Randy Gaugler & Ary Faraji
Center for Vector Biology – Rutgers University / NJAES
Zika virus by Cynthia Goldsmith, CDC Public Health Image Library
Zika virus is not a new disease. Zika virus has been largely confined to equatorial Africa in the tropics where it circulated predominately between forest dwelling mosquitoes and wild primates. The virus was actually discovered from a sentinel monkey that had been placed in a cage in the Zika forest of Uganda in 1947. But the virus rarely spilled over into human populations even in highly endemic areas of Africa. The explosive reemergence we are current witnessing is truly extraordinary. Human activities are the greatest factor attributing to this spread because of rapid changes in land use and globalization leading to rapid increases in the movement of goods and people.
Zika is a pandemic because the virus is no longer confined to Africa but has spread to Asia, the Pacific Islands, and now the Americas where the World Health Organization is predicting several million infections and classifying Zika a ‘global health emergency’. The virus is not continent-hopping via the spread of mosquitoes, but because of the frequency and rapidity of air travel by humans. An individual can be bitten by an infected mosquito where the virus is circulating, and then fly long distances within a short span of time. Since the incubation period in humans usually lasts several days, if that infected individual is bitten by a local mosquito that can replicate and transmit the virus (i.e. vector competency in the host mosquito), then local infections in a new area may occur. However, only a handful of Aedes mosquitoes are vector competent for Zika virus. The primary vector is the yellow fever mosquito, Aedes aegypti; a highly invasive urban species. Their eggs may remain dormant for months in small containers, which contribute to a wide geographical distribution. Not coincidentally, these mosquitoes are abundant in the areas where Zika virus is currently circulating in the Americas. In short, humans are responsible for the transportation of Zika virus, whereas mosquitoes are responsible for transmission of the virus to humans.
What is New Jersey’s vulnerability to Zika virus? The answer is unclear at present. In the U.S., Aedes aegypti is a tropical species reported along the Gulf Coast, Florida and parts of California. Amazingly this species was just reported to be established in a Capitol Hill neighborhood in Washington DC, where it likely overwinters below ground where temperatures remain above freezing. New Jersey does not have established populations of Aedes aegypti, but the Asian tiger mosquito, Aedes albopictus is a resident. This mosquito has been shown to carry the Zika virus and is highly suspect as a vector in some locations. Aedes albopictus, like Aedes aegypti, is an invasive species that prefers small water-holding containers (tires, pots, bird baths, gutters, etc.) in close proximity to people in urban centers and preferentially feeds on humans.
Unfortunately, mosquitoes are not the sole means of Zika virus transmission. As with any blood-borne pathogen, transmission through contaminated blood is a serious possibility. The infectious phase of the virus is usually only a few days, but may last longer in some individuals. Consequently, many blood collection agencies are already screening donors to assure they have not traveled from an outbreak area within the previous month. An additional concern is sexual transmission, presumably due to survival of the virus in semen. A case of Zika virus transmission in the U.S. was recently reported in Dallas where an individual became infected after having sex with someone returning from South America. The prevalence of sexual transmission is as yet unknown, but we can be certain that this will greatly complicate efforts to contain outbreaks. Condoms are recommended for men returning from Zika endemic areas.
The good news is that four out of five persons infected with the virus show no symptoms. When the disease is symptomatic it tends to be mild and characterized by fever, rash, joint pain, and conjunctivitis (red eyes). Severe disease may require hospitalization but this is uncommon and death is rare. The bad news, of course, is the potential link between infected pregnant women and birth defects in their infants. Brazil has reported a 20-fold increase in microcephaly (reduced brain size) in newborns and has attributed this to Zika virus. Although not scientifically proven, circumstantial evidence strongly implicates the virus, and authorities are urging women in outbreak areas to delay pregnancy. The U.S. Centers for Disease Control and Prevention is recommending special precautions to pregnant women to postpone travel to areas with Zika virus activity or to take extra steps for the prevention of mosquito bites if travel is unavoidable. If microcephaly were not enough, the virus had earlier been implicated with another neurological disease, Guillain-Barre syndrome, in which the immune system attacks peripheral nerves. Studies are underway to examine these associations, albeit researchers are hampered by the lack of a good non-primate model.
It is worth noting here that ultrasound can detect microcephaly. The ultrasound test should be conducted late in the second trimester or early in the third trimester.
Vaccines for Zika virus are currently under development, but will not be available (if ever) for human use for many months if not years. Most mosquito-borne viruses do not have a vaccine or preventative medication available. The best prevention is the avoidance of mosquito bites. If travel to epidemic areas is not avoidable, travelers should take extra protection from mosquito bites by wearing long sleeve shirts and pants, staying indoors with air conditioning when mosquitoes are most active, and using a DEET-based mosquito repellent.
What is New Jersey’s response to the outbreak? There is concern among state, county and university public health workers but not alarm. The closely related chikungunya virus ran amok in the Caribbean last year, but this did not translate into significant outbreaks in the U.S. where window screens, air conditioning, and local mosquito control agencies are prevalent. If outbreaks of Zika virus should occur in New Jersey they are likely to be limited. In the event of an outbreak, suppressing vector populations is the only effective means of reducing disease transmission. The control of the most probably New Jersey vector, the Asian tiger mosquito, is difficult because this species thrives in residential backyards where access is difficult and habitats are scattered. New Jersey’s authorities are currently making plans to incorporate Zika virus into the current arboviral surveillance and to respond aggressively to suspected cases with intensive targeted mosquito control in those localities.