Malathion immunotoxicity in the American lobster (Homarus americanus) upon experimental exposure

A lobster die-off reduced the 1999 fall landings in western Long Island Sound by up to more then 99%. The die-off corresponded in time with the application of pesticides for the control of mosquitoes that carried West Nile virus, a new emerging disease in North America at the time. In order to determine the possible implication of pesticide application as a direct cause or contributing factor in the die-off, we studied the effects of experimental exposure to malathion on the health of lobsters. Lobsters were exposed in 20 gallon tanks, and the direct toxicity as well as sub-lethal effects on the immune system were determined. The 96 h LC50 for malathion upon single exposure was 38 microg/l. Malathion degraded rapidly in sea water, with 65-77% lost after 1 day and 83-96% lost after 3 days. Phagocytosis was significantly decreased 3 days after a single exposure to initial water concentrations as low as 5 ppb, when measured water concentrations were as low as 0.55 ppb. Similarly, effects on phagocytosis were observed at 1, 2 and 3 weeks after the initiation of weekly exposures. Cell counts did not differ significantly upon exposure to malathion. Malathion was not detected in muscle and hepatopancreas of exposed lobsters. Evaluation of phagocytosis is a sensitive indicator of subtle sub-lethal effects of malathion, and relatively small concentrations of malathion (6-7 times lower than the LC50) can affect lobster defense mechanisms.