MOSQUITO DISEASES

Many of the above mosquito species are very important disease vectors for many different arboviruses (arthropod-borne virus) here in Michigan. These viruses are cycled by many different mosquito species within various Michigan bird and mammal populations. Mosquito-borne encephalitis is not common annually, but because it may occur in epidemics and localized areas in certain years, it remains a true concern for human and animal health. West Nile virus (WN) has been the most notable as of late and is responsible for hundreds of deaths and thousands of sicknesses in the United States, since it was first detected in 1999. St. Louis encephalitis (SLE), eastern equine encephalitis (EEE), and different types of the California group of encephalitis also occur in Michigan on occasion. Dog heartworm, a concern for pet owners, is common in Michigan and is transmitted by a variety of Aedes mosquitoes. Prophylactic drugs are available from veterinarians to prevent dogs from contracting this disease.

As mosquito control personnel, it is important to monitor and respond to mosquito-borne disease threats. Disease surveillance, such as mosquito sampling, wild and sentinel bird sampling, and monitoring dead bird reports, is invaluable to mosquito control programs. Programs that incorporate one or more surveillance tool will be most effective in disease prevention. Early detection and control strategies can greatly reduce the arbovirus infection risk to humans and animals. Early detection concentrates resources and control measures to best address the disease threat. Control measures taken early in a disease threat will be less costly with respect to human health and economic cost than a potential epidemic.

MMCA Zika Statement.pdf - April 19, 2016

West Nile virus in Michigan was first detected in bird populations in 2001. It has since been responsible for many human deaths (51 in 2002), and averaging just below 30 cases of human infection a year between 2003 and 2011. The epidemic nature of this arbovirus was noted recently in 2012, responsible for over 200 human cases and 13 deaths. Historically this disease is most prevalent in Michigan’s urban and suburban areas; an average of 85% of all human WN cases occur in Kent county and the Detroit metro area. This arbovirus has also been responsible for a large number of deaths in horses. Michigan’s main WN vector is Cx. pipiens with birds as the primary reservoirs and amplifying hosts for this disease. Several species of Culex and Aedes have been found positive for WN in nature. These mosquitoes likely serve as important vectors within bird populations as well as bridge vectors in human and horse infections. Certain species of birds, especially the Corvids (crows, ravens, and blue jays) are highly susceptible to infection and are useful as wild sentinels for West Nile virus surveillance.

St. Louis encephalitis, like WN is caused by a virus that has a "natural" transmission cycle involving several species of wild birds and mosquitoes. Cx. pipiens is believed to be Michigan’s main vector in birds and bridge vector in humans. Epidemics have produced human fatality rates of 4 to 20 percent with most deaths occurring in people over 50 years of age. In 1975 an epidemic of SLE resulted in 1,967 confirmed or probable human cases and 4 deaths here in Michigan.

Eastern equine encephalitis also has a natural transmission cycle involving different species of wild birds and mosquitoes. This arbovirus is more prevalent in rural areas where the endemic transmission cycle occurs. The mosquito vector in EEE’s natural cycle is Culiseta melanura, which is found in freshwater swamps and blood feeds almost exclusively on birds. In Michigan, Cq. perturbans is the main bridge vector responsible for human and horse infections. The EEE fatality rate ranges from 50 to 70 percent in humans and in horses, 90 to 95 percent. Unlike SLE, EEE may produce severe disease and deaths in some exotic bird species like the pheasant. The first known Michigan resident to die of EEE was a 10-year old boy in 1980. The most recent epidemic occurred in 2010, with 3 human and 58 horse cases. In 2005, the first reported cases of EEE in whitetail deer were found in both Kent County and Montcalm County.

The status of the California group of encephalitis viruses, another group of arboviruses, is less well known in Michigan compared to other nearby states where it is more of a public health concern. These viruses are more rural and suburban based the natural cycle which most often involves woodland mosquitoes and mammals. La Crosse encephalitis virus, the most important of this group in Michigan is transmitted by Aedes triseriatus mosquitoes. La Crosse encephalitis is more serious in children, with less than 1% of human cases resulting in fatalities. This arbovirus is more prevalent in the states surrounding Michigan. The last human fatality in Michigan transpired in 2006. Jamestown Canyon, another virus in this group, is widespread in distribution and is mainly transmitted by spring Aedes mosquitoes with whitetail deer as the vertebrate host of the virus. However, it is not a major cause of human illness and does not affect domestic animals.