|Photo by Ryan Somma|
Incidentally, catfish farms also make ideal habitats for the rams-horn snail (Planorbella trivolvis) and they are commonly found at fish farms. These snails also happens to be host to a variety of trematode flukes, some of which happen to infect fish as the next host in their life-cycle. In the study we are looking at today, a group of researchers at Mississipi State University examined rams-horn snails from a catfish farm and found the snails there were shedding at least four different species of trematode flukes, with D. spathans being the most abundant species.
|Photo of D. spathans from the paper|
Other animals that frequent catfish farms are fish-eating birds, and ram-horn snails become infected by D. spathans from the parasite's eggs which are shed in the faeces of such birds carrying the adult fluke - in this case the Double-crested cormorant (Phalacrocorax auritus). The parasite undergoes asexual proliferation inside the snail to produce the larval stages that then go on to infect the catfish. So the cormorant is a key source of the parasite; but it is also a protected species under the Migratory Bird Treaty Act, so getting rid of birds "terminally" was not really an option. They also usually feed at night when no one would be around to try and scare them off.
Therefore, the key to breaking the life-cycle of this parasite lies with finding a way of controlling the population of snails at the farm. Flukes with similar life-cycles are common to fish farms and also cause fish diseases in other parts of the world such as Taiwan, Vietnam, and Finland, therefore this is not a problem that is restricted to the fish farms of United States. Understanding the life-cycle of the parasites and how they use each of their hosts is an important step in figuring out how to control disease outbreaks - whether in aquaculture or other contexts where infectious disease is a major problem.
Griffin MJ, Khoo LH, Quiniou SM, O'Hear MM, Pote LM, Greenway TE, Wise DJ. (2012) Genetic sequence data identifies the cercaria of Drepanocephalus spathans (Digenea: Echinostomatidae), a parasite of the double-crested cormorant (Phalacrocorax auritus), with notes on its pathology in juvenile channel catfish (Ictalurus punctatus). Journal of Parasitology 98: 967-972.
P.S. Speaking of aquaculture, I have a new paper on the global pattern of disease outbreak in aquaculture in Journal of Applied Ecology. It has been selected as the Editor's Choice; you can read a summary of the findings and a link to a free copy of the paper here.