History of Rabies in Michigan
Human cases of rabies in the United States have declined from 100 or more each year in the early 20th century to an average of 2-3 cases each year today. During the same period, the incidence of rabies in domestic animals has decreased dramatically in Michigan and the U.S. These improved statistics can be traced to two developments in the past century; the efforts, begun in the 1940's, to control the dog population and vaccinate dogs against rabies, and the development of improved human rabies vaccines and immunoglobulins used to treat people exposed to rabies.
While rabies is rarely diagnosed in humans in Michigan, cases do occur. In 2009, a 55 year old man died of rabies due to a bat strain of the virus. The patient did not seek treatment following an exposure to a bat months earlier. Prior to this, the last human case in Michigan was reported in 1983, also likely acquired from contact with a rabid bat.
Wildlife species including bats, raccoons, skunks and fox are now the primary source of rabies virus circulating in the U.S. today. In Michigan , bats and skunks are the animals that harbor rabies virus. Sometimes they transmit the virus to other wild animals such as fox, or domestic animals such as unvaccinated cats, horses, cattle, sheep and dogs. Bats are the species most often tested for rabies in Michigan , followed by cats and dogs. While dogs and cats are frequently tested for rabies following bite incidents involving humans, they rarely test positive.
Rabies virus is found in the saliva and neural tissue (brain, spinal cord) of an infected animal. Bites are the primary way in which the virus is spread from animal to animal or animal to person. In addition, introduction of saliva or neural tissue into broken skin or onto mucous membranes of the eyes, nose and mouth can also provide a route for infection to occur. Any animal bite represents a potential exposure to rabies and as such, animal bites must be reported to public health authorities. Public health authorities are then tasked with evaluating the bite for rabies risk, coordinating testing or observation of an animal, and recommending post-exposure treatment PEP. Rabies infection is preventable if treatment is received soon after the exposure occurs.
The graphs below illustrate trends in animals testing positive for rabies in Michigan.
[Left] Animals testing positive for rabies from 1938 to 2005, which includes individual animal species. [Right] Animals testing positive for rabies from 1950 to 2005, which is classified by whether the animal is wild or domestic.
Bats are the animal most frequently testing positive for rabies in Michigan. The prevalence of rabies in Michigan's bat population is probably less than 1%, however, 4-6% of bats submitted for testing following a potential human or unvaccinated pet exposure test positive for rabies [See Graph Below].
The number of animals submitted for testing following a potential rabies exposure increases in the spring. The highest submissions occur in the summer months of June, July, and August, followed by a period of decreasing submissions [See graph below]. In the winter, potential rabies exposures and laboratory submissions are at their lowest levels. Geographically, bat rabies is generally wide spread across the Lower Peninsula of Michigan, although cases do occur in the Upper Peninsula. As can be seen in the map below, terrestrial rabies in wild animals is mainly restricted to the southeastern region of the state.