A friend writes the following: "the
public health info released by the Brazilian government on zika and
microcephaly is actually pretty good. [Still t]here is a fundamental methodological
problem with all of this… zika may not be the source of the issue. The rise in
microcephaly is correlated with a spike in zika, but it may not be the factor
causing the condition [or it may be a] catalyst of some underlying aetiology."
He is certainly right on the mechanism:
nobody knows yet. Maybe something else is causing the surge in microcephalia
cases, which is the only really big news here: from 147 to 1248, for Brazil,
and from 12 to 646, for Pernambuco, as of November 28 and 30). [References here and here].
For the rest, what we have is less
compelling: in an interview, the head of the Dengue and Arbovirus society spoke
of experimental results showing Zika to damage the brain of mice, but I have
not seen—and I have looked for it—the research he was referring to. For its part, the government’s
acknowledgement of a link is based on the presence of the Zica virus in the
blood and tissue of a single new born baby who died in Ceará. The
virus was also found in the amniotic liquid of pregnant women in Paraiba (Pernambuco's northern neighbour state).
The broader nervousness is based on a
significant number of interviews of mothers of newborns with microcephalia, a
large proportion of whom reported suffering from Zika-like symptoms (which
could have been dengue) during the first months of pregnancy, and on a time
correlation: the surge in mosquito infections, as proxied by diagnosed dengue cases, began 10 months ago and went on for 20 weeks (week 5 to 25 of 2015, see
the graph below), which corresponds to the beginning of the surge in affected
newborns’ beginning on week 40-41 (beginning of October) and quickly increasing
after that, which is more or less what appears to be happening. Nobody knows
precisely if, for how long, and where Zika had been transmitted to people by
aedes aegypti, but if the hypothesis is right, the next 15 weeks will tell. Moreover,
many of the zika cases identified over the last year may have been misdiagnosed
as dengue. Carlos Brito, a member of the Health Ministry's Arbovirus technical
committee considers for instance that "of the dengue cases identified in
the State (of Pernambuco), 80% were, in fact, zika cases."
Now, 80% may look like a lot, so let's do
the math. As of November 16, according to the Health Ministry, there had been 83,601
"probable cases of dengue" in Pernambuco.
Most of the cases of dengue being
asymptomatic (in a proportion of between 1.8:1 13:1), this must
be considered an absolute floor. But to play "safe," let's stay with
the cases identified. Zika cases misdiagnosed as dengue would thus number
around 67,000 for the state or about 0.8% of the its population. Assuming that Pernambuco's share of birth is
proportional to its share of Brazil's population (4%), it would have had about
120,000 births in 2014 (Brazil's total was about 3 million, or 1.5% of its
population). Using this as a proxy for the number of pregnancies, and factoring
the fact that the peak infection period is about four months-long, this would
imply a floor of about 350 pregnant mothers infected by the Zika virus in Pernambuco
over the course of 2015. As noted above, there are already more than 600 cases
of microcephalia this year in the state…
In other words: if there is indeed a causal link between zika and
microcephalia, the number of zika infections that should be used as a basis of
impact assessment has to be much larger than even 80% of the suspected number of
dengue cases. In addition, the calculations made above and leading to the projected 350
cases assumes that 100% of pregnant mothers transmit the virus, and that 100%
of the foetuses are affected by it. Anything lower than that would
massively increase the hypothetical number of infections.
