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Infection and immunity to toxoplasmosis induced by the RH strain of Toxoplasma gondii was compared in Sprague-Dawley (SD) and Wistar rats and in outbred Swiss Webster mice. All rats injected with up to 1,000,000 RH-strain tachyzoites remained clinically normal, whereas mice injected with only 1 live tachyzoite died of acute toxoplasmosis. Rats could be infected with 1 tachyzoite of the RH strain as shown by antibody development and by bioassay in mice. However, after 8 days, RH-strain organisms were recovered only inconsistently from SD and Wistar rat brains. Contrary to a report of sterile immunity to T. gondii infection in rats after immunization with live RH tachyzoites, we found infection immunity after challenge with the VEG strain. Toxoplasma gondii tissue cysts of the VEG strain could be recovered from most SD and Wistar rats, first injected with live RH-strain tachyzoites and then challenged with oocysts of the VEG strain. Our RH strain, and probably many others, passed for 50+ yr as tachyzoites has lost not only the capacity to form oocysts, but also shows a marked reduction or absence of tissue cyst (bradyzoites) formation.

Antibodies to Toxoplasma gondii were determined in sera from 222 coyotes (Canis latrans), 283 red foxes (Vulpes vulpes), and 97 gray foxes (Urocyon cinereoargenteus) from Indiana, Kentucky, Michigan, and Ohio during 1990-1993. Sera were examined in 1:25, 1:100, and 1:500 dilutions by the modified direct agglutination test (MAT) with formalinized whole tachyzoites plus mercaptoethanol. Antibodies were found in 131 (59.0%) of 222 coyotes, 243 (85.9%) of 283 red foxes, and 73 (75.3%) of 97 gray foxes. Antibodies were also measured by different serologic tests in 4 littermate T. gondii-free red foxes fed T. gondii tissue cysts or oocysts; the fifth littermate fox was not fed T. gondii. Antibodies were measured in fox sera obtained 0, 14, and 36-55 days after infection with T. gondii. All 4 foxes fed T. gondii developed MAT and dye test antibody titers of 1:200 or more 14 days later. The latex agglutination test (LAT) and indirect hemagglutination test (IHAT) were less sensitive than MAT for the diagnosis of T. gondii infection in foxes. Antibodies were not detected by LAT (titer 1:64) in the 2 foxes fed tissue cysts nor by IHAT in 1 of the foxes fed tissue cysts. Toxoplasma gondii was isolated by bioassay in mice from tissues of all 4 foxes fed T. gondii. The control fox had no T. gondii antibodies detectable by any of the serologic tests.

Immunity to toxoplasmosis was investigated in outbred pigs after feeding Toxoplasma gondii oocysts inactivated by irradiation. In 2 experiments 23, 2-3-mo-old pigs were fed (12 pigs only once and 11 pigs twice) approximately 10(5) oocysts irradiated at 0.3 or 0.4 kGy of 137Cs and 11 pigs served as unvaccinated controls. Eleven to 13 wk later, 15 vaccinated pigs and 5 nonvaccinated pigs were challenged orally with a high dose (10(5) or 10(6)) of oocysts, and 7 vaccinated and 6 nonvaccinated pigs were challenged orally with a low dose (10(2) or 10(3)) of oocysts; 1 vaccinated pig was not challenged. All nonvaccinated pigs challenged with a high dose became ill starting 4-5 days postchallenge (p.c.). One pig died day 9 p.c., 1 died day 16 p.c., and 1 was killed day 10 p.c. because of weakness. All vaccinated pigs, nonvaccinated pigs challenged with a low dose, and the 1 vaccinated unchallenged pig remained clinically normal. Bioassay of pig tissues (tongue, diaphragm, brain) in mice indicated fewer tissue cysts in tissues of vaccinated pigs compared with unvaccinated pigs following challenge with live oocysts. However, vaccination with 1 or 2 doses of irradiated oocysts did not prevent formation of tissue cysts even in pigs challenged with 10(2) oocysts.

The development of sporozoites to tachyzoites and bradyzoites was studied in mice after feeding 1-7.5 x 10(7) Toxoplasma gondii oocysts. Within 2 hr after inoculation (HAI), sporozoites had excysted and penetrated the small intestinal epithelium. At 2 HAI, most sporozoites were in surface epithelial cells and in the lamina propria of the ileum, and by 8 HAI, T. gondii was also seen in mesenteric lymph nodes. At 12 HAI, sporozoites had divided into 2 tachyzoites in the lamina propria of the small intestine. By 48 HAI, there was a profuse growth of tachyzoites in the intestine and mesenteric lymph nodes of mice fed 7.5 x 10(7) oocysts. Parasites had disseminated via the blood and lymph to other organs by 4 days after inoculation (DAI). Toxoplasma gondii was first isolated from peripheral blood at 4 HAI. Tissue cysts were visible histologically in the brain at 8 DAI. By using immunohistochemical staining with anti-bradyzoite-specific (BAG-5 antigen) serum, BAG-5-positive organisms were first seen at 5 DAI in the intestine and at 8 DAI in the brain. Using the bioassay in cats, bradyzoites were first detected in mouse tissues between 6 and 7 DAI, and they were found in intestines before they were found in the brain. Cats fed murine tissues containing bradyzoites shed oocysts in their feces with a short (10 days) prepatent period, whereas cats fed tissues containing tachyzoites did not shed oocysts within 3 wk. Using a pepsin-digestion procedure and mouse bioassay, bradyzoites were first detected in brain tissue at 7 DAI and in many organs of mice at 51 and 151 DAI. Individual bradyzoites, small and large tissue cysts, and tachyzoites were seen in the brains of mice at 87 and 236 DAI.

To follow antibody responses measured by various serologic tests in pigs orally inoculated with low (or = 10 oocysts) numbers of Toxoplasma gondii oocysts.24, 2- to 3-month-old pigs.Pigs (n = 42) were inoculated orally with 10 (14 pigs) or 1 (28 pigs) infective oocysts, and 6 pigs served as uninoculated controls. Blood (serum) samples were obtained at 1- to 3-week intervals until euthanasia. At necropsy, the brain, heart, and tongue of pigs were bioassayed in mice and cats for isolation of T gondii. Modified agglutination test (MAT), using whole, fixed tachyzoites and mercaptoethanol; latex agglutination test (LAT); indirect hemagglutination test (IHAT); Sabin-Feldman dye test (DT); and ELISA were used to evaluate serologic responses to T gondii.T gondii was isolated from tissues of 13 of 14 pigs each fed 10 oocysts, 17 of 28 pigs each fed 1 oocyst, and 0 of 6 control pigs. 29 of 30 T gondii-infected pigs developed antibodies when measured by MAT, DT, and ELISA; the 1 seronegative-infected pig had been fed 10 oocysts and was euthanatized 69 days after inoculation. LAT detected antibodies in 26 of 30 T gondii-infected pigs. IHAT detected antibodies in 11 T gondii-infected pigs.MAT, DT, and ELISA were more sensitive serologic assays than LAT and IHAT for detecting antibodies induced by low numbers of T gondii in pigs.

Toxoplasma gondii oocysts are highly resistant to environmental influences. To study the effect of alpha-irradiation on the viability of T. gondii oocysts, 1 million sporulated oocysts of the VEG strain were irradiated to absorbed doses of 0.10, 0.15, 0.20, 0.25, 0.30, 0.40, or 0.50 kGy at 5 C with a 137Cs gamma-irradiation source. Treated oocysts were bioassayed for viable T. gondii in mice. Viable T. gondii was not found in brains of mice 2 mo after oral inoculation with oocysts irradiated at levelsor = 0.25 kGy. Therefore, alpha-irradiation is an effective means of killing T. gondii oocysts. Mice inoculated orally with oocysts irradiated at 0.20 and 0.40 kGy were partially protected when challenged orally with lethal doses of nonirradiated oocysts.

To define the infectiousness of the VEG strain of Toxoplasma gondii, 42 pigs were fed doses estimated at 10, 1, or1 mouse infective oocysts. They were killed 38-99 days after inoculation and 50 g of tissues from their tongue, heart, and brain were individually homogenized in acidic pepsin solution and bioassayed in mice. Pools of brain, heart, tongue, and skeletal muscle (total 500 g) were bioassayed in cats. Toxoplasma gondii was isolated by bioassays in mice and in cats from 13 of 14 pigs fed 10 oocysts, 13 of 14 pigs fed 1 oocyst, and 4 of 14 pigs fed "less than" 1 oocyst, indicating high infectivity of VEG strain of T. gondii to pigs. All infected pigs developed modified agglutination test antibodies (1:50). Control pigs (n = 6) remained seronegative (1:20) and T. gondii was not isolated from their tissues. Toxoplasma gondii was isolated from tongues of 27 (93%), brains of 21 (72%), and hearts of 13 (45%) of 29 experimentally infected pigs by bioassay in mice. The number of T. gondii-positive mice after inoculation of tongue, brain, and heart from infected pigs was 240 (80%), 84 (28%), and 36 (12%) of 300 mice inoculated with each organ, respectively. Thus, the VEG strain of T. gondii was localized more often and in higher numbers in the tongue than in the brain and the heart of pigs. The apparent muscle localization after infection with the low dose of the VEG strain of T. gondii agrees with other studies in livestock that suggest T. gondii is more neurotropic in mice than in livestock.

Two epidemiologic studies of risk factors for transmission of Toxoplasma gondii to swine were conducted for farms in Illinois. The first study was a cross-sectional survey of swine farms from the state of Illinois pseudorabies testing program, in which farm owners or managers were interviewed by telephone regarding presence of risk factors for transmission of T. gondii on the farm. There were 123 farms surveyed that provided blood samples for at least 30 sows. The mean sow seroprevalence was 19.5% (median = 10.0%). Multiple regression analysis of the association of sow seroprevalence with outdoor housing of sows, cat access to sow areas, number of sows, open feed storage and water delivery, delayed removal of carcasses, and presence of rodents on the farm indicated that higher sow seroprevalence was associated with cat access to sows (P = 0.009) and fewer sows in the herd (P = 0.05). The second study was a field investigation of 47 swine farms (37 from the cross-sectional study). Data collection included obtaining blood samples from swine, cats, and rodents, and fecal samples from cats, heart and brain tissue from rodents, and feed, water, and soil samples for T. gondii examination. The risk of T. gondii transmission from cats and rodents to sows and finishing pigs was evaluated, taking into account housing conditions and herd size. Multiple regression analysis indicated that T. gondii seroprevalence in finishing pigs increased with more seropositive juvenile cats on the farm (P0.0001) and higher seroprevalence in house mice (P = 0.0023).(ABSTRACT TRUNCATED AT 250 WORDS)

Field studies were conducted on 47 swine farms in Illinois during 1992 and 1993 to identify sources and reservoirs of Toxoplasma gondii infection. Blood samples were obtained from swine and from trapped wildlife. Serum antibodies to T. gondii were determined using the modified agglutination test, incorporating mercaptoethanol. Antibodies to T. gondii (titeror = 25) were found in 97 of 4,252 (2.3%) finishing pigs, 395 of 2,617 (15.1%) sows, 267 of 391 (68.3%) cats, 126 of 188 (67.0%) raccoons, 7 of 18 (38.9%) skunks, 29 of 128 opossums (22.7%), 6 of 95 (6.3%) rats, 3 of 61 (4.9%) white-footed mice (Peromyscus sp.), and 26 of 1,243 (2.1%) house mice (Mus musculus). Brains and hearts of rodents trapped on the farm were bioassayed in mice for the presence of T. gondii. Toxoplasma gondii was recovered from tissues of 7 of 1,502 (0.5%) house mice, 2 of 67 (3.0%) white-footed mice, and 1 of 107 (0.9%) rats. Feces of 274 cats trapped on the farm and samples of feed, water, and soil were bioassayed in mice for the presence of T. gondii oocysts. Toxoplasma gondii was isolated from 2 of 491 (0.4%) feed samples, 1 of 79 (1.3%) soil samples, and 5 of 274 (1.8%) samples of cat feces. All mammalian species examined were reservoirs of T. gondii infection. All farms had evidence of T. gondii infection either by detection of antibodies in swine or other mammalian species, or by detection of oocysts, or by recovery from rodents by bioassay. The possibility of transmission of T. gondii to swine via consumption of rodents, feed, and soil was confirmed.

Fourteen 2-3-wk-old turkeys were inoculated orally with 10(5) or 10(4) infective oocysts of the ME 49 strain of Toxoplasma gondii. Of the 8 turkeys given 10(5) oocysts in experiment 1, 3 died or were killed 12 or 14 days after inoculation (DAI) because of respiratory distress associated with a concomitant Aspergillus-like fungus infection. The remaining 5 turkeys remained normal and were killed 62 DAI. Toxoplasma gondii was isolated in mice from the heart of all 5, from the breast muscles of 2, leg muscles of 3, and from the brains and livers of none of the turkeys. All 6 turkeys fed 10(4) oocysts in experiment 2 remained clinically normal until necropsy on 41 DAI; T. gondii was isolated from pooled tissues from each turkey. All 14 turkeys developed high antibody titers to T. gondii in the modified agglutination test (MAT) using formalinized tachyzoites. The enzyme-linked immunosorbent assay (ELISA) was as sensitive as MAT for detecting T. gondii antibodies in turkey sera. The latex agglutination and indirect hemagglutination tests were less sensitive than the MAT and ELISA. No dye-test-measurable antibodies were found in sera of any turkey.