Sources of Toxoplasma gondii infection in pregnancy

Humans commonly acquire Toxoplasma gondii infection by ingesting food and water contaminated with the resistant stage of the parasite (oocyst) shed in the faeces of infected cats or by ingesting the encysted stage of the parasite (tissue cysts) in infected meat.¹ Cats can rapidly shed millions of oocysts after eating rodents, birds, or other animals infected with T gondii, and these oocysts can remain viable in the environment for many months.¹ Toxoplasma, listeria, and salmonella are the three most important pathogens carried by food in terms of illness and death in the United States and perhaps in Europe. Pork, lamb, and mutton are the most important sources of T gondii infection, along with game meats such as bear and feral swine.^2–4

There is no test to distinguish infections from oocysts as opposed to tissue cysts. Therefore, epidemiological surveys remain the most useful way of assessing the relative importance of different sources of T gondii infection in human beings. This approach is not so effective when subjects are chronically infected and infections might have occurred many years before. The paper by Cook et al in this issue (p 142) reports risk factors for acute toxoplasmosis in women who acquired infection during pregnancy.⁵ A total of 252 women with toxoplasmosis, along with 748 controls from Naples, Lausanne, Copenhagen, Oslo, Brussels, and Milan, were interviewed by telephone or in person. Overall, eating raw or undercooked beef, lamb, or other meats; contact with soil; and travel outside the country were major sources of infection.⁵ Surprisingly, the risk of T gondii infection was increased in women who reported tasting or eating raw or undercooked beef, lamb, or game meats, but not pork. However, it is well documented that lamb and goat meat are sources of T gondii.⁶ Finding beef as a source of infection is unexplained because T gondii has never been isolated from edible beef in Europe or North America.¹ Adulteration of beef by cheaper meats is not uncommon in grocery stores, and this is one possible explanation for the association of infection with beef. Further studies are needed in both beef and poultry, in live animals and retail meats, to adequately assess the role of these species in human infections.

In the past, pigs have been regarded as the most important meat source of T gondii infection in humans.¹ Fortunately, the prevalence of T gondii in market pigs (six month old pigs) is declining drastically in Europe and North America.⁶,⁷ A higher risk of T gondii in women who ate raw sausages, salami, and cured meats is easily explained because the prevalence of the organism is high in older animals; for example, it was isolated from 17% of 1000 sows from one abattoir in the United States.⁸ The authors point out that indoor pigs are less likely to harbour T gondii, and this is true if good biosecurity is practised. Consumers tend to thoroughly cook pork, a holdover from fears of acquiring trichinellosis. The consumption of unpasteurised milk or milk products, whether from goats, sheep, or cows, was an unexplained risk because tachyzoites, the stage most likely to be present in milk, are thought to be destroyed immediately by gastric juice.⁵ However, recent evidence indicates that ingested tachyzoites can cause infection.⁹ There has also been a case of acute toxoplasmosis in a breast fed infant.¹⁰

The association of cats and human toxoplasmosis is difficult to assess by epidemiological surveys because soil, not the cats, is the main culprit. Oocysts are not found on cat fur and are often buried in soil along with cat faeces.¹¹ Therefore, direct contact with cats is irrelevant with respect to T gondii transmission, and soil contact is universal and difficult to avoid.

The study by Cook et al is informative and is useful to the public health community. However, risk assessments such as this may be influenced by the small sample size and reliability of personal interviews. Therefore, additional studies of this type should be encouraged to add confidence to our understanding of risks of human exposure to T gondii. At the same time, there is an urgent need to determine the prevalence of the organism in edible meats in grocery stores so that relative risks of acquiring toxoplasmosis from different types of meats can be accurately assessed. The authors rightly point out that although prevalence of T gondii has declined in adult human beings and pigs by advocacy of control measures, the incidence of congenital toxoplasmosis in children has not decreased in the past decade.⁵,⁷,¹² Therefore, health education and control measures (eating frozen, well cooked meat, keeping cats away from pig housing) must continue.

Papers p 142