Native Diseases

The Intermountain Western environment has numerous zoonotic diseases present in it. With more folks moving into areas shared by wildlife, the potential for the transmission of these diseases rises. Taking reasonable precautions can dramatically reduce the chances of contracting any of these diseases. Following is a brief summary of the most common zoonotic diseases in Colorado and advice for treatment and control. For more detailed information, contact your local or state health department.

Arthropod-borne viral encephalitis

The two types of encephalitis commonly found in the Intermountain states are Western Equine Encephalitis (WEE) and St. Louis Encephalitis (SLE). They usually occur in humans from late July through September. The viruses are transmitted through the bite of an infective Culex tarsalis mosquito. These mosquitoes acquire the infection from rodents or other mammals and wild birds, and remain infective for life. In people, the disease can range from a mild case of aseptic meningitis to a severe infection with acute headaches, high fever, stupor, disorientation, meningeal signs, coma, spasticity, tremors, convulsions and spastic paralysis. There is no specific treatment but supportive care is given to ease any severe complications. You're best off to avoid exposure to mosquitoes: use repellents and manage the immediate environment to eliminate calm pools of water and any other mosquito breeding habitat.


In 1993, an unknown strain of Hantavirus popped up in the four corners region of Colorado. Since then, the virus has been discovered all across America. The nastiest part of this disease is the 60% fatality rate. While other rodents have been found carrying the virus, its primary vector is the deer mouse. The virus spreads through contact with the urine, saliva and feces of infected rodents. Humans can be infected through breathing in the dust from dried feces, rodent nests or areas contaminated with rodent droppings. Early symptoms are much like the flu and appear 3 to 8 days after exposure. These symptoms may include tiredness, cough, vomiting, headache, and stomach pain. In later stages, the disease causes respiratory distress with the lungs filling with fluid causing difficulty in breathing. Supportive care is given to ease the symptoms as there is no specific treatment available. To control exposure, eliminate rodent living spaces near living and working areas, avoid inhaling the dust from dried rodent nests and droppings, and if you have to clean these things, disinfect them first with a spray made of Lysol or bleach mixed with water at a 1 to 10 ratio.


Norwegian rats brought plague to the Americas about 1900 and it is now firmly entrenched in the rodent populations of Colorado and New Mexico. The causative bacteria is spread primarily by the bites of infective fleas. It can also be contracted through direct contact with infected animals and by inhalation of infective droplets. 2 to 5 days after exposure, symptoms start with a sudden onset of high fever, chills, headache and prostration. Extreme pain and swelling of the lymph glands can also occur. With prompt teatment, antibiotics are extremely efficient at curing plague. To minimize exposure, take steps to prevent flea bites (use personal repellents and use flea spray or powder on your pets), avoid contact with sick and dead rabbits and rodents, protect your hands and face when handling or skinning carnivorous animals, and rodent-proof your house and the area around it.


Whenever someone is bitten by a carnivorous animal, rabies infection has to be considered. If not treated quickly, this acute encephalitis is nearly always fatal. The virus is most commonly transmitted in the saliva of a biting infected animal. Other routes of infection are possible but are very rare. Beginning with headache, fever and sensory changes, the disease progresses into paralysis, muscle spasms, delirium and convulsions. Death is usually caused by respiratory paralysis. A post-exposure prophylaxis series of five injections has proven highly successful in saving people infected with rabies but treatment must be prompt. An immediate, thorough cleansing and flushing of the bite wound with soap and water is also quite effective in preventing rabies. To avoid the problem: vaccinate all your cats and dogs and avoid any animals that look sick or act strange. If you are bitten, try to safely catch or control the animal for rabies testing. If you can't catch the suspect animal, presume you have rabies in your system and get ready for the prophylaxis treatment.

Tick fevers

Between 100 and 300 cases of Colorado Tick Fever are reported annually. Lyme Disease, Rock Mountain Spotted Fever, Tick-Borne Relapsing Fever, and Tick Paralysis are relatively rare. Infective ticks transmit viruses, spirochetes, and rickettsia through their bites. In most cases, the tick must remain attached and feeding for several hours before the disease is transmitted. Incubation periods range from 3 to 32 days. First, a rash will usually appear in the area of the bite. Then flu-like symptoms (fever, aching muscles, headache and fatigue) appear. Tick Paralysis is a progressive muscle weakness caused by a substance toxic to the nervous system. In the case of Tick Paralysis, removal of the tick normally leads to total recovery. For the other diseases, broad spectrum antibiotics are effective. To avoid the problem: avoid tick infested areas. When in tick infested areas, use tick repellents, search your whole body for ticks every few hours and remove ticks promptly (slowly and firmly pull the tick out of the wound to avoid leaving mouth parts behind).


Tularemia is found in rodents, rabbits, domestic animals and humans. The annual number of cases ranges from 3 to 20 in Colorado. The disease is caused by a rod-shaped bacterium. Usually, it is inoculated into the skin while skinning or dressing infected animals (especially cottontail rabbits). It can also be transmitted by eating under-cooked rabbit meat, drinking contaminated water, inhaling contaminated soil, hay or grain, and sometimes through the bites of infected insects and carnivores. The disease starts with a sudden, dramatic onset of chills and fever. Specific antibiotics are usually effective in curing the disease. Untreated cases experience a 5% fatality rate. To avoid the problem: wear rubber gloves when skinning or dressing potentially infected animals, thoroughly cook the meat of wild rabbits, avoid drinking raw water where infection may be present in the wild animal population, and avoid bites and contact with ticks, flies and mosquitoes in areas where Tularemia may be present. logo
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