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Mystery Still Surrounds Occurrence Of Zika In Northeast Brazil

More than two years after Zika virus was linked to microcephaly cases in northeast Brazil, there is still plenty of mystery surrounding why the outbreak was so severe and did not spread in the in the same way to other countries of the region. Also, cases did not materialize in a second wave in 2016 in northeast Brazil as anticipated.

Recent articles in the Globe and Mail entitled “Zika virus: still no clear answers” and at entitled “Two Years Later, Zika Virus Is Still A Big Mystery” highlight the unanswered questions and describe some of the hypotheses epidemiologists and other have advanced to explain the Zika observations.


The first key question in whether or not the apparent increase in microcephaly cases in northeast Brazil was a real increase or an artifact of reporting.

While the number of reported cases of microcephaly was in the low thousands, there are no reliable date on the number of affected persons in the population and no reliable data on the number of Zika infected pregnant women.  If the number of infected pregnant women was in fact very large because the virus was newly introduced to the population, then the rate of microcephaly cases may not have been higher than what was seen in other populations. Also, it is possible there was overreporting of cases once the alarm about the risk of microcephaly was recognized.

Gut Feeling

Laura Rodrigues, a Brazilian epidemiologist at the London School of Hygiene and Tropical Medicine, told the Globe and Mail “My personal gut feeling is that we had large Zika epidemics with lots of mosquitoes and most people getting infected, in northeast Brazil, and in Cape Verde and French Polynesia…and in other places transmission was much slower, with fewer mosquitoes, more air conditioning, and so on, so it will take years for the same proportion of people to get infected, and microcephalic babies will not peak but are spread over years…[but] a gut feeling is not science.”

Another opinion expressed in the articles is that the lower attack rate in less affected areas may have been reduced because of effective mosquito control efforts, or more widespread use of abortions in populations which had more advance warning than in northeast Brazil.

Data from a registry of pregnant women with Zika infection in the US has shown a rate of approximately 5%.


Fatima Marinho, Brazil’s coordinator of epidemiological analysis and information in the Ministry of Health, told the Globe and Mail “It is becoming increasingly clear that something beyond Zika virus occurred in a part of the northeast region and was one of the causative factors of microcephaly. We have always been been betting on an environmental co-factor, like water, which could carry various contaminants, even biological ones.”

Assuming there was a real increase in the rate of birth defects, why was the rate higher than reported in other areas? Was there some other environmental co-factor that could explain the unusual number of microcephaly cases?


Multiple hypotheses have been advanced or are being investigated including lower than average pre-existing immunity to other arboviruses acquired naturally or from vaccination which could have provided cross-protection against Zika.

Another hypothesis is derived from the observation that there is a high seroprevalence of dengue antibodies in the north of Brazil. Did possible previous exposure to a related arbovirus cause the Zika virus to be more virulent?

From the observation that more cases occurred in less populated small cities and rural areas, some investigators speculate that prior exposure to livestock viruses could have enhanced the response to Zika infection.

Still others have considered the role of the public health system itself in intensifying the outbreak since many of the cases occurred in poor, younger mothers who access these services.

No Answers Soon

The difficulties of conducting the research needed to answer these questions are being compounded in Brazil by budget cuts, competing priorities because of other outbreaks, political turmoil in Brazil, and the difficulty of finding new cases to study. Yale university epidemiologist Albert Ko told the Globe and Mail, “The virus is obviously circulating in Africa and Asia causing outbreaks. It may be a large proportion of women of child bearing age who are susceptible and we need to get a handle on what the risk is.”


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