How soon will Zika disease spread to New England?

Zika disease, at epidemic levels in Brazil for more than a year, has come to Miami, FL. Although often described as a “tropical disease,” it has escaped the tropics, and people are keeping a greater distance. This month, the Miami Herald quoted the operator of a Florida travel business, saying, “I had to cancel eight out of my 12 weekly summer season tours.” In recent days, several locally transmitted Zika cases were reported in Miami Beach, and the danger zone was expanded from 1-1/2 square miles to most of the community.

Origin of the threat: Zika is not a new threat. It was first found almost 70 years ago as a disease of rhesus monkeys in the Ziika Forest–for which the disease was named–located near Lake Victoria in Uganda. The cause is a flavivirus (“yellow virus”). That virus family and genus includes the agents of yellow fever, dengue fever, chikungunya and West Nile fever. The diseases have mostly been transmitted by aggressive species of mosquitos common in the tropics. Some of the diseases have migrated to temperate regions, and some infect wild and domesticated animals–including goats, sheep and mice–as well as humans.

The flaviviruses are single-strand RNA viruses, like the virus that causes AIDS. Lacking stabilizing effects of DNA-based genetics, they mutate relatively often, sometimes producing new, persistent strains. Research shows that happened in recent years with Zika. The original strain found in Africa caused mostly mild, brief illness in humans. The common symptoms were low fever, sometimes with skin rash or joint pain, that lasted up to a week.

The disease spread from Africa into south and southeast Asia. A 2007 outbreak on Yap and nearby islands of Micronesia drew attention because it seemed very widespread, even though it caused no deaths or long-term health problems. A survey using immunology tests suggested that about three-quarters of the population had been infected. Those tests encounter cross-reactions among the flaviviruses. A previous infection by dengue or chikungunya may produce a positive result. Since dengue is often present where Zika strikes, estimates of infections using immunology tests can be clouded by errors.

Growth of the threat: Starting in 2013, another flavivirus epidemic occurred in Tahiti and nearby islands of French Polynesia. This time health centers had genetics tests available when live virus could be sampled. They distinguish more clearly among viruses, and Zika was soon identified as a main cause of the epidemic. However, the virus had mutated, producing new strains. Some victims had more severe symptoms than previously reported for Zika disease. A small fraction of the victims developed long-term problems including profound muscle weakness, known as Guillain-Barré syndrome.

After the epidemic in French Polynesia, unusual problems began to be found in newborns: smaller heads than normal, called microcephaly. While such symptoms occur without Zika, they occurred more often in births from pregnancies during the epidemic. Other severe problems began to be found, including defects in the brain, eyes and spinal cord. Immunology tests associated a high proportion of newborn victims with Zika exposure.

During 2014, newer strains of Zika spread eastward, appearing in other Pacific islands and then in South America. During 2015, the disease spread through most of Brazil, then appeared in neighboring countries and Central America, including the Caribbean. Windblown mosquitos helped spread the disease, but epidemiologists also attribute the spread to infected people traveling to places where aggressive species of mosquitos are common. Cabo Verde, near the west coast of Africa, recently reported cases involving newer strains of Zika.

As of 2012, only five strains of Zika had been reported. By early spring, 2016, about 60 Zika strains had been identified by gene sequencing. Comparisons found two main groups: one common in Africa, the other common in south and southeast Asia. Strains responsible for the 2013 outbreak in French Polynesia and the recent outbreaks in South and Central America had developed from previous Asian strains. As with older strains, many people apparently infected by newer strains did not seek care for relatively mild symptoms, while the virus was infecting cells and multiplying.

During the past year, publications surged. By mid-September, 2016, gene sequences for almost 100 strains had been reported. Compared with other diseases, however, research on Zika immunology and therapeutics remains poorly developed. According to a recent review of the science, researchers “currently lack major basic tools for [Zika vaccine] development, including reliable animal models, reference reagents and assays.” In Congress, for months Republicans driven by reactionary agendas failed to act on President Obama’s request of February, 2016, seeking $1.9 billion in emergency funds for applied research on Zika.

Dangers and precautions: Soon after an infection has taken hold, Zika has been found in many body tissues and fluids. It may persist for months after symptoms of an infection–if there were any–have gone away. Laboratory measurements found that newer Zika strains are highly infectious; just a few copies of the virus may be needed to transmit the disease. Although apparently not contagious, the disease is transmitted by intimate contact, including sex. Since current genetics tests cannot insure that levels of Zika virus are below an infectious threshold, major health organizations have been recommending long delays between potential Zika exposure and pregnancy.

It is not yet known whether antibodies produced during infection by one Zika strain can prevent infection by other strains. A pattern from the closely related dengue virus is troubling. A previous infection involving one class of dengue virus does not prevent infection by strains belonging to another class and may worsen health hazards. Early indications, still controversial, suggest Zika infections might behave similarly.

There is no approved vaccine against Zika. One candidate vaccine recently began the first of three stages in clinical trials: testing for safety. The first vaccine approved against dengue began marketing just this year, after over 80 years of experiments, and already it has been clouded with safety issues–potentially worsening health hazards, including those from Zika.

Spreading disease: Mosquitos, notably those in the Aedes genus, have been the main vectors for Zika and other flaviviruses. The Aedes aegypti species is adapted to humans and their habitats. Other Aedes species are also frequent carriers, helping to infect wild and domesticated animals as well as humans. Although often called “tropical,” Aedes mosquitos live throughout the southern half of the United States. They are also key vectors for yellow fever virus, which became a scourge of East Coast and Mississippi River cities during the late 1600s through the late 1800s. New England is already visited by dengue fever, the flavivirus most closely related to Zika.

New England dengue fever cases

denguefevercasesnewengland2009
Source: Natural Resources Defense Council, 2009

The Aedes aegypti mosquito range extends into New England, including at least the western seacoasts of Connecticut. However, laboratory experiments show that mosquitos in the Culex genus can also carry Zika. They are common back-yard and house mosquitos throughout New England, with ranges extending well into Canada. During the last few decades, they have become vectors in the region for West Nile virus, and they may be vectors for dengue virus. Although the region is not likely to see Zika epidemics as widespread as those in the tropics, New England remains under threat.

– Craig Bolon, Brookline, MA, September 20, 2016


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