Zika continues its romp around the world. In its wake, controversy erupted over the Olympic Games in Brazil, with some calling to move or postpone the Games – but is that really justified? Zika has already moved outside of Brazil in a big way.
To be clear, the Zika epidemic is dramatic and awful. Mosquito-borne transmission of this previously obscure and seemingly wimpy virus is ongoing in 60 countries, and it has already left a trail of dire neurological and developmental disruptions that will hobble families and communities for decades. This is a virus that has been full of surprises – a virus that is spread by a mosquito and also by sex! There aren’t many precedents for that, and no one knows what may come next.
Why Zika? Why now? And, by the way, what can an individual who plans to attend the Olympic Games do?
Why did Zika explode onto the world stage? Did humans create the conditions to allow this virus to escape Africa and Asia to become established in tropical and subtropical areas throughout the world? Massive urban growth continues, especially in low income, low latitude areas, places that are hot and poor. Explosive growth in international travel connects large, dense, tropical urban centers and adjacent slum areas with all parts of the world. Mosquito vectors, competent to transmit multiple viruses that can sicken humans, have been introduced via trade (e.g., along with used tires and plants) into many areas. Vector control programs have failed for many reasons, including lack of support, poor infrastructure, and resistance of mosquitoes to insecticides. Infected humans on international flights transport viruses in blood and other tissues. Warmer temperatures may have been an accelerant in some areas. We have created conditions conducive to Zika virus movement and establishment.
Other hypotheses about the reasons Zika virus spread rapidly include mutation in the virus to allow it to replicate to higher levels, to allow more efficient spread, and to become more virulent and attack neurologic tissues. Zika epidemics have appeared in areas pummeled by repeated dengue epidemics. Does prior infection (or multiple prior infections) with dengue virus, a related flavivirus, lead to antibody-dependent enhancement? Under these circumstances, prior infection with a related virus, instead of protecting the host, paradoxically leads to higher virus replication and more severe disease.
These and other key questions will be answered by research in progress, but we have already learned a lot about this virus. It can be transmitted sexually via semen. It is also present in saliva, urine, and breast milk, but transmission via these fluids has not been documented, to date. The virus has been found in wild populations of Aedes aegypti and Aedes albopictus in the Americas. Based on abundance and distribution, Aedes aegypti is thought to be the most important vector in the Americas. Bad news!
Aedes aegypti originated in Africa and is now found in tropical and subtropical areas worldwide. It is extremely well adapted to contemporary urban life. It can breed in small collections of water (plastic trash, bottle caps, used tires), bites during the daytime, lives in homes (can be found in closets, under beds), prefers human blood, and may feed on multiple humans if its feeding is interrupted. It is competent to transmit many viruses, including dengue, yellow fever, and chikungunya. This means that any areas with dengue transmission (encompassing almost half the global population) are also at risk for Zika virus transmission. More bad news!
Aedes aegypti has been extremely difficult to control especially in dense urban communities, such as those that lack reliable piped water and have lots of trash. The good news is that controlling Aedes mosquitoes would also help control other infections like dengue and chikungunya.
Fortunately the Olympic Games will take place during the cooler and drier months in Rio de Janeiro, a time when mosquito populations are low and dengue transmission limited. Vector control interventions in Rio have also been expanded in preparation for the Olympic Games. Some of the football (soccer) matches will occur at other venues, like Manaus and Salvador, where conditions may be more favorable than Rio for mosquitoes in August.
Mosquito populations in Rio may be low during the games, but sexual transmission of Zika is unaffected by temperature and rainfall. Persons who attend the Olympic Games could acquire infections through sexual contact with other attendees who come from areas with ongoing Zika transmission. Those attending the games should protect themselves by using mosquito repellents – but they should also protect themselves against sexual transmission by using condoms. Sexual transmission from male-to-female and male-to-male has been reported and possible female-to-male transmission. A man who is infected can potentially transmit infection in two ways: via a mosquito vector during the few days the virus is in the bloodstream and sexually via semen, which may contain virus for a month or longer. One report found viral RNA in semen 62 days after infection. Virus has also been found in the female genital tract. Because the majority of Zika infections are asymptomatic, precautions should be taken for sexual contact with anyone who is from or has recently visited an area with active Zika transmission. The use of barrier protection should take account of the potential long persistence of virus in semen (longer than a month).
Attendees also need to be mindful of their potential to transmit after they return home. A person who becomes infected (with or without symptoms) could be a source of virus for a mosquito that could transmit infection if it is hot enough in that area for the virus to replicate and disseminate in the mosquito before it dies or is killed. Typically it takes at least a week or two (in hot weather) for this to occur. In cooler areas, a mosquito, even if it bit someone with virus in the bloodstream, would likely die before it could transmit the virus. Persons who recently visited Zika-transmission areas should not donate blood.
Those going to the Olympic Games can prepare by understanding routes of transmission and by using repellents and condoms during the games. After the Games, they also should understand how to prevent onward spread if they live in an area with competent mosquito vectors or are sexually active (men). That being said, accumulated evidence suggests the risk of a major Zika outbreak during the Olympic Games in Rio is low. Zika has already delivered many surprises. There is reason to believe the Olympic Games will not deliver one more.
Featured image credit: Mosquito by Mikadago. CC0 Public Domain via Pixabay.