Como funciona o vírus Zika

No final de 2015, a mídia americana começou a se preocupar com uma doença transmitida por mosquitos que era notícia antiga na África e na Ásia, mas nova no Ocidente. Para os médicos familiarizados com esse primo relativamente fraco da dengue, chikungunya e febre amarela, a preocupação poderia parecer infundada, não fosse por uma série de casos aparentemente relacionados de microcefalia que coincidiram com a chegada do vírus ao Brasil.

Os medos aumentaram quando imagens de crianças com cabeças anormalmente pequenas começaram a aparecer nos noticiários, acompanhadas de palavras como "extraordinariamente grave" e "desenvolvimento cerebral incompleto". E quando ficou claro que as Américas, cuja falta de imunidade ao zika era igualada pela abundância de habitats propícios aos mosquitos, estavam enfrentando uma epidemia, não importava mais que o vírus normalmente causasse apenas sintomas leves de gripe em uma pequena porcentagem da população. adultos infectados. Comprovada ou não, compreendida ou não, a ameaça de crianças nascerem com microcefalia – ou com síndrome de Guillain-Barré, outro distúrbio neurológico ligado ao Zika – logo gerou pedidos de opções “nucleares”. Os mosquitos, diziam alguns, tinham que morrer, e o diabo arcaria com as consequências.

Mas espere um minuto, disseram outros. O que realmente está acontecendo aqui? Esses distúrbios neurológicos são realmente causados ​​pelo Zika? Em caso afirmativo, qual mecanismo liga os dois fenômenos e por que nunca ouvimos falar dessa conexão antes? É possível que a ligação tenha sido mera coincidência ou que estejamos negligenciando algum fator vital, como dieta ou meio ambiente? E o fato de que o Zika pode ser transmitido pelo sangue, sêmen ou possivelmente outros fluidos corporais?

Seja qual for o caso, onde o Zika levou, o pavor se seguiu. Em 1º de fevereiro de 2016, a Organização Mundial da Saúde declarou o Zika uma Emergência de Saúde Pública de Importância Internacional devido à sua ampla e rápida disseminação nas Américas e sua possível relação com complicações neurológicas. Uma semana depois, o governo Obama disse que pediria ao Congresso mais de US$ 1,8 bilhão em fundos de emergência para apoiar testes, vigilância, resposta e contenção, e buscar uma vacina. Alguns países, incluindo os EUA, recomendaram que seus atletas pulem os Jogos Olímpicos de Verão no Rio de Janeiro. Alguns também sugeriram que as mulheres adiassem ter filhos por alguns anos.

No início de fevereiro de 2016, o Zika atingiu níveis epidêmicos no México, no Caribe e nas Américas Central e do Sul [fonte: Ungar ]. A doença está se espalhando por 33 países, que abrigam um total de 600 milhões de habitantes, sugerindo a potencial infecção de dezenas de milhões [fonte: McNeil et al. ]. Não há vacina no horizonte imediato, então, como sempre, nossa melhor arma é o conhecimento.

1. Origens do Zika
2. Zika e microcefalia
3. Sexo e Gravidez
4. Protegendo-se - e lutando para trás

Zika é um arbovírus transmitido por mosquito (abreviação de vírus transmitido por artrópodes ) do gênero Flavivirus , família Flaviviridae — o mesmo gênero da febre amarela, dengue e vírus do Nilo Ocidental [fonte: CDC ]. Ele vem em dois sabores principais, ou linhagens – africana e asiática, cada uma com várias cepas. As Américas são anfitriãs da linhagem asiática. O zika pode infectar vários animais, como elefantes, leões, roedores e zebras, mas humanos e primatas não humanos parecem ser seus principais reservatórios [fonte: Rogers ].

Como muitos de seus primos Flavivirais, os mosquitos preferidos do Zika vêm do gênero Aedes , particularmente as espécies aegypti nativas da África. Nos EUA, esses skeeters podem ser encontrados nos estados da Costa do Golfo, Geórgia e Carolina do Sul, na metade sul do Arizona, e espalhados pela Califórnia e Novo México. O vírus provavelmente faz uma segunda casa no homólogo asiático do aegypti , o Ae. albopictus , que tem escavações americanas até o norte de Nova Jersey e até o oeste do Texas, com faixas que se estendem até o terço sul do Arizona e pedaços da Califórnia e Novo México [fontes: CDC , McNeil et al. , Rogers]. Outros mosquitos aedinos também podem transmitir a doença. Por exemplo, alguns acreditam que o surto de Zika na Polinésia Francesa em 2013 se espalhou pelas probóscides de Ae. polynesiensis [fonte: Rogers ].

O zika transmitido por mosquitos começa seu ciclo quando uma fêmea, que precisa de sangue para botar ovos, cutuca uma fonte de refeição e suga o vírus junto com seu Slurpee de plasma. O zika viaja no sangue para o intestino da fêmea, para depois retornar pelo sistema circulatório à saliva. As propriedades anticoagulantes de seu cuspe são úteis quando ela o esguicha – e o vírus – em uma refeição posterior [fonte: McNeil et al. ].

Na tradição de que isso fez tanto para impulsionar o turismo no rio Ebola, o Zika recebeu o nome da floresta de Uganda em que foi isolado em 1947, primeiro em um macaco rhesus e depois em mosquitos. Os anticorpos para zika foram encontrados pela primeira vez em humanos no início da década de 1950 [fontes: McNeil et al. , Rogers ]. Infelizmente, a convenção de nomenclatura não é tudo que o Zika tem em comum com o Ebola. Assim como a febre hemorrágica, o zika pode "se esconder" e se replicar em áreas do corpo — como cérebro, olhos, placenta e testículos — que bloqueiam a resposta imune [fonte: Steenhuysen ].

Antes confinado à África e à Ásia, em 2007 o Zika começou a se espalhar pelo Pacífico. No início de 2015, ele já rodava pelo Brasil há pelo menos um ano. Embora o surto tenha começado em maio de 2014, os casos já haviam aparecido em Natal, capital do estado do Rio Grande do Norte, no início do mesmo ano. Esse momento se alinhou aproximadamente com a Copa do Mundo de 2014, que foi parcialmente realizada em Natal, embora várias outras fontes propostas sejam igualmente possíveis [fontes: McNeil et al. , Wade ].

Agora, em um país com condições propícias para a explosão populacional de mosquitos, situado em um hemisfério praticamente sem imunidade, o Zika se espalhou rapidamente em 2015. E foi aí que o problema realmente começou.

Sintomas e Tratamento

Only about 1 in 5 people infected with Zika virus develop symptoms. These generally include fever, rash, joint pain and/or red eyes (conjunctivitis) lasting a few days to one week. Headaches or muscle pain can also occur. Often these are so mild as to go unnoticed, but in rare cases, they can require hospitalization. Zika is only rarely deadly. Its incubation period remains unknown, but current estimates peg it at a few days to a week. No specific medicines exist — just treat the symptoms with rest and fluids, and use acetaminophen (not aspirin or other non-steroidal anti-inflammatory drugs [NSAIDs]) for fever and pain [sources: CDC, Rogers, McNeil et al.].

Mild concern over Zika's advent in the West turned to dismay when Brazilian doctors began to find record numbers of microcephaly cases in the maternity wards of Pernambuco. The northeastern state of 9 million people typically saw just nine cases of microcephaly out of 129,000 births each year. In November 2015, they reported 646 such births, with neighboring states Bahía and Paraíba soon following suit [sources: McNeil et al., Wade]. Brazilian health officials were facing 4,000-plus possible cases; by mid-February 2016, they had verified around 400, compared to the country's average rate of 150 to 163 microcephaly cases per 3 million annual births [sources: Berkrot and Boadle, McNeil et al., Rampton and Hirschler, Wade].

The sheer prevalence was enough to shock physicians, but doctors were taken aback by the severity of the cases as well. Beyond the characteristic small heads and brains , they found eye malformations, intracranial calcifications (aka "brain stones"), malformed cerebral cortexes, or abundant spinal fluid suggesting that the brain had grown and then abruptly shriveled [sources: CDC, Berkrot and Boadle, Rogers]. In many cases, mothers were confirmed to have Zika or Zikalike symptoms while pregnant [sources: CDC, Wade]. Zika had also been found in placentas, amniotic fluid and fetal brain tissue, proving that the virus could cross the placental barrier [sources: Steenhuysen, Wade].

Meanwhile, another disorder began spiking in suspected Zika patients — Guillain-Barré syndrome (GBS), a slowly paralyzing attack on the body's nervous system by its own immune system [sources: CDC, NINDS, Rogers]. One Pernambuco neurologist reported 50 patients in 2015, compared to 14 the year before [source: McNeil et al.]. A similar uptick had occurred before, during the Polynesian outbreak of 2013, but the Zika link in that case, if true, remains unknown [sources: CDC, Rogers].

Indeed, tracing a clear causal link between Zika and microcephaly has proven tricky — in part, because it's baffling. As one Pan American Health Organization epidemiologist told The New York Times, no one has ever seen a "congenital malformation by mosquito before." Moreover, the microcephaly link is unconfirmed and largely circumstantial — a matter of co-occurrence in some regions but not others. Further blurring the picture: false positives, varying diagnostic standards and spikes in diagnoses thanks to heightened awareness [sources: CDC, McNeil et al., Wade].

Fire or no, there is plenty of smoke. In 17 of the 400 cases mentioned above, Zika has been confirmed in mother or baby. In a different study of 35 Brazilian babies born with microcephaly, all of the mothers involved had spent time in a known Zika area while pregnant [sources: Berkrot and Boadle, Rampton and Hirschler].

Clearly, caution is called for, as well as a good set of guidelines for avoiding spreading the disease to women who were, or might become, pregnant. If only it were that simple.

An X Factor?

Even as evidence mounts to support a Zika link, room remains for a third element — a microorganism, a nutritional factor or something environmental — to explain the microcephaly increase in infants [sources: CDC, Wade]. Fringe groups have put forward herbicidal or larvicidal causes, most famously pyriproxyfen larvicides, which a group calling itself Physicians in the Crop-Sprayed Villages claims were added to the drinking water of affected populations. Brazil's Ministry of Health, as well as independent experts, unequivocally rejected the pyriproxyfen argument, citing a lack of evidence and pointing to the fact that such an effect should not be physiologically possible [source: Welch].

Zika's second avenue of transmission has less to with bugs than with the birds and the bees. Evidence suggests that a man can transmit the virus to his sex partners, although this is probably not common, and in most known cases the men experienced symptoms such as genital pain and bloody semen. We know that the virus persists in semen longer than in blood (a few months versus 7 to 10 days), but we don't know how long semen remains infectious. We also don't know whether a woman can transmit Zika to a sexual partner, or what types of sexual contact (e.g., anal or oral) could spread it [sources: CDC, McNeil et al., Steenhuysen].

It's also possible that Zika can spread through blood transfusions and that the virus lives on in saliva and urine, although its transmissibility via these fluids remains an open question [sources: CDC, McNeil et al., Rampton and Hirschler].

Although cases remain rare, mothers can transmit Zika to fetuses, typically at or near the time of birth. Infection might also occur during pregnancy, but the jury is still out on this [sources: CDC, pregnancy; CDC, transmission]. If true, then infection earlier in pregnancy is likely more dangerous [source: McNeil et al.]. Researchers have detected Zika RNA in breast milk but have yet to see transmission through that route, so the benefits of breastfeeding still outweigh the risks [source: CDC].

According to the CDC, EPA-registered insect repellents that contain DEET, picaridin and IR3535 are safe for use during pregnancy.

Given the sexual knowns and unknowns, the safest plan for people at risk of having, getting or giving Zika lies in abstinence or in using condoms the right way every time. Along similar lines, countries such as Brazil, Colombia, El Salvador and Honduras have called for women to put off having children for a few years [sources: CDC, McNeil et al., Wade]. Initially, Latin American Catholic bishops balked at the mention of contraceptives, but Pope Francis has since relaxed the church's stance in cases of likely Zika infection [sources: Goodstein, NPR]. The Pope still strongly denounces abortion, however, which means that pregnant women affected by these issues will face significant resistance from both the Catholic Church and, in many countries, the legal system if they decide to terminate their pregnancies.

Further complicating matters, Zika is hard to diagnose in adults, let alone fetuses. There's no widely available test, and procedures such as amniocentesis carry risks of injury to the baby and rendering false positives. In part, this is because we can only detect Zika in the first week — after that, we're detecting antibodies, which closely resemble the antibodies of Zika's cousins (and their vaccines) [sources: CDC, McNeil et al.].

Bottom line: If you got pregnant before/during recent travel to a country with Zika, see a doctor and get a blood test 2 to 12 weeks after returning. Have ultrasounds done, too, but prepare yourself for the fact that your doctor won't be able to spot microcephaly before the end of the second trimester. If you get pregnant after travel to a country with a Zika outbreak, your risk is much lower, but you should still have your newborn tested. Even if your child does not have microcephaly, other birth defects (e.g., vision and hearing) might still occur [sources: CDC, McNeil et al.].

Microcephaly

Microcephaly typically results from genetic causes or from one of the following: craniosynostosis (a premature fusing of the skull's plates that prevents further brain growth); chromosomal abnormalities, such as Down syndrome; decreased oxygen (cerebral anoxia) from pregnancy or delivery problems; infections, such as toxoplasmosis (parasitic disease) or herpes (virus-caused); alcohol, drugs and other toxins; severe malnutrition; or uncontrolled phenylketonuria in the mother [sources: Mayo Clinic, McNeil et al., Steenhuysen].

We said above that the obvious answer to avoiding mosquito-borne viruses is to avoid mosquito bites. This is easier said than done with Aedes, which bite aggressively, mainly (but not exclusively) during the day, and live and feed both indoors and out [source: CDC]. But mosquitos are only half of the equation. The other half is their preferred meal — us [source: CDC].

In many areas, including the U.S., mosquitos do not yet carry Zika, and all cases come from human travelers. We want to keep it that way. So if you think you have Zika, see a doctor, and try to avoid being bitten by any mosquitos, especially during the first week of illness [source: CDC]. Also, as mentioned previously, you should really abstain from sex. But if you can't fight the urge to merge, at least inform your partners, and use condoms the right way every time [source: McNeil et al.].

Bite avoidance is mainly a matter of wearing the right clothes, controlling your environment and living better through chemistry. Make long-sleeved shirts and long pants your new fashion statement, and treat your togs with permethrin. Spend some quality time in air-conditioned spaces, deck out your domicile with screens and screen doors, and sleep under a mosquito net when camping or traveling in Zika-infested countries [source: CDC].

Obviously, travelers should pay attention to health notices and avoid traveling to places where Zika exists, if possible [sources: CDC, McNeil et al.].

States and countries are fighting Zika through a combination of tracking populations and denying the critters their preferred habitat. You can help by organizing your neighborhood to cover, get rid of and dry out areas where water collects. Aedes mosquitos lay their eggs in standing water, and even a bottle cap's worth can be enough. During mosquito season, officials will sample adult mosquitos for evidence of infection and apply adulticides around homes known to have Zika [sources: CDC, McNeil et al.].

More radical solutions with potentially unforeseeable and dangerous consequences involve wiping out the mosquitos themselves. One company is offering to spread genetically modified mosquitos to wipe out Aedes vectors. These male mosquitoes are engineered to have offspring that die before adulthood [source: Barker]. But the wrong approach could have disastrous ecological consequences. Whether that will stay our hands for long remains to be seen.

Zika Areas

Prior to 2015, Zika was confined mainly to areas of Africa, Southeast Asia and the Pacific Islands. Today, many countries are seeing outbreaks, including all of Central America; South America north of Chile, Argentina and Uruguay, and east of Peru; and parts of the Caribbean, including Barbados, Curacao, Dominican Republic, Guadeloupe, Haiti, Jamaica, Martinique, Puerto Rico, Saint Martin and the U.S. Virgin Islands [source: CDC]. As of Feb. 17, 2016, 22 U.S. states and Puerto Rico had at least one case, totaling 82 cases of travel-acquired Zika [source: CDC].

Author's Note: How Zika Virus Works

De muitas maneiras, o Zika é um pesadelo para os epidemiologistas, e não apenas porque está se espalhando por uma área do mundo com muitos habitats de mosquitos e sem imunidade à doença. Não, o que torna o Zika realmente terrível é sua ligação horrível, mas não comprovada, com a microcefalia. A emoção é uma má companhia para a tomada de decisões em crises e – após um surto de Ebola que muitos viram como mal administrado e confrontados com imagens de bebês e mães em sofrimento – estamos preparados para uma reação exagerada. A questão é: sabemos o suficiente sobre o Zika para medir nossa abordagem, para colocar recursos onde eles podem fazer o melhor, ou vamos deixar nossos medos tomarem conta de nós?

For now, Zika will spread — or, perhaps not. It's possible the virus has already peaked, at least in the region where it hit most intensely [source: McNeil et al.]. The only thing that is truly clear is that scientists need to understand how Zika affects developing fetuses, whether through mouse models, "brain balls" grown from stem cells or some other means [source: Wade]. We simply lack too many essential answers.

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