West Nile Encephalitis

Illustration by Shawneen Finnegan

It's late August, and your son is having a sleepover. He wants to "rough it" in the backyard, but you've heard news reports about West Nile encephalitis, a virus transmitted by mosquitoes. How do you assess the risks to decide whether to grant your son's request?

West Nile encephalitis (WNE) arrived in New York City sometime during the summer of 1999. This virus is transmitted by mosquitoes, with a complex ecology involving birds and mammals, including humans. It can cause an inflammation of the brain in humans and horses. The illness is usually mild, but a small proportion of cases (less than one percent) can be serious.

Before frost halted mosquito activity for the 1999 season, WNE sickened and/or killed birds (primarily jays and crows), horses, and humans in New York. Plus, the virus was found in areas of New Jersey and Pennsylvania as birds migrating south for the fall carried the virus with them. The virus managed to survive the winter, causing additional cases during the summer of 2000. All signs indicate it will become well established on the continent in coming years.

ECOLOGY OF WNE

The life cycle of West Nile encephalitis starts in the salivary gland of a mosquito. When this mosquito takes a blood meal (only females do this - separate story), saliva, along with the virus, is injected into the host. This host is usually a bird and, in North America, the mosquito is usually the common house mosquito, Culex pippiens, although other species of mosquitoes have been found with the virus. The virus reproduces inside this host, greatly increasing its numbers, and when mosquitoes bite this infected host, they pick up the virus. The newly infected mosquitoes that survive the approximately ten days it takes to digest the blood meal and lay eggs will transmit the virus to new hosts as they take a second blood meal to fuel the production of a second batch of eggs.

Over the course of a summer season, WNE levels in bird populations (the primary hosts) become greatly amplified, reaching their peak at the end of the summer season. The virus is only active in host birds for one to four days. After that time, the host either develops an immunity, becomes sick, or dies. Either way, that host is no longer a source for the virus. Activity ceases for the season after the first hard frost, when mosquitoes go dormant for the season.

Exactly how WNE survives the winter is unknown. Several wintering pools of common house mosquitoes were found with active virus in New York City during the winter season of 1999/2000. This was unexpected, since only mosquitoes that had not taken a blood meal were thought to overwinter, and the only way for a mosquito to pick up the virus is during a blood meal. There are still holes in human understanding of the encephalitis life cycle at this point; exactly how the virus survives the winter is unknown.

Bird migration is the engine that carries WNE to new parts of the continent. As infected birds move south, they carry the virus with them. Mosquitoes continue to plague birds during this movement, picking up the virus and continuing the amplification process. It is unclear at this time what the eventual range of WNE will be in North America. Data from the 2000 season show that birds infected with the WNE virus were found in most of New York, New Jersey, and Massachusetts; all of Connecticut and Rhode Island; and parts of Vermont, New Hampshire, Pennsylvania, Maryland, the District of Columbia, Virginia, and North Carolina. This large range expansion is from the initial 1999 outbreak in New York City.

WNE causes much greater mortality among bird populations in North American than in the Middle East and Africa, where it is native; encephalitis viruses native to North America do not cause illness in North American bird populations. Birds in areas where the virus is native have had time to evolve an immunity to the virus. North American bird populations were exposed for the first time in August of 1999 - small wonder crows were dropping from the skies.

THE PROBLEM FOR HUMANS

If the mosquitoes harboring the WNE virus fed only on birds, the sole health concerns regarding the virus would be with regard to avian populations. Unfortunately, that is not the case. Sometimes, mosquitoes that feed primarily on birds will take a meal from a mammal. When a mosquito infected with WNE bites a mammal, the virus enters its system. If it manages to survive and multiply, it can cause disease in humans and horses. The virus has also been isolated from cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and racoons, although disease has not been found in these species.

The risk of developing illness from a single mosquito bite is small, and an individual's chance of getting bitten by an infected mosquito is much less than one percent. Of those unlucky individuals that do get bitten by an infected mosquito, fewer than one percent will develop severe illness. Most people will experience mild, flu-like symptoms, if anything. Severe illness occurs most frequently in the elderly, the very young, and those with immune system impairments; of those individuals, mortality rates range from three percent to fifteen percent. Symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, and paralysis. Treatment is supportive; there are no medicines effective against West Nile. Individuals who have recovered from West Nile encephalitis are assumed to have lifelong immunity, regardless of whether their illness was mild or severe.

Worth noting, WNE is not the only and certainly not the most serious mosquito-borne disease affecting New Jersey residents. Two other forms of encephalitis, St. Louis encephalitis and eastern equine encephalitis, have a long history in North America. Eastern equine is present in bird populations every year and is much more deadly to humans, causing mortality in approximately one out of three diagnosed cases.

WHY NOT CALL IN THE CAVALRY -
i.e., THE COUNTY MOSQUITO COMMISSION

In the summer of 1999, when the first human cases of WNE were diagnosed, the City of New York instituted widespread aerial applications of Malathion, a broad-spectrum, carbamate-based insecticide. The problem with this method is that the effects are not limited to the mosquitoes. Malathion affects all adult insects and crustaceans that come in contact with the substance, causing widespread environmental disruption.

If you enjoy seeing butterflies in your garden, and if you want birds to be able to forage in your yard, you probably don't want anybody to apply Malathion or other insecticides that target adult insects.

Also worth noting, aerial spray did not eradicate the disease or prevent its spread.

IS THERE ANYTHING GOOD ABOUT THIS?!?

From the human standpoint - no. From a bird's standpoint - no. From the bird populations' standpoint - perhaps. Diseases are one way nature controls animal populations (for another, see Suburban Survival Guides: Subject #1). If every baby bird hatched in a given year survived to breed, the continent would soon be waist high in birds, habitat and food would be in extremely short supply, and birds would start to starve. The example is a bit extreme, but the point, that WNE may serve as a factor keeping bird populations at a manageable level, stands.

Looking at WNE from the naturalist's standpoint, the picture is bleak, and it is difficult to put a positive spin on yet another disease to worry about contracting outdoors. However, as diseases go, West Nile encephalitis is relatively mild, no bird species seem to be threatened with massive mortality, and nature has an amazing ability to reset its balance. It is also fairly simple to minimize one's risks.

HOW TO KEEP RISKS AT A MINIMUM

Nobody wants to be one of the unlucky few who has a serious reaction to WNE. However, there are many actions individuals can take to reduce their exposure to the virus. These actions fall into two basic categories: minimize exposure to bites, and eliminating breeding habitat around the home.

Minimizing Bite Risk

1. Stay indoors at dusk, dawn, and the early evening, since mosquitoes feed most actively during these periods.

2. Wear long-sleeved shirts and long pants when you are outdoors to minimize skin exposure.

3. Spray clothing with repellents containing pyrethrin or DEET, since mosquitoes may bite through thin clothing.

4. Apply an insect repellent sparingly to exposed skin. An effective repellent will contain thirty-five percent DEET, according to the Centers for Disease Control. However, studies conducted at the University of Guelph in Ontario indicate that Bite Blocker, a plant-based repellent (available at both CMBO bookstores) had an effectiveness comparable to DEET-based products. Personally, I have found that the stuff works great. It feels oily, since its base is soybean oil, but I would rather feel oily than put a chemical like DEET on my skin. If you, too, have concerns about DEET, Bite Blocker is worth a try.

ELIMINATING BREEDING HABITATS

Mosquitoes can breed in less than a half-inch of standing water, and it takes eight to fourteen days for a brood to go from egg to biting adult. With those two facts in mind, scout your property for potential breeding sites. The common house mosquito generally flies less than 100 yards from its larval habitat. Common mosquito breeding habitats around homes include, but are not limited to: clogged rain gutters, pool and spa covers, inverted trash can lids, folds in tarps, old tires, buckets - anything that can hold standing water. It doesn't take much time or much water to produce a crop of mosquitoes. Policing your property for these types of mosquito breeding habitats prior to the onset of mosquito activity, and several times during the breeding season, can do much to reduce the mosquito population around your home. If you have standing water that can't be avoided, use of a product containing Bacillus thuringensis (BT) is a way to kill mosquito larvae with minimal impacts on other animals. BT is a species of bacteria that kills insects after they ingest it, by destroying the lining of their gut. There are varieties of BT specific to mosquitoes.

Garden ponds are a special case. Most NJAS members install ponds in their yards to provide habitat for wildlife. Killing mosquitoes does not fit in with this overall theme. Luckily, this is not a problem. Garden ponds will attract dragonflies that lay eggs in the pond. Their larvae are the best mosquito control available. They need no encouragement from the pond owner to consume every mosquito larvae they can get their jaws on. In the unusual event that no dragonflies find your pond, a few feeder goldfish from the pet store are almost as good. Just give the dragonflies a few months to become established before panicking and throwing fish in, because fish will eat the dragonfly larvae.

West Nile encephalitis is a problem. There are risks to humans, but in this case knowledge is the best weapon. The defensive and preventive measures you take will greatly reduce the risks.