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Wildlife Diseases

and

Health Concerns

Rabies

Rabies is an acute disease, caused by a virus (rhabdovirus), that can infect all warm-blooded animals, and is usually fatal. Certain carnivorous mammals and bats are the usual animal hosts. Rabies occurs throughout most of the world; only Australia and Antarctica are free of it. Most human cases have been contracted from rabies-infected dogs. In the United States, human cases have decreased to an average of one person per year (75% of cases are acquired outside the United States). Reduction in human rabies is likely linked with the intensive control of dog rabies during the 1950s and 1960s through massive vaccination campaigns, stray dog control programs, and improvement in human treatment following exposure. Nevertheless, thousands of people in the United States continue to receive treatment every year for possible exposure to rabies virus by animal bites. Most of the treatments are still due to dog and cat bites; however, these pet species have the lowest occurrence of reported rabies among all animal species tested.

Rabies in wildlife increased dramatically during the 1960s and now accounts for most of the reported animal rabies cases (91% in 1991). Some of the increase in reporting was due to real increases in the number of cases, and some was due to an increased awareness of wildlife rabies, particularly in striped skunks, raccoons, and bats. In 1991, 6,975 cases of animal rabies were reported in 49 states, the District of Columbia, and Puerto Rico. Raccoons (44.2%), striped skunks (29.7%), and various species of bats (9.9%) continued to be the major hosts. Red and gray foxes (4.6%), other wildlife species (2.8%), and domestic animals (8.9%) comprise the remainder of hosts. During the last 2 years, raccoons replaced striped skunks as the major wildlife host in the United States because of the continued expansion of raccoon rabies in the northeastern United States. Animal cases are reported throughout the year, although the number of cases reported reaches a seasonal peak for skunks in March and April, for raccoons in April, and for bats in August.

Clinical Signs. Rabies is considered almost 100% fatal once clinical signs develop. The disease progresses rapidly following the appearance of clinical signs, and the animal dies within a few days. Although abnormal behavior is not diagnostic for rabies (other diseases, like distemper, cause similar behavioral changes), atypical behavior and signs develop following brain infection, and rabies should be suspected whenever wild animals display unusual behavior.

Infected animals usually display either “furious” or “dumb” rabies, although some animals progress through both stages. Skunks, raccoons, foxes, and other canids usually have furious rabies and are unduly aggressive before convulsions and paralysis set in. Some animals, however, have dumb rabies and proceed to tremors and convulsions without agitation or aggression. Other behavioral changes include friendliness or loss of fear, appearance in the daytime for some typically nocturnal species (skunks, bats), unprovoked attacks on anything that moves (including inanimate objects), bewilderment, and aimless wandering. Unusual barking, crying, and frothing at the mouth are additional signs, which are the result of paralysis of the throat muscles. Occasionally, rabid bats are encountered prostrate or fluttering on the ground, unable to fly; they should be handled with care because they can still bite and transmit rabies. Some rabid bats, particularly solitary species like the hoary bat, are aggressive and have been known to attack people. In domestic animals, rabies should be suspected if there is any change in normal habits, such as sudden change in disposition, failure to eat or drink, running into objects, or paralysis.

Transmission. Rabies virus is transmitted primarily via the saliva during the bite of a rabid animal. However, other methods of transmission are possible. Accidental exposure of wounds or cuts to the saliva or tissues of infected animals can occur. The virus is also present in various body organs of infected animals, especially the brain and salivary glands, which poses a health hazard to persons who are field dressing or performing necropsies on these animals. In addition, aerosol exposure has occurred, although rarely, in caves containing very large populations of infected bats. Transmission between animals also occurs by ingestion of infected tissues and by transplacental passage to offspring.

Avoid exposure to any sick or dead animals that are suspected to have rabies. Handle any dead animal with gloves or with a plastic bag that can be turned inside-out to cover and contain the animal. Avoid direct skin contact with the animal. For large animals such as skunks and raccoons, remove the head cautiously and seal it in a plastic bag, avoiding contact or aerosol exposure. Seal the whole animal or head inside an additional plastic bag (double) and keep it cool at all times. Do not freeze the specimen unless a delay of several days is anticipated before it is examined for rabies. Disinfect gloves or knives that were in contact with the animal with a strong detergent or bleach or dispose of them.

Prevention and Treatment. The best treatment for rabies is prevention. Individuals at high risk of exposure to rabies, such as wildlife biologists, game wardens, animal control officers, animal handlers, and veterinarians should be vaccinated before potential exposure. Safe and highly effective vaccines are available through a physician or the local health department.

First aid should immediately be provided to a person who has been bitten by or had contact with a potentially rabid animal. Scrub the exposed site, including bite wounds, with soap and water or water alone and flush thoroughly. Then apply a strong first aid solution (iodine) or cream. First aid treatment is the most effective method of preventing infection by the rabies virus but should not preclude medical attention from a physician, hospital emergency room, or the local health department. Contact your physician or health department as soon as possible to determine dosage of rabies vaccine and whether antirabies serum is required. Inform the health care professionals about the rabid animal and the circumstances of the exposure (species of animal involved and its behavior, if the attack or bite from the animal was provoked, and what type of first aid was administered).

Hantavirus

Hantavirus includes a group of viruses that can cause a febrile illness in humans which can be accompanied by kidney, blood, or respiratory ailments and can sometimes be fatal. The febrile illness includes fever, headache, muscle aches, nausea, vomiting, and lower back pain. Field and commensal rodents are the natural reservoirs for viruses in this group and these viruses are found worldwide. Infected rodents shed virus in their urine, feces, and/or saliva and can remain chronically infected. The contaminated excreta from infected rodents are thought to be the source of virus for aerosol and direct (animal bite) transmission to other rodents and humans.

The recent discovery of a possible new hantavirus in the southwestern United States and its apparent increased virulence, has heightened the awareness of and concern for rodent-associated diseases. It produces produces respiratory distress and potential death in humans. Human cases and deaths from this viral infection were first reported in 1993 in the Four Corners area of Arizona, Colorado, New Mexico, and Utah and, more recently, throughout the United States. Preliminary information has incriminated the deer mouse (Peromyscus maniculatus) as the natural reservoir and source of human infection in that region. Individuals trapping and handling small rodents in this region should take increased precautions to reduce their exposure to this virus. They should at least wear surgical gloves and masks when processing rodents (contact CDC Hotline for more detailed and thorough safety information). Rodent control with careful handling and disposal of carcasses should be instituted at campsites or in cabins before they are occupied. The premises should be sprayed with detergents or diluted bleach before thorough cleaning. Wet-mopping is recommended. Dry sweeping and vacuuming may increase risk of producing airborne particles. Rodent harborage should be removed from premises and from the surrounding area. Exclude rodents where possible.

Trichinosis

Trichinosis may result in diahrrea, sudden edema of the upper eyelids, photophobia, muscle soreness and pain, skin lesions, thirst, sweating, chills, and weakness. Other respiratory and neurological symptoms may appear if treatment is delayed.

Trichinosis is contracted by eating infected meat which contains the encysted parasites. The parasites may remain infectious in meat which is raw or poorly cooked.

Trichinosis is caused by a nematode parasite which produces the disease in humans and domestic and wild animals. Evidence indicates that nearly all mammals are susceptible to infections with this parasite, which encysts in the muscle of the host and is then transmitted through consumption of infected flesh. As would be expected, the disease is most common in wild carnivores and scavengers.

As with other wildlife diseases, trichinosis is difficult to control in nature. However, certain steps can be taken to decrease the problem. Carcasses of carnivores and other meat-eating species should not be discarded in the fields or woods, but should be made unavailable by burying or other means. These carcasses also should not be fed to swine, dogs, or other domestic animals. Open garbage dumps should be replaced by the landfill type or other methods of disposal where wildlife will not have access to meat scraps. If open garbage dumps cannot be eliminated, rodent control programs should be initiated and the areas fenced to prevent scavenging by larger animals such as foxes. These steps would markedly reduce the problem of trichinosis in wildlife in the United States.

If carnivorous or omnivorous wildlife such as bears, bobcats, opossums, raccoons, or feral pigs are consumed by humans, the meat should be properly prepared by cooking, freezing, or curing to destroy any viable trichinae. Cooking to an internal temperature of 137oF is deemed sufficient for pork, while freezing at 5oF for 20 days, -10oF for 10 days, or 20oF for 6 days will kill trichinae. Curing should follow approved government regulations.

Mosquito-borne Encephalitis

Encephalitis is a disease caused by mosquito-borne viruses (arboviruses) that affect the central nervous system. Infections range from unapparent to mild, nonspecific illnesses (fever, headache, musculoskeletal pain, and malaise) to occasionally severe illness of the central nervous system resulting in permanent neurologic damage and possibly death. The four major types of encephalitis in the United States include St. Louis encephalitis (SLE), California encephalitis (CE primarily includes the LaCrosse virus [LAC]), eastern equine encephalitis (EEE), and western equine encephalitis (WEE). The distribution of these arboviruses varies (Fig. 2). SLE occurs throughout the United States (an epidemic occurred in central Florida in 1990 and Arkansas in 1991), WEE occurs west of the Mississippi River, EEE occurs east of the Mississippi River but mostly along the Atlantic and Gulf coasts and north-central states, and CE occurs in California and the eastern United States (LAC type). Human cases of arbovirus infection have a seasonal occurrence from mid- to late summer.

These distinct viruses naturally infect a variety of birds and mammals and are transmitted between animals by mosquito vectors. Occasionally, infected mosquitoes will feed on human or equine hosts that are “dead ends” for the viruses, with little or no chance of subsequent transmission to other mosquitoes. These viral infections may, however, result in severe illness or death in humans or horses (EEE and WEE). Only EEE and occasionally WEE viruses adversely affect wild vertebrates; for example, EEE causes death in ring-necked pheasants and other exotic game birds, house sparrows, red-winged blackbirds, whooping cranes, and other species. The wildlife hosts for LAC virus are the eastern chipmunk, tree squirrels, and foxes. The natural hosts for the other three viruses are mostly songbirds, although squirrels and jackrabbits may be involved in WEE transmission.

No treatment or commercial vaccine is available for humans, but vaccines for WEE and EEE are readily available for horses. The best preventive measures are personal protection against mosquito bites, especially avoiding exposure to mosquitoes during early evening hours, and the use of repellents. Mosquito populations can be reduced in an area by eliminating breeding sites for vector species. Killing adult mosquitoes with areawide applications of insecticides has been most effective in preventing epidemics.

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