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Wildlife Diseases

and

Health Concerns

Colorado Tick Fever

Colorado tick fever (CTF) is an acute and rather benign disease caused by a virus (coltivirus) that is transmitted to humans by ticks. Symptoms are usually limited to high fever, headache, muscle aches, and lethargy, but the symptoms are frequently biphasic and recurring. The disease is confined to the mountains or highland regions of eight western states and western Canada (Fig. 3). About 150 to 200 cases are reported each year; 1,438 cases were reported from 1980 to 1988 in eight western states, 63% of them in Colorado. CTF is transmitted to humans during the spring and early summer by the bite of the adult stage of the Rocky Mountain wood tick (Dermacentor andersoni) or by D. occidentalis in California. The virus is maintained in nature through transmission by immature stages of ticks to various species of small mammals, particularly chipmunks, ground squirrels, and deer mice during the spring and summer months. The virus survives the winter in infected tick nymphs and adults. The habitats that support the rodent hosts and tick vectors of the virus in the disease endemic region contain rocky surfaces with moderate shrub cover and scattered pines.  Avoid tick-infested habitats during spring and early summer and use personal protection against ticks. No vaccines or treatment are available.

Rocky Mountain Spotted Fever (Tick-borne Typhus)

Rocky Mountain spotted fever (RMSF) is a moderate to severe illness caused by a rickettsia (Rickettsia rickettsii). The disease is distinguished by a sudden onset of high fever, severe headache, muscle pain, and a red rash starting on the extremities about 3 to 6 days after onset of symptoms and extending to the palms of hands and soles of feet and then to the rest of the body. Delirium, coma, and death occur in about 1% to 2% of cases (15% to 20% in untreated cases). The disease is transmitted to humans in the United States by several hard tick (Ixodidae) species;

D. andersoni in the Rocky Mountain region, D. variabilis in the east and southeast, and Amblyomma americanum in the south-central states. In 1990, 649 cases of RMSF were reported from all regions of the United States, although more cases were reported in the south-Atlantic and south-central states. The natural hosts for the rickettsia are a variety of wild rodents, although rabbits and wild and domestic carnivores are involved in some cases. The rickettsia survive the winter months in the tick vector and may be maintained by transovarial transmission from the female adult tick to its offspring.  Avoid tick-infested areas and use personal measures to protect against tick bites. No vaccine is presently licensed for public use, but antibiotic treatment is effective and should be initiated without waiting for laboratory confirmation of clinical diagnosis.

Lyme Disease

Lyme disease is caused by a spirochete bacterium (Borrelia burgdorferi) that is transmitted to humans by hard ticks. Early symptoms include a flu-like illness with headache, slight fever, muscle or joint pain, neck stiffness, swollen glands, jaw discomfort, and inflammation of the eye membranes. A diagnostic rash, erythema migrans (EM), occurs in 65% to 75% of the cases. The rapidly expanding red rash starts at the tick bite site and expands to a nearly circular lesion of about 1 to 8 inches (2 to 20 cm). It often has a bulls-eye appearance with central clearing and/or darkening around the edge. Additional smaller skin lesions may appear at other sites of the body and may last for days or weeks. Later symptoms, including heart, nervous system, and joint manifestations, may develop in untreated individuals. The joint pain and swelling usually occur one or more months after infection, may involve one or more joints, and may recur in different joints; the knee joint is most frequently affected. Domestic animals may be affected as well.

In 1992, 9,695 cases of Lyme disease were reported in 44 states Most cases were reported in the northeastern and upper midwestern states where the vector is the deer tick (Ixodes scapularis) and where transmission is predominately in residential communities.

Transmission in these other regions of the United States may be more sporadic and occur during outdoor activities related to recreation and occupation. Acquisition of Lyme disease by humans peaks during the summer months when the tick nymphs are feeding on hosts. Because of its small size, the attached nymph frequently goes unnoticed and is not removed. The transmission cycle of Lyme disease begins when larvae acquire spirochetes while feeding on infected white-footed mice, chipmunks, other rodents, and birds. Engorged larvae drop to the ground, molt to the nymphal stage, and wait until the following summer to attach to and transmit spirochetes to susceptible rodents, birds, larger mammals, and humans. Uninfected larvae subsequently feed on these wild vertebrate hosts to complete the transmission cycle. The engorged nymphs drop to the ground and molt into adult ticks which are active during the fall and following spring and feed on large mammals, primarily deer. Deciduous forest is the predominant habitat for the tick vector and vertebrate hosts in the Northeast and Midwest. Other prime habitats include forested areas interspersed with residential development and grass and shrub areas, particularly along forest edges.

Patients treated with appropriate antibiotics during the early stages of the disease usually have rapid and complete recovery. Even patients treated during later stages generally respond well and recover. No vaccine is available except for domestic dogs. Avoid locations with ticks during seasonal activity periods, use personal measures to protect against ticks, become knowledgeable about the symptoms of Lyme disease, and seek medical care and treatment if infected.

Tularemia

Tularemia is caused by the bacteria Francisella tularensis and is characterized by sudden onset of high fever and chills, joint and muscle pain, and prostration. Slow-healing sores or lesions develop at the site of entry of the bacteria (or arthropod bite). Inflammation and swelling of nearby lymph nodes follow.

Tularemia is endemic throughout North America. Most of the 100 to 300 cases reported each year are from the area between the Rocky Mountains and the Mississippi River (especially Arkansas and Missouri). Most cases are acquired during the summer months from vector transmission; however, a second peak of cases occurs during the winter and is probably associated with rabbit hunting and carnivore trapping.

The bacteria is maintained in rabbits, hares, rodents, and birds by tick transmission. The natural reservoir for the bacteria includes infected ticks and animal species that are less susceptible and thus survive acute infections. Hard ticks, primarily D. andersoni, D. variabilis, and Haemaphysalis leporispalustris, and some flies, especially the deerfly (Chrysops discalis), can subsequently transmit the disease to humans. Tularemia can also be transmitted directly to humans. Transmission routes include drinking contaminated water; eating contaminated food or improperly cooked game meat; inhaling aerosols contaminated with rodent urine, feces, or dust; cuts from contaminated knives or other instruments; and scratches or bites from infected animals. Use personal protection measures against ticks and practice good sanitation procedures when handling wild animals, especially rabbits. Promptly seek medical care and treatment if symptoms develop.

Relapsing Fever

Relapsing fever can be caused by several Borrelia spirochete bacteria, which are related to the Lyme disease spirochete and are transmitted by soft ticks (Argasidae). Symptoms resemble Lyme disease except for the absence of the diagnostic rash and the presence of recurring fever. The most common type is caused by B. hermsii. Most human cases of this type of relapsing fever have been associated with log cabins or houses containing rodent nests (particularly of chipmunks and pine squirrels) and Ornithodoros hermsi ticks. This species of tick is active at night. Since it feeds rapidly and its bite is relatively painless, it may go unnoticed. The ticks feed on humans when the rodents disappear from the cabin nests because of rodent control measures or death from other diseases. Most human cases occur during the summer months when the cabins are in use. Sporadic cases are reported primarily in the mountainous regions of the western United States and British Columbia; 159 cases were reported during 1985 to 1991 in 10 western states. Two outbreaks occurred among tourists and staff staying in cabins at the Grand Canyon in Arizona in 1973 and 1990. Inspect cabins for rodent use and nests, promptly remove nests, and treat cabins with insecticides or fumigate to kill any remaining ticks. Rodent-proof cabins to prevent rodent entry.

Two other species of relapsing fever spirochetes are transmitted occasionally to humans in the western United States by Ornithodoros ticks. The spirochete B. parkeri is transmitted by O. parkeri, mostly in California, and B. turicatae by the tick O. turicata. Five humans were infected with B. turicatae in Texas in 1990 following exploration of a cave containing infected ticks. For prevention, use personal protection against tick exposure. If sick with relapsing fever, seek medical care and appropriate antibiotic treatment.

Other Tick-borne Diseases

Three other tick-borne diseases occur in the United States. Human ehrlichiosis is a recently recognized disease caused by a rickettsia, Ehrlichia chaffeensis. It is probably transmitted by ticks. Symptoms are similar to those of RMSF: an acute fever with headache, muscle ache, and nausea. A rash appears less frequently and for a much shorter duration. From 1986 to 1991, 262 cases and 4 fatalities were reported in 23 states, the majority occurring in Missouri and Oklahoma. Use personal protection against ticks and seek medical care and treatment if sick.

Powassan encephalitis is caused by a virus (flavivirus) which is transmitted by the ticks I. cookei, D. andersoni, and other Ixodes spp. Symptoms include the sudden onset of fever, sore throat, sleepiness, headache, and disorientation. Encephalitis, meningitis, and, occasionally, partial paralysis may develop. Natural hosts are marmots, sciurid rodents, rabbits, hares, carnivores, and possibly birds. Only 19 cases have been reported, all in New York, Pennsylvania, Ontario, and Quebec. Use personal protection to reduce exposure to ticks. No treatment is available.

Babesiosis is a protozoan disease with gradual onset of fever, sweating, loss of appetite, fatigue, general muscle ache, and possibly prolonged anemia. The disease can be severe and sometimes fatal. A protozoan, Babesia microti, is transmitted among wild rodents, particularly white-footed mice, by the tick I. scapularis along the coastal areas of New England and on adjacent offshore islands. This tick may be infected occasionally with both B. microti and the Lyme disease spirochete. Use personal protection measures to prevent tick exposure and seek medical care if sick.

Personal Protection

The following personal measures can protect against tick-transmitted diseases:

• When possible, avoid tick-infested areas.
  

• To better see crawling ticks, tuck pant legs into socks and tape the tops of socks over pant legs. Wear light-colored clothes.
  

• Use tick repellent on exposed skin (DEET) or treat clothes with permethrin. Follow label instructions for use.
  

• Check yourself frequently for ticks and remove them.
  

• After outdoor activity, remove and wash field clothing promptly and dry clothes at a high temperature.
  

• Inspect your body carefully and remove attached ticks with a pointed tweezers. Grasp ticks as close to the skin as possible and pull them loose with a slow, steady motion.
  

• Inspect pets carefully for ticks and remove ticks soon after returning from the outdoors.
  

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