma gondii in the diaphragm was correlated with the dye test titres in 174 sheep. In 94 of these the presence of the parasite was also correlated with the haemoglobin (Hb) type. T. gondii was recovered from 3 % of the sheep with titres < 1/16, compared with 30 % of those with titres 1/16 and 70 % of those with titres ≥ 1/32. The results indicate that the distinction between serologically positive and negative individuals at a final serum dilution of 1/16 is justified. Some evidence was found that the parasite is easier to recover from dye test positive mature sheep than from dye test positive lambs. Of the 174 sheep, 143 were examined at random among 186 sheep culled or cast for age during a 4-year period from 1 flock in which the prevalence of Toxoplasma antibodies was representative for flocks in the southern Norway, and T. gondii was recovered from 53 (37 %) of these. It was concluded that 10—15 % of the lamb carcases and 25—37 % of the carcases from mature sheep in this country have T. gondii in their muscles detectable by the peptic digestion technique. A possible genetical influence on the infection was indicated by the higher frequency of recoveries of T. gondii from sheep with Hb type B than from sheep with the Hb types A or AB, but the number of individuals with Hb type B was too small to demonstrate statistically significant differences. The epidemiological importance of infected sheep carcases is discussed.     

Effect of Age and Sex on the Acquisition of Immunity to Toxoplasmosis in Cats*

SYNOPSIS. The effects of age and sex of the cat on oocyst shedding, multiplication of Toxoplasma gondii in tissues of cats, and acquisition of immunity were investigated after oral inoculation of cats with Toxoplasma cysts. Twenty-five cats varying in age from 1 week to 39 months were killed 7-97 days after inoculation with T. gondii. Homogenates of brain, heart, mesenteric lymph nodes, retina, and blood from these cats were inoculated into mice to test for Toxoplasma infectivity. Toxoplasma was isolated more frequently and in higher titers in mice receiving inocula from cats of the youngest age group (1 week old). Toxoplasma gondii was isolated from tissues of only 2 of 21 cats older than 2 months (at the time of inoculation), although all of the animals shed oocysts within 1 week after ingesting the parasites. The number of oocysts shed varied among littermates of the same sex and between sexes. Generally, cats younger than 12 months shed more oocysts than older cats. The number of oocysts shed by older cats varied considerably; males generally shed more oocysts than the females. However, the numbers of cats examined were too small for statistical comparison. Nevertheless, the observations suggest that cats older than 12 months should not be used in experiments where numbers of oocysts shed is critical.     

Ovine Toxoplasmosis in Sweden

Sera from 155 ewes representing 21 different sheep flocks from the central parts of Sweden were examined for the presence of antibodies to Toxoplasma gondii by the indirect fluorescent-antibody test. Seropositive animals were found in 14 flocks, which corresponded to 55 % of the tested sera. Animals from the same flock were with few exceptions either all positive or all negative. Some ewes with a history of abortion a few months before blood sampling showed high titers indicating causative correlation with toxoplasmosis. These are the first verified cases of ovine toxoplasmal abortions in Sweden.     

Experimental Toxoplasmosis in Chimpanzees

Two chimpanzees were given by mouth large numbers of viable oocysts of Toxoplasma gondii obtained from the faeces of experimentally infected cats. Before the experiment the first chimpanzee had a positive dye test reaction (1:250), an indication that it had undergone an earlier infection of toxoplasmosis; the serum antibody titres remained unchanged, no evidence of illness was found, and oocysts did not appear in its faeces during the subsequent six weeks. The second chimpanzee showed a negative dye test reaction before infection, and this converted to positive on the 7th day, rose to a peak on the 35th day, and remained high for six months. This animal appeared unwell during the first week, and on the 7th day its blood proved infective to mice; on the 40th day the lymph nodes became enlarged and biopsy specimens of a node and muscle in the 11th week were also infective to mice. No oocysts were passed in the faeces. The presumed cycle in the chimpanzee and in man and the relationships between Toxoplasma and Isospora are discussed.     

Chorio-Retinitis in Porcine Toxoplasmosis

Porcine toxoplasmosis generally occurs as a latent disease in adolescent and adult pigs, but now and then also manifests itself as a fatal congenital disease in piglets. It is known to occur in USA (Farrel et al. 1952), Germany (Becker 1954), Denmark (Momberg-Jørgensen 1956), Mexico (Varela et al. 1956), Japan (Sato et al. 1958), England (Harding et al. 1961) and Sweden (Hansen et al. to be published).