Seasonal and spatial variation in Salmonella infections, rates in quokkas (Setonix brachyurus) on Rottnest Island, Western Australia, in areas of human contact

A sampling programme to determine the seasonal incidence of Salmonella infections in the marsupial quokka, Setonix brachyurus, on Rottnest Island, was carried out in public contact areas over the period March 1984 to March 1985. The study followed an unusual rise in the frequency of Salmonella adelaide infections in quokkas foraging in the Thompson Bay settlement area and isolations from catchment waters and sewage effluent. A total of 670 tagged quokkas was swabbed in the island's three settlement areas, plus Kingston Barracks, Garden Lake and Lighthouse swamp. A total of 1,139 duplicate swab samples was collected from quokkas and 713 swabs from 244 animals recaptured more than once. Twenty five Salmonella serotypes were identified, including Salmonella javiana and S. typhimurium Phage Type 202 associated with human cases previously traced to the island. Significant differences were found in rates of infection in quokkas and these peaked at approximately 40% in the resident population at Lighthouse swamp, and at 30% in settlement areas during the summer months. Based on population estimates, a minimum of 200 infected animals are likely to frequent the Thompson Bay settlement area during the peak summer vacation period. Infection rates fell below 5% in settlement areas during the winter period but remained around 20% in the Lighthouse Swamp population. The majority of Salmonella infections in recaptured animals was classified as transient and few quokkas showed evidence of a carrier state, or absence of infection throughout the sampling period. Salmonella infections were higher in juvenile quokkas than adults in settlement areas and in adult females at Lighthouse Swamp. The large population density of quokkas foraging nocturnally for food in settlement areas maintained high levels of ground contamination with infected droppings. However, evidence of progress in long-term management strategies to promote natural behaviour in the island's wildlife and ameliorate public health problems is consistent with the absence of cases of salmonellosis traced to the island during the monitoring period and in subsequent years marked by significant increases in visitors to the island.