Toxoplasmosis in sheep II. The ability of a live vaccine to prevent lamb losses after an intravenous challenge withToxoplasma gondii

Abstract

Two-tooth ewes (n=48) were immunized pre-tupping with a live Toxoplasma gondii vaccine. At midpregnancy these ewes were challenged intravenously with 1 x 105 live T. gondii tachyzoites. The strain of T. gondii used for vaccination was an incomplete strain that did not produce oocysts. It was derived by continuous twice weekly passage in mice. The lambing percentage for ewes immunized with the live vaccine was significantly higher (P<0.001 normal score) than non-vaccinated control ewes. However, vaccination did not prevent foetal or placental infection. The serological response to vaccination and challenge was measured by both the Dye test and the Indirect Haemagglutination test. No significant relationship between titre of antibody and protection in the vaccinated ewes was observed.

Increased Toxoplasma gondii positivity relative to age in 125 Scottish sheep flocks; evidence of frequent acquired infection

“…Toxovax ® was licensed in 1988 in New Zealand and was launched in 1992 in UK and Ireland [12]. Toxovax ® consists of a live attenuated T. gondii strain (S48) that will not cause a persistent infection [13,14]. The efficacy and safety of this vaccine were tested by Intervet B.V. in collaboration with the Moredun Research Institute in Scotland, where it was shown that a single vaccine dose will induce an effective immune response that will persist for at least 18 months without further challenge [15].…”

Induction of CD4+ and CD8+ T cell responses in efferent lymph responding to Toxoplasma gondii infection: analysis of phenotype and function

“…Innes et al Parasite Immunolo,Cv T. gondii occurs through successive generations (Beverly 1959) whereas in women and sheep congenital infection occurs only if a primary infection takes place during pregnancy. Also, the S48 strain of T. gondii which causes very mild clinical responses in sheep (Buxton et al 1991) is lethal if inoculated into mice (O'Connell, Wilkins & Te Punga 1988). Studies of T. gondii infection in sheep and mechanisms of protective immunity in that species may offer a more relevant model for human infection as well as being of direct interest due to the significance of ovine toxoplasmosis to the farming industry.…”

“…(1994) Seventy-two h after surgery five of the sheep (la-le) were infected with 10' tachyzoites of the S48 strain of T. gondii(O'Connell, Wilkins & Te Punga 1988) in 1 ml of phosphate buffered saline (PBS) by subcuta-…”

Veterinary vaccines against Toxoplasma gondii

“…Another challenge would be to devise methods of antigen delivery that would allow the processing and presentation of antigens within the correct MHC background in order to effectively stimulate the required T-cell responses. This is easier to achieve using a live vaccine preparation and may help to explain the success of the S48 strain vaccine used successfully to protect sheep and goats against congenital toxoplasmosis (O'Connell et al 1988, Buxton & Innes 1995.…”

“…Researchers then observed that the parasites had over time lost the ability to differentiate into bradyzoites or oocysts and had become effectively an incomplete strain (O'Connell et al 1988). The S48 strain tachyzoites induce a short-lived infection in sheep of around 14 days before being eliminated from the host by the sheep's immune system (Innes & Wastling 1995).…”