UK Schistosomatidae: wildlife parasites or zoonotic threat?

Schistosome flukes (Trematoda: Schistosomatidae) are important aetiological agents of human and animal schistosomiasis and cercarial dermatitis. Molecular discrimination of species and hybrids is in progress to investigate geographical diversity and predict emergence of medical and veterinary diseases, but schistosomes are insufficiently studied in the UK. Recent molecular studies on the invasive larval stages (ocellate furcocercariae) and their aquatic snail intermediate hosts have identified four schistosome taxa in the UK: Trichobilharzia franki from Radix auricularia and R. balthica, T. regenti from R. balthica, T. szidati from Lymnaea stagnalis and Gigantobilharzia sp. from Planorbis carinatus. These avian schistosomes can invade humans as accidental hosts and cause cercarial dermatitis, an inflammatory response provoked by antigens of cercariae penetrating the skin. Cercarial dermatitis is manifested by maculo-papulo-vesicular eruptions with oedema and intense itching. Although juvenile worms are known to die in human skin, the possibility of visceral migration has not been excluded. The potential risks of cercarial dermatitis as a zoonotic threat and re-emerging disease are discussed, particularly in relation to leisure and sporting activities such as open water swimming.