Comparison of the immune response and pathogenesis in sheep and cattle to Toxoplasma gondii infection

Abstract

Toxoplasma gondii is an extremely successful parasite capable of infecting all warm blooded animals. However, there is a wide variation between different animal species in their vulnerability to infection, providing a fascinating opportunity for comparative studies on the host/parasite relationship.

The main purpose of this thesis was to compare primary oral Toxoplasma gondii infection in two animal species, sheep and cattle, which are thought to differ in their respective vulnerability to infection. The parameters examined were the immune responses, pathogenesis and persistence of the parasite within tissues.

The rationale behind this study was based on reports that cattle seem to be more resistant to the outcome of Toxoplasma infection. Congenital disease is rarely reported and cattle are thought to harbour fewer parasite tissue cysts which may not persist for the life time of the host. Whereas in sheep, a primary infection usually results in the host remaining chronically infected for life and may result in abortion if primary infection occurs during pregnancy. It was hoped that these studies may indicate the critical factors involved in controlling T. gondii infection and increase our understanding of the events leading to persistence of the parasite within tissue cysts in the muscles and central nervous system of infected food animals. T. gondii tissue cysts in animal meat are an important source of infection for people, and recrudescence of parasites within tissue cysts in the central nervous system of immunosuppressed people, is one of the most common causes of death in AIDS patients. Host immune responses, in particular T cells and IFNγ, are known to play an important role in determining the outcome of Toxoplasma infection. Much of the data on host immune responses is derived from the study of laboratory mice which are extremely vulnerable to infection. There is comparatively little data from other animal species.

An initial experiment was conducted to determine an appropriate dose of T. gondii oocysts which would consistently produce detectable amounts of parasite cysts in sheep tissues. Groups of sheep were orally infected with a titrated dose of oocysts ranging from 10³ to 10⁵ and their tissues were examined 6 weeks later at post mortem. An infective dose of 10⁵ sporulated oocysts gave the best result with T. gondii parasites being detected consistently in samples of heart and brain tissues. Two further experiments were conducted to compare T. gondii infection and host immune response in groups of sheep and cattle which were orally infected with either 10³ or 10⁵ T. gondii oocysts. Clinical responses, humoral and cell mediated immune responses were examined along with parasitaemia and the presence of T. gondii in various tissues at post mortem. The most striking difference between the sheep and the cattle was that T. gondii was more frequently and consistently detected in ovine tissues, in particular within brain and heart tissues, whereas parasites were not detected in the cattle samples. Although there was a marked difference in parasite detection, negative results do not necessarily exclude the possibility of persistent infection due to the limitations of sample size. Infection of sheep and cattle with either a low (10³) or high (10⁵) dose of oocysts provoked no more than a mild clinical response. The sheep developed an earlier febrile response which persisted longer than in the cattle. T. gondii was more frequently detected in the blood of sheep and cattle given the high dose of oocysts (10⁵). Both, cattle and sheep, seroconverted following oral infection with antibody titres appearing to correlate with the infective dose. The higher the infective dose the higher the antibody titre. Specific antibody was detected earlier in sheep given 10⁵ oocysts compared with 10⁴ or 10³. In agreement with previous studies on the immune response of sheep to T. gondii, an increase in the ratio of CD8⁺: CD4⁺ T cells occurred in peripheral blood following Toxoplasma infection. In cattle there was an early, brief increase in the percentage of CD4⁺ T cells. The cytokine IFNγ was more frequently detected in blood plasma samples from cattle compared to sheep in the 3 weeks following infection with either 10³ or 10⁵ oocysts. In addition when in vivo primed peripheral blood lymphocytes were stimulated in vitro with specific T. gondii antigen the cells from cattle showed stronger proliferative activity which was detected more frequently compared with the sheep cells under the same conditions. There was more IFNγ present in supernatants from activated cattle cells compared to sheep cells.

It is difficult to draw any precise conclusion as to why cattle may be less vulnerable to T. gondii infection than sheep. In general the cattle appear to be better able to control the parasitaemia resulting in fewer T. gondii cysts being detected in their tissues compared with sheep. Interestingly the cytokine IFNγ was more frequently detected either from plasma or activated cell supernatants in the infected cattle compared to the sheep. This cytokine is known to play an important role in protection against T. gondii and has been implicated as a factor involved in the differentiation of tachyzoites to bradyzoites (tissue cyst stage). Recent data would suggest that production of IFNγ can occur via NK cells very early in infection and that this may have a profound effect on the ability of the host to withstand infection. Future studies should perhaps focus on local responses at the very early stages of infection as measurement of peripheral immune responses may not be sufficiently sensitive.