What are Arboviruses and Where do Arboviruses come from?

The group of viruses that are transmitted by insects are called (and this is not a technical term) “arboviruses.” The term is a coinage based on “ARthropod BOrne Virus.” West Nile, transmitted by a mosquito, is one of these.

There are several other viruses transmitted by insects to humans, all of them quite rare in the United States. La Crosse encephalitis was discovered in 1963 in La Crosse, Wisconsin (another of those unfortunate cities that has lent its name to a disease).

It has appeared since then in several Midwestern and Mid-Atlantic states. Unlike many viruses, it affects children more often than adults, and there are about 70 cases reported per year.

It is transmitted by a mosquito called Aedes triseriatus and survives the winter in the mosquito’s eggs, which grow into infected adults. Kids can get quite sick with this, and some of them wind up hospitalized for central nervous system infections. The virus can also cause neurological problems that last for several years. The illness can kill, but it rarely does: the fatality rate is under 1 percent.

St. Louis encephalitis is caused by a flavivirus closely related to West Nile. It’s transmitted by several different species of mosquito, depending on the region of the country, and there are an average of 128 cases per year. The disease occurs all over the country except in northern New England. The virus is carried by birds, but unlike West Nile virus, it doesn’t make the birds sick.

Normally the virus travels back and forth between birds and mosquitoes, people and other animals are “dead-end” hosts, not part of the normal life cycle of the virus. Most infections result in either mild symptoms or no symptoms at all, but the cases that have symptoms are overwhelmingly central nervous system problems, including neck stiffness, stupor, disorientation, tremors, convulsions, paralysis, and coma. The illness in its serious form affects mainly the elderly. You can only get St. Louis encephalitis from the mosquito, it isn’t transmitted person-to-person or by contact with any other animal. There’s no vaccine, and no treatment except supportive therapy.

Two other arboviruses that are carried by mosquitoes and cause encephalitis are western equine encephalitis (WEE) and eastern equine encephalitis (EEE), caused by different types of a virus of the Togaviridae family, genus Alphavirus. These infections are rare, but they can be deadly.

Even though it is very uncommon, EEE is the most severe of the arboviral illnesses, in 1996–1997 there were 19 cases of EEE, and five of them were fatal. It is thought that the virus may be amplified in emus, large flightless birds that are raised commercially for meat and other purposes, making humans more susceptible to the illness, even though there is no evidence that an emu has ever directly infected a human. There are more cases of EEE in Florida than in all other states combined.

EEE and WEE, as their names suggest, both infect horses. The CDC doesn’t track the horse version of the disease, but some state health departments do, and there are more than a hundred cases each year, with 23 states reporting. Humans can’t get the disease from a horse, only from a mosquito. There is neither a drug to treat nor a vaccine to prevent either EEE or WEE.

The man from Kennebec County was a real Down Easter who hadn’t left Maine since he was 45, a quarter-century earlier. He’d been spending most of his time doing yard work and lying on the ground repairing the hull of a boat, when in June 2001 he got really sick: muscle weakness, somnolence, diarrhea, and a fever of 104.7°F. His kidneys weren’t working right, either, and he was anemic.

Shortly after hospitalization, the left side of his body became paralyzed, and his mental state became completely confused.

He’d had a stroke, clearly, but no one could figure out why. His cerebrospinal fluid contained 10 times as many white blood cells as normal, suggesting a severe infection of some sort. A serum test found antibodies to a very unusual virus, the one that causes Powassan encephalitis. He was hospitalized for three weeks until his acute symptoms subsided and was then discharged to a rehabilitation facility. Three months later, in September 2001, he was still paralyzed on his left side.

Powassan encephalitis has been known since 1958, but in the 40 years that followed its discovery, there were only 27 known cases of the illness.

Then, between September 1999 and June 2001, there were four new cases reported in Maine and Vermont, including the one described above. Each of the patients suffered symptoms similar to the man from Kennebec. The Powassan virus is carried by four different species of tick in North America, and 38 mammal species have been found to be infected, mostly woodchucks. Unlike the tick that carries Lyme disease, the ones that carry Powassan virus rarely look for hosts walking through the brush.

Instead, they are found near the nests and burrows of medium-sized mammals. Although two of the victims didn’t recall any tick bites, they all lived where the ticks flourish, and the insects are easily missed. Powassan is rare but deadly: there is a case-fatality rate between 10 percent and 15 percent, and long-lasting neurological deficits are common. There’s no vaccine and no treatment, so the only protection is prevention.

The CDC believes that because of lack of awareness and the need for specialized laboratory tests to confirm a diagnosis, the illness may be more common than previously thought.