Introduction

West Nile Virus (WNV) is an emerging infectious disease. The virus was first identified in the West Nile district of Uganda in 1937. Until 1999 the disease was found in Africa and Eurasia. Some of the countries where the virus is found in are Algeria, Russia, Botswana, Cyprus, Egypt, India, France, Spain, Italy and Israel. In 1999 the virus was introduced into New York City. No one knows how the virus was imported into the country. The imported strain of WNV is genetically similar to virus from Israel .

Since WNV was imported into the U.S. it has spread to 48 states, Puerto Rico, Canada and the Caribbean. Some evidence suggests that it may also be in Central and South America. The largest human outbreak was in 2003 there were a total of 9,862 confirmed cases nation wide. During the same year there were 264 deaths. Colorado experienced the largest outbreak with a total of 2,947 human cases. Also in 2003 there were 5,181 horses diagnosed with WNV.

In 2000 the virus was found in Massachusetts . Between 2000-2004 there have been 42 human cases of West Nile state wide. However, there have been no human cases of West Nile in Plymouth County. There has also been over 50 infected horses in the state, most of the horses have been from Plymouth County. The number of infected horses has decreased significantly since the introduction of a vaccine for horses.

Virus Cycle

WNV primarily circulates between birds and mosquitoes with mammalian infections being incidental. The CDC reports that 60 mosquito species nationwide have been infected with WNV. However, the virus is largely transmitted by Culex mosquitoes. In Massachusetts WNV is transmitted mainly by Cx. pipiens and Cx. restuans which obtain the virus from birds. These mosquitoes prefer to blood feed on birds. As the summer progresses the amount of virus circulating between these mosquitoes and birds increases. Toward the end of the summer the virus begins to infect mammal biting mosquitoes. Mammal biting mosquitoes become infected when they mistakenly blood feed on an infected bird. Once this happens, people, horses and other mammals begin to get WNV. There is also increasing evidence that Culex pipiens and Culex restuans can mistakenly bite mammals thus transmitting the virus directly to people and other mammals.

Human infections of WNV often occur in urban areas. For example, in 2002 both Chicago and Detroit experienced large epidemics and in Massachusetts most human infections occurred in and around Boston. This is because Culex pipiens and Cx. restuans are closely associated with people. When a female Cx. pipiens or restuans is ready to lay eggs she seeks out a container with water and organic material inside. Often this container is man made such as a tire or a bird bath. These containers are also home to several mammal biting mosquitoes. Thus all the mosquitoes needed to create human WNV infections can be found in your yard!

The impact of WNV on bird populations is unknown. In the U.S. 285 different species of birds have died from WNV infections (includes captive birds). The greatest mortality of birds has been with birds in the family Corvidae. This family includes crows, blue jays and ravens. Research has shown that crow mortality from WNV is near 100%. Blue Jays have also been experiencing high mortality from WNV. Due to the large numbers of crows dying from WNV monitoring the number of dead crows has become a way to gauge the amount of virus activity in a given area (for more information on the dead bird reporting program in MA please see Massachusetts Department of Public Health’s web page).

WNV and People

While, WNV can infect people it is important to remember that most human infections do not result in serious illness. It has been estimated that about 20% of infected people will experience flu like symptoms (West Nile Fever) and that less than 1% will become seriously ill (neuroinvasive disease). Age is the most important risk factor for contracting WNV. People over the age of 50 have the greatest risk of getting sick. In 2002 the median age for fatalities was 78. Among those with serious illness the fatality rate ranges from 3 – 15%. The highest mortality rate is among the elderly. It is not known if people with compromised immune systems have a greater risk for serious illness.

The incubation period for people is 3 -14 days. Symptoms include fever, headache, sore throat, backache, fatigue, conjunctivitis, rash spreading from trunk to extremities, nausea and abdominal pain.

While the risk is small WNV can also be transmitted from person to person via blood, organ transplants, breast milk and from mother to fetus. Blood donations are currently tested for WNV.

In 2002 a Michigan mother who contracted WNV from a blood transfusion appears to have passed the virus on to her infant through breast milk. The child is healthy. Currently, the risk of transmitting the virus through breast milk is unknown. However, the benefits of breast feeding are well documented. The CDC is recommending that women who are breast should continue to breast feed their infants through the first year. Women who are ill or are having difficulty breast-feeding for any reason should consult their physicians.

Horses and other mammals

Horses and other mammals can get WNV but do not play a role in the amplification of the virus. In other words an infected animal can not infect other mammals or mosquitoes. Just like people they are incidental infections and tend to occur later in the season when there is more virus circulating in the mosquito population.

Other mammals have gotten the virus such as dogs, cats, squirrels, bats, deer, bear and a llama. A survey of the dogs and cats in the New York City area suggests that while they were exposed to the virus it is unlikely that infected pets will show clinical signs of the virus. Experiments show that sheep, chickens and pigs could be affected by WNV. In these experiments the virus caused pregnant sheep to abort. However, it is largely unknown what the effect of WNV will be on mammals other than horses.

Many horses that get WNV probably never show symptoms. However, of those that show symptoms about 40% are either euthanized or die from the infection. WNV affects the nervous system of horses. Early symptoms of the disease are often indistinguishable from other equine encephalitides including rabies, equine herpesvirus-1, equine protozoal myeloencephalitis, and eastern, western, or Venezuelan equine encephalomyelitis. Some of the symptoms are listlessness, stumbling and incoordination, weakness of limbs, partial paralysis, knuckling over, head tilt, muscle twitches or tremors, inability to stand, circling, weakness or paralysis of limbs, apparent blindness, lip droop, grinding teeth and death.

There are two vaccines approved for horses. The vaccine is available only for horses. One vaccine is a killed WNV vaccine and the other is a live vectored recombinant vaccine. The vaccine is safe for pregnant mares. The vaccine should be given every year.

What we’ve been doing

Plymouth County Mosquito Control Project recognizes that WNV is a complex problem and has taken several approaches to control and monitor WNV. When the virus was first introduced we helped Massachusetts DPH design a plan for controlling an epidemic. We have also been working with Massachusetts Department of Public Health to monitor mosquito populations for WNV. We have been treating catch basins to reduce the number of Cx. pipiens and Cx. restuans. Finally, we have been working to educate the public about the virus.

It is important to monitor the amount of WNV circulating in the mosquito population. This allows us to determine the risk for human infections in a given area. We have been running traps designed to collect gravid (pregnant) Cx. pipiens and Cx. restuans in areas that we feel have a risk for an epidemic or epizootic of WNV. The traps consist of a pan of water infused with organic material, which acts as an attractant, on top of the pan rests a fan that sucks up mosquitoes into a collection chamber. The mosquitoes are taken back to our office, identified and put into vials. The vials are then brought to Massachusetts DPH for testing.

Cx. pipiens and Cx. restuans larvae are found in containers or in polluted water such as sewage. Most containers are in yards and are the responsibility of homeowners. However, it is well documented that catch basins can produce large numbers of these mosquitoes. As a result we have begun a program to treat catch basins with a larvacide. Working with local DPWs, we have treated most catch basins in the county with the larvacide since WNV was introduced.

Since Cx. pipiens and Cx. restuans need containers to reproduce we have been working to educate the public about the issue. We have been visiting schools and clubs to discuss WNV. We have also been visiting farms to discuss mosquito control on their property. If you are interested in having us come visit your organization or home, please give us a call at (781)585-5450.

What you can do

There three things an individual can do to protect themselves from a WNV infection. 1. Prevent mosquito bites. 2. Rid your property of any containers. 3. Use repellents.

Mosquito bites can be prevented by taking several actions. First avoid going outside at dawn and dusk. Many mosquitoes are most active at this time. Second wear loose long sleeve shirts and pants. The clothing creates a physical barrier. Third use repellents containing DEET. This chemical is found in many common insect repellents. When purchasing a repellent look at the list of active ingredients for the chemical N-N-diethyl-meta-toluamide, DEET is an abbreviation for this chemical. However, more DEET is not better because high concentrations may cause an adverse reaction and does not improve repellency. In general adults should use products containing between 10 and 35% DEET . Children should not use a product containing more than 10% DEET and infants should not use any. Always follow labeled directions. There are several new repellents on the market, you can find information about them at repellents .

The common name for Cx. pipiens is the common house mosquito. One of the reasons for this is because the mosquito will readily enter buildings and bite people inside their home. It is therefore important that the screens in your home are in good working order.

As stated several times in this article Cx. pipiens and Cx. restuans larvae live in any object that holds stagnant water, except a properly maintained swimming pool. It has been our experience that when asked most people say that they have no containers on their property. A quick inspection of the yard often reveals at least one or two containers producing thousands of mosquitoes! Some of the containers we commonly find are tires, bird baths, pools, children’s toys, plastic tarps, buckets, gutters and food dishes. We recommend inspecting your property for any containers.