Mosquito borne diseases have been in the news quite a bit lately, so what’s going on and how can you protect yourself and your family?
First, we found out that Dr Fauci was hospitalized with West Nile (WNV). Then we heard about a man in New England dying from Eastern Equine Encephalitis (EEE). And then, Maryland confirmed its first case of WNV for the season in the Baltimore area.
First, let’s just note that these are both very rare viruses to find in humans, especially in Maryland. There are an average of 11 human cases of EEE and around 2000-3000 cases of WNV annually, though these numbers can fluctuate considerably.
Both viruses cause Encephalitis, inflammation of the brain. They are both predominantly avian diseases as well, even EEE, despite Equine being in the name. For WNV and EEE, humans are what’s called a “dead end host”. This is because humans do not develop sufficient quantities of the virus in their blood to pass the disease on to a mosquito. Each transmission of WNV or EEE to a human resulted from a mosquito acquiring the disease from an infected bird, and then infecting a human.
Both viruses have very high mortality rates in humans, about 10% for WNV and about 30% for EEE. This is why each case generates so much media attention, especially compared to other much more commonly spread mosquito borne diseases such as heartworm in cats and dogs.
Thankfully, by far the most prevalent mosquito in our service area, the tiger mosquito (Aedes albopictus), is not viewed as a significant vector of either WNV or EEE. This largely makes sense as Aedes albopictus only very rarely uses birds as a host, preferring mammals instead. The primary vectors of WNV are all in the genus Culex, with the most common being Culex pipiens, the northern house mosquito. This mosquito bites approximately 75% birds and 25% mammals. The primary vector of EEE is Culiseta melanura, which bites almost exclusively birds. This is believed to be the reason that EEE cases in humans are so rare.
Adult female Culex pipiens, the northern house mosquito
Both Culex pipiens and Culiseta melanura are active between dusk and dawn, so avoiding being outside unprotected, especially in the 2 hours after dusk, can go a long way towards protecting you. Both lay their eggs in rafts, laying all eggs for that brood in a single raft in a single location.
Culiseta melanura is a permanent water species, primarily using swamps for its breeding habitat. As a result, most EEE cases are found within 5 miles of a swamp, as 5 miles is their approximate flight range. If you live close to a swamp, taking extra precautions in the 2 hours after dark may be warranted.
Culex pipiens is much more versatile in its selection of breeding sites, and thus much more common overall. Because of this, and because its host preference includes more mammals, WNV is much more common than EEE. While it can and will use permanent water sites, it is also a container breeder, like the tiger mosquitos. Because it lays only one egg raft per brood, thus putting all of its eggs in one basket, Culex pipiens is very susceptible to being targeted at the larval stage.
Culex pipiens egg raft right after being laid, the eggs will turn darker in a few hours
When we are monitoring our baiting stations, we often find Culex egg rafts early on, but seldom find them after a customer has been with us for a while. This is likely because their populations are substantially reduced by the baiting stations, as well as larviciding. A properly larvicided backyard pond can all but eliminate Culex pipiens locally. They do have a longer flight range than tiger mosquitos, between 0.25 and 2.0 miles, but their susceptibility to targeting at the larval stage far outweighs the flight range.
While the headlines can be scary, we don’t want to drum up fear over these diseases, as our customers are unlikely to fall victim to them. EEE is simply too rare and too specific to certain areas, while the risk of contracting WNV can be significantly reduced by managing Culex pipiens populations. The most important recommendation we can offer is just to avoid being outside for dusk and the 2 hours after, especially in August and September. If you must be outside at that time, use a picaridin or lemon eucalyptus oil based mosquito repellent. Picaridin based lotions are the most effective and long lasting.