Toxoplasmosis is a disease caused by the parasite Toxoplasma gondii, a microorganism that completes its life cycle exclusively inside cells. An important part of this protozoan's life cycle takes place in the intestines of felines, cats included. In fact, if a cat ingests infected meat containing the parasite, it becomes a carrier, and its feces become infectious for humans as well as for all other animals. It's different if an animal other than a feline ingests the parasite: in that case, the parasite cannot be transmitted through the animal's feces, but only through the consumption of its raw or undercooked meat. This is, in fact, the main route of transmission of this disease, along with eating fruit and vegetables that have not been properly washed under running water and handling soil or gardens where infected felines may have passed. Today, direct contact between a pet cat and a human is fairly rare, mainly because pet cats are generally fed canned food and their litter box is cleaned every day (the parasite living in feces needs about 3 days to become infectious). The real reservoir for this parasite is stray cats, which pick up the infection from contaminated birds or mice and, by defecating on the ground, can release the parasite for several weeks.
Infection with Toxoplasma gondii can occur at any point in a person's life. Immunocompetent individuals generally do not develop symptoms, or show only nonspecific symptoms such as fever, headache, muscle aches, and swollen lymph nodes. For this reason, although a third of the world's population is infected with the parasite, a specific diagnosis of toxoplasmosis is often never made.
The level of attention required is different when it comes to a pregnant woman, which is why one of the tests a pregnant woman undergoes is precisely the toxoplasmosis test. At this stage, it becomes extremely important for the woman to find out whether she has contracted the parasite at any point in her life. The test consists of a blood sample in which the antibodies produced in response to infection, or the parasite's genetic material (DNA) circulating in the bloodstream, are looked for, and it is carried out to detect an active or past infection. For a pregnant woman, or one planning a pregnancy, having contracted the parasite in the past is actually an advantage, since Toxoplasma gondii infections occurring before pregnancy do not pose a risk to the fetus if contracted up to 3 months before conception; furthermore, as a general rule, once the body has been infected, the acquired immunity protects against further Toxoplasma gondii infections.
If a woman contracts the parasite during pregnancy, conditions such as hydrocephalus, intracranial calcifications, and chorioretinitis can occur in the newborn. The timing of infection during pregnancy plays a key role in the onset of these conditions: infection early in pregnancy carries a small risk of fetal transmission (under 6%), while transmission rates range between 60% and 81% in the third trimester. Conversely, although transmission of Toxoplasma gondii during embryogenesis is rare, the effects that can appear on the fetus are far more severe than those seen when the maternal infection occurs in the third trimester; in the latter case, newborns are in fact often asymptomatic (though it's important that they still be treated appropriately to prevent them from developing neurological deficits in childhood or early adulthood). This condition shouldn't be a cause for alarm if handled correctly: taking the test allows the pregnant woman to know how much attention to pay to activities such as washing vegetables and fruit if eaten raw and unpeeled, or to caring for her pet cat, including cleaning its litter box. It's important to understand that there is no need to send your cat away when you're pregnant; a few small precautions are enough if the woman has not contracted the parasite in the past. For the more cautious, it's also possible to ask your vet for a blood test on the cat to rule out any infection.
Finally, it's worth noting that dogs can contract the disease but cannot spread it in any way, just as is the case for humans, so dogs are not a cause for concern when it comes to toxoplasmosis and pregnancy.
When planning a pregnancy, it's advisable to check anti-toxoplasmosis antibodies through a blood test. The test checks for immunoglobulin G (IgG) and immunoglobulin M (IgM). IgG antibodies are also known as «memory antibodies» because they remain elevated after the first contact with toxoplasma, ensuring long-lasting immunity; IgM antibodies, on the other hand, are antibodies whose levels rise during acute infection (allowing diagnosis) and then fall, returning to normal after a period that varies from person to person. If a recent infection is suspected and the blood sample tests positive for Toxoplasma, the binding avidity of the IgG antibodies can be checked. This value increases the further away one is from the acute infection. In cities, infection with toxoplasma is not very common, so most women will need to pay attention to stray cats and food during pregnancy and have their anti-toxoplasma antibodies checked at regular intervals throughout it.
References
- Bollani, 2014, Journal of Pediatric and Neonatal Individualized Medicine
- Shahnaz Akhtar Chaudhry, 2014, Can Fam Physician
The public health epidemiology portal, curated by the Istituto Superiore di Sanità (Italian National Institute of Health): www.epicentro.iss.it/toxoplasmosi/