Tularaemia is a zoonotic disease caused by Francisella tularensis, a gram-negative, aerobic, intracellular bacterial rod. The four subspecies are F. tularensis tularensis (Type A), F. tularensis holartica (Type B), F. tularensis novicida and F. tularensis mediasiatica. These subspecies vary epidemiologically and in virulence , .
Tularaemia is an OIE-listed disease and is currently an emergency animal disease in NSW and Australia. The Biosecurity Act (2015) has tularaemia listed as prohibited matter.
Francisella tularensis has been found in a wide range of species, including mammals (including people), birds, reptiles, amphibians and invertebrates. Tularaemia however, is predominantly a disease of rodents and lagomorphs .
F. tularensis is highly virulent and can be spread by (i) biting arthropods, including ticks and mosquitoes, (ii) direct contact with blood, fluid or tissue of infected animals, (iii) inhalation of infective aerosols, or (iv) ingestion of contaminated water or food. There is no transmission between people . The bacteria is viable in mud and water for several months .
The highly virulent subspecies F. tularensis tularensis is only found in the USA and infection with this subspecies in people may lead to mortalities. The infective dose of this strain is very low with only ten bacteria required subcutaneously and 25 when aerosolised. F. tularensis holartica is found across the northern hemisphere and in Australia and is less virulent than F. tularensis tularensis. F. tularensis mediasiatica has been isolated in central Asia and has similar virulence to F. tularensis holartica .The least virulent subspecies, F. tularensis novicida has not been isolated from animals, but has been implicated in water-borne transmission to people in Australia, Spain and USA , .
Tularaemia has been associated with mass mortality events in animals, such as hares and mink, overseas. It is thought, however, that the main form of spread in the environment overseas is from low levels of the bacteria cycling in rodent populations .
Signs of tularaemia in animals are vague and variable and include acute onset anorexia, depression, vomiting, diarrhoea, pyrexia, ulceration, lymphadenopathy, hepatomegaly and sudden death , , .
Gross pathology reveals splenomegaly with pinpoint pale foci of necrosis in the liver, spleen and bone marrow. The lungs are often congested and lymph nodes may have areas of caseous necrosis. Granulomas may be found in the liver, spleen, lungs, kidneys and bone marrow in chronic cases .
Clinical signs in people usually develop between three to five days post exposure to the organism (range 1 – 21 days) .
Definitive diagnosis of F. tularensis is culture but the organism is slow growing and requires special culture media (except for F. tularensis novicida). F. tularensis infection can also be confirmed by PCR, immunofluorescence or immunohistochemistry. Impression smears of the liver, spleen, lung, kidney or bone marrow show large numbers of very small gram negative organisms .
There have been three reports of Francisella tularensis infections in people in Australia.
The first case was in the Northern Territory in 2003, where a man cut his toe in brackish water and then his toe became infected. A F. tularensis novicida- like organism was identified .
The second case was in Tasmania in 2011 and a woman was scratched by a ringtail possum (Pseudocheirus peregrinus) and diagnosed with F. tularensis holartica.
The third case was also from the same area in Tasmania in 2011 and a woman was bitten by a ringtail possum and subsequently diagnosed with F. tularensis holartica .
All three patients recovered uneventfully with antibiotic treatment.
Despite sampling of ringtail possums in Tasmania after the diagnosis of two human cases in 2011, no evidence of infection or exposure to F. tularensis was found in association with the two human cases .
In September of 2016, tularaemia infection was confirmed in historical samples from two separate clusters of deaths in ringtail possums, from the Sydney basin in 2002 and 2003. The evidence of the tularaemia infection was initially detected using Next Generation sequencing by researchers and was confirmed with PCR testing and culture at the Australian Animal Health Laboratory in Geelong. This is the first diagnosis of tularaemia in an Australian animal. The infective agent in the Sydney ringtail possums was F. tularensis holartica and is very similar to the genomic material identified in the 2011 Australian human cases.
As for all zoonotic disease, when tularaemia is on the differential list of diseases, appropriate infection control measures should be followed. When handling suspect cases cover cuts and abrasions with a waterproof dressing, wear gloves, use sedation or appropriate constrain to minimise risk of scratches and bites when handling animals and observe good hygiene. If a person is bitten or scratched by wildlife direct them to their local health authority.
Animals that are suspected of having tularaemia should not be incised and the whole carcase should be sent to the NSW DPI Laboratory Services for exclusion testing. The carcase should be stored in the fridge and not frozen. Prior to submission of samples please contact the NSW DPI Wildlife Coordinator.
As F. tularensis is highly infectious to people, suspect cases should only be necropsied and cultured in a Biosafety Level 3 facility.
As wildlife is the main source of infection, controlling the disease in the source population is not possible. When human outbreaks have occurred overseas, reducing the numbers of reservoir hosts and vectors has been used, along with good hygiene and infection control precautions .
With the diagnosis of F. tularensis holartica in ringtail possums in Australia, tularaemia should be included in the differential diagnosis list in acute sudden death events in wildlife, particularly possums, rabbit, hares and rodents. Due to the zoonotic potential of this disease, appropriate hygiene and infectious disease control measures should be undertaken when handling wildlife. If people are bitten or scratched by wildlife they should seek assistance from their local health authority. Suspect tularaemia infected animals should be sent whole to NSW DPI Laboratory Services for testing, after consultation with the NSW DPI Wildlife Coordinator.