Abbey Olsen: Seroepidemiology of Toxoplasma gondii in Danish pigs & the zoonotic potential from pork consumed in Denmark

PhD student: Abbey Olsen

Project background
Among various sources of human toxoplasmosis, infection from consuming undercooked meat is an important one, as the meat may contain viable Toxoplasma gondii parasites. Acquired toxoplasmosis is rarely reported in healthy humans because the symptoms are either mild or absent. But toxoplasmosis in immunocompromised individuals like those with HIV, infection can be life-threatening. In pregnant women, infection may lead to abortions or infection may be passed on to the fetus resulting in brain damage or blindness from congenital toxoplasmosis.

Like humans, pigs rarely show signs of illness. However, after picking up an infection from the environment or from preying on infected rodents or birds, pigs may develop cysts in their muscle tissues. These cysts are not macroscopically visible, and therefore a previous or an active infection in a pig is detected by identification of IgM or IgG antibodies produced against the parasite. Currently, pigs are not surveyed for T. gondii infections. As the disease burden from toxoplasmosis is estimated to be high worldwide, the World Health Organization identifies T. gondii as an important foodborne parasite. In Western and Northern Europe, pigs are, for welfare reasons, increasingly raised outdoors. However, outdoor raising is a risk factor for T. gondii infection. To enhance food safety, the European Food Safety Authority recommends surveillance on high-risk farms, where biosecurity could be improved. This PhD aimed to evaluate whether and where there might be need for risk mitigation to reduce exposure to T. gondii from consuming pork in Denmark.

Purpose of the project 
The project aimed to generate knowledge and results that could be used by the risk manager to evaluate whether there was a need to control exposure to T. gondii in Danish pork either through reducing the prevalence of infection in the Danish pig population or to mitigate the risk directly in the pork. If surveillance was deemed necessary in future, then the aim was to provide decision supporting information that would enable implementation of a surveillance system, capable of identifying high-risk herds through serology, for example using meat-juice samples from the existing Salmonella surveillance program. An additional aim was to identify potential high-risk products for T. gondii in Denmark.

T. gondii infection was present in indoor- and outdoor-raised finishers reared for pork and in sows reared for producing piglets. Serological surveys indicated a low prevalence in indoor-raised finishers, suggesting biosecurity measures for Trichinella control are also effective in controlling T. gondii. In outdoor-raised finishers, prevalence was higher probably because biosecurity is lower than seen indoors. In sows, the prevalence was generally higher than in finishers, among other reasons, because of increased chance of becoming infected during the longer lifespan.

The results of the thesis support EFSA’s recommendation for auditing of the requirement of controlled housing conditions (high biosecurity) in indoor-raised finishers and surveillance in outdoor-raised finishers for identification of high-risk herds. Sows are more likely to carry cysts than finishers. But their meat is used for making processed products that are treated with high concentration of salt, which is known to kill cysts. Hence, a socio-cost-benefit analysis looking into the effect of freezing all sow meat versus surveying is needed for decision-making. In a close follow-up of sows on four farms for one year, a lower level of infection was observed compared to the slaughtered sows possibly related to slaughter of sows with T. gondii related reproductive disorders. Also, antibodies in many newly infected sows lasted for a short duration only, raising questions regarding the true infection status. This makes surveillance of sows using serology challenging. In Denmark, the zoonotic risk from T. gondii was associated with products that were either insufficiently treated with heat or salt or both.