A rare tick-borne disease that resembles West Nile virus but seems more likely to cause severe illness is on the rise.  

The number of confirmed and suspected human cases of the tick-borne Powassan virus reached nearly 50 by the end of last year, more than half of the cases occurring since 2010.

The states most affected are Minnesota, with 19 cases since 2001; New York, with 13; and Wisconsin, with 10. Maine, Michigan, Pennsylvania and Virginia have reported one or two cases each during the past 12 years.

Like the more common West Nile virus, which is spread by the bite of infected mosquitoes, Powassan virus can infect the central nervous system and cause encephalitis and meningitis. Symptoms include fever, headache, vomiting, weakness, confusion, loss of coordination, speech difficulties and seizures.

"There's no way to clinically differentiate the two," Dr. Scott Weese, a veterinary internist and microbiologist who heads infection control at the Ontario Veterinary College teaching hospital, said by email. "However, when Powassan causes disease, it is much more likely to be severe. West Nile Virus tends to cause mild disease more often, with severe disease occurring in a small percentage of diseased individuals. Obviously, the epidemiology of disease is different because of different vectors and ranges. Human infections (of Powassan virus) are very rare but when they occur, neurological disease is severe, mortality rates are high and survivors often have residual neurological problems. It's an interesting virus."

The virus is named for the Ontario town of Powassan, where the first recognized case of disease occurred in 1958, resulting in the death of a boy from encephalitis. According to the Canadian Cooperative Wildlife Health Centre, the virus has since sickened people in Quebec and New Brunswick as well as Ontario.

A variety of small mammals, including woodchucks, squirrels and chipmunks, have been found with the virus “but disease appears to be unusual” in wildlife, the agency states.

Whether domestic dogs and cats are susceptible is unknown. Said Weese: “I’m not aware of any confirmed cases, but you have to consider how many dogs and cats with neurological disease get tested for this virus (very few, if any). Encephalitis in dogs and cats is not always well explored (beyond rabies), and testing for Powassan is not commonly available.”

He added: “There was one small study here in Guelph where they tried to experimentally infect cats, and they couldn’t cause disease (although there were some histological changes in the brain of one cat). However, diseases can sometimes be hard to reproduce, so that can’t be taken as confirmation that it can’t infect dogs and cats. The evidence is still lacking either way.”

There are two recognized types of Powassan virus, both linked to human disease but which are genetically and ecologically distinct. The first, dubbed lineage 1 POW virus, is associated with Ixodes cookei, also known as the woodchuck tick, and Ixodes marxi, also known as the squirrel tick.

The second type, referred to as lineage II POW virus or deer-tick virus, is associated with Ixodes scapularis, also known as deer ticks or blacklegged ticks. Blacklegged ticks may transmit the bacterium that causes another well-known illness affecting people and pets alike: Lyme disease. They also may transmit organisms that cause anaplasmosis and babesiosis.
 
Responding to a surge of cases of Powassan encephalitis in seven counties of New York’s Hudson Valley during the past 15 years, the New York State Department of Health conducted a large sampling study — involving the collection of more than 13,000 ticks from mammals, birds and vegetation — to determine the prevalence of the virus in in the region. The study, published in July in the peer-reviewed online journal Parasites and Vectors, did not detect lineage I POW virus. The researchers concluded that lineage II POW virus is responsible for the cases in New York.
   
Researchers also found the virus more common east of the Hudson River, leading them to surmise that the waterway provides something of a natural barrier to the westward movement of the virus.

Noting, however, that the river barrier has been breached by the Lyme disease bacterium as well as the Powassan virus, research leader Laura Kramer said, “I think it's more of a hindrance than a barrier. An infested animal crosses the river, like a deer, and birds will disperse the tick."

Kramer, who is director of The Arbovirus Laboratories at the state health department and a professor of biomedical sciences at the State University of New York at Albany School of Public Health, suspects that wherever there are Ixodes cookei and Ixodes scapularis and Lyme disease, Powassan virus may be found.

From a veterinary standpoint, Weese said, companion animals need appropriate tick preventive treatments, whether or not they’re vulnerable to the Powassan virus.

“Overall, the risk to pet owners and pets posed by Powassan virus is very low,” he said. “Taking measures to avoid ticks is the key, and such precautions should be taken for many reasons beyond Powassan virus exposure."