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Rare Powassan virus is just one of many tick-borne diseases that Minnesotans should be aware of

Of the 75 cases reported by the CDC between 2006 and 2015, 20 were in Minnesota. And five more cases occurred in the state in 2016, according to MDH.

Blacklegged ticks
Minnesota Department of Health
Blacklegged ticks

In late April, the Centers for Disease Control and Prevention (CDC) reported that a 5-month-old baby had become the first person in Connecticut to be diagnosed with the Powassan virus, a rare but often serious disease that is transmitted to humans by ticks.

Just how rare can be seen in the numbers: There have been a total of 75 confirmed cases of Powassan virus in the United States during the past decade, according to the CDC. That compares with about 30,000 reported cases each year of Lyme disease, a tick-borne bacterial infection.

Despite its rarity, Powassan is still worrisome. It’s fatal in about 10 percent of cases, and leaves many of its survivors with lifelong neurological problems, the CDC says.

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Powassan may also be particularly concerning for Minnesotans. Of the 75 cases reported by the CDC between 2006 and 2015, 20 were in Minnesota. And five more cases occurred in the state in 2016, according to preliminary data compiled by the Minnesota Department of Health (MDH). Still, that number is dwarfed by the 1,176 confirmed cases of Lyme disease that occurred in the state in 2015.

With Powassan virus in the news, however, MinnPost decided to find out more about the disease — and about what Minnesota health officials are predicting regarding the coming infectious tick season, which is just entering the period when the risk to humans is at its highest (mid-May to mid-July). We spoke with David Neitzel, an MDH epidemiologist who specializes in tick-borne diseases. An edited version of that interview follows.

MinnPost: What is the Powassan virus?

David Neitzel: It’s a virus that is maintained in nature in ticks — between ticks and certain mammal species. It’s a cousin of West Nile virus, but is transmitted by ticks instead of mosquitos.

MP: Which ticks carry it here in Minnesota?

DN: The main ticks we’re concerned about in Minnesota are the blacklegged ticks, which most people still call deer ticks. There’s a different type of Powassan virus that can be carried by another species of tick, which we also have here in Minnesota, but we have no evidence that that tick really feeds on people that much.

MP: Twenty of the 75 cases reported to the CDC over the past decade were in Minnesota. Is the disease more common here, or are we just finding and reporting it better than other states?

David Neitzel
Macalester College
David Neitzel

DN: That’s certainly part of the equation. Science has known about the Powassan virus since the 1950s, but testing for human specimens hasn’t been available widely until just the last few years. We had our first Powassan case in Minnesota in 2008, and to get testing for that patient we had to go the CDC and also to the New York Department of Health. Those were the only two labs in the country that could even test for this virus at that time. Since then we’ve built testing in our own laboratory – the Minnesota Department of Public Health Laboratory – and we’ve been testing specimens submitted by doctors from suspect cases. It’s one of those deals where the more you look, the more you’re going to find.

That said, Powassan virus is another one of the many diseases transmitted by the blacklegged tick, and we’re one of the states that has lots of blacklegged ticks. So, it’s a combination of being in a place where the ticks and the diseases they transmit are found, and also just more aggressive looking for cases here.

MP: It seems like this is a more serious tick-borne disease, even though it’s rare.

DN: It’s potentially very serious. We haven’t released our 2016 numbers yet, but it looks like we’re going to have five additional Powassan cases [for that year]. One of those was a fatality, and we had at least one other fatality in that [earlier] group of 20 cases. So, we’re right there at roughly that 10 percent [fatality] mark. It’s very similar to West Nile virus. Some patients will have a very severe disease of their central nervous system, encephalitis or meningitis, and both of those can be life threatening.

MP: But the bigger concern for Minnesotans in terms of tick-borne diseases is Lyme disease, correct?

DN: Yeah. In our average year we’ll have anywhere from 1,000 to 1,400 Lyme disease cases reported in Minnesota residents. And when we go out into the field and collect tick specimens…we don’t have to go far to find ticks that are infected with Lyme disease bacteria. But when we look for Powassan virus — and we’ve found it widespread across much of the forest in parts of the state — the prevalence of ticks is down in the 1 percent to 4 percent range, depending on the site and the year.

MP: You said the state had five diagnosed cases of Powassan infection in 2016. That’s a quarter of what was reported in the previous 10 years. Why so many last year?

DN: It looks like we just had a lot of tick-borne disease in general last year. It was a good year for ticks to be out and feeding, and unfortunately that means that a lot of people get exposed to these ticks and the diseases they can transmit.

MP: What are your predictions for tick-borne diseases, particularly Lyme, this year? Are you expecting more cases? Fewer cases?

DN: It’s hard to predict with a lot of certainty because there are variables that really make a big difference, especially the weather conditions – in particular the weather conditions that we get in the latter half of May and through June. That’s typically our highest-risk time of year because that’s when the smaller nymph stage of the blacklegged tick comes out, and that’s the life stage that’s so small that people have a hard time seeing them. Those ticks are very susceptible to drying out, so if we have weather conditions that are a lot hotter and drier and windier than normal, it makes it hard for the ticks to come out and feed, and if they can’t come out and feed, they can’t bite people and transmit disease agents. If, however, we have a warm, humid spring – if it’s nice and moist out in the woods where these ticks live — we, unfortunately, have a higher-risk year.

Powassan virus neuroinvasive disease cases reported by state, 2006–2015
Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention
Powassan virus neuroinvasive disease cases reported by state, 2006–2015

MP: And what about other tick-borne diseases, such as babesiosis and anaplasmosis?

DN: Anaplasmosis is our second-most commonly reported tick-transmitted disease here in Minnesota. We typically have several hundred cases reported every year, somewhere in the 400 to 600 range. It can also occasionally cause fatality, especially in folks who are elderly or have a compromised immune system. Babesiosis is a little less common. We have generally 50 to 60 cases reported per year now, but that one can also cause very serious illness, including fatality. So those are both very serious diseases, although less common than Lyme. But we want Minnesotans to know that there is much more than just Lyme disease out there as far as tick-borne diseases. They are all good reasons to take precautions against ticks.

MP: What are those precautions?

DN: First, know when and where you’re at risk. The “when” is mid-May through mid-July. That’s when we have our highest risk for tick-borne disease, when that small nymph stage of the tick is out. The “where” is basically forested regions of the state, especially east central Minnesota between the Twin Cities and Duluth over to Bemidji and back down. But things have been changing in recent years. The ticks are being found a lot further north and west than they ever have been before. We’re basically at a point now where any forested regions of the state are considered to have some risk, so if you’re going out in the woods, you should take precautions.

The main precaution is using a good repellent. You can use repellents on your skin that contain DEET, the same stuff you use to scare away mosquitos. There are also repellents that are labeled for use on clothing. They contain permethrin. It’s not only a strong repellent, but it also kills any ticks that come into contact with a treated clothing item. It’s a very good option for people who live right out in the woods amongst the ticks.

After people have been out in the woods, they should make sure to take a shower to get any ticks that haven’t attached off of them right away. Then do a very good inspection to find and promptly remove any ticks that have gotten on and started to feed. The sooner you get them off, the more likely you are to prevent disease transmission.

FMI: For more information about tick-borne diseases — including how to recognize and watch for early symptoms — go to the MDH website.