Are pesticides causing the birth defects in Brazil, not Zika? originally appeared on Quora - the knowledge sharing network where compelling questions are answered by people with unique insights.
Answer by Tirumalai Kamala, Immunologist, Ph.D. Mycobacteriology, on Quora.
Pesticides are unlikely to be directly involved in the Brazil birth defect issue. If they are, link is more likely indirect. By now, several groups in different countries have found Zika but not other known pregnancy-associated viruses in placenta and brain of several newborns with microcephaly as well as in miscarriages where the mothers were known to have had Zika infections in their 1st trimester. These data strengthen the link between Zika and birth defects. Proof is still a ways away, requiring larger studies, more numbers, expanded diagnostic tools to confirm Zika, not just Reverse transcription polymerase chain reaction but also culture, etc. Meantime, other questions about Zika remain unanswered, and that's where the role of environmental factors comes into the picture. Discovered in the Zika forest in Uganda in 1947, Zika never caused outbreaks until recently.
Starting in 2007, Zika's been cutting a swath through human populations in its path as it spread first from Africa to Asia then across the Pacific and now into Latin America. First, the outbreak in the Micronesian island of Yap in 2007, then another larger one in French Polynesia in 2013-2014. Symptoms as well becoming more severe. After all, Guillain-Barré syndrome was first documented in that French Polynesia Zika outbreak, ~40 GBS cases out of a total of ~8510 suspected Zika cases there (~0.5%) (1, 2, 3). Clearly, Zika's changing as it moves across the world (4, 5, 6). These studies show there are now at least two lineages, the original African and the newer Asian. The latter seems to be the one currently sweeping across Latin America. Two questions emerge at this point.
- What drove these changes in Zika?
- Do these changes make it more virulent, neurotropic, embryotoxic, teratogenic?
Potential role for pesticides/insecticides emerges as a possibility for question 1, specifically as one of several environmental triggers that could have placed selective pressure on Zika or the mosquito carrying it (7).
As to question 2, French Polynesia GBS as well as emerging GBS reports from Brazil and El Salvador (paho.org 8, 9) suggest Zika may have newly developed either neurotropicity or molecular mimicry to peripheral nerves.
Among environmental triggers, one feature often overlooked in vector-borne diseases is the response these vectors make against the virus. Mosquitoes too have a well-developed immune system and make well-characterized responses against the disease agents they transmit to humans (10). When we use chemical or genetic arsenal to try to wipe them out, we're applying tremendous selection pressures on these mosquitoes. In turn, mosquito responses triggered by such pressures could impose selection pressures of their own on the disease agents they carry. This is at least one plausible scenario to explain how pesticides/insecticides could indirectly have a role in the Brazil birth defect issue, i.e, in helping increase virulence of Zika or some other mosquito-borne virus.
The other unexplained feature of the outbreak in Brazil is the possibility raised by at least one group studying the issue on the ground that milder forms of microcephaly may have been rising in some of the regions in question already (11) even before Zika got there. This too raises the question of what's been changing in this environment to make that happen.
Bibliography
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- Musso, D., E. J. Nilles, and V‐M. Cao‐Lormeau. "Rapid spread of emerging Zika virus in the Pacific area." Clinical Microbiology and Infection 20.10 (2014): O595-O596. Rapid spread of emerging Zika virus in the Pacific area
- Ioos, S., et al. "Current Zika virus epidemiology and recent epidemics." Medecine et maladies infectieuses https://www.researchgate.net/pro...
- Enfissi, Antoine, et al. "Zika virus genome from the Americas." The Lancet 387.10015 (2016): 227-228.
- de Melo Freire, Caio Cesar, et al. "Spread of the pandemic Zika virus lineage is associated with NS1 codon usage adaptation in humans." bioRxiv (2015): 032839.http://biorxiv.org/content/biorx...
- Faye, Oumar, et al. "Molecular Evolution of Zika Virus during Its Emergence in the 20 th Century." PLoS Negl Trop Dis 8.1 (2014): e2636. http://www.ncbi.nlm.nih.gov/pmc/...
- Diniz, Simone G. "Zika virus and pregnancy: The perspective from Brazil." Midwifery (2016).
- Pan American Health Organization. 17 January 2016: Neurological syndrome, congenital malformations, and Zika virus infection. Epidemiological Update.http://www.paho.org/hq/index.php...
- WHO 2016. WHO statement on the first meeting of the International Health Regulations (2005) (IHR 2005) Emergency Committee on Zika virus and observed increase in neurological disorders and neonatal malformations. WHO statement on the first meeting of the International Health Regulations (2005) (IHR 2005) Emergency Committee on Zika virus and observed increase in neurological disorders and neonatal malformations
- Kramer, Laura D., and Alexander T. Ciota. "Dissecting vectorial capacity for mosquito-borne viruses." Current opinion in virology 15 (2015): 112-118.
- Soares de Araújo, J. S., et al. "Microcephaly in northeastern Brazil: a review of 16 208 births between 2012 and 2015." Bull World Health Organ E-pub (2016).http://www.who.int/bulletin/onli...