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We may not have lifelong immunity to toxoplasmosis

Medical students are taught that once infected with Toxoplasma gondii — the "cat parasite" — then you are protected from reinfection for the rest of your life. This should be questioned.

Their concerns stem from a handful of case studies in which expectant mothers in their late 20s and early 30s were known to have been infected by T. gondii at birth but were actually found to in screening to lack immunology protection. The research was published December 8 in an Opinion paper in Trends in Parasitology.

There is a global decrease in the number of people who test positive for toxoplasmosis immunity. In the 1960s, surveys of expectant mothers in France found that 80% or more had antibodies to the parasite. This dropped to 30% in 2010 and is expected to continue to fall. Also observed in the United States, this change could be due to improved food hygiene — particularly better preparation and quality of meat as beef, lamb, and venison are especially likely to carry the parasite. Also, fewer domestic cats consume raw rodent meat, thus transmitting T. gondii to humans.

"We put forward the hypothesis that, in the past, people kept their antibodies to T. gondii because they were very likely to become re-infected. Now that the parasitic pressure has gone down, I think people are less stimulated and they lose their immunity — it's exactly what we see for malaria."

François Peyron, parasitologist, Hospices Civils de Lyon, Institut de Parasitologie et Mycologie Médicale, Hôpital de la Croix Rousse, Lyon, France and lead author.

The uncertainty about the rate of toxoplasma infection is partly due to how infrequently it is reported. Aside from those immunocompromised, only a minority of people will experience side effects, typically flu-like symptoms, after coming into contact with this parasite. Once inside the body, the single-celled parasite travels through the blood into the brain and muscles, where it forms cysts. Researchers believe these cysts remain in an infected person for life and that their presence retriggers the immune system. But Peyron and his co-authors, Solène Rougier of Hospital Croix-Rousse and Jose Montoya of the Stanford University School of Medicine, also challenge this idea.

Toxoplasmosis becomes a problem when a mother is infected while pregnant. Undetected and untreated, the parasite spreads to the developing child and can end the pregnancy or cause the baby to develop brain or eye abnormalities. Countries, such as France, regularly screen expectant mothers with no previous exposure to T. gondii to ensure infection is caught early. But this standard might need revision if there is evidence that previous infection doesn't prevent re-infection.

Peyron: "We have to be concerned by T. gondii, but it has been clearly demonstrated that clinical treatment is very effective at preventing fetal infection and reducing associated conditions."

It is our opinion that this aspect of public health has not been well investigated in many countries, especially in the United States, and provided we keep the price of testing low, the cost of regularly screening pregnant women for toxoplasmosis is less expensive than the cost of caring for a child with disabilities as a result of the infection."

It is possible lifelong immunity is via repeated parasite infections when parasite surface proteins become altered to evade a host immune response.

The research group plans to follow up with hundreds of patients to identify when and for whom T. gondii immunity might be decreasing. In advance of evidence to support their hypothesis, they recommend pregnant women continue to follow guidelines for avoiding infection, such as practicing safe food habits and frequent hand washing.

Researchers also caution those who are pregnant not to believe themselves protected and to speak with a doctor about how often to screen for the parasite.

It is believed that infection by Toxoplasma gondii triggers a lifelong protective immunity due to the persistence of parasitic cysts which induce immunoprotection against reinfection. A review of the scientific literature since the 1950s did not yield any definitive data regarding the duration of cysts in the host or the presence of lifelong protective immunity, which led us to question this dogma. We put forward the hypothesis that sustained immunity to T. gondii requires repeated antigenic stimulations. The decline of seroprevalence recently observed in many countries might contribute to explain the loss of immunity. We address the potential consequences of this phenomenon, should it persist and worsen. Trends Infection with Toxoplasma gondii is believed to induce lifelong protective immunity through long-term persistence and regular rupture of T. gondii cysts. Lifelong persistence of cysts and immunity remains to be demonstrated. We propose that sustained immunity to T. gondii requires repeated antigenic stimulations.

Trends in Parasitology, Rougier et al.: "Lifelong persistence of Toxoplasma cysts: a questionable dogma?" http://www.cell.com/trends/parasitology/fulltext/S1471-4922(16)30190-8

Trends in Parasitology (@TrendsParasitol), published by Cell Press, is a monthly review journal that reflects the global significance of medical and veterinary parasites. It aims to provide a point of access for communication between researchers in all disciplines of parasitology, bringing content that is authoritative and cutting edge, yet accessible to a wide audience of readers. Visit: http://www.cell.com/trends/parasitology. To receive Cell Press media alerts, please contact press@cell.com.
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Dec 21, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   

The uncertainty about the rate of toxoplasma infection is partly due to how infrequently it is reported.
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