Investigation of the viability and national serological prevalence of Toxoplasma gondii in Australian sheep

Summary

Toxoplasma gondii is a significant zoonotic parasite (30-40% of Australians are seropositive), which can often be foodborne.

The aim of this project was to fill data gaps regarding toxoplasmosis in sheep and goats to better determine the magnitude of the risk associated with the consumption of undercooked sheep meat.

The survey estimated the national seroprevalence in mutton to be 11.5%.

Objectives

The objectives of this project were to:

• establish gold standard positive and negative controls for sheep and goats to calibrate T. gondii serological tests
• estimate a prevalence of T. gondii infection in sheep through a national survey
• genotype and preserve Australian T. gondii isolates collected during the project to provide a valuable library of Australian strains for future research into risk mitigation.

Key findings

• Eighteen T. gondii isolates were obtained from 18 seropositive sheep, demonstrating potential infectivity in meat.
• A national survey indicated a nation-wide serological prevalence of 11.5% (46/401), with a range by state of 0 to 44%. This compares with the 31.9% prevalence estimated in 2009.
• An analysis of all the Australian T. gondii strains isolated in this study indicate that all 18 isolates are genetically highly similar (clonal), regardless of geographic location, yet distinct from most global strains to which they were compared. In particular, they were not clustered with any of the three South American isolates examined that may be more virulent in immune competent humans.

Benefits to industry

• The results of this project fill a number of existing data gaps with regard to toxoplasmosis in Australian sheep and highlight the importance of developing a risk management strategy to deal with this issue.
• The demonstration of T. gondii viability in sheep meat combined with an improved seroprevalence estimate, allows industry to more effectively assess consumer risk.
• The library of cryo-preserved Australian isolates will provide a useful resource for developing future mitigation capability.

MLA action

MLA will carefully consider the risks posed to sheep and to humans from toxoplasmosis, as well as the most likely approaches to control and their cost before embarking on further research. The size of the risk, and therefore cost, of toxoplasmosis in terms of sheep reproductive efficiency and human health impacts due to sheep meat consumption are not known. Despite not knowing, with any precision, the effects on the sheep industry and public health burden, investigation of strategies to reduce the risk could be conducted.

Future research

• Although mature sheep are likely to have higher prevalence of infection with T. gondii, due to the life-time persistence of viable infection, it is lamb that is most likely to be eaten rare and therefore provide the greatest foodborne risk to consumers. A lamb seroprevalence survey should be conducted.
• It is uncertain to what extent normal sheep meat storage parameters reduces that viability. Freezing kills the organism but the effect of chilling over time is less understood.
• An understanding of the ecological situation that leads to infection could also be useful to define how to prevent infection.