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A recent outbreak of the Zika virus in Brazil has caused alarm world-wide, although the relationship between the virus and birth defects is not certain.
What Is the Zika Virus?
Zika virus is a member of a group of viruses known as arboviruses. These viruses maintain themselves in a cycle traveling from vectors, the insects and ticks that carry them, to hosts, the people they make sick. A blood-feeding arthropod (mosquito, tick, or fly, for example) acquires the virus when feeding on the blood of a human who is already infected. The arthropod regurgitates the virus when it feeds on the next human or animal, causing the infection.
Zika isn't really new. It was first discovered in 1947 in Uganda in the Zika forest near Lake Uganda. The first medically identified victim of the virus was a monkey, not a human being. Prior to 2007, a worldwide total of only 14 cases of humans infected with virus were known to medical researchers. However, these cases occurred far outside the Zika forest. There were people infected in Uganda in 1969 and 1970, Nigeria in 1971 and 1975, Sierra Leone in 1972, Gabon in 1975, Central African Republic in 1979, Senegal from 1988 to 1991, and Côte d'Ivoire in 1999. Recently, researchers report anecdotally, Zika virus was detected in Senegal in 2011 and 2012. Zika virus infections were reported in Pakistan, Malaysia, and Indonesia in 1977 and 1978, Micronesia in 2007, and Cambodia in 2010. There was a small epidemic in April 2007, when a Zika fever epidemic occurred in Yap island in Micronesia, where there were 49 confirmed cases and 73% of the residents older than 3 years tested positive for recent Zika virus infection.
All of these outbreaks were recorded in the medical literature, but public health officials weren't especially alarmed. Before 2015, Zika virus fever caused relatively mild symptoms, a little like the flu. There would be fever, maybe a rash, conjunctivitis, and joint pain. Pregnant women did not transmit the virus to their children before they were born. There were no reports of birth defects.
By 2015, however, something about the virus had changed. In Paraiba state in northeastern Brazil, a number of women who had seemed fine during pregnancy delivered babies with severe birth defects. These babies had deformed bones and joints, and the bones in their skulls had joined together prematurely so that the head was unusually small. The deformation of the skull, microcephaly, has been getting the most media attention, but it's not the only problem facing these infants. In many of these infants, not only is the skull too small, there are multiple calcifications in the brain, suggesting the brain had become infected and inflamed before the baby was born. When public health officials were informed of the growing numbers of cases, they sent blood samples to the Oswaldo Cruz Foundation in Rio Janeiro, and the results showed that in some of the cases of microcephaly, the mother and child had been infected with Zika virus before birth.
Why would a relatively harmless virus start causing devastating birth defects? It's possible there were actually many more cases in Africa that simply didn't get reported. It's possible that people in Uganda have been exposed to the virus so often that they built up a resistance to it. However, molecular geneticists have found a specific mutation in the Zika virus that seems to have occurred, or to have occurred in observed cases, only since 2012.
Zika virus infection is not the only explanation of microcephaly. West Nile virus, Rubella, cytomegalovirus, and toxoplasmosis are known causes of microcephaly. There are at least eighteen genetic mutations that can cause the condition. The death of an identical twin while still in the womb can cause microcephaly, as can fetal alcohol syndrome or the mother's use of the anti-epileptic drug phenytoin (Dilantin). Malnutrition, exposure to radiation, the mother's hypothyroidism, poorly controlled gestational diabetes, phenylketonuria in the mother, ischemic or hemorrhagic stroke while still in the womb can also cause microcephaly.
In Brazil, there seem to be many more cases of microcephaly than Zika virus. Here are the statistics as of February 1, 2016:
- There have been 4,180 cases of suspected microcephaly (that is, the baby's skull appeared small), but only 732 babies have been referred to specialists.Of those 732 babies, 460 have been found not to have microcephaly and only 272 have the condition.
- Of the 272 babies who have confirmed microcephaly, only six have microcephaly clearly due to Zika virus. The other babies were were found to have microcephaly due to some other cause.
Six cases of Zika-virus induced microcephaly is a tragedy for the babies and their families, but that doesn't make Zika virus a global health emergency. However, thousands of cases of suspected microcephaly without knowing the cause most certainly is an emergency.