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Is there a link between the Zika virus and toxoplasmosis?

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Since each can cause microcephaly, is there any correlation between Zika [which everyone seems to fear] and toxoplasmosis [which most seem to have forgotten]? —Maja Ramirez

Are Zika and toxoplasmosis related? Biologically speaking, that one's easy: no. In seeing them as similar, are you nonetheless onto something? Nice work, Maja—the medical world sees it the same way.

Zika needs little introduction, as you suggest, having vaulted into public awareness a few years ago following scary outbreaks in South America. It's a mosquito-borne virus, kin to yellow fever and dengue. Toxoplasmosis is another, possibly weirder story: Linked to schizophrenia, it's caused by a parasite, Toxoplasma gondii, that lives in cats and spreads via their feces. As discussed here in 2006, one theory is that T. gondii evolved to cause rodents to hallucinate and behave irrationally, increasing their likelihood of being caught by cats and thus the parasite's likelihood of reproduction.

So although infection with Zika or toxoplasmosis during early pregnancy can each indeed result in microcephaly—an unusually small head or brain in the developing fetus—that doesn't make them any more related than two random diseases that might cause blindness.

But since the early '70s, doctors have grouped toxoplasmosis with a few hard-to-distinguish but otherwise unrelated in-utero infections that share some grim traits: they might cause only mild illness (or none at all) in the pregnant mother but severe problems in the fetus, and treating the mother prenatally doesn't usually improve the outcome for the child. This original group of pathogens—toxoplasmosis, rubella, cytomegalovirus, and herpes simplex—was given the acronym TORCH; the O later came to stand for "other" infections that might present similarly, notably syphilis.

Use of TORCH as a diagnostic tool varies from region to region. U.S. medical societies don't recommend full prenatal screening; doctors typically check pregnant patients for rubella antibodies, but even if they're not there, it's too dangerous to administer a live-virus vaccine with a fetus in the picture—all you can do is keep an eye out for symptoms in the baby once it's born, and vaccinate the mom later so it's not an issue again. Most often, infants are tested for the TORCH agents if they display certain telltale indicators: microcephaly is the most dire, but the list also includes hearing loss, cataracts, jaundice and others.

In the worst cases, of course, there might be little to be done for the newborn, but the screening's valuable in any event. Doctors need to establish whether congenital issues are the result of heredity, meaning there's a high risk of recurrence in the mother's future pregnancies, or if an infection was the cause instead. Also, as clinicians will tell you, generally parents just want to understand what happened to their baby. TORCH screening can't always help with the problems of a particular infant, but from a public health standpoint it's important to know what caused the problem so we can prevent it from afflicting future children.

And in the last few years, doctors have come to recognize Zika as the newest member of the TORCH group. The virus was first identified in Uganda in 1947, and for decades wasn't thought to be a big deal: Only 20 percent of those infected experienced symptoms, and these were things like fever and achy joints—nothing too heinous. This view prevailed until a series of outbreaks in the 2000s caught everyone's attention—particularly in Brazil, where an explosion of Zika infections beginning in 2013 coincided with a terrifying 20-fold increase in fetal microcephaly. In 2016, the Centers for Disease Control concluded that Zika was a cause of microcephaly and other severe brain defects.

The story of rubella, seemingly the best analogue to Zika among the TORCH group, is a pretty close match: it wasn't considered dangerous for about 50 years after its discovery, until it began to be linked with congenital defects in the 1940s. In the American rubella epidemic of 1964-65, an estimated 50,000 pregnant women were among the 12.5 million new cases, and the result was a tragedy: 20,000 babies with serious birth defects, thousands more dead in infancy or during pregnancy. By 1969, we'd found vaccines and started administering them to children; now annual U.S. cases are in the single digits.

Rubella, says the author of one 2017 paper on Zika, "can be viewed as a model for a TORCH virus that has been controlled through the widespread development of an efficacious vaccine." Again, you can't vaccinate already-pregnant women against rubella without endangering the fetus; to beat the disease, we had to vaccinate the whole population. Presumably that's how a Zika vaccine would work, too.

Of course, we don't have a Zika vaccine yet—and, in the U.S. at least, neither have we acquired any natural immunity via exposure. As of the mid-1960s epidemic, rubella had been kicking around long enough that most American women had developed antibodies; things could have been a lot worse. Zika, by contrast, hasn't really shown up here at all thus far. Vaccine researchers are on the case, needless to say, but the clock's ticking.

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