Shellfish may be the cause of typhoid fever (see ENTERIC FEVER) when contaminated by sewage. Paralytic shellfish poisoning is caused by saxotoxin, which is present in certain planktons which, under unusual conditions, multiply rapidly, giving rise to what are known as ‘red tides’. The toxin then accumulates in mussels, cockles and scallops which feed by filtering plankton. The symptoms are loss of feeling in the hands, tingling of the tongue, weakness of the arms and legs, and difficulty in breathing. (See also FOOD POISONING.)