Studies of antibody titres in experimentally induced ovine toxoplasmosis

Acute phase toxoplasma abortions in sheep

Within 13 days of the experimental infection of 15 oestrus-synchronised ewes with 2000 sporulated oocysts of Toxoplasma gondii at 80 to 90 days of gestation 11 had aborted. The infection induced pyrexia and specific antibody in all the ewes. One ewe resorbed its fetus, 11 ewes aborted and three delivered, at full term, live congenitally infected lambs whose pre-colostral serum was antibody-positive. Tissues from the aborted fetuses and placentae from the live lambs were examined for toxoplasma infection by polymerase chain reaction (PCR) amplification of the B1 gene and by mouse inoculation. The live lambs were all shown to be infected by both methods, but there was no evidence of infection in any of the tissues from the acute phase abortions, suggesting that these fatalities occurred before the placenta or the fetus had been invaded by T gondii. Such toxoplasma-induced, acute-phase abortions are likely to be impossible to diagnose from fetal tissues. These results have implications not only for the diagnosis of naturally occurring ovine abortions but also for the understanding of the pathogenesis of toxoplasma-induced abortion.

Toxoplasma gondii infection in sheep and cattle

Toxoplasma gondii is a protozoan parasite that can infect all warm-blooded animals. Sheep and cattle show different susceptibilities to T. gondii infection. Primary infection in pregnant sheep can result in abortion or the birth of weak lambs but they are then protected against further challenge by the development of an effective immunity. Cattle on the other hand can be readily infected, but abortion or perinatal mortality have not been recorded. The evidence suggests that cattle develop a more effective immune response to T. gondii infection than sheep. Potential mechanisms to explain these differences are discussed in this paper.

Enzyme-linked immunosorbent assays for the measurement of specific antibodies in experimentally induced ovine toxoplasmosis

Tachyzoites of the RH strain of Toxoplasma gondii were inoculated intravenously into sheep following which serum samples were collected at approximately weekly intervals for 9 months. The sera were examined by the toxoplasma dye test and two enzyme-linked immunosorbent assays (ELISA) specifically developed for investigations of ovine toxoplasmosis. One was an antibody class capture assay for the detection of anti-toxoplasma specific IgM, the other an indirect assay which detected anti-toxoplasma IgG. Some of the sheep had antibodies to toxoplasma prior to inoculation but none had specific IgM. Sera collected 17 days after inoculation showed that all had raised specific antibody levels but the only sheep that produced specific antitoxoplasma IgM were those that were initially without any antibody. Specific IgM could be detected in all these particular sheep for at least 1 month after infection and up to 3 months in some. Specific IgG persisted at high levels for at least 3 months and could still be detected at moderate levels for at least 9 months. The ELISA methods described are simple to perform and could clearly distinguish between previous infection and this experimental infection with Toxoplasma gondii.

Studies on ovine efferent lymph following infection with Toxoplasma gondii

Cell output was monitored in efferent popliteal lymphatics of sheep following subcutaneous injection into the lateral tarsus of either 100 T. gondii tissue cysts or control inoculum. Sheep undergoing a primary infection developed a substantially increased lymphoblast output which reached a peak of more than 50 per cent of all cells. The majority of lymphoblasts did not contain Ig and may have been T cells. IgG antibody against T. gondii was detected in lymph by day 9. Animals with naturally acquired antibody to T. gondii developed a more rapid lymphoblast response to the parasite which, at its peak, accounted for 27.5 per cent of all cells. Many of these lymphoblasts contained Ig, presumably specific antibody, which, by increasing phagocytosis and intracellular killing of the T. gondii, would prevent multiplication and dissemination of the parasite.

Evaluation of a solid-phase immunoassay (DIG-ELISA) for the serodiagnosis of ovine toxoplasmosis

The antibody response to Toxoplasma gondii was studied in 5 experimentally infected sheep by means of the indirect fluorescent antibody test (IFAT) and the diffusion-in-gel enzyme-linked immunosorbent assay (DIG-ELISA). A significant increase in the antibody levels was registered by both methods as early as one week after subcutaneous inoculation with the RH strain of T.gondii. Maximal antibody levels were recorded 4 to 8 weeks after the inoculation. Analysis of a total of 135 sera obtained during different stages of experimental or natural toxoplasma infection revealed close agreement between the results of the two methods used. The expediency of regular DIG-ELISA as well as of a modified procedure for the direct analysis of blood sampled onto filter paper discs was also demonstrated. There was a close correlation between the results obtained by analysis of serum and of blood adsorbed onto filter paper.

Serological survey for Toxoplasma infections in sheep

A factorial-design survey was performed to determine the prevalence of specific antibody against Toxoplasma in young and adult sheep from 6 areas in 3 different geoclimatic zones in South Australia. Serum samples obtained from 1,159 sheep belonging to 59 flocks were tested by a conventional indirect haemagglutination test (IHAT) as well as by an enzyme-linked immunosorbent assay (ELISA) which used class-specific conjugates to detect both IgG and IgM against Toxoplasma. Titres greater than 64 were detected in 7.4%, 9.2% and 25.2% of the sheep by the IHAT, IgG-ELISA and IgM-ELISA respectively. A significant positive correlation was found between the results of the IgG-ELISA and the IHAT with identical results obtained for 1,050 samples. Antibody detected by all 3 tests was more prevalent and higher in titre in adult sheep than in lambs as well as in sheep from Kangaroo Island than in those from mainland South Australia. Although the regional differences in prevalence suggested a relationship with climate, no significant correlations were detected between the prevalence results and any single climatic factor; namely, average annual rainfall, average annual evaporation or mean temperature range.