Toxoplasmosis is an important infectious disease of sheep and humans that may result in abortion, stillbirth or foetuses born with congenital infection.
The disease is caused by Toxoplasma gondii, one of the most successful parasites worldwide, as it can infect all warm blooded animals. It is very small, consisting only of a single cell and is transmitted through ingestion of oocysts shed by cats or by the consumption of undercooked infected meat containing bradyzoite infected cysts. Contamination of the environment (farmland, gardens, rivers and coastal waters) by Toxoplasma oocysts is widespread and the parasite can survive for up to 18 months in cool and moist conditions. The extent of the environmental contamination with oocysts has been highlighted by some recent work showing that Toxoplasma oocysts contaminating sea water are being filtered and concentrated within shellfish. The marine mammals eating the contaminated shellfish were then overwhelmed by acute Toxoplasma infection.
Toxoplasma gondii is one of the most important causes of abortion in sheep and goats worldwide and in the UK it is the second most frequently diagnosed cause of ovine abortion, only surpassed by Chlamydophila abortus. Moredun was involved in conducting the efficacy testing of the only commercially available vaccine worldwide to protect against ovine toxoplasmosis, using a live attenuated “incomplete” strain of T. gondii. Research at Moredun showed that immunisation of sheep prior to mating with the S48 strain of T. gondii induced both humoral and cell-mediated immune responses and gave solid protection against an oral challenge with T. gondii oocysts. This protective immunity was found to be effective for up to 18 months after the initial immunization. The vaccine is currently marketed as Ovilis Toxovax ™ by Intervet Schering Plough Animal Health.
Disease in humans, due to T. gondii infection, is usually attributed to infections of the foetus if the mother has contact with the parasite for the first time during pregnancy. Immunosuppressed individuals can also suffer serious disease as a result of Toxoplasma infection where problems can arise from a recrudescence of a previous latent infection. However, new data from South America has identified novel strains of the parasite with unusual allele combinations that are causing eye disease and blindness in people through acquired infections. Parasites with unusual allele combinations have been observed in 5% of oocysts shed by cats in a recent study in Germany. This has raised concerns that Toxoplasma gondii may be a re-emerging zoonotic pathogen. The European food safety agency has recently highlighted that there is a need to conduct studies to determine the prevalence of T. gondii in food animals in Europe and to apply molecular diagnostics to determine genetic diversity within strains present.
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