Mosquito-borne Diseases in Michigan
Mosquito-borne disease probably have been present in Michigan since before the arrival of European settlers, and in the early years they created many more serious interruptions to daily activities than they do now. The extent of these problems was underlined in 1898 by one of the speakers at the 50th anniversary celebration of Michigan Agricultural School, now Michigan State University. "In 1858, the most notable and impressive event of the season was the fever and ague (malaria) . . . In the latter part of August and fore part of September, there were 70 out of 100 students unable to attend classes, at least they could come only every other day, as the fever was mainly intermittent."
Mosquito-borne diseases are not as prevalent in Michigan today as many other pathogenic infections, but the emotional stress associated with their occurrence and the media publicity that inevitably has been associated with them create intense public concern that must be dealt with by health professionals.
West Nile Virus (WNV)
West Nile (WN) virus was first isolated from a woman in the West Nile province of Uganda in Central Africa. It was first reported in the United States in 1999. It has since spread throughout much of the U.S.
West Nile virus exists primarily as an infection of bird populations is transmitted from bird to bird by several species of mosquitoes. Humans become infected through the bites of mosquitoes which have previously fed on birds carrying the virus. The birds most commonly associated with WNV in the U.S. are members of the Corvid family (Crows and Blue Jays).
Mosquitoes serve as primary vectors of WN virus. These include Culex pipiens in the northern half of the country, its close relative Culex quinquefasciatus in the south, Culex tarsalis in the west, and Culex nigripalpus in the deep south. Other mosquito species which have been implicated as possible bridge vectors in the United States include the container mosquitoes Aedes albopictus, Aedes atropalpus and Aedes japonicus and the saltwater mosquito Ochlerotatus sollicitans. West Nile Virus in Michigan and Midland County.
Eastern Equine Encephalitis (EEE)
Eastern equine encephalitis (EEE) virus causes acute encephalitis in horses and man. The clinical disease is severe with a case fatality rate of 50 to 75 percent (90 to 95 percent in horses). The case fatality rate is especially high in children. The disease occurs in horses each summer along the Gulf Atlantic coasts as well as in such states as Massachusetts, New York, New Jersey, Maryland, and Michigan. Human cases are rare. Most human cases in Michigan have been in the southern Lower Peninsula. Eastern Equine Encephalitis occurs regularly in Michigan horses. For more information on EEE Horse Cases in Michigan by County.
EEE virus is maintained in nature through a cycle involving primarily the fresh water swamp mosquito Culiseta melanura and many species of birds.
Mosquitoes which feed on both birds and mammals are responsible for transmitting the disease to horses and humans. Horses, and probably humans, rarely develop viremia high enough to infect mosquitoes. Horses and humans are considered dead end hosts not involved in the cycle of transmission. Human and Equine cases usually appear relatively late in the season.
Eastern equine encephalitis virus has been isolated from several species of mosquitoes. The swamp-breeding Culiseta melanura, the summer flood-water breeder Aedes vexans, and the cattail mosquito Coquillettidia perturbans are probably most important. Aedes canadensis and Culiseta minnesotae may also be involved.
Coquillettidia perturbans seems to be the major vector of EEE to horses and humans in Michigan. Cq. perturbans breeds prolifically in permanent bodies of water with emergent vegetation such as cattails. These mosquitoes are relatively strong flyers and may migrate some distance from their breeding area. Peak feeding occurs at dusk and dawn. Aedes vexans is not an efficient vector of EEE but it could possibly become involved when populations are extremely high.
Jamestown Canyon Virus
Jamestown Canyon is an “emerging” disease, possibly expanding with the increasing deer populations in many regions of North America. The first recognized case of primary encephalitis due to Jamestown Canyon virus was seen during 1980 in an eight-year-old girl living in rural southwest Michigan. Subsequent human infections with JC virus have been recognized from Michigan, New York, Wisconsin, Indiana, Illinois, Ohio and Canada.
With Jamestown Canyon, the white-tailed deer is the most important amplifying host.
In the western U.S. Jamestown Canyon virus has been isolated almost exclusively from Culiseta inornata. Throughout most of the remainder of its range, though, it has been isolated primarily from woodland mosquitoes such as Aedes stimulans. Anopheles punctipennis and Anopheles quadrimaculatus are potential late season vectors.
During the past several years there has been increasing recognition that LaCrosse represents a serious and growing infectious disease problem in the United States. In many years LaCrosse accounts for more human disease than SLE, WEE, and EEE combined. The disease is most prevalent in the midwest and mid-Atlantic states.
Unlike the other arboviral encephalitides that occur in the U.S., relatively few deaths have been associated with LaCrosse virus.
The natural transmission cycles of LaCrosse virus involves mostly small mammals such as squirrels, chipmunks, field mice, and rabbits as the primary vertebrate hosts and mosquitoes of the genus Aedes, as the vector.
The primary vector of LaCrosse virus is Aedes triseriatus. This species is the most widely distributed tree hole mosquito in North America. Adults normally found in deciduous forests and woodlots, although they may become serious pests in residential areas near wooded localities and tire piles.
Dog heartworm, Dirofilaria immitis appears to be a relative newcomer to Michigan, although it has been a major problem in the southeastern United States for many years.
The severity of the disease symptoms in dogs is highly variable, depending upon the intensity of infection, the age, breed, and health of the animal, and a variety of other factors.
Transmission occurs only through the bite of a mosquito and involves a rather complicated cycle of development.
Approximately one quarter of the species of mosquitoes known to occur in Michigan are biologically capable of transmitting dog heart worm.
Malaria was endemic through much of the United States into the 1900’s. During the 1940’s a combination of improved economic conditions, water management, vector control and case management was successful in interrupting transmission throughout the U.S. However, hundreds to thousands of cases of malaria continue to be reported from the U.S. annually. With the continuous reintroduction of malaria parasites each year infection of native Anopheles species mosquitoes and subsequent local transmission remains a possibility. Cases of local transmission have occurred in Michigan as recent as 1995.