Studies on the ecology of Austrobilharzia terrigalensis (Trematoda:Schistosomatidae) in the Swan Estuary, Western Australia

Christopher Charles Appleton

Velacumantus australis is the snail intermediate host for the avian schistosome Austrobilharzia terrigalensis in the Swan estuary. This snail lives for two and a half to three years and breeds during its third summer. It is a dominant member of the estuarine benthos, particularly in the belt of Halophila ovalis which grows on the terrace. It occurs here at a mean density of 14.75 snails per 0.25m2. The transmission of A. terrigalensis in the Swan estuary is a seasonal phenomenon. The drastic seasonal fluctuation in salinity is shown to influence profoundly both the activity of V. australis and the emergence of A. terrigalensis cercariae from the snail. When the salinity falls below approximately 15°/oo in winter, V. australis becomes dormant and cercariae no longer emerge for the three to four month duration of the fresh-water flush. There is evidence that during this period, the development of cercariae within the sporocysts is suppressed. With the return of the saline water, usually in late spring, V. australis resumes activity and at the salinities above 25 ° / oo that generally prevail at this time of year, A. terrigalensis cercariae are able to emerge again. At this time, too, there is evidence that the number of gull definitive hosts (Larus novaehollandiae), having direct contact with the water, increases by more than 100% over the winter number. Coincident with the return of the gulls is the arrival of migratory waders predominantly at the sandflat areas of Alfred Cove, Como and Pelican Point. Although no waders from the Swan estuary have been examined for A. terrigalensis infection, various migratory species have been found infected in other parts of the world. The influence of water temperature on the transmission of A. terrigalensis is less clear. There is no doubt that, within the range of temperatures normally encountered in the estuary, cercarial output from V. australis decreases with decreasing temperature and that this effect is superimposed upon that due to the decreasing salinity which occurs at approximately the same time. Very few cercariae emerged at temperatures below 15°C, even at a salinity of 35°/oo. These data predict a seasonal break in transmission, at least from snail to gull, of up to 15 weeks in winter (depending on the distance from the mouth of the estuary). Approximately 2% of the V. australis population in the Swan estuary is infected with A. terrigalensis. These infected snails occur within approximately 65m of the shoreline at an average density for the terrace as a whole of 0.4 per 0.25m2. The majority of infected V. australis are sexually mature or spent adults. They are more common in summer than in winter. A. terrigalensis can, however, only develop successfully in prior-infected V. australis and the density of prior-infected snails is therefore important. The present data suggest that, in the Swan estuary, the critical density of such snails, below which A. terrigalensis does not become established, is approximately three prior-infected snails per 0.25m2 . The prepatent period for A. terrigalensis development in V. australis is not known but daughter sporocysts lodge between the lobules of the digestive gland, gonad and in the connective tissue around the rectum and kidney in the mantle cavity. Cercariae emerge from infected V. australis according to a daily rhythm comprising a major peak between 08.00h and 10.00h and a minor one between 16.00h and 18.00h. This corresponds to the daily water-contact pattern shown by the gulls. The cercariae may live for up to 50 hours, with a half life of 30 hours. Approximately 81% of L. novaehollandiae in the Perth area were infected and harboured an estimated median burden of five pairs of worms. The development of A. terrigalensis in the bird host is rapid. Eggs were recovered from the faeces of infected birds between 11 and 17 days postinfection and were voided according to a diurnal rhythm. The mean egg-output of naturally-infected birds in the Perth metropolitan area was 44.4 eggs/g while at localities further from the Swan estuary this fell to a mean of 22.7 eggs/g. No reliable estimate can be given of the longevity of A. terrigalensis, but few worms are thought to live longer than four months. Peak egg-production was achieved about three weeks after the first eggs appeared in the host's faeces. Paired adult worms generally occurred in the venules of the mesenteric vein draining the duodenum and intestine whi1e most unpaired worms were found in the lungs. The liver and intestine were the organs most heavily involved in avian schistosomiasis due to A. terrigalensis but although typical schistosome-egg granulomata were common in the liver, there was little evidence of hepatic damage. Eggs deposited. in the lungs elicited no histological response while those passing through the gut wall were surrounded mostly by ·heterophils. Schistosome dermatitis due to the penetration of human skin by the cercariae of A. terrigalensis in the Swan estuary was investigated and found to be a condition mainly affecting children. Other aspects of its epidemiology were also defined and an attempt to measure its public health importance suggested that only about 7% of users of the estuary were affected.

Studies on the ecology of Austrobilharzia terrigalensis (Trematoda:Schistosomatidae) in the Swan Estuary, Western Australia

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