We have 48 species of mosquitoes in New Hampshire, but most of them do not pose any health risk to people. Some species won’t even bite people, but instead bite turtles, frogs or birds. We have two mosquito-spread human diseases in New Hampshire: Eastern Equine Encephalitis and West Nile Virus. Both are primarily diseases of birds, with mammals as “accidental” victims. People ask me about Zika virus, but we do not anticipate having locally-acquired cases in New Hampshire. The two vector species do not occur here.
When describing the risk from various mosquito species, entomologists use two terms: vectors and bridge vectors. Vectors are mosquito species that have the ability to spread the disease from bird to bird. Bridge vectors present a bigger threat to people --- they are species that can spread the disease from birds to mammals. A third term sometimes appears in reports and can confuse people. Sometimes we say that “a pool of mosquitoes tested positive for EEE”. Some people assume that it means a pool of water. No, it does not. Adult mosquitoes are collected in traps, and painstakingly sorted by species into groups. A group of mosquitoes (perhaps 10) of the same species, from the same site and date is ground up together and tested. So ten mosquitoes are “pooled” and tested as one. This greatly reduces costs. In recent years, we have tried to use the term “batch” so it doesn’t create confusion.
Typically the human disease risk in New Hampshire becomes detectable some time in July or August, and rises to its highest in September. If the first killing frost is late, high risk continues into early October. Historically, the greatest risk is in the southeastern part of the state, but a disease case could turn up almost anywhere.
Here you will find regular posts characterizing the mosquito-spread disease risk in NH as the data become available.
Most Recent Risk Update:
October 19, 2017
This will likely be my last report of the season. We had light to moderate frost reported earlier this week in much of the state, and weather forecasts suggest more is coming. Heavy frost should stop all flying mosquito activity for the year… but the last part of the state likely to get frost is right where we had two more West Nile Virus-positive mosquito batches detected. Those were from East Kingston and Danville, a region where historically we have found both WNV and EEE. In both cases, the mosquitoes were Culiseta melanura. This is a bit reassuring, because Cm is strictly a bird biting species. It shows us that west nile virus is circulating in the bird population, which is expected for late in the year, before the first heavy frost.
So my recommendation is to use precautions to prevent mosquito bites (repellants, covering clothing), especially in the southeast ¼ of our state. After we have had a good killing frost there, the risk of new mosquito-borne human infections should drop to zero.
- Dr. Alan Eaton, UNH Cooperative Extension
September 29, 2017
Today I learned that another batch of mosquitoes has turned up positive for West Nile virus. This time it was from a trap in Madbury. The species is Culiseta melanura, which is strictly a bird biter. So this may raise the risk slightly, this time in the seacoast region. This is pretty much what we’d expect for this time of year. Overall, the risk isn’t very high this year.
What is surprising is that a second New Hampshire person has been detected with Jamestown Canyon Virus. We had one person from Grafton county earlier this summer, and now one from Hillsborough county with this very rare (for us) mosquito-spread disease. To my knowledge, the disease is so rare here that the state public health lab doesn’t even test our mosquitoes for it. (Testing is very expensive.) So we really don’t know much about the risk for this in NH, other than the cases may have been acquired in those two counties. The disease is widespread in the USA, but is very rare. Initial symptoms sound pretty similar to WNV and EEE: fever, severe headache, dizziness, etc.
- Dr. Alan Eaton, UNH Cooperative Extension
September 21, 2017
This is the time of year when the risk of mosquito-spread diseases is highest… until a hard frost kills the adults and stops the risk for the year. Today we learned that another batch of mosquitoes was positive for West Nile virus. This time they were Culex salinarius from Rye. This species usually feeds on birds, but sometimes bites mammals. It makes the 5th batch that was WNV positive this year, and it is the first one that involves a mixed feeder. The other positives this year were mosquito species that bite birds, not mammals. So this represents a slight increase in risk, especially for the Rye region.
- Dr. Alan Eaton, UNH Cooperative Extension
September 7, 2017
I have just received the NH DHHS state lab arbovirus testing results through September 2nd. There are no further positive reports of any kind. That is a relief, but we are not out of the woods yet. September (and early October if we don’t get a killing frost early) is usually when the risk of getting West Nile Virus or Eastern Equine Encephalitis is the highest in the year. So far, the risk has been low.
- Dr. Alan Eaton, UNH Cooperative Extension
August 17, 2017
In addition to the two WNV-positive mosquito batches reported recently, another positive mosquito batch has turned up from Keene. As before, the species is one that almost exclusively bites birds (Culex resutans). That does not change the risk picture too much, except that it now suggests that people in the Keene area increase their use of anti-mosquito behaviors, like covering up (especially around dusk) and considering using repellants. If we begin finding the virus in mosquitoes that bite mammals, that will increase the risk.
We also learned that NH has a case of Powassan virus. This is a tick-borne disease, and constitutes the third human case ever found in New Hampshire. The other cases were in 2013 and 2016. This one was from Strafford County.
We also learned of a New Hampshire person with Jamestown canyon virus. Human cases of this disease are pretty rare, especially in the eastern USA. It is spread by mosquitoes, especially spring Ochlertatus species.
- Dr. Alan Eaton, UNH Cooperative Extension
August 10, 2017
Today I received word of the first virus-positive mosquitoes for this year. Two batches of Culex pipiens and/or restuans were collected from Manchester that were positive for West Nile virus. One was taken Monday July 31st; the other was Tuesday August 1st. Both of these species rarely bite people, but readily bite birds. To put it in perspective, that’s Two batches out of 240 mosquito batches of various species submitted to NH DHHS this week, from Cheshire, Hillsborough, Rockingham and Strafford counties. It tells us that the virus is circulating in the bird population, but the risk to humans is still probably quite low.
For people in the Manchester area, it is wise to take normal precautions regarding mosquitoes: cover bare skin, use repellants if you are outdoors, especially around dusk or dawn. The risk will likely go higher as the season progresses. Eastern Equine Encephalitis (a more serious disease) has not turned up yet.
- Dr. Alan Eaton, UNH Cooperative Extension
July 25, 2017
This spring we had abundant rainfall, and many areas had high mosquito numbers. Biting and annoyance levels were high in many places, but the risk of disease from mosquitoes is so low in the spring and early summer that it is extremely difficult to measure. Now it is July, and often that is when disease risk from mosquitoes starts to rise and indicators of risk begin to appear. Usually the highest risk in our state is in late summer and early fall. We have had good rainfall recently, so mosquito numbers and annoyance is likely to remain high in many places.
There have been no positive West Nile Virus or Eastern Equine Encephalitis indicators yet this year in New Hampshire. In Massachusetts, there have been multiple recent detections of mosquitoes infected with WNV in Essex, Middlesex and Suffolk Counties.
- Dr. Alan Eaton, UNH Cooperative Extension
October 1, 2016
As of October 1st, there have been no additional positive indicators of either EEE or West Nile virus in New Hampshire for 2016, so this continues as a pretty low-risk year for us. The period of mosquito-borne disease risk will end when we get a hard frost that extends through all of New Hampshire. As I write this on Friday October 7th few sites in the state have had a fall frost.
- Dr. Alan Eaton, UNH Cooperative Extension
September 20, 2016
The first positive indicator of mosquito-borne disease is really late this year. A batch of Culex pipiens from Nashua, collected on September 13th has tested positive for West Nile Virus. This far north, this species is almost exclusively a bird biter, but occasionally (esp farther south) it will bite mammals.
There have been no other positive indicators (mosquito batches, veterinary samples, human tests) in New Hampshire so far, but others could develop. Typically the risk is highest for the year in late summer and early fall before the first killing frost.
In our neighboring states, there have been 2 human WNV cases detected in Windsor County, VT and one in Middlesex County, MA. There have been no positive indicators of any kind for Eastern Equine Encephalitis.
It is wise for people who will be active outdoors (especially in the Nashua area) to wear clothing that exposes little skin, and consider using mosquito repellant, especially within an hour or two of sunset.
- Dr. Alan Eaton, UNH Cooperative Extension
September 6, 2016
I have just received the testing report for the period of August 28 to Sept 3. There have been no positive detections of West Nile virus or Eastern Equine Encephalitis in New Hampshire this year. That includes mosquito testing, human tests, or animal tests. There have been human cases of West Nile virus detected in Middlesex County, MA and Windsor Co, VT. The only eastern equine encephalitis detected in New England this year has been a mosquito batch in Washington Co, Rhode Island.
- Dr. Alan Eaton, UNH Cooperative Extension
August 23, 2016
I just received the NH testing report through August 20th. There have been no positive indicators of West Nile virus or Eastern Equine Encephalitis in NH so far, and no additional discoveries in neighboring states. September is coming, and that is typically when the risk is highest.
- Dr. Alan Eaton, UNH Cooperative Extension
August 9, 2016
The New Hampshire testing report for July 31 through Aug 6 just arrived. There have been no positive indicators in New Hampshire so far this year. The Centers for Disease Control & Prevention report there has been one human case of West Nile Virus in Windsor County, Vermont. So far this year, there have been no indicators of Eastern Equine Encephalitis anywhere in New England.
- Dr. Alan Eaton, UNH Cooperative Extension
August 2, 2016
I just received the testing report through July 30th, and there are no positive indicators of either EEE or West Nile virus yet in New Hampshire. Yes, that includes human, animal and mosquito testing.
- Dr. Alan Eaton, UNH Cooperative Extension
July 27, 2016
As of July 26, we have seen no positive indicators of mosquito-spread diseases (West Nile Virus, Eastern Equine Encephalitis) in New Hampshire this year. This includes testing batches of mosquito adults, plus testing animals and people. In a drought year, sometimes West Nile Virus can occur at higher-than-average levels, but typically we do not see positive indicators of either WNV or EEE until August.
- Dr. Alan Eaton, UNH Cooperative Extension
October 6, 2015
This week we learned of our second positive indicator for Eastern Equine Encephalitis this year: another batch ofCuliseta melanura mosquitoes tested positive, this time from Candia. Both Candia and Newton (site of last week’s positive mosquito batch) are places where historically the risk of EEE has been elevated in our state. Other positive EEE indicators in New England include positive mosquito batches in York County Maine, and Franklin County, Vermont.
We had no additional West Nile Virus indicators announced this week.
So this raises the disease risk slightly for Rockingham County, an area that has experienced elevated EEE risk in the past. Culiseta melanura is a bird-biting species, so this shows the virus is being spread among birds there now. The risk for WNV stays roughly the same as last week for us. Last week’s announcement of a WNV-positive bird from Holderness points out that the risk is not completely restricted to those towns with positive mosquito batches (in Cheshire, Hillsborough and Rockingham Counties). Additionally, WNV-positive mosquito batches have now been found in every county of Massachusetts, and in every county of Vermont except Orange, Caledonia and Essex. At least one human case was announced from Cumberland County, ME. When a hard frost kills off the adult mosquitoes, the risk should plummet. A light frost is insufficient to do this.
Protecting yourself from mosquito bites is your best defense from acquiring one of these diseases. That includes minimizing going in areas with lots of mosquitoes (esp within an hour or two of dusk), wearing clothing that exposes little skin, and using repellants if you are outdoors in mosquito-prone places.
- Dr. Alan Eaton, UNH Cooperative Extension
September 30, 2015
This week we learned of our very first positive indicator for Eastern Equine Encephalitis this year: a batch of Culiseta melanura mosquitoes from Newton that tested positive. The only other positive indicators anywhere in New England were announced recently: positive mosquito batches in York County Maine, and Franklin County, Vermont.
We had one additional West Nile Virus indicator announced this week: a raven from Holderness.
So this raises the risk slightly for Rockingham County, an area that has experienced elevated EEE risk in the past. C. melanura is a bird-biting species, so this shows the virus is being spread among birds there now. The risk for WNV stays about the same, but the discovery of a WNV-positive bird from Holderness points out that the risk is not completely restricted to those towns with positive mosquito batches. When a hard frost kills off the adult mosquitoes, the risk should dramatically drop.
- Dr. Alan Eaton, UNH Cooperative Extension
September 25, 2015
The risk of acquiring a mosquito-spread disease in New Hampshire is unusually low for this time of year. This week we learned that two additional batches of mosquitoes tested positive for West Nile Virus. One was a batch of Culiseta morsitans from East Kingston. That species is a mixed feeder, meaning that it sometimes bites mammals. The second was a batch of Aedes vexans from Keene. That species mostly bites mammals. So now there is a slightly elevated risk of acquiring WNV in three areas that we know of… the areas around East Kingston, Manchester and Keene.
So far there are still no positive indicators for Eastern Equine Encephalitis in New Hampshire. Positive mosquito batches have now been detected in York County, ME and Franklin Co, Vermont.
- Dr. Alan Eaton, UNH Cooperative Extension
September 9, 2015
As of today, New Hampshire has had no positive indicators of mosquito-borne diseases this year. That includes mosquito and bird testing, veterinary cases or human cases. In bad years for West Nile Virus or Eastern Equine Encephalitis, we would have found positive mosquito batches, and several veterinary or human cases by now. The risk is always highest in September, and sometimes it extends into early October if we don’t get a good, killing frost until then. But I am pleased to note that no EEE positives have been reported anywhere in New England.
For West Nile Virus, there have been no human cases so far, but positive mosquito batches have turned up in several states: Vermont: Addison Co, Franklin Co, Rutland Co, Washington Co, Windham Co, Windsor Co. Massachusetts: Barnstable, Berkshire, Bristol, Middlesex, Norfolk, Plymouth, Suffolk, and Worcester Counties. Connecticut: Fairfield, Hartford, New Haven, New London and Middlesex Counties. Rhode Island: Washington Co. Maine: No positives.
So, I am pleased to report that overall the risk in New Hampshire is low now. That could change. During periods of elevated risk, you affect your risk by where you go, what time of day you are active, how much exposed skin you have, and whether or not you use insect repellant.
