Six months ago, few people had heard of Zika virus. Now, not a day passes without new revelations about the virus.
Let’s put Zika virus infection in some context. John Donne wrote, in the 16th century, “No man is an island.” Aside from the fact that we would now want Mr. Donne to say “no person is an island” he was correct, especially as it concerns infectious diseases.
About 60 years ago, we began to see mosquito-borne virus diseases that had never before been recognized in South, Central or North America. The first of these was dengue. Relatively common in Africa and locally known there as breakbone fever, dengue (pronounced “den-gee”) is caused by a member of the flavivirus family of viruses and is transmitted from person to person by two specific species of mosquito: Aedes aegypti and Aedes albopictus. Unfortunately, both of these mosquitoes are present in South America, the Caribbean and, importantly for us, the southern U.S.
Initially, all cases of dengue diagnosed here were imported by travelers from the Caribbean and Central America. But all that’s needed to turn a new geographic area into an endemic one (where the disease can be transmitted from person to person) are two factors: 1) A high-enough prevalence of infected people with the virus circulating in their bloodstream (initially, returning travelers); and 2) the prevalence of the appropriate mosquito, which can pick up the virus during a blood meal, allowing it to multiply within the mosquito and then be deposited in the next person (or animal) the mosquito bites. At some point several years ago, those factors came together, and dengue is now endemic in some subtropical areas in the U.S., particularly in parts of Florida.
A couple of years ago, a different virus, called chikungunya, began to make its appearance in the Caribbean, and the cycle described above has been repeated with the same result. Now it’s the Zika virus’s turn, and it’s highly likely that the wheel will come ’round again.
Common symptoms, different outcomes
All three virus infections commonly present with some of the same initial symptoms: fever, a rash, muscle and bone pain and malaise (feeling sickly). Other symptoms may include conjunctivitis (eye inflammation) and, occasionally, temporary paralysis. Actually, dengue has the highest morbidity and mortality; repeated infection with one of the four types of this virus may occur and can lead to dengue hemorrhagic fever—a potentially lethal condition. Symptoms of dengue may last for months or years.
For the majority of Zika-infected patients, the initial illness is perhaps the mildest and least consequential of the three and there may be no symptoms at all. However, for one group of infected patients, this is certainly not the case. Zika infection, which was first described in the 1940s and then pretty much forgotten about, achieved its current prominence because health officials in Brazil in 2015 epidemiologically linked a marked increase in the incidence of the birth defect of microcephaly (small head and brain) with the mothers of these infants having symptoms (and later laboratory confirmation) of recent Zika virus infection. Microcephaly is a lifelong, devastating condition, and acquiring Zika during pregnancy has become the principal public health concern with this infection.
Zika and the Rio Olympics
Although recent mathematical modeling has estimated that no more than 80 cases of Zika infection will be imported back to the United States as a result of attending the ongoing Olympics in Brazil, this may be an underestimate. We now know that although the Zika virus is transmitted primarily through the bite of a mosquito, sexual transmission also occurs from men to women and vice versa. This phenomenon would probably increase the predicted importation rate. Therefore, any importation can result in new cases here, even in places where the mosquito does not exist. Furthermore, although there are current guidelines for how long sexual partners should avoid pregnancy, we really can’t be certain at this time how long the virus can persist in blood, serum or vaginal fluid. These factors, and the difficulty in eradicating all mosquitoes, may make the projections that endemic infection will be limited to a small geographic area in the U.S. a bit premature and optimistic.
So, while Zika infection should not pose a great health risk to most people, those who are pregnant, or who might become pregnant, must protect themselves from mosquitoes and from the risk of sexual transmission.