Tularemia

Gyuranecz, Miklós (2018) Tularemia. In: Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. Office International des Epizooties, Párizs, pp. 675-682.

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Abstract

Description of the disease: Tularemia is a zoonosis caused by Francisella tularensis. The causative bacterium is a Gram-negative coccoid rod, 0.2–0.5 μm × 0.7–1.0 μm, non-motile and non-spore-forming organism that is an obligate aerobe with optimal growth at 37°C. It is oxidase-negative, weakly catalase-positive, and cysteine is required for growth. Tularemia is primarily a disease of the orders Lagomorpha and Rodentia, but a wide range of other mammals and several species of birds have also been reported to be infected. Haematophagous arthropods have a substantial role both in the maintenance of F. tularensis in nature and in disease transmission. The disease is characterised by fever, depression and often septicaemia. In humans, there may be ulcers or abscesses at the site of inoculation (this is rarely seen in animals), and swelling of the regional lymph nodes. On post-mortem examination, lesions may include caseous necrosis of lymph nodes and multiple greyish-white foci of necrosis in the spleen, liver, lungs, pericardium, kidneys and other organs. The spleen is usually enlarged in septicaemic cases. It is important to understand that there is a high risk of direct infection of humans by direct contact with this organism. Special precautions, including the wearing of gloves, masks and eyeshields, are therefore recommended when handling infective materials. All laboratory manipulations with live cultures or potentially infected or contaminated material must be performed at an appropriate biosafety and containment level determined by biorisk analysis (see Chapter 1.1.4 Biosafety and biosecurity: Standard for managing biological risk in the veterinary laboratory and animal facilities). Identification of the agent: Polymerase chain reaction is a safe and convenient way for the detection and identification of F. tularensis in clinical samples. The bacterium can be demonstrated in impression smears or in fixed specimens of organs by the fluorescent antibody test or immunohistochemistry. With Gram staining, the bacteria appear as very small punctiform Gram-negative rods, often difficult to distinguish as bacteria. The organism is highly fastidious. For growth it is necessary to use Francis medium, McCoy and Chapin medium, or Modified Thayer-Martin agar. In certain cases selective medium or mouse inoculation is needed to aid successful isolation. The colonies are small, round and transparent, and do not appear before 48 hours incubation at 37°C. If transportation is necessary, samples should be inoculated into sterile nutrient broth and stored at 4–10°C for a few hours or on dry ice if transit is likely to be prolonged. Serological tests: Serological tests are useful diagnostic aids in human infection, but are of limited value in the more susceptible animal species that usually die before developing antibodies. Epidemiological surveys can be conducted in domestic animals, in relatively resistant species that survive the infection, such as sheep, cattle, pigs, dogs, cats, wild ungulates, foxes and wild boars as these species develop antibodies. Relatively resistant species of rodents and lagomorphs (e.g. European brown hare in Central Europe) can also be used in epidemiological surveys. Requirements for vaccines: The attenuated F. holarctica live vaccine strain (LVS, NCTC 10857) has been used for decades as a tularemia vaccine, especially in laboratory workers handling large volumes of F. tularensis cultures. This vaccine is no longer used because of its overall limited efficacy and concern about reversion to virulence, although a derivative of LVS is still used to immunise people in endemic regions of Russia. Novel vaccines against tularemia are under development but not yet licensed for human or animal use.

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