Paragonimus westermani infection of freshwater crab Sundathelphusa philippina and melaniid snails in Cadacan River in Irosin, Sorsogon, Philippines

Paragonimiasis, the disease caused by Paragonimus westermani, is transmitted primarily by freshwater crabs Sundathelphusa philippina in the Philippines. Human infection has been recorded, but there is a dearth of published information on the extent of infection in animal reservoirs, particularly in crabs and snails. This study aimed to investigate the infection status and risk factors of P. westermani in freshwater crabs and melaniid snails collected in an endemic village along Cadacan River in Irosin, Sorsogon, where human cases of paragonimiasis were previously reported. A total of 246 freshwater crabs (118 females, 128 males) were dissected, and the gills, muscles, gonads, and viscera were examined for the presence of metacercariae; of which, 41.87% were found infected. The metacercariae were recovered from the gills (100%) and muscle tissues (7.3%) of infected crabs. Male crabs were more likely to be infected (49.22%) than female crabs (33.90%) (p < 0.05). Moreover, 70.87% of crabs showed low parasite intensity levels at ≤ 30 metacercariae/g tissue. A negative weak correlation was observed between parasite intensity and crab weight and carapace length, i.e., highly infected crabs were found to be smaller in size. Meanwhile, only 12% of the 150 melaniid snails collected were positive with cercariae with Tarebia granifera and Jagora asperata as the most infected species. Household survey conducted revealed that some knowledge, attitudes, and practices of the locals contribute to the sustained transmission of the parasite in this endemic area. These findings revealed that P. westermani is still prevalent among intermediate hosts and that some social and environmental factors contributed to the sustained parasite transmission in this endemic community.