Suspected M&CD cases...
"At this stage, based on the evidence available,
WHO does not see an overall decline in the outbreak."
That statement refers to current data whereas Brazil's M&CD report #23 is describing events that may have occurred approximately 20 to 40 weeks ago.[3] I picked that range because foetal disorders with preceding ZIKV detection have been reported to occur very rapidly, between 17 and 20 weeks in one case study.[4]
Confirmed and discarded M&CD diagnoses...
M&CD cases under investigation decreased by 31 to 3,710 this week - the fifth consecutive decrease.
In the graph above, we can see that 79 (blue bars; right hand axis) suspected M&CD diagnoses were discarded upon closer investigation.The rate of these resolved diagnoses (line with blue dots, left-hand axis) is slowing but still seems to be outpacing the rate of confirmed M&CD diagnoses (red dots, left-hand axis). The cumulative number of M&CD diagnoses does continue its climb this week (+30), but this is the smallest addition of new diagnoses on record (red bars; right-hand axis).
The number of these M&CD diagnoses to be confirmed with a ZIKV infection also grows (green dots; left-hand axis) but by just 2 new detections to 194 this week (green bars; right-hand axis). That's the smallest rise on record. Those confirmed ZIKV infections represent 16% of all confirmed diagnoses and 3% of all suspect M&CD cases - but these are not fair comparisons for a range of reasons I won't go on about here.
The number of these M&CD diagnoses to be confirmed with a ZIKV infection also grows (green dots; left-hand axis) but by just 2 new detections to 194 this week (green bars; right-hand axis). That's the smallest rise on record. Those confirmed ZIKV infections represent 16% of all confirmed diagnoses and 3% of all suspect M&CD cases - but these are not fair comparisons for a range of reasons I won't go on about here.
Why are M&CD diagnoses slowing...?
It's not precisely known why the numbers of M&CD diagnoses are slowing in Brazil. Whatever their cause(s), the trigger(s) for the anomalies occurred in the past - as far as 9 months or as close as perhaps 20 weeks. One suggestion which ties in with the timing of arbovirus epidemics in Brazil, is the weather and its impact on the mosquito breeding cycle. (Thanks Luis F. B. Correia) Mosquitoes are assumed to be the major vector for transmitting ZIKV, wherever in the world it may be. That's a safe bet as far as we know. Undoubtedly, reporting on mosquito detections is not a priority. Only Mexico has reported infected wild-caught mosquitoes to date.[5] There have been more monkeys identified as ZIKV-infected than mosquitoes![5,6] For now at least.
It's not precisely known why the numbers of M&CD diagnoses are slowing in Brazil. Whatever their cause(s), the trigger(s) for the anomalies occurred in the past - as far as 9 months or as close as perhaps 20 weeks. One suggestion which ties in with the timing of arbovirus epidemics in Brazil, is the weather and its impact on the mosquito breeding cycle. (Thanks Luis F. B. Correia) Mosquitoes are assumed to be the major vector for transmitting ZIKV, wherever in the world it may be. That's a safe bet as far as we know. Undoubtedly, reporting on mosquito detections is not a priority. Only Mexico has reported infected wild-caught mosquitoes to date.[5] There have been more monkeys identified as ZIKV-infected than mosquitoes![5,6] For now at least.
The rainy season begins in the north-east (where M&CD has mostly been diagnosed) in May. Infections are likely to pick up from then - if we crudely add 6-9 months to May 2015 we get to October-2015 - January-2016.
August saw a rise in microcephaly diagnoses in Pernambuco State in the north-east.[6]
November is when the Brazil ministry declared a link between M&CD and ZIKV.[7]
It all kinda fits.
None of these numbers are precise though because when and where ZIKV was becoming established in Brazil is not known. It is predicted to have been as early as May-December 2013 based on analysis of viral genome sequences to hand,[9] but local transmission was not detected (looked for?) until April/May 2015.[9,10] It may have been there earlier.
Solid data and answers continue to evade trapping. As do ZIKV positive mosquitoes apparently.
References...
- http://portalsaude.saude.gov.br/images/pdf/2016/abril/27/COES-Microcefalias---Informe-Epidemiol--gico-23--SE-16-2016--25abril2016-20h07.pdf
- http://portalsaude.saude.gov.br/index.php/cidadao/principal/agencia-saude/23386-saude-confirma-1-198-casos-de-microcefalia-no-pais
- http://apps.who.int/iris/bitstream/10665/205686/1/WHOsitrep_28Apr2016_eng.pdf?ua=1
- http://www.nejm.org/doi/full/10.1056/NEJMoa1601824?query=featured_zika
- http://virologydownunder.blogspot.com.au/2016/04/biting-assumptions.html
- http://virologydownunder.blogspot.com.au/2016/04/three-monkey-species-and-some-labs.html
- http://www.theguardian.com/global-development/2016/jan/25/zika-virus-mosquitoes-countries-affected-pregnant-women-children-microcephaly
- http://wwwnc.cdc.gov/eid/article/22/6/16-0062_article
- http://portalsaude.saude.gov.br/index.php/cidadao/principal/agencia-saude/23384-saude-divulga-primeiro-balanco-com-casos-de-zika-no-pais
- http://science.sciencemag.org/lookup/doi/10.1126/science.aaf5036
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