Pseudaliid lungworms infect the lungs and sinuses of cetaceans. Information on the life cycle and epidemiology of pseudaliids is very scarce and mostly concerns species that infect coastal or inshore cetaceans. Available evidence indicates that some pseudaliids are vertically transmitted to the host, whereas others are acquired via infected prey. We documented pseudaliid infections in an oceanic cetacean, the striped dolphin (Stenella coeruleoalba) in the western Mediterranean, and investigated the possibilities of vertical vs. horizontal transmission and the potential influence of host body size, sex, and season on infection levels. We found two species of lungworm in 87 dolphins that stranded along the Spanish Mediterranean coast between 1987 and 2018. One or two larvae of Stenurus ovatus were found in three adult dolphins. Larger numbers of larvae and adults of Skrjabinalius guevarai were collected in 51 dolphins, including unweaned calves. These observations suggested that Skrjabinalius guevarai could be vertically transmitted. The abundance of Skrjabinalius guevarai increased significantly with host size, which suggested that it could be trophically transmitted, as well, with larger hosts consuming more infected prey. Infection levels peaked in spring, outside of the calving season, which is likely a reflection of a seasonal shift in dolphin diet. In summary, results indicate that Skrjabinalius guevarai was capable of both vertical and horizontal transmission, but future research should be directed at clarifying the potential mechanics behind transmission and intermediate hosts.

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