What is tularaemia?
Tularaemia is an infection caused by a bacterium, called Francisella tularensis, which is transmitted by ticks and other animals. Various tick species are responsible for 9% to 57% of transmission. Other animals that can transmit F. tularensis include rabbits, squirrels, opossums, cats, muskrats, and mosquitoes.
F. tularensis is mainly found in the Northern hemisphere and has been reported in North America, Russia, Europe, the Middle East, China, and Japan. Tularaemia has recently received attention due to its potential as a biological weapon.
Two predominant strains of F. tularensis have been isolated; type A and type B. Type A causes more severe disease, causing death in up to 5% to 7% of untreated patients. Type B generally causes a milder illness, which can occasionally be symptom-free.
What are the clinical features of tularaemia?
- In adults – fever, headache, skin changes, malaise, and enlarged lymph nodes of the head and neck.
- In children – fever, sore throat, enlarged liver and spleen, fatigue and malaise.
Various clinical subtypes of tularaemia have been described depending on the mode of transmission and organ systems involved.
|F. tularensis penetrates the skin through a scratch, graze, or tick or insect bite. Most common form of tick-borne tularaemia. Causes a painful skin ulcer, with enlarged, inflamed nearby lymph nodes, or nodular lymphangitis (swellings beneath the skin that track along the course of lymph channels, swellings may be painful and ulcerate). Lymph nodes may become fluctuant (soft, fluid-like) and rupture.
|Causes enlarged lymph nodes without an ulcer.
|Due to direct contamination of an eye, e.g. from a squeezed tick spraying blood directly into the eye.
Other clinical subtypes include typhoidal (most lethal), pneumonic, oropharyngeal, and gastrointestinal.
What are the skin manifestations of tularaemia?
Tularaemia can cause primary skin lesions (seen in ulceroglandular subtype) and secondary skin lesions (seen in all forms of tularaemia).
This develops at the point of entry after a 2 to 5 day incubation period. The lesion is a painful red papule (lump) that slowly enlarges and ulcerates within a few days. The ulcer has raised, hardened, ragged edges, and a sensitive base. A discharge may be present, and the ulcer may be covered by an eschar (scab) and/or be itchy. Over weeks to months the ulcer heals and is replaced by scar tissue.
The primary lesion caused by type B tularaemia infection may be less severe, e.g. crusting, but no ulcer.
Secondary lesions (called tularemids) develop in 8% to 20% of cases of tularaemia and take various forms:
- The most common secondary lesions are papular (small elevated lumps) or papulovesicular (small elevated lumps and blisters). Occasionally the lesions are macular (flat discolouration), maculopapular (small discoloured lumps), vesicular, pustular (pus-filled blister), pimple-like, nodular (larger, solid papule), or plaque-like (broad, flat lesion). These lesions appear around 11 days after the onset of symptoms and may be widespread, symmetrically distributed on both sides of the body, and itchy.
- Erythema nodosum occurs in 1% to 13% of cases and appears at the end of the second week of illness.
- Erythema multiforme occurs in 0.5% to 2.0% of cases and predominantly affects the trunk and extremities. Erythema nodosum may also be present.
- Less common secondary skin lesions include herpes simplex labialis (cold sores), urticaria, and lymphangitis (red streaks visible along the path of lymph channels) and swollen, tender lymph nodes.
|Transmission due to other animals
|Usually a single primary lesion at the site of the bite. Lesion occurs on a part of the body commonly affected by tick bites.
|Multiple primary lesions, often on the arms and hands.
How is tularaemia diagnosed?
- The diagnosis of tularaemia is usually made by serologic testing (detection of antibodies against F. tularensis in the blood). A rising level of antibodies between the acute and convalescent stages of disease confirms the diagnosis. Because this process can take 2 to 4 weeks, treatment may be started based on the patient’s history and clinical features.
- F. tularensis can be isolated from blood, biopsy samples, or other bodily fluids and tissues. A specific culture medium is required to grow this organism in the laboratory. F. tularensis is highly infectious to laboratory staff so infection control precautions are required.
- Blood tests show disturbed liver function in around half of all patients.
What is the treatment for tularaemia?
Tularaemia is treated with antibiotics such as streptomycin, gentamicin, tetracyclines, chloramphenicol, and tobramycin. Streptomycin is generally the drug of choice, as it appears to offer the highest rate of cure with the lowest rate of relapse.
Secondary skin manifestations may require topical corticosteroids.