Written collaboratively by Russ Daly and Marlene Braun.
Tularemia is a rare, but potentially severe, disease that can develop in many species of wild and domestic animals, but especially rabbits, rodents, cats, as well as humans. Diagnoses of tularemia in animals are noted every year at the South Dakota State University Animal Disease Research and Diagnostic Laboratory (abbreviated as SDSU ADRDL).
What causes Tularemia?
Tularemia is caused by a gram-negative bacterium called Francisella tularensis. The germ can affect many species of animals, but in the United States, rodents and rabbits (particularly cottontails) are the important hosts. Various species of ticks (American dog tick, lone star tick, and the Rocky Mountain wood tick) are important maintenance hosts and biologic vectors.
Francisella tularensis is highly infective. As few as 10 to 50 organisms inhaled or otherwise introduced into the body can cause illness in humans. The organism, besides surviving in its maintenance hosts and vectors (including ticks, deerflies, and possibly mosquitoes), can survive for long periods of time (weeks to months) in water, soil, and dead animal carcasses.
Dogs and cats often become infected through the bite of an insect carrier or from eating or contacting infected prey, and livestock usually are infected through bites from carrier insects. As such, conditions that promote tularemia in an area can often be related to an increase in rabbit or rodent populations (or a decrease in their predators), along with environmental conditions that promote survival of the vectors.
Of the domestic animals, cats and dogs are the most commonly affected species, although severe clinical signs are more common in cats. In livestock, sheep are the most commonly affected species, but horses and calves have also been affected. In these species, young animals (including lambs, foals, and calves) seem to be more frequently affected.
People become infected usually by contact with insect vectors or through contact with an infected animal. Cats and dogs can infect people without showing clinical signs themselves. People have also been infected after mowing over dead rabbits with lawn mowers and inhaling the agent! Because it is so highly infectious, F. tularensis is classified as a Category A agent of bioterrorism. In South Dakota, tularemia is a reportable disease for both people and animals.
A variety of clinical signs may result from tularemia infections in people and animals. Signs of tularemia in cats can range from inapparent infection to mild illness with swollen lymph nodes and fever, to severe overwhelming infection and death. In dogs, sheep and other livestock, general, non-specific signs are commonly noted. These include fever, sluggishness, stiffness, and decreased appetite. Clinical signs in rabbits and rodents are lethargy and sluggishness, making these animals easy prey for cats and other predators.
Diagnosis of tularemia in animals is achieved by demonstration of F. tularensis in blood, tissues, or exudates from affected animals, or by demonstration of a specific antibody response (see below).
Tularemia in people takes form in any one of six syndromes, the most common of which is the “ulceroglandular” form, where a sore develops at the point of entry of the bacteria, accompanied by swelling in the local lymph nodes. The most severe form is the primary pneumonic (lung infection) form, but it is relatively rare.
Tularemia is diagnosed yearly in people in South Dakota. A recent peak in human tularemia occurred in 2015, when 25 cases were reported. Most cases were from central and western South Dakota.
A. Demonstration of F. tularensis in affected animals (Use appropriate personal protective measures when obtaining samples for diagnosis).
- Bacterial culture (collect samples before antibiotic treatment if possible)
- a) Clinically affected animals
- i) Swab of exudate from draining lymph nodes or lesions
- ii) Aspirate/biopsy of lymph node or lesions
- b) Post-mortem samples (Submit fresh and formalin-fixed tissues)
- i) Spleen
- ii) Liver
- iii) Lung
- iv) Kidney
- a) Clinically affected animals
- PCR testing of tissues (of particular use when tissue or sample quality is poor; SDSU ADRDL currently forwards tularemia PCR testing to outside labs)
- a) Tissues as described above under (1) Bacterial culture
B. Demonstration of antibodies specific to F. tularensis in serum
- Card agglutination test (SDSU ADRDL currently forwards to outside labs; 1 milliliter serum required)
- Demonstrable antibodies are normally not present for 2 to 3 weeks following initial infection
- Titers greater than 1:80 are presumptive; otherwise a 4-fold rise in titer between acute and convalescent samples is diagnostic
Recent Cases in South Dakota
Examples of representative animal cases diagnosed at SDSU’s ADRDL:
- Cat from west-central South Dakota.
A cat with a history of routine hunting of rabbits and rodents. Lately, a die-off of rabbits in the immediate area had been noted. Cat was presented with signs of incoordination, dehydration, and lack of appetite and died soon afterward. Findings included abscesses in lungs, swollen lymph nodes, and areas of necrosis (dead tissue) in the spleen. The tissues were positive by culture and PCR (polymerase chain reaction: a very sensitive method of bacterial detection) for F. tularensis. Later, from the same locale, a dead cottontail was submitted, which had an enlarged spleen and white pinpoint spots on the liver. F. tularensis was isolated from lung, liver, kidney, and spleen.
- Cat from southern South Dakota.
A cat near the Missouri River was another case. Its owner had reported losing other cats with similar signs for over three years. Clinically, the cat was emaciated, and had jaundice and diarrhea. Inflammation and swelling of the lymph nodes and liver were seen. The diagnosis of tularemia was made by using special stains on microscope slides from the submitted tissue.
- Cat from central South Dakota.
This cat had “eaten a tame rabbit that had died.” Clinical findings included swollen lymph nodes in the jaw area. There were small tan spots in and swelling of the lymph nodes. F. tularensis was confirmed by culture from lymph nodes and PCR.
Treatment of tularemia in animals centers on use of an appropriate antibiotic, along with supportive care. An animal is considered infectious until it has received 72 hours of appropriate antibiotic treatment and shown clinical improvement (without fever).
Tularemia is a relatively rare, but serious, disease that has potential to cause disease in people through their contact with infected animals or insect vectors. Animal owners, and cat owners especially, should, as always, observe their animals closely for signs of illness and contact their veterinarian as soon as possible when unusual signs develop. Proper use of tick and insect repellants on pets and livestock is another good practice to prevent transmission of this and other vector-borne diseases. Cat owners should use caution and consult their veterinarian when using many of the insect repellants that are available. Many products that are labeled for use on dogs oftentimes will be toxic for cats. People themselves should also use insect repellants when outside with their animals and take proper precautions when disposing of dead animals that are found (wearing gloves, double-bagging).
- Feldman KA. Tularemia. Tularemia. J Am Vet Med Assoc 2003; 222: 725-730.
- Infectious Disease Surveillance Dashboard. South Dakota Department of Health.
- Kwit NA, Middaugh NA, VinHatton ES, et al. (2020). Franciscella tularensis infection in dogs: 88 cases (2014-2016). J Am Vet Med Assn 256(2):220-225.