Killer whales are top predators in most marine ecosystems and impact the populations of common prey, such as seals and sea lions in breeding areas. Killer whales are host to some endoparasites and ectoparasites. They are host to killer whale lice (Cyamus orcini), trematodes (Fasciola skiranini), cestodes (Trigonocotyle spasskyi), and nematodes (Anasakis simplex).

A disease that affects killer whales and is often studied is toxoplasmosis (Toxoplasma gondii). While this parasite is often benign, it can have serious and fatal effects. (Chadwick, 2001; Murata, et al., 2004; Estes, et al., 1998; Heyning and Dahlheim, 1988; Mann, et al., 2000)

Cetaceans include the largest living animals on the planet, but some of the smallest can be found living on and in whales, dolphins, and porpoises. Small crustaceans called whale lice live in folds and rough patches of their skin, as well as in their lesions and nostrils. Most whale louse species have an exclusive corresponding cetacean species that acts as the host for their entire life cycle. Male and female sperm whales even have different unique lice species that they host. 

Barnacles mooch off of whales in a commensal relationship—hitching a ride on their backs. The barnacles don’t harm the whale but take advantage of the moving home that brings meals directly to them. Remora fish also take advantage of the ride, attaching to the large mammals with a modified fin. They don’t seem to harm cetaceans—research indicates that they likely help whales remove other external parasites, although any substantial benefit remains unclear. Some seabird and fish species that follow whales take advantage of these leftovers. 

A large diversity of internal parasites also infest cetaceans. A variety of parasitic worm species can be found in the blubber, intestines, stomach, liver, and lungs of whales. One roundworm species lives in the placenta of sperm whales and reaches lengths of over eight meters, and pygmy sperm whales host a similarly long roundworm in the tissue under the whale’s skin.