Tularemia

Tularemia

Causative Agent

A tick-borne, potentially zoonotic disease, primarily of rodents and lagomorphs . Caused by the bacteria, Francisella tularensis.
Two different sub-species of F. tularensis:
- one holarctic (F. t. palaearctica);
- one restricted to North America (F. t. tularensis).

Click on a photo to enlarge.

Small white dots of necrosis on the liver of this beaver are typical of tularemia.

Enlargement of the spleen is typically observed in animals with tularemia.

Distribution

Geographic:

In North America:
- F. t. tularensis occurs in terrestrial habitats;
- F. t. palaearctica occurs most often in aquatic habitats (wetlands
  and boreal conditions).
F. t. tularensis does not seem to occur in Canada and may be limited to the lower 48 contiguous United States.
F. t. palaearctica infection primarily occurs in Canada.

Seasonality:

Ticks are only present on hares from May to September (Alaska data); as a result, the disease essentially disappears from October through April.
Human cases may also occur throughout the fall and winter following exposure to rabbits during rabbit-hunting season.

Hosts, Transmission and Life Cycle

Hosts:

F. t. tularensis: rodents and lagomorphs.
F. t. palaearctica: aquatic rodents.
In North America, tularemia occurs most commonly in cottontail rabbits, muskrats, ground squirrels, and beavers.
It is thought that tularemia is often fatal in the above species, but not so for the snowshoe hare.

Transmission:

F. t. tularensis: transmission occurs among hosts primarily through ticks, but also by mites, mosquitoes, fleas, lice and biting flies (Tabanidae).
F. t. palaearctica: transmission occurs directly through water, which may remain infectious for weeks to months following contamination.
For both species, transmission may also occur through contact with feces, urine or body parts of infected animals.

Life Cycle:

Although often fatal, the bacterium can infect both lagomorph and rodent hosts without apparent ill-effects. These hosts can then remain infected for extended periods serving as “reservoirs of infection” for other animals and biting arthropods.
Ticks act not only as vectors of transmission but also as reservoirs of the bacterium which can live in certain tick species for months.
F. t. palaearctica may also be transferred among voles (Microtus spp.) through cannibalism of infected individuals.
The mammal and arthropod associations differ from location to location.

Signs and Symptoms

Animals:

In the most sensitive species, clinical signs are not often observed due to the short duration of infection before death occurs. These animals are usually in good body condition at death.
In less sensitive species, during the latter stages of the disease, animals may become lethargic or depressed and have elevated body temperature.
Tularemia is most often recognized during examination at a diagnostic laboratory.
Tiny, pale spots on the liver, spleen or lung are typical lesions of tularemia.
Spleen or liver may become enlarged.
As in humans, an ulcer may form where the bacteria have entered the body.
Thin, whitish strands of material may be present in the abdominal cavity.
Above lesions are not unique to tularemia: see also plague.

Humans:

Initially, symptoms are: generalized fever-like illness (e.g., fever with chills, headache, vomiting) beginning 1-10 days after infection.
Confirmation of the disease is usually accomplished using blood tests.
The course of the disease depends on the route of infection:
- arthropod bite: an ulcer forms at the bite wound followed by enlargement of the lymph nodes draining the area of the bite wound.
- inhalation: inhalation of infected material results in pneumonia.
- ingestion: of infected water or meat leads to inflammation of the posterior portion of the oral cavity and intestines.
Disease resulting from F. t. tularensis is more serious than that caused by F. t. palaearctica.
40-60% of untreated cases of pneumonia or inflammation of the inner surface of the intestines and 7% of all forms of untreated infection with F. t. tularensis have resulted in fatalities. In contrast, 1% overall fatality in untreated infections with F. t. palaearctica have been reported.
Human to human transmission is rare.
F. t. palaearctica is common in trappers.

Meat Edible?

Normal cooking temperatures destroy bacteria in the meat – it is therefore safe to eat when thoroughly cooked.
Human exposure typically results from preparing carcasses.

Human Health Concerns and Risk Reduction

Human infection in North America prior to 1950 has been closely associated with exposure to cottontail rabbits infected with the F. t. tularensis strain.
After 1950, the major risk factor to humans has switched to muskrats infected with the F. t. palaearctica strain.
Taking precautions such as: basic hygiene, use of insect repellents and protective clothing to avoid arthropod bites, inspection and removal of ticks, use of gloves when handling and dissecting wild animals, particularly rodents and lagomorphs, should help to reduce the chance of exposure to the tularemia bacteria.
Vaccination against tularemia may be warranted in high-risk areas.
Dogs and cats can die from tularemia. Since infected animals are easier to catch, pets may become infected after eating the internal organs of the diseased animal. Keeping pets from roaming free should help to reduce the spread of tularemia.
Tularemia is readily treated with antibiotics if treatment is started early in the course of the disease.
Thus, symptoms of general malaise and fever should not be ignored and medical attention sought. Medical personnel should be advised that you may have been exposed to wildlife and so may be at risk with respect to various wildlife diseases including tularemia.

Samples for Diagnosis

Similar Diseases

Lesions in animals infected with plague, yersiniosis and other bacterial infections may be similar to that of tularemia.