Nematode: Philonema agubernaculum
Volume 4, Issue 11
Figure 1. Brook trout female with severe visceral adhesions involving all internal organs including complete liquifactive necrosis of its left ovary and scar tissue preventing spawning of eggs in the right ovary, caused by Philonema agubernaculum. This fish has lost 100% of its reproductive ability.
Landlocked Atlantic salmon and brook trout in some waters are infected by the animal parasite, Philonema agubernaculum. This nematode parasite can cause severe visceral adhesions.
Severe infections bind all the internal organs into a single mass. Affected fish loose the ability to extrude egg/sperm at spawning time and become functionally sterile. Retained eggs/sperm rot in the fish’s abdomen forming a larger liquid abscess (Figure 1).
The life cycle of Philonema agubernaculum is not well documented; however, it is thought that the adult nematode’s eggs are either extruded from the brook trout’s (or salmon’s) gastrointesinal tract and/or coelomic cavity into the water. Eggs are eaten by freshwater crustaceans (copepods). Brook trout become infected directly when they eat infected copepods or indirectly when they eat rainbow smelt that have eaten infected copepods.
Figure 2. Male brook trout with Philonema cysts.
Philonema agubernaculum can be recognized in brook trout and salmon without special equipment. The nematodes cysts are imbedded amongst the adhesions and beneath the mesenteric lining. They are whitish in color, flat and thin, and the round worm is visible through it. The small worm is very delicate and will rupture quickly if placed in water spilling thousands of eggs.
Some species of round worms can be transmitted to humans. They can cause severe intestinal upset. Cook fish to kill the nematodes before eating it.
Special points of interest:
Some nematode can be transmitted to people if the is not cooked before eating.
Philonema is a parasitic animal.
Do not put fish entrails back into the water it can spread this parasite’s eggs.
For more information read: G. Hoffman 1999 Parasites of North American Freshwater Fishes.
Images in this document were made possible by a grant from the Maine Outdoor Heritage Fund.