SAN ANTONIO — The incidence of Powassan encephalitis, a tick-borne cause of long-term neurologic problems, disability, and death, has recently been increasing in the northern United States and in Canada, and a second virus lineage found in the deer tick might someday enable the disease to spread even further, a researcher from the New York State Department of Health reported.
The U.S. infection rate, which had held at 0.7 cases per year for 30 years, has climbed to 1 case per year since 1998, Susan Wong, Ph.D., said at the Southwest Conference on Diseases in Nature Transmissible to Man.
According to the Centers for Disease Control and Prevention, mild cases of arboviral encephalitis may present with only a slight fever and/or headache and body aches. Severe infections, however, occur with rapid onset and usually feature high fever, headache, and disorientation; they can include tremors, convulsions, paralysis, coma, and death. There is no effective treatment for the disease, which is fatal in 10%–15% of cases.
The primary lineage of Powassan virus is carried by a tick (Ixodes cookei) that feeds on the woodchuck (groundhog) and other small mammals, including squirrels and chipmunks, Dr. Wong said. Persons at greatest risk of infection are residents of the aforementioned regions who have close exposure (e.g., via gardening) to woodchuck burrows. Most cases occur from May to December, with the peak incidence from June to September.
During 1999–2001, four cases of Powassan encephalitis were reported in New Hampshire and Maine. During 2002–2006, there were two cases in New York (one in Westchester County, a suburb of New York City), as well as one in Wisconsin and one in Michigan, she said. This contrasts with the 1958–1998 period, wherein there were only 27 human cases in the United States and Canada.
Moreover, a second strain of the virus was found in the deer tick (Ixodes scapularis) in the 1990s, she noted, adding that in this second lineage there appears to be “potentially perhaps a greater epidemic potential,” because this tick species is the one that carries Lyme disease, ehrlichiosis, and other more common tick-borne diseases.
Dogs and horses can also become infected by ticks carrying the virus and can spread it to humans, she noted at the meeting, which was held in conjunction with the International Conference on Diseases in Nature Communicable to Man.
Powassan viral isolates have been found in California, South Dakota, New York, West Virginia, Connecticut, Massachusetts, and Maine, as well as Ontario. Moreover, serologic testing has identified human infections in most other Canadian provinces bordering the United States, Dr. Wong said.
The virus until recently was considered a low threat to human health. In North America, the disease first appeared in Canada in 1958, in a patient from Powassan, Ont. The first U.S. case occurred in New Jersey in 1970.
The Powassan virus is the least common cause of arboviral encephalitis in North America, whereas West Nile virus is the most common.
The incidence of another often lethal arbovirus, Eastern equine encephalitis virus, has been increasing in New England, according to the CDC.