Tularemia, sometimes called rabbit fever, is an uncommon but debilitating disease spread from animals to man caused by the bacteria Francicella tularensis. It can be found in a variety of animal hosts, notably lagomorphs (rabbits and hares), aquatic rodents (muskrats, beavers, and water voles), other rodents (water and wood rats and mice), squirrels, and cats. In the United States, an outbreak involving commercially distributed prairie dogs occurred in 2002.
Human infection occurs sporadically throughout the continental U.S. Historically, cases have been concentrated in the south-central states, but distribution of human tularemia cases in the U.S. has moved progressively northward since 1965. The disease is now endemic in Colorado, Nebraska, South Dakota, and Wyoming, enhanced by increased rainfall promoting vegetation growth, pathogen survival, and increased rodent and rabbit populations. Disease trends are independent of changes in human population and reflect shifts in environmental factors and arthropod vector abundance. Tularemia cases have doubled and tripled in the above states.
Tularemia is a classic zoonosis, capable of being transmitted by aerosol droplets, direct contact, ingestion, or through bites of arthropods — primarily ticks. Inhalation of aerosolized organisms can produce a pneumonic form. Direct contact with or ingestion of infected carcasses of wild animals (such as rabbits) can produce swollen glands (lymph nodes) and ulcers. Systemic infections (typhoid form) are common. Ingestion of contaminated water can result in infection in aquatic animals. Ticks maintain infection through their different life stages, which makes them an efficient reservoir for infection as well as a vector. Recognized tick vectors include Dermacentor andersoni (wood tick), Amblyomma americanum (lone star tick), and Dermacentor variabilis (dog tick). Large biting flies like deer flies also transmit infection.
Tularemia is highly infectious, with as few as 10 organisms needed to cause disease. Humans can develop severe and sometimes fatal illness but do not transmit the disease to others. The typical incubation period is three to five days, with a range of one to 14 days.
F. tularensis is considered a category A bioterrorism agent. F. tularensis is highly infectious, occurs widely in nature, and can be isolated and grown in quantity in the laboratory. During the Second World War, the Japanese conducted research on F. tularensis as a biological weapon. During the 1950s and 1960s, the United States developed weapons that could deliver aerosolized F. tularensis organisms. The United States government stockpiled weaponized tularemia until stockpiles were destroyed in 1973. The former Soviet Union also weaponized F. tularensis; the Soviet program included development of antibiotic and vaccine-resistant strains. In 1969, the World Health Organization estimated that an aerosol dispersal of 50 kg of virulent F. tularensis over a metropolitan area with five million inhabitants in a developed country would result in 250,000 illnesses, including 19,000 deaths.
Although tularemia is uncommon, physicians, laboratory staff, public health workers and veterinarians should be alert to the possibility of disease caused by F. tularensis. The disease is endemic in British Columbia and other parts of Canada. Most cases are likely acquired in rural areas. Skin lesions, often accompanied by swollen regional lymph nodes, are the most common clinical signs.
In endemic areas, members of the public and individuals who handle wild animals should be aware of the disease. An unexpected case of tularemia diagnosed in rural B.C. in October 2006 led to a review of reported cases in the province to further define the clinical and public health importance of this infection. Laboratory results confirmed tularemia in an adult resident in the Greater Sudbury area (2015), the first human case of tularemia in Sudbury since 2003. It is believed the Sudbury case resulted from contact with wild game. Elsewhere, there have been several reports of tularemia in humans following bites from infected domestic cats.
There are a number of clinical presentations and diagnosis may be difficult.
Although tularemia is a potentially serious and life-threatening disease, treatable with appropriate antimicrobial agents, early clinical suspicion and appropriate diagnostic testing are required. Serology and culture are used to diagnose tularemia. F. tularensis is commonly isolated in culture or detected by polymerase chain reaction from patients’ blood specimens, but can also be identified in specimens from skin lesions, spinal fluid, lymph nodes, and respiratory secretions. Specimens suspected of containing F. tularensis should be handled safely. Biosafety level 3 containment is recommended when handling live cultures.
F. tularensis can be recovered from contaminated water, soil, and vegetation. It can persist for weeks under ideal environmental conditions. F. tularensis can also be found in amoebas (small waterborne organisms) which become airborne in some settings, and represent a significant environmental reservoir for this bacterium.
Typically, humans become infected through:
- Bites or licks of an infected animal.
- Handling or cleaning the carcass of an infected animal, especially through contact with the skin or meat.
- Eating inadequately cooked wild game.
- Wound infections with contaminated soil.
- Contaminated water.
- Bites of an infected tick or deer fly.
- Small domestic pets like hamsters have been a source of tularemia for humans.
Hunters are at higher risk of exposure because of the handling of wild game carcasses. Transmission of tularemia from person to person has not been reported.
The clinical presentation of tularemia depends on the route of exposure. The onset of tularemia is usually abrupt, with fever, headache, chills, and generalized body aches (often prominent in the low back), coryza (inflammation of the mucous membranes lining the nasal cavity), and sore throat. Nausea, vomiting, and diarrhea may occur. Sweats, fever, chills, progressive weakness, malaise, anorexia, and weight loss characterize chronic illness. Gentamicin, doxycycline, ciprofloxacin and streptomycin are used to treat humans.
Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected])