Continuing with viruses that have been newsworthy in recent years,
it’s time to discuss Zika virus. Even though Zika virus was initially isolated over 70 years ago,
it hadn’t been studied all that closely or really considered to be a relevant human pathogen
for the majority of that time. We knew that the virus was isolated in the Zika forest
of Uganda during a routine surveillance check for yellow fever back in 1947. We
also knew that Zika virus can be transmitted to humans and nonhuman primates by mosquitoes,
and that it typically causes sporadic and mild viral infection.
It really wasn’t until 2007 or so that Zika virus started causing alarming outbreaks. First,
almost three-fourths of the population of Yap, an island in the Federated States of Micronesia,
was infected by Zika in 2007. In 2013, Zika spread through French Polynesia,
infecting about 11% of the population, then to other islands in Oceania. Then in 2016,
the World Health Organization declared a Public Health Emergency of International
Concern when Zika infection was associated with clusters of neurological disorders in Brazil.
So, what’s this virus all about? Zika virus is a mosquito-transmitted Flavivirus in the
Flaviviridae family, along with yellow fever, West Nile, and Dengue. As a brief refresher,
members of the Flaviviridae family are positive, single-stranded, enveloped RNA viruses. Like other
flaviviruses, the Zika virus genome is enclosed within an icosahedral capsid. When we covered West
Nile, we learned the details of how flaviviruses replicate and cause disease during an infection,
which are essentially the same here. In the case of Zika, after the bite of an infected mosquito,
proteins on the surface of the virus attach to a host cell membrane. The virus gets taken into the
host cell by endocytosis, the capsid is broken apart in the cytoplasm to release viral RNA,
and the host cell creates more copies of the virus, which get released to infect more cells.
As with West Nile, about 80% of Zika infections are asymptomatic altogether. For those that do
show symptoms, this will typically include a fever, headache, joint pain, muscle pain,
or a rash. But alarmingly, in infected pregnant women, Zika virus can pass from mother to baby
in utero or after birth, either through breast milk, placenta, or direct contact during delivery.
Even though infection in the mother is usually mild, the Zika outbreak in Brazil taught us that
vertical in utero transmission of Zika virus can sometimes cause birth defects. More specifically,
Zika can cause fetal or newborn microcephaly, which is a condition where a baby’s head is
much smaller than it should be, either because the baby’s head did not develop properly or it stopped
growing after birth. We don’t fully understand every aspect of how this works, and newborns with
microcephaly can have a wide range of symptoms, from mild to severe. Zika infection during
pregnancy can also cause miscarriage, stillbirth, or yet other birth defects. Another interesting
aspect we don’t fully understand is that in areas affected by Zika, there have been increased
reports of Guillain-Barré syndrome, an affliction of the nervous system that is typically rare.
Diagnosis of Zika infection isn’t entirely clear cut, since symptoms of Zika aren’t much different
from other mosquito-spread illnesses. But there are both blood and urine tests that can confirm
Zika, and a person’s recent travel history can help in figuring out if they’ve been
exposed or not. Since there isn’t a vaccine to protect against Zika, or specific treatment,
for that matter, Zika infections are usually treated with extra rest, fluids, or fever reducers
like acetaminophen. As far as prevention, the primary recommendations are to protect yourself
and loved ones from mosquito bites, either by protective clothing, insect repellant, or making
sure mosquitoes can’t get inside the home. Since Zika can be transmitted sexually, condoms are
recommended for prevention as well. Pregnant women are advised to check the CDC and WHO
websites to see where Zika outbreaks are before traveling and weigh the risks with their doctor.