Zika and Correlation vs. Causation

Zika and Correlation vs. Causation

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Last week I told you of my admiration for Dr. Mona Hanna-Attisha, the Michigan pediatrician and epidemiologist whose strong research and advocacy was able to finally bring a shining light to the problem of lead in the water supply of Flint.

Continuing with a theme, I now bring you the story of Dr. Adriana Melo of Campina Grande, Brazil.

Dr. Melo is an OB-GYN who subspecializes in Maternal-Fetal Medicine (MFM), the branch of obstetrics that deals with high-risk pregnancies.

She lives and works in northeast Brazil, which is less populous and more economically challenged than the southern, more well-known parts of the country (including Rio de Janeiro and Sao Paulo).

Dr. Melo noted an uptick in the number of fetuses with small heads on ultrasound — which is the main tool used by MFM doctors to diagnose babies in utero.

How much of an uptick? A rough look at the statistics shows ONE HUNDRED times the ‘normal’ rate of babies born with microcephaly, the medical name for the condition.

Dr. Melo had a suspicion that the mothers giving birth to these babies all had a common trait: they’d all told her that they’d had the characteristic rash associated with the mosquito-borne Zika virus.

When she tested the mothers for evidence of the Zika virus in their blood, the tests were negative. Not deterred, she convinced public health authorities to test the amniotic fluid of mothers carrying microcephalic fetuses. And indeed a strong correlation was found between exposure to Zika and microcephaly.

It’s this story of a doctor in a somewhat out-of-the-way place using her clinical insight to ‘prove’ a correlation which I find inspiring.

Dr. Melo could have been content to merely diagnose and treat these poor mothers and babies, perhaps simply ‘reporting up’ her findings on the increase in microcephaly. Instead, she decided to push against the inertia of daily medical practice because what she was seeing really bothered her — and as a mother of young children herself, she felt the urge to get to the bottom of the new trend.

If you follow health news, you no doubt have heard a lot about the Zika virus in the last few weeks, including warnings from both the CDC and the World Health Organization. As is often the case with warnings from these organizations, a certain amount of panic ensues — such as women in Latin America feeling that they’re being told not to get pregnant, for example.

I want to make it very clear that though there is a STRONG ASSOCIATION between the rise in cases of Zika in the tropics of the Western Hemisphere and a concurrent rise in babies born with microcephaly, we must remember: Correlation does not equal causation. The public health agencies issuing travel and birth control warnings, while sounding dire, are making best guesses for us all to minimize our chances of harm. But drowned out in the response is the fact that we don’t yet know for certain that Zika is the cause of microcephaly. That work is ongoing.

For example, many experts think something else may be the cause — perhaps the use of dangerous pesticides in Brazil (that are banned elsewhere). That also sounds plausible since pesticides are used to “control the mosquito vector.” It’s entirely possible that microcephaly is occurring because of a chemical effect.

For now, we must wait and hope that science can show us the true cause of the uptick in microcephaly.

I also think it’s important to remind ourselves of two things about Zika virus: We’ve known about it since the 1940s, when it was discovered in Africa — so though if feels new, it’s really not. Secondly, at least for non-pregnant people, it only appears to cause mild flu-like symptoms and be a self-limited illness (not more than a few days at most).

My advice: Better to save your anxiety for the never-ending Presidential race.

John Henning Schumann, MD, is President of the University of Oklahoma – Tulsa. He blogs at GlassHospital.com and is on Twitter: @GlassHospital.

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