It is not clear how the pigs reported in our study became infected with
Toxoplasma. Earlier studies in the United States have analyzed the risk factors involved in the occurrence of
Toxoplasma infection in pigs.
11 Major reported risk factors were farm size, presence of cats, and method of rodent control. Our farmer interviews also point to the same risk factors. Three routes of infection can be envisaged for a pig to become infected with
Toxoplasma. The first is the transfer of infection during pregnancy. Although congenital
Toxoplasma infection has been described in pigs, no data are available concerning its incidence. A second route of infection is through ingestion of another intermediate host or through meat products in the feed of the animals. The latter option is unlikely, since pig feed in the European Union is not allowed to contain animal products. Moreover, high temperatures are used in the processing of pig feed, leading to an inactivation of the parasite. Infection due to the uptake of an intermediate host such as a rodent is a possibility that should not be ruled out. Various studies have shown that a small percentage of wild rodents are infected with the parasite.
12 Of interest is the fact that, unlike in humans, vertical transmission in mice can go on for several generations.
13 This implies that rodents can form a reservoir of
Toxoplasma that can be very difficult to control. A third route is an infection due to the uptake of oocysts shed into the environment through the feces of infected cats. From the farmer interviews, it became clear that both free-range and organic farmers kept a large number of cats on their farms and often relied on cats for rodent control. Cats on these farms often had access to the straw bedding and the sites on the farms where feed was stored. These findings suggest that cats could be a major source of
Toxoplasma infection on these pig farms. Despite the presence of various risk factors such as outdoor access, presence of cats, and inappropriate rodent control, the number of animals that became infected was low, often only a small percentage. This could be due to other additional factors that we did not investigate, such as the
Toxoplasma infection status of the rodents and cats on the farm. Small changes in these risk factors may already result in a farm with pigs being free from
Toxoplasma.