Rabbit fever

Tularemia Truth by Ruby Mosher, DVM
Rabbit Fever still occurs. Hunters and outdoors enthusiasts should take basic precautions.


Rabbits. Ticks. Bacteria. Add a hunter or other outdoor recreationist to the mix, and there’s a chance to come down with tularemia. But awareness of the disease and proper precautions can help people remain disease-free while enjoying the outdoors.

Tularemia, commonly known in the U.S. as “rabbit fever” or “deerfly fever,” is not a new disease but is in the news again because of recent epidemics. It is highly infectious but not common in modern society. During the 1930s and ‘40s, the U.S. averaged 1,000 cases of tularemia per year. Decline of the disease was partly due to society’s residence shift from rural areas to cities in the early 1900s. Public education about tularemia and how to prevent it also helped decrease the number of cases.

As cases diminished, tularemia was dropped from the federal government’s mandatory disease reporting list in 1995. However, its highly infectious nature makes it one of six top diseases for potential development as a biological weapon in terrorism. Therefore, in 2000, tularemia was reinstated as a disease that must be reported to state and federal officials when diagnosed.

While tularemia has declined in the U.S. as a human disease, locations of reported cases have also shifted over time. Tularemia used to be more prevalent in the eastern U.S. but now, with the exception of Martha’s Vineyard in Massachusetts, it occurs mostly in southern, central, and western states. Missouri, Arkansas, South Dakota, Oklahoma, and Kansas report the most human cases of tularemia. Kansas reported 637 cases from 1924-1949; 328 cases from 1950-1975; 96 cases from 1976-1994; and 73 cases from 1991-2004. Since then, approximately five cases of tularemia have been reported each year in Kansas. Most Kansas tularemia cases occur in eastern counties.
The Disease
Tularemia gets its name from Tulare County in California, where the causal bacteria was first isolated in 1912 in a study of ground squirrels with a plague-like disease. Tularemia’s common names, such as rabbit fever and deerfly fever, are derived from the type of animal most-associated with the disease.

Tularemia is caused by the bacterium, Francisella tularensis, one of the most highly infectious organisms known to affect humans and animals. People can get tularemia in all sorts of ways, ranging from contact with infected wildlife or contaminated water, to bites from infected ticks, flies, and mosquitoes, to simply breathing in the bacterium. In North America, tularemia is primarily a tick-borne disease, although biting flies, mosquitoes, gnats, and midges can transmit the disease to people after feeding on infected animals. The disease itself is variable, causing symptoms that range from mild to life-threatening.

Tularemia occurs in patchy locations throughout the world, though it tends to occur mostly in the northern hemisphere. On the North American continent, tularemia has been reported in every state in the mainland U.S., in every Canadian province, and as far south as Guadalajara, Mexico. In Eurasia, the disease is reported to occur mostly in Northern and Central Europe, as well as Japan.

The tularemia bacterium can infect nearly any creature, including mammals, birds, bugs, amphibians, reptiles, and fish. The disease affects animals differently, though, depending upon the susceptibility of the species. Wild animals most commonly associated with tularemia are rabbits, hares, squirrels, muskrats, beavers, mice, voles, chipmunks, woodchucks, and prairie dogs. Less commonly, mule deer and white-tailed deer can be sources of tularemia for humans. Domestic animals most commonly associated with tularemia are sheep and cats.

Two types of tularemia exist, designated Type A and Type B, depending upon the severity of disease caused in humans. In the U.S., most tularemia cases are Type A – a tick-borne syndrome associated with rabbits. Type B, a waterborne syndrome associated with rodents, is most common overseas. Both tularemia strains are virulent in wild animals, but Type A bacteria are most dangerous to humans.

To better understand how people can become infected with tularemia, let’s take a closer look at how these two different types of the disease occur in nature.
Type A Tularemia

Type A tularemia has a terrestrial, or land, cycle and is maintained in the wild primarily because of ticks. Hard ticks, such as the dog tick and the lone-star tick, are able to carry F. tularensis in their guts throughout their lives. They are even able to continue the infection, through their eggs, to the next generation of ticks. Thus, some ticks maintain a reservoir of tularemia bacteria always ready to infect the mammals they feed on. This is why tularemia is endemic, or always present, in certain areas.

Maintenance of the tularemia cycle in nature also requires a mammal susceptible to the disease. In Kansas, cottontail rabbits and jack rabbits are the primary victims, though squirrels and other small mammals serve as well. Tick bites may transmit F. tularensis into the mammal’s bloodstream where the bacteria then multiply into great numbers and spread throughout the body. Other ticks feeding on the infected rabbit then pick up the bacteria and spread it to more animals. Flies, mosquitoes, and fleas feed on the host and vector bacteria to other animals in the same way. Local outbreaks or widespread epidemics can then occur if enough susceptible animals are present.

Rabbits with acute tularemia infection may be depressed, uncoordinated, or in a stupor. These are not diagnostic symptoms, but simply indicate that the animal is ill. Internally, the disease usually causes small white spots on the liver and spleen where the bacteria have caused cellular destruction. Rabbits may also have enlarged lymph nodes and, more rarely, pneumonia. Many infected rabbits will die in an outbreak of tularemia, but some will continue to live and become chronic carriers of the bacteria.

Enter man. With infected ticks, flies, mosquitoes, and rabbits in an area, there are numerous opportunities for an outdoors person to encounter F. tularensis. Bacteria can be transferred to people through bug bites, or by even squashing a bug and releasing the infected body fluid. Hunters can encounter tularemia when field-dressing an infected rabbit or squirrel, or by eating undercooked game meat. Less commonly, the bacteria can be acquired by simply breathing airborne infective particles.

When humans become infected with Type A tularemia, the resulting disease depends on how the bacteria entered the body. Within 3-5 days after exposure, a person may feel flu-like symptoms including a sudden fever, chills, headache, muscle aches, joint pain, weakness, diarrhea, and a dry cough. If bacteria entered through a bug bite or open wound, the entry area will usually redden and form an ulcer or crater. If bacteria entered through the mouth when eating improperly cooked infected meat, a sore throat often develops along with swollen lymph nodes and sores in the mouth. A serious infection possibly leading to blindness may develop if the bacteria is rubbed onto or otherwise contacts the eye. If airborne bacteria are inhaled, a particularly deadly pneumonia may develop: this is the delivery method that security officials believe terrorists might exploit. In all cases, Type A tularemia may progress to a fatal disease if left untreated.

The good news is that timely antibiotics can easily and effectively treat Type A tularemia. Early diagnosis is the key, which is why outdoors enthusiasts and small game hunters in particular should be aware of dangers and symptoms of tularemia.
Type B Tularemia
Type B tularemia is primarily a waterborne disease of true rodents such as beavers, muskrats, voles, and mice. This was the type of tularemia involved in the 2002 disease outbreak in Texas among wild prairie dogs that had been captured for sale as pets.

Worldwide, more human cases of tularemia are actually caused by rodents than by rabbits. Fortunately, Type B tularemia causes a much milder infection in man than does Type A. In wildlife, however, Type B tularemia is just as deadly. Although this type of tularemia occurs in North America, it is more prevalent overseas.

Rodents are very susceptible to Type B tularemia and are vital to the ecology of the disease. Rodents are hosts to ticks that vector the disease. Additionally, rodents shed bacteria in their urine and feces and thus contaminate water sources as well as hay and grain stores.

Infected beavers and muskrats tend to die in epizootics, or outbreaks. Because of this, they are not likely to be responsible for maintaining tularemia in the environment. Instead, voles or meadow mice are believed to be the reservoirs for continual contamination of certain water sources. Some of these individuals can live and become chronic carriers harboring the bacteria.

People can become infected with Type B tularemia in the same ways as Type A. Direct contact with infected beavers or muskrats is most likely through trapping. Additionally, Type B tularemia can be acquired through ingestion and inhalation of contaminated water. Infective airborne particles can also be inhaled while handling hay or grain that voles or mice have infested. Fortunately for man, this form of the disease runs a milder course in humans.
Preventive Measures

In the U.S., human cases of tularemia tend to occur in small sporadic clusters and can usually be linked to outdoor activities. During summer months, the disease is caused mainly from insect and tick bites, whereas in winter, infections usually involve hunters and trappers who handle infected animals. Outdoor pets can also be sources of infection. Dogs and cats may bring the tularemia bacteria home after eating dead animals or through tick-borne infections. A number of recent Kansas cases have occurred through cat bites and scratches.

Since tularemia cases are not common, the disease may not be high on your doctor’s lists of suspicions. If you become ill with tularemia-like symptoms, be sure to alert your medical professional of possible recent contact with the bacteria. This would likely involve bug bites or handling wild game, though any outdoor activity, including gardening, hiking, camping, hunting, trapping, and fishing could be responsible for infection.

You should seek medical attention promptly if any of the following conditions occur anytime within a few weeks of possible exposure:

if a rash or sore develops around a tick or insect bite
if a sore or ulcer begins to form on the skin or in the mouth after preparing or eating wild game
if you develop flu-like symptoms such as fever, muscle aches, nausea, or vomiting.
Awareness of tularemia should not lead to fear of the outdoors. Not every tick or rabbit you encounter will be infected with the disease. In fact, the vast majority of outdoors enthusiasts will go a lifetime without contracting tularemia. Even so, it’s wise to know the potential dangers. Be aware of tularemia and use precautions. Then you can relax and enjoy the great Kansas outdoor experience.

*ALWAYS WEAR RUBBER GLOVES WHEN CLEANING RABBITS*