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Outbreak of Powassan Encephalitis --- Maine and Vermont, 1999--2001

Powassan (POW) virus, a North American tickborne flavivirus related to the Eastern Hemisphere's tickborne encephalitis viruses (1), was first isolated from a patient with encephalitis in 1958 (1,2). During 1958--1998, 27 human POW encephalitis cases were reported from Canada and the northeastern United States (3). During September 1999--July 2001, four Maine and Vermont residents with encephalitis were found to be infected with POW virus. These persons were tested for other arbovirus infections found in the northeast after testing for West Nile virus (WNV) infection was negative. This report describes these four cases, summarizes the results of ecologic investigations, and discusses a potential association between ticks that infest medium-sized mammals and the risk for human exposure to POW virus. The findings underscore the need for personal protective measures to prevent tick bites and continued encephalitis surveillance.

Case Reports

Case 1. In June 2001, a 70-year-old man from Kennebec County, Maine, was taken to a local hospital with generalized muscle weakness, somnolence, diarrhea, and anorexia. On clinical examination, he had a fever of 104.7 F (40.4 C), leukocytosis of 11,500/mm3 (normal: 4,300--10,800/mm3), decreased renal function, and anemia. He subsequently developed left-sided hemiplegia and marked confusion. Cerebrospinal fluid (CSF) contained 40 white blood cells (WBCs)/mm3 (normal: <4/mm3) (87% lymphocytes) with elevated protein (96 mg/dL; normal: 20--50 mg/dL). Magnetic resonance imaging (MRI) revealed parietal changes consistent with microvascular ischemia or demyelinating disease. No causes for his apparent stroke were found. After 22 days of hospitalization, he was discharged to a rehabilitation facility. Nearly 3 months after symptom onset, he remains in the facility and is unable to move his left arm or leg. Serum specimens and CSF collected 3 days after hospitalization revealed POW virus-specific IgM; neutralizing antibody (1:640 titer) also was found in serum specimens. Although some cross-reaction with WNV and St. Louis encephalitis (SLE) virus occurred in the IgM assay, no neutralizing antibody was found.

The patient had not left Maine for 25 years. On ecologic investigation, overgrown bushes, leaf piles, and stacks of old lumber and scrap metal covered his property. Family members reported seeing woodchucks, skunks, and squirrels on the property. During the 2 weeks before illness, the patient's main activities were lying on the ground repairing a boat hull and yard work. Approximately 6 weeks after illness onset, nine medium-sized mammals were trapped on or near the patient's property. Collections from these mammals and the grassy and brushy areas of the property yielded 31 ticks (Ixodes cookei). Tests for POW virus infection were conducted at CDC. Of the nine mammal serum samples, four (two woodchucks and two skunks) contained neutralizing antibody to POW virus, but no virus was isolated from the ticks.

Case 2. In September 2000, a 53-year-old woman from York County, Maine, sought medical care at a local hospital for loss of balance, visual disturbance, and fever of 103 F (39.4 C). Her clinical examination showed agitation without confusion, ataxia, bilateral lateral gaze palsy, and dysarthria. CSF contained 148 WBCs/mm3 (46% neutrophils, 40% lymphocytes). During hospitalization, she developed altered mental status, generalized muscle weakness, and complete ophthalmoplegia. An electroencephalogram (EEG) indicated diffuse encephalitis, and a MRI showed bilateral temporal lobe abnormalities consistent with microvascular ischemia or demyelinating disease. After 13 days, she was transferred to a rehabilitation facility where she remained for 2 months. Nine months after onset of symptoms, she was walking and had regained her strength, but the ophthalmoplegia continued. A serum specimen collected 19 days after illness onset was positive for POW virus-specific IgM and neutralizing antibody (1:640 titer) and negative for WNV and SLE virus antibodies.

The patient had not left Maine in several months before illness onset. During two visits to a rural vacation home in the month before illness onset, the patient removed several squirrel nests but reported no contact with ticks or rodents. One month after illness onset, an ecologic evaluation of her primary home noted a well-manicured suburban property near brush and woodlands. No evidence of medium-sized mammals was found, and only three Ix. scapularis were collected; no POW virus was isolated. Nine months after illness onset, an ecologic evaluation of the patient's vacation home found several mammals, but none had ticks, and no serology samples were collected.

Case 3. In July 2000, a 25-year-old man from Waldo County, Maine, sought medical care at a local hospital for fever of 101.3 F (38.5 C), headache, vomiting, somnolence, and confusion. On clinical examination, the patient had difficulty answering simple questions and was intermittently uncooperative. He had bilateral hand twitching, muscle weakness, and pronounced lip smacking. CSF contained 920 WBCs/mm3 (74% lymphocytes) with elevated protein (77 mg/dL). EEG showed diffuse background slowing consistent with encephalitis. After 11 days of hospitalization, he was transferred to a rehabilitation facility. When discharged home 44 days later, the patient required assistance to stand and perform daily activities. Serum specimens and CSF collected 3 days after illness onset were negative for antibody to WNV and SLE virus but positive for POW virus-specific IgM antibody. The serum sample also had neutralizing antibody (1:80 titer) to POW virus. At the time of illness onset, the patient worked as a logger and lived in rural Maine where he raised livestock.

Case 4. In September 1999, a 66-year-old man from Washington County, Vermont, sought medical care at a hospital for somnolence, severe headache, increasing confusion, and bilateral leg weakness that developed over 6 days. On clinical examination, he was afebrile but had slow speech, memory loss, a wide-based gait, and bilateral weakness in proximal lower extremities. CSF contained 54 WBCs/mm3 (95% lymphocytes) and elevated protein (67 mg/dL). An EEG showed diffuse background slowing consistent with encephalitis. When discharged home 11 days later, he could walk but had cognitive difficulties, including severe memory lapses. Serum specimens collected 19 days after illness onset contained POW virus-specific IgM and neutralizing antibody (1:640 titer) but no antibody to WNV and SLE virus. During the month before illness onset, the patient traveled frequently to a vacation home where he saw numerous squirrels and skunks.

Reported by: T Courtney, MD, Southern Maine Medical Center, Biddeford; S Sears, MD, J Woytowicz, MD, MaineGeneral Medical Center, Augusta; D Preston, MD, MaineGeneral Medical Center, Waterville; R Smith, MD, P Rand, MD, E Lacombe, M Holman, C Lubelczyk, Lyme Disease Research Laboratory, Maine Medical Center Research Institute, Portland; G Beckett, MPH, E Pritchard, MS, K Gensheimer, MD, State Epidemiologist, Maine Dept of Human Svcs. A Beelen, MD, Veterans Affairs Medical Center, White River Junction; P Tassler, PhD, Vermont Dept of Health. Arbovirus Diseases Br and Bacterial Zoonoses Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note:

These four cases of POW encephalitis are the first reported in Maine and Vermont and the first in the United States since 1994 (4). Since the introduction of WNV into the northeastern United States in 1999 (5), testing for POW virus and other arboviruses that cause encephalitis has increased (CDC, unpublished data, 2001). These cases were identified as a direct result of requests for WNV testing. As surveillance continues, knowledge of the epidemiology of POW virus in the United States may increase.

In North America, POW virus has been isolated from four tick species, including Ix. cookei, Ix. marxi, Ix. spinipalpus, and Dermacentor andersoni; a variant POW virus also has been isolated from Ix. scapularis; and evidence of infection has been found in 38 mammal species, primarily woodchucks (1,6). Unlike Ix. scapularis, the primary vector for Lyme disease, Ix. cookei rarely search for hosts on vegetation and are often found in or near the nests or burrows of medium-sized mammals. Infections have occurred from May to December, with a peak during June--September when ticks are most active (1). Although neither the first or second patients recalled tick bites, ecologic investigations suggest that their illnesses resulted from visiting or living in areas where ticks are common. As with many infectious agents transmitted by Ixodid ticks, few infected persons recalled tick bites because these ticks are small and can be easily missed (3).

POW encephalitis is associated with significant long-term morbidity and has a case-fatality rate of 10%--15% (1,3). Because there is no vaccine or specific therapy for POW encephalitis, the best means of prevention is protection from tick bite. This includes using insect repellents, wearing light-colored clothing with long sleeves and pants tucked into socks or boots, avoiding or clearing brushy areas, and removing ticks before they attach or as soon after attachment as possible. Checking family pets also can prevent ticks from entering the home. Because Ix. cookei are often found on woodchucks and skunks and may be the primary vector of POW virus, environmental controls reducing human contact with small and medium-sized mammals should reduce risk for exposure to POW virus-infected ticks. Persons should keep areas adjacent to their home clear of brush, weeds, trash, and other elements that could support small and medium-sized mammals. When removing rodent nests, avoid direct contact with nesting materials and use sealed plastic bags for disposal and to prevent direct contact with ticks.

Because of the lack of awareness and the need for specialized laboratory tests to confirm diagnosis, the frequency of POW encephalitis may be greater than previously suspected. POW encephalitis should be included in the differential diagnosis of all encephalitis cases occurring in the northern United States, especially the northeast. Laboratory tests for POW virus infection are not commercially available but can be requested through state public health laboratories for testing at CDC. Awareness should be promoted among clinicians and public health staff, and tick-bite prevention strategies emphasized for the general public.


  1. Artsob H. Powassan encephalitis. In: Monath T, ed. The arboviruses: epidemiology and ecology. Volume IV. Boca Raton, Florida: CRC Press, 1988:29--49.
  2. McLean DM, Donohue WL. Powassan virus: isolation of virus from a fatal case of encephalitis. Can Med Assoc J 1958;80:708--11.
  3. Gholam BIA, Puksa S, Provias JP. Powassan encephalitis: a case report with neuropathology and literature review. Can Med Assoc J 1999;161:1419--22.
  4. CDC. Arboviral disease---United States, 1994. MMWR 1995;44:641--4.
  5. Nash D, Mostashari F, Fine A, Miller J, O'Leary D, Murray K. The outbreak of West Nile virus infection in the New York City area in 1999. N Engl J Med 2001;344:1807--14.
  6. Ebel GD, Spielman A, Telford SR. Phylogeny of North American Powassan virus. J Gen Vir 2001;82:1657--65.

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