Powassan virus/deer tick virus

This pathogen is a tick-transmitted virus that rarely causes illness in humans in Wisconsin.  Cases of Powassan/deer tick virus infections have been detected because of surveillance for West Nile virus, a mosquito transmitted pathogen.  This section will cover the transmission, clinical signs and symptoms and biology related to Powassan/DTV virus.  This virus has been found in approximately 1.3% of adult deer ticks in the northwestern part of the state (Washburn County) and about 2-3% of adult deer ticks in Marathon County, in an area where high tick densities have been documented.  The infection rates in nymphal deer ticks are not known.

Figure 1.  Map of Powassan virus infections in humans. See https://www.dhs.wisconsin.gov/tickborne/powassan.htm for the most up to date version of this map and other information.

Powassan 2003-2014 Map courtesy of Wisconsin Department of Health

What is Powassan/DTV Virus?  

Powassan virus (POW) is a rare but serious tick-borne illness in North America caused by a virus. Infection with POW virus can cause encephalitis or acute infection and inflammation of the brain. It is classified as a “flavivirus” which is related to some mosquito-borne viruses such as West Nile virus, dengue fever virus, yellow fever virus and several other viruses which can cause encephalitis. POW virus is named after Powassan, Ontario, Canada where it was first identified in 1958 in a five year-old child with encephalitis (McClean and Donahue 1958). The first recognized case of Powassan encephalitis in the United States was reported in 1970 in New Jersey, although the virus was first isolated from Rocky Mountain wood ticks (Dermacentor andersoni) collected from Colorado in 1952 (Ebel 2010). The first reported case of Powassan encephalitis in a Wisconsin resident occurred in 2003 (Hoang-Johnson et al. 2010). DTV (deer tick virus) is a related virus (sometimes called lineage II).  DTV may be responsible for most of the human disease in Wisconsin.

How is Powassan Virus Transmitted?  

Powassan virus is considered an “arboviral” disease or a virus that is transmitted by arthropods such as mosquitoes and ticks. Powassan virus is transmitted to humans by the bite of a tick infected with POW virus. Not all tick species can transmit the virus. In North America, POW virus as been isolated from several tick species– Ixodes cookei,  Ixodes marxi, Ixodes spinipalpus and Dermacentor andersoni–although I. cookei is suspected to be the primary vector or transmitter of POW virus (Ebel 2010). I. cookei ticks are primarily associated with small to medium sized mammals–groundhogs (Marmota monax) in particular (Pesko et al 2010). Although I. cookei ticks prefer wild mammalian hosts, they will occasionally bite humans (Brackney et al. 2008). I. cookei does not typically “quest” or search for hosts on vegetation like Ixodes scapularis– the tick vector primarily associated with transmitting the bacteria which causes Lyme disease–but is more commonly found near the nests or burrows of its preferred hosts. POW-lineage viruses have been associated with at least 38 small and medium size wildlife species as well as domestic animals (Hoang-Johnson et al. 2010).

In Wisconsin, a variant or different type of Powassan virus, called the “deer tick virus” (DTV) or “lineage II”, has been detected in Ixodes scapularis (blacklegged ticks) while the “lineage I” type of POW virus is mainly carried by Ixodes cookei ticks (Brackney et al. 2008; Ebel et al. 2001). DTV appears to be primarily maintained in nature between I. scapularis ticks and white-footed mice (Peromyscus leucopus) (Pesko et al. 2010; Ebel et al. 2000; Telford et al. 1997). At least three cycles of POW virus appear to be present in nature according to literature including (Ebel 2010):

(1) Ixodes cookei and groundhogs and small carnivores,

(2) Ixodes marxi and squirrel species,

(3) Ixodes scapularis and white-footed mice 

The vector competence– or the ability of an arthropod such as a tick to become infected with, support replication and/or development of, and transmit a pathogen to humans or other animals– is currently under investigation for both POW and DTV viruses. Laboratory studies determining the vector competence of blacklegged ticks (Ixodes scapularis) and Rocky Mountain wood ticks (Dermacentor andersoni) found vertical transmission (transfer of a pathogen from the female to her offspring) of POW virus did not occur in infected D. andersoni females but did occur in infected I. scapularis females (Ebel 2010; Costero and Grayson 1996). In a recent study to determine the duration of tick attachment necessary for the efficient transfer of POW virus from blacklegged ticks (I. scapularis) to the host found transmission occurred rapidly–nymphal phase deer ticks efficiently transmitted POW virus to laboratory mice within 15 minutes of attachment (Ebel and Kramer 2004).

It is important to keep in mind many experimental studies to determine vector competence or duration of attachment necessary for transmission have been conducted with laboratory animals and with only a single strain of the virus, therefore it is still unclear the minimum length of time necessary to transmit POW virus from infected ticks to humans or the minimum amount of virus necessary to cause clinical illness in humans. 

In Wisconsin, the tick vector most likely to transmit POW/DTV infections to humans is the blacklegged or deer tick, Ixodes scapularis


Where is Powassan Virus Found in the United States?

 Powassan virus primarily occurs in northeastern and upper Midwestern states (Ebel 2010).

Figure 2. Powassan virus neuroinvasive disease cases reported by state, 2004–2013.  Figure from CDC http://www.cdc.gov/powassan/statistics.html

CDC Pow surveillance map

What are the Clinical Signs and Symptoms of Infection with Powassan Virus?  

There are no clinical signs or symptoms that are unique to infection with Powassan virus. Clinical signs and symptoms of human infection with Powassan virus may vary from a mild, febrile illness to severe encephalitis. The virus can affect the central nervous system of some people causing encephalitis or inflammation of the brain.

Clinical signs and symptoms of infection with POW virus may include but are not limited to: 

Fever, headache, muscle and/or joint aches, body aches, stiff neck, vomiting, weakness, loss of coordination, confusion, speech difficulties or loss of memory. 

About 10-15% of cases of POW encephalitis are fatal but severe, long-term neurological deficits and/or complications may occur in approximately 50% of survivors (Ebel 2010; Hoang Johsnon et al. 2010). 

How is Powassan Encephalitis Diagnosed?  

If you think you may have clinical signs or symptoms suggestive of a tick-borne illness or encephalitis or may have been exposed to a tick vector of POW virus such as the blacklegged or deer tick (Ixodes scapularis), please contact your physician or medical provider. Diagnosis of Powassan virus encephalitis and other tick-borne and arboviral diseases are based on a thorough medical history, possible exposure to established high-risk areas for tick-borne diseases where you live and physician observed clinical signs and symptoms as well as accompanying diagnostic laboratory methods to support the diagnosis. 

Laboratory Detection:

Symptoms of infection with Powassan virus are not specific and can be similar to symptoms of infection with other arboviral infections, therefore if infection with an arbovirus is suspected in a patient, arboviral panel testing is often necessary. Validated blood tests to detect antibodies to POW virus are not available through commercial laboratories but arboviral testing can be requested through the Wisconsin State Laboratory of Hygiene. The arboviral panel includes testing for: West Nile, California/La Crosse, St. Louis, Eastern equine, and Western equine encephalitis viruses. The Wisconsin State Lab of Hygiene can forward a POW virus test request to the Arbovirus Diagnostic Laboratory with the Centers for Disease Control and Prevention (CDC).

Laboratory diagnosis of Powassan virus is generally accomplished by testing a sample of blood (serum) or cerebrospinal (CSF) fluid from a patient for the detection of virus-specific antibodies–a measure of the reaction of the body’s immune system to a disease agent. 

Serological diagnosis is not genotype specific–therefore serological testing cannot distinguish between POW lineage I and POW lineage II or “deer tick virus” (Ebel 2010).

How is Powassan Encephalitis Treated? 

There are not specific medications, antibiotics or vaccines for the treatment and/or prevention of Powassan virus encephalitis. Supportive care is recommended to help relieve the symptoms associated with POW encephalitis.

Currently the most effective way to prevent infection with Powassan virus is to avoid being bitten by ticks or exposure to known tick habitats by taking proper precautions when spending time outdoors where ticks may be present.

Proper precautions for avoiding tick-borne illnesses include:

  • Walking on mowed or cleared trails,  
  • Avoiding areas with overgrown grasses or brush,
  • Wearing long pants, long-sleeved shirts, and socks when outdoors, 
  • Applying appropriate repellents to skin and/or clothing and showering shortly after spending time outdoors
  • Checking your entire body for ticks shortly after spending time outdoors 


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Brackney DE, Nofchissey RA, Fitzpatrick KA, Brown IK, Ebel GD. (2008). Stable prevalance of Powassan virus in Ixodes scapularis in a northern Wisconsin focus. Am J Trop Med Hyg; 79(6): 971-3. 
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Hoang Johnson DK, Staples JE, Sotir MJ, Warshauer DM, Davis JP. (2010) Tickborne Powassan virus infections among Wisconsin residents. WMJ; 109(2): 91-7.

McClean, D. M., and W. L. Donahue. 1958. Powassan virus: isolation of virus from a fatal case of encephalitis. Can. Med. Assoc. J. 80: 708-711.
Pesko KN, Torres-Perez F, Hjelle BL, Ebel GD. (2010). Molecular epidemiology of Powassan virus in North America. J Gen Virol; 91(Pt 11): 2698-2705. Epub 2010 Jul 14.
Telford SR III, Armstrong PM, Katavolo P, Foppa I, Garcia ASO, Wilson M, and Spielman A. (1997). A new tick-borne encephalitis-like virus infecting New England deer ticks, Ixodes dammini. Emerg. Infect. Dis; 3:165-170.