The epidemiology of Salmonella serovars in Tasmania

The incidence of human salmonellosis and the distribution of Salmonella serovars in Tasmania was investigated to provide epidemiological information on Salmonella occurrence in Tasmania. This baseline data was not available previously and could contribute information to Australia-wide surveys of Salmonella and related enteric pathogens and, more specifically, aid in the prevention of enteric diseases in Tasmania. In addition, the relationship between Salmonella and indicator organisms in drinking water was investigated. The incidence of human salmonellosis in Tasmania is 2% in patients with diarrhoea and 0.13% of the general (asymptomatic) population. This is comparable to rates on the Australian mainland and in other developed countries. The disease is most prevalent in summer which is also similar to seasonal distribution patterns elsewhere. However, the distribution of salmonellae in Tasmania is unusual in the prevalence of Salmonella mississippi, a serovar rarely encountered in mainland Australia. This serovar is not particularly invasive and is unremarkable in the age and sex distribution of its human hosts or its seasonal variation. Epidemiological investigation of foods, domestic and wild animals, reticulated and natural waters, sea water and effluent were undertaken. No particular food type was implicated as a major source of Salmonella mississippi. Domestic animals, while having many Salmonella serovars in common with those seen in the human population, are not a significant source of S.mississippi. Human infection is likely to be frequently water-borne as 1.6% of 500 reticulated drinking waters and 8.2% of 250 natural fresh waters contained Salmonella of which 53% were S.mississippi. The peak incidence of Salmonella in water occurs in early summer and precedes the summer maximum of human cases. Salmonella mississippi was isolated from several species of carnivorous and insectivorous mammals and reptiles but not herbivores. Fifty percent of 120 native cats (Dasyurus viverrinus) sampled were infected with Salmonella and S.mississippi comprised 97% of these. There was no apparent seasonal variation of the presence of S.mississippi in native cats. This serovar persisted in native cats for at least 2-3 months while on a Salmonella-free diet. During this time no symptoms or ill effects were apparent. To establish how native cats became infected, components of their diet were tested. Salmonella mississippi was not isolated from the common pasture pests which compose the bulk of their diet. However, it was isolated from 62% of 34 metallic skinks (Niveoscincus metallicus) sampled on which they sometimes prey. This is the commonest of 16 Tasmanian skink species and is endemic to Tasmania and south east Victoria. The reservoir of Salmonella mississippi in Tasmania appears to be the native animal population which contaminates water supplies leading to sporadic human infections.