April 20 - Nasitrema globicephalae

Helpless whales and dolphins stranded on a beach are always a dismaying sight for any animal lover, and the causes of stranding can often be varied and mysterious, but who would have thought a little worm, hidden from view could be a contributing factor?

Nasitrema globicephalae and other species of that genus are trematodes that inhabit the heads and air sinuses of small cetaceans such as dolphins and pilot whales. It is unclear how dolphins become infected by these endoparasitic flukes, though seeing how it is a trematode, it is quite likely the host become infected through eating prey items which contain the larval stages. Nasitrema is definitely not a very well-behaved parasite because once it is inside the host, it tends to roam around a lot, ending up in all kinds of organs it is not supposed to and causing terrible damage in its path. Sometimes Nasitrema ends up in the brain tissue causing massive necrotic lesion and inflammation that can lead to secondary infections.

It is unknown why Nasitrema would migrate to the brain, and even though the worm can develop to full maturity and even produce eggs within the brain (see picture) this does not benefit the parasite in any way as the eggs have no way of leaving the host through the brain tissue. A study found that high percentage of the cetaceans stranded along the Southern Californian coastline were found to harbour massive infestation of Nasitrema, with mature, gravid (egg-bearing) worms in the brain tissue. Because of the injuries this parasite can cause to its host, it has been suggested as a contributing factor to the stranding of small cetaceans.

It must be noted that Nasitrema is not responsible for all or even most strandings. While they are frequently found associated with stranded dolphins and porpoises, the actual role they play in contributing to that outcome is still uncertain within the context of other factors. And fortunately, at least for dolphins in captivity, it has been found that the same anthelminthic drugs used for treating human lung fluke infection can also be use to treat bottlenose dolphins with Nasitrema infection.

References:

Dailey, M.D. and Walker, W. A. (1978). Parasitism as a factor (?) in single strandings of southern California cetaceans. Journal of Parasitology, 64: 593-596.

O'Shea, T.J., Homer, B.L., Greiner, E.C. and Layton, AW (1991). Nasitrema sp.-associated encephalitis in a striped dolphin (Stenella coeruleoalba) stranded in the Gulf of Mexico. Journal of Wildlife Diseases, 27: 706-709.

Contributed by Tommy Leung.
Photo by William Walker, from "Diseases of Marine Animals" Volume 4 edited by Otto Kinne.