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 Vocativ.com 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.
Why?
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%.
Co-Factors
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?
Hypotheses
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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