Tree holes may harbor disease vectors

July 1, 1993

A hole in a living tree is usually created by an injury that breaks through the bark and exposes the inner sapwood. This is followed by weathering and subsequent infection of the raw wood with fungi and bacteria. Sometimes, before the wound is closed by the healing overgrowth of bark, boring beetles and carpenter ants, which enhance the damage, attack the softened wood. Some tree holes are started by ground fires that scorch or by wind-broken branches. However, most tree holes are started by natural pruning in which the lower branches of a tree become starved by a lack of sunlight, die, and break off. Most heal over naturally, but some develop into rot cavities that may hold water for a considerable time.

Tree holes that don’t hold water offer ideal habitats for a variety of birds and mammals. As a matter of fact, when you think about it, birdhouses are merely imitations of tree holes. If the tree hole happens to be water tight, another microenvironment is created that breeds a variety of insects, including at least six species of mosquitoes.

Of the six species of mosquitoes that routinely breed in tree holes, only two are of medical importance to man in this country, Three of the tree hole species are not known to feed on man, and the larvae of one of these types are carnivorous and voraciously devour the larvae of the others, keeping their numbers in check.

Two of the medically important tree-holers, as entomologists call them, are native species while the other is a fairly recent import from the Far East and not yet widely distributed in the United States. This foreign invader is called the tiger mosquito and has not yet been incriminated in disease transmission in the U. S. but is a potent vector of dengue fever in other parts of the world.

The two native species of importance are the closely related Aedes triseriaus and Aedes hendersoni, which are the transmitters of La Crosse encephalitis that is sometimes called California encephalitis. Triseriatus was first recognized in 1823, and hendersoni as a variety of its sibling in 1918. It was not until decades later that hendersoni was recognized as a separate species. This writer was the first to report the occurrence of hendersonii in Illinois (Cook County) in 1965.

La Crosse virus was first isolated from a fatal case of encephalitis in La Crosse, Wisconsin in 1960, and the tree hole breeding Aedes were soon identified as the vectors. Since that time the virus has been found to be endemic with about 100 clinical cases being reported annually, but surveys indicate that about 300,000 human infections occur each year throughout the Midwest. More than 90 percent of all cases in the U. S. are from Minnesota, Wisconsin, Illinois, Indiana, and Ohio during the months of July, August, and September. Serological studies suggest that about 20 percent of Midwesterners have been exposed to the virus by age 60, and that for every reported case of La Crosse encephalitis in children under the age of 16, there are more than 1,000 unreported cases

Children are most affected by the La Crosse virus; but, fortunately, it is much less virulent than the West Nile or St. Louis viruses. The first symptoms are nonspecific and are followed by the onset of signs and symptoms of involvement of the central nervous system. As with West Nile encephalitis, many adults may contract the disease but experience only mild symptoms and recover rapidly. The mortality rate from La Crosse encephalitis is about 3 percent.

One reason La Crosse encephalitis is not as prevalent as West Nile is the number of mosquito vectors are relatively few and more or less confined to wooded areas and specific breeding sites. The species of Culex mosquito that is responsible for West Nile is much more prolific in numbers and breeds in almost any type of water container.

The Aedes vectors of La Crosse bite during daylight hours, whereas the Culex are mainly dusk, night, or dawn feeders. The La Crosse transmitting mosquitoes feed mainly on small woodland mammals such as chipmunks, squirrels, and mice that are also the reservoirs of the virus in nature. Birds are the reservoirs for the West Nile virus and the preferred source of blood for the Culex vectors.

An important aspect of the epidemiology of La Crosse encephalitis is that the infected female may transmit the virus to her offspring through her eggs. This phenomenon is known as transovarial transmission and rarely occurs in insects. Ticks and mites, however, routinely pass infectious agents that cause such diseases at lyme, tularemia, Colorado tick fever, scrub typhus, and rocky-mountain spotted fever through their eggs to the next generation.

Though La Crosse encephalitis is not nearly so important to us in the Rock River Valley as West Nile, we should be aware of its presence. If you find a tree hole on your property that contains water, it’s a good bet that it is a breeding site for Aedes mosquitoes. Drilling can drain the cavity, or it can be filled in with a suitable substance.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960-1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

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