The Zika Virus& Microcephaly

Dr. Allen Cherer

Zika virus is an arthropod-borne flavivirus discoveredin Africa in 1947. When infected by the virus, most

persons are asymptomatic or demonstrate generallymild, self-limited illness characterized by fever, rash,

arthralgia, and conjunctivitis. The first widespreadoutbreak of the Zika virus was recognized on YapIsland, Federated States of Micronesia, in 2007,followed by outbreaks in Southeast Asia and the

Western Pacific, including a large outbreak in FrenchPolynesia in 2013-14.

Since the identification of the Zika virusinfection in Brazil in May, 2015, the virus hasspread rapidly throughout the Americas, andas of February 2016, thirty-one countries and

territories had reported cases.

A bite of the Aedes aegypti mosquito is the mainroute of exposure, but sexual, maternal-fetal, and

intrapartum transmission have all beendocumented. To date, all cases reported in the

continental United States have been travel-associated, whereas in the U.S. territories

(American Samoa, Puerto Rico, and U.S. VirginIslands), the vast majority of cases have been

locally acquired vector-borne.

Although infection with the Zika virus generally leads tomild disease, its emergence in the Americas has

coincided with a marked increase in babies being bornwith microcephaly, a neurological disorder present atbirth and defined as head circumference at least 2 SDsmaller than the mean for sex, age, and ethnicity andwith head circumference at least 3 SD smaller being

deemed severe. Congenital microcephaly is a conditionassociated with a reduction in brain volume and is oftencaused by genetic or environmental factors that affect

fetal brain development.

Prenatal viral infections, such as rubella andcytomegalovirus, hypertensive disorders, andmaternal alcohol have also been associated

with the condition. Cases have been reportedafter intrauterine infection with West Nile

virus (also a flavivirus) and Chikungunya virus.

Given the widespread nature of the Zika virusepidemic in the Americas, the temporally

associated increase in microcephaly cases inBrazil, and the retrospective findings of acluster of microcephaly and neurologic

disorders associated with the Zika virus inFrench Polynesia, the WHO declared Zika

virus a Public Health Emergency ofInternational Concern on February 1, 2016.

In order to reduce the risk of microcephaly,recommendations included avoidance of travel to

affected countries by pregnant and childbearing agedwomen, use of condoms with partners returning from

affected countries, and pregnancy delay.

In order to better quantify the risk of microcephalyassociated with the Zika virus infection, a retrospective

study based on data from the completed Zika virusoutbreak in French Polynesia in 2013-14 was reported

in The Lancet by Simon Cauchemez, PhD, andcolleagues from the Institut Pasteur.

Based on four datasets providing information on allcases of microcephaly, weekly number ofconsultations for suspected Zika infection,

seroprevalence for Zika virus antibodies, and thenumber of births during the outbreak, the

researchers developed a mathematical andstatistical model to illustrate the association

between the Zika virus and microcephaly anddemonstrated the risk for microcephaly to be

greatest during the first trimester of pregnancy.

According to the analysis, it is estimated that the riskfor microcephaly for mothers with the Zika virusinfection during the first trimester is about 1%.

Although the risk appears low compared to otherintrauterine viral infections (e.g., rubella,

cytomegalovirus), the incidence of the Zika virusinfection is very high during outbreaks (eg, 66% in

French Polynesia and 73% in the island of Yap).Therefore, despite the relatively low fetal risk,

infection with the Zika virus is an extremely importantpublic health matter.

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Dr. Allen Chererdrallencherer.net