Tularaemia

Tularaemia, also known as rabbit fever or deer fly fever, is an infectious disease caused by the bacterium Francisella tularensis. This bacterium is typically spread by ticks, deer flies, or through contact with infected animals.

It can also be transmitted by drinking contaminated water or inhaling contaminated dust, but it does not spread directly between people. Hunters are particularly at risk due to potential inhalation of the bacteria during the skinning process.

Signs and Symptoms

Tularaemia presents with various clinical forms depending on the site of infection, which include:

1. Ulceroglandular: The most common type (75% of cases) characterised by skin ulcers and swollen lymph nodes.
2. Glandular: Similar to ulceroglandular but without skin ulcers.
3. Oropharyngeal: Affects the mouth and throat.
4. Pneumonic: Affects the lungs, potentially leading to severe respiratory issues.
5. Oculoglandular: Affects the eyes.
6. Typhoidal: A severe form causing systemic illness.

The incubation period ranges from 1 to 14 days, with most infections becoming apparent within three to five days. Common symptoms include fever, lethargy, loss of appetite, and signs of sepsis. The fever can be moderate to very high, and the disease often involves lymph node inflammation and enlargement, mimicking bubonic plague.

Cause

Tularaemia is caused by the bacterium Francisella tularensis, a facultative intracellular bacterium that primarily infects macrophages. This allows it to evade the immune system and spread to multiple organs, including the lungs, liver, spleen, and lymphatic system. Different exposure routes result in varying disease courses, with mortality rates in untreated pneumonic and typhoidal forms reaching up to 50% historically.

Diagnosis

Diagnosis is typically confirmed through blood tests or microbial cultures. The bacteria require special media such as buffered charcoal yeast extract agar for isolation. Serological tests are also available but can be confounded by cross-reactivity with Brucella. Molecular methods like PCR are used in reference laboratories to confirm diagnosis.

Prevention

Preventative measures include using insect repellent, wearing long pants, rapidly removing ticks, and avoiding disturbance of dead animals. Currently, there are no approved vaccines for tularaemia, though live attenuated and sub-unit vaccines are under investigation. When handling potentially infected animals, using gloves and face masks is advised.

Treatment

If infection occurs, treatment is generally with antibiotics such as streptomycin or gentamicin. Doxycycline can also be used, and there is tentative evidence supporting the use of quinolone antibiotics. Gentamicin may be more readily available than streptomycin.

Prognosis

The prognosis for tularaemia is generally good with treatment. Before the advent of antibiotics, the mortality rate was as high as 60%, but it has decreased to less than 4% with modern medical care.

Epidemiology

Tularaemia is most common in the Northern Hemisphere, especially in North America, Europe, and parts of Asia. In the United States, it is rare with around 200 cases reported annually, primarily affecting males and occurring most frequently in the summer. In Europe, Finland and Sweden report the highest incidence rates, with periodic outbreaks.

Outbreaks

Notable outbreaks include an incident in Martha's Vineyard in 2000 linked to lawn mowing, and instances in Kosovo (1999–2000), Germany (2005), and Spain (2007). The disease has also been reported in feral pig populations in Texas and Tasmania.

History

The bacterium was first isolated by G.W. McCoy in 1912. Historically, tularaemia has been associated with hunters and agricultural workers. It has also been considered a viable biological warfare agent due to its high infectivity and ability to be aerosolized, having been included in the biological warfare programmes of several nations.

Other Animals

Cats and dogs can contract tularaemia through tick or flea bites from infected hosts. Treatment includes antibiotics like tetracycline, chloramphenicol, or streptomycin, with long treatment courses necessary due to common relapses.

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Self-assessment MCQs (single best answer)

What is the causative agent of tularaemia?

What is the most common clinical form of tularaemia?

Which method is NOT a typical way tularaemia is transmitted?

What is the first step in treating tularaemia?

Which antibiotic is commonly used to treat tularaemia?

How is tularaemia diagnosed?

Which form of tularaemia has the highest historical mortality rate if untreated?

What is a common preventative measure against tularaemia?

In which regions is tularaemia most commonly found?

Which type of professional is at particularly high risk of contracting tularaemia?