Thursday, 31 January 2019

"Zika Virus as a Possible Risk Factor for Autism"?

The paper by Vianna and colleagues [1] provides the blogging fodder today, discussing a potentially important topic around whether "the fetal brain infection caused by ZIKV [Zika virus] could predispose to ASD [autism spectrum disorder]."

Just before anyone gets the impression that a *link* has been found between Zika virus exposure and autism or autism spectrum disorder (ASD), I'm going to say no, not yet. The current peer-reviewed evidence at the time of writing this post has not identified autism as an outcome following exposure to the Zika virus. That's not to say that various other neurodevelopmental issues have not been associated with Zika virus exposure (see here for example) or that Zika virus exposure might not have biological consequences akin to that noted in some autism [2]. But as far as I'm aware, there is no peer-reviewed science yet suggesting that autism is for example, over-represented among those exposed to Zika virus in-utero. Not yet anyway.

Nonetheless, Vianna et al go through a number of lines of evidence suggesting how autism *could* be an outcome of Zika virus exposure. They start by talking about how Zika virus exposure during pregnancy can have a devastating effect on the developing child (see here) as a function of its teratogenic status. Severe microcephaly (small head size), a cardinal feature linked to infants' Zika virus exposure during pregnancy, has notable effects on the brain and its development, and is one of the more noticeable effects associated with Zika virus exposure in-utero. But further: "this phenotype is now considered only “the tip of the iceberg” and there is a spectrum of less severe abnormalities after congenital Zika infection."

Authors go on to talk about how pregnancy is a time of 'change' when it comes to maternal immune system functions, as a reprogrammed immune system has to become 'tolerant' of the developing foetus. Such reprogramming means that the foetus can survive and thrive. It also however means that the maternal immune system might be more susceptible to certain infections, where for example "opportunistic infections... take advantage." This is also the point where autism enters into the conversation, and the idea that exposure to various infections (viral and bacterial) occurring during pregnancy "can alter [offspring] brain development and are associated with alterations, such as brain calcifications, microcephaly, and neurodevelopmental disorders." I've covered a few possible examples on this blog with autism in mind (see here and see here).

Putting all this together, as well as talking about some of the immune system chemistry that *might* link Zika virus exposure and autism, and the authors come up with a model of 'neuroimmunomodulation' talking about "an ineffective anti-viral response" and increased levels of pro-inflammatory cytokines as being potentially important. I'll also provide another quote from Vianna which is similarly intriguing: "Moreover, in the case of ZIKV, previous infections with other flaviviruses, such as dengue virus and yellow fever, may trigger a secondary immune response of differential magnitude given the great molecular similarity of some immunogenic epitopes among these correlated viruses."

Reiterating that there is currently no link between Zika infection and risk of autism, I do find the Vianna paper interesting. It offers some testable hypotheses that could examined in the lab and beyond. It also provides some further support for the various surveillance and monitoring initiatives that remain in place with regards to Zika virus and the promise of further important data to come from them.

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[1] Vianna P. et al. Zika Virus as a Possible Risk Factor for Autism Spectrum Disorder: Neuroimmunological Aspects. Neuroimmunomodulation. 2019 Jan 10:1-8.

[2] Beys-da-Silva WO. et al. Zika Virus Infection of Human Mesenchymal Stem Cells Promotes Differential Expression of Proteins Linked to Several Neurological Diseases. Mol Neurobiol. 2018 Oct 30.

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