Tick-borne Powassan virus spreads in US
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Cases of Powassan virus disease, a potentially fatal tick-borne illness for which there is no vaccine or treatment, have increased in recent years in the United States, with infections reported from an increasing number of states.
Powassan virus can cause encephalitis, which is fatal in approximately 10% of reported cases, according to the CDC. The virus can be transmitted in as little as 15 minutes by several tick species, including Ixodes cookei, Ixodes marxi and Ixodes scapularis, also known as the blacklegged tick, a common vector of Lyme disease.
According to Carolyn Gould, MD, MSCR, a medical epidemiologist in the CDC’s Division of Vector-Borne Diseases, some patients with Powassan virus infection do not develop symptoms. However, early signs can include fever, headache, vomiting and generalized weakness. Over time, the disease progresses to meningoencephalitis, which can cause altered mental status, seizures, paresis, movement disorders or cranial nerve palsies, with approximately half of survivors having permanent neurological symptoms such as recurrent headaches, muscle wasting and memory problems.
“Diagnosis is based on a combination of signs and symptoms and laboratory tests of blood or spinal fluid. These tests typically detect Powassan virus-specific [immunoglobulin (IgM)] and neutralizing antibodies,” Gould told Infectious Diseases in Children. “Immunoassays for Powassan virus IgM antibodies are available commercially and through state public health laboratories and CDC. A positive Powassan virus IgM antibody test result should be confirmed by neutralizing antibody testing at a state public health laboratory or CDC.”
Most cases of Powassan virus occur in the Northeast and Great Lakes region, and infections typically happen in late spring, early summer or mid-fall when ticks are most active.
“The geographic expansion of Ixodes scapularis has led to the expansion of Powassan virus, and human cases have been detected along the Eastern seaboard from Canada to North Carolina, and in the north-central Midwest, mainly in Michigan, Minnesota and Wisconsin,” Lyle R. Petersen, MD, MPH, director of the CDC’s Division of Vector-Borne Diseases, said in an interview.
The U.S. has experienced a spike in Powassan virus starting in 2016, when the number of reported cases climbed to 21 from six the previous year. In 2017, 33 cases were reported — the highest number so far. There were 21 cases reported in 2018.
According to experts, the rise in cases can possibly be attributed to increased awareness and testing for Powassan virus infection in certain states, especially where the risk is higher. Since 2009, Minnesota and Wisconsin have seen the most cases — 34 and 25, respectively — with Massachusetts (22) and New York (19) reporting the next highest numbers.
Petersen said doctors should consider Powassan virus in patients with a febrile or acute neurologic illness who have possibly been exposed to ticks in areas where Powassan virus is endemic.
There is no treatment for Powassan virus. According to the CDC, patients with severe Powassan virus disease often need to be hospitalized and may receive respiratory support, IV fluids and medications to reduce brain swelling.
“They either survive or die,” Infectious Diseases in Children Editorial Board Member Eugene D. Shapiro, MD, professor of pediatrics and epidemiology at Yale School of Public Health, said in an interview.
In November 2016, a 5-month-old infant became the first recorded patient in Connecticut to contract the rare virus after he was bitten by a tick carried into the house by a relative, according to an MMWR report. The attending physician who ordered that the boy be tested for Powassan virus said several clues indicated he had been infected with the virus, including that it was past mosquito season, that the boy had a confirmed tick bite, and that his MRI results were more consistent with a diagnosis of Powassan virus disease than other causes of encephalitis.
According to Shapiro, Powassan virus seems to be related to tick-borne encephalitis, which is “fairly common” in Western Europe and for which there is a widely used vaccine. There have not been any studies testing if the vaccine would work on Powassan virus.
Gould said the CDC is not aware of or working on any preventive vaccine for Powassan virus and Petersen suggested that, with the still-low incidence of the infection, the development of a commercially available vaccine is unlikely.
With no available vaccine, experts agree that the best way to prevent Powassan or any other tick-borne virus is by preventing tick bites by avoiding high brushy areas, wearing long sleeves and pants when possible, using repellent and checking for ticks after being outdoors.
Powassan virus disease has been a nationally notifiable condition since 2005.
“How big of a threat is Powassan virus? Well, if you’re infected with it — it’s a very big threat,” Shapiro said. “It appears that there has been a spike or increase, so it may be that there has been inadequate surveillance. We may have just been identifying the most severe cases, not the subclinical. There needs to be more research on this, clearly.”– by Caitlyn Stulpin
- References:
- CDC. Powassan virus. https://www.cdc.gov/powassan/index.html. Accessed September 23, 2019.
- CDC. Preventing tick bites. https://www.cdc.gov/ticks/avoid/on_people.html. Accessed September 23, 2019.
- CDC. Tickborne diseases of the United States. https://www.cdc.gov/ticks/tickbornediseases/index.html. Accessed September 23, 2019.
- CDC. Ticks. https://www.cdc.gov/ticks/index.html. Accessed September 23, 2019.
- Ebel GD and Kramer LD. Am J Trop Med Hyg. 2004;doi:10.4269/ajtmh.2004.71.3.0700268.
- Tutolo JW, et al. MMWR Morb Mortal Wkly Rep. 2017;doi:10.15585/mmwr.mm6615a3.
Disclosures: Gould, Petersen and Shapiro report no relevant financial disclosures.