Biological Control of Proliferative Gill Disease in Farm-Raised Catfish

Proliferative gill disease (PGD) in catfish is caused by the myxozoan parasite Henneguya ictaluri. The complex life cycle requires Dero digitata as the oligochaete host. Endemic in most catfish ponds, efforts to control PGD by eradicating the oligochaete host have been unsuccessful. Smallmouth buffalo (Ictiobus bubalus; SMB) are opportunistic bottom feeders and a putative option for the biological control of D. digitata. The study was conducted in 14 ponds that were stocked with 8,000 channel catfish/pond. Catfish in one-half of the ponds in were polycultured with SMB stocked at a rate of 300 SMB/acre.  In the first year of the study, pond muds were sampled regularly monthly for the presence of benthic invertebrates including D. digitata. After the first year ponds stocked with SMB had lower oligochaete numbers compared to ponds without SMB.   Ponds were harvested at the end of the production season and re-stocked similarily.in a similar manner the following year. There were no differences in percent survival, total weight, total feed fed or feed conversion between the mono- and polyculture ponds. In the second and third year of the study, benthic populations were again sampled, pond water was collected for qPCR estimation of H. ictaluri actinospore concentrations and sentinel fish were used to estimate disease severity., in addition to benthic samplin There were no differences in numbers of benthic organisms, actinospore concentrations, or disease severity in sentinel fish among treatments. Under these study conditions, the presence of SMB does not have a measureable effect on PGD incidence, severity of disease or overall catfish production.  As a result SMB are no longer recommended as a biological control for PGD, saving farmers between $200-300/acre on an ineffective treatment.  

Introduction

The commercial catfish industry is the largest aquaculture industry in the United States and is a vital component to the economy of several southern states  Disease related losses account for nearly half of the losses experienced by commercial producers The three most common diseases in catfish aquaculture are bacterial septicemias caused by the gram negative bacterium Edwardsiella ictaluri and Flavobacterium columnare, and proliferative gill disease (PGD) caused by the myxozoan parasite Henneguya ictaluri. Proliferative gill disease is the most prevalent parasitic disease associated with the commercial production of channel catfish.  The complex parasitic life cycle involves a myxospore stage in the channel catfish and an actinospore stage in the ubiquitous benthic oligochaete Dero digitata. Prolonged exposure of catfish to this actinospore stage results in a severe inflammatory response at thesevere inflammation of the gills, dramatically impairing the fish’s ability to carry out key physiological functions, namely osmoregulation, acid/base balance and gas exchange end result isresulting in decreased production and , with mortality rates approaching 100% in some severe outbreaks.  The exact economic impact is unknown but is much greater than what can be attributed to direct losses associated with infection.  Producers report significant losses in production due to poor feed consumption resulting from sublethal infections.  On a 1000 acre farm, a conservative 50% reduction in feed consumption, typically lasting for 21 days between April-May, would result in a loss of fish growth of approximately 840,000 ponds, representing resulting in a potential reduction in gross sales of $1.1 million. Currently there are no effective treatments for the disease and management practices designed to curb disease related losses are associated with reducing additional losses of re-stocked fish following catastrophic losses.

A potential strategy for disease control is through disrupting the parasite life cycle by reducing the oligochaete host in the pond environment.   Several producers in the Mississippi Delta who raise smallmouth buffalo in polyculture with channel catfish have reported a lower incidence of PGD, although these claims are anecdotal and have not been substantiated.  Smallmouth buffalo (SMB), a native species in Mississippi, are omnivorous opportunistic forage feeders with a subterminal mouth, feeding primarily on benthic invertebrates, zooplankton and algae. As such, they are a prime candidate for the biological control of the benthic oligochaete D. digitata.  Previous research has shown polyculture of smallmouth buffalo with channel catfish does not have any measurable effects on water quality, nutrients or phytoplankton communities.  Preliminary data collected from initial experimental trials conducted at our research facility demonstrated a significant reduction in benthic oligochaetes in pond sediments. This suggests smallmouth buffalo have the potential to dramatically reduce oligochaete populations in catfish ponds, indirectly reducing PGD incidence within the system by reducing the oligochaete host.  Since low infestation levels have no measureable effect on fish health and production, SMB do not need to completely eradicate D. digitata from the pond environment to be effective.  As long as oligochaete populations are kept below concentrations that allow for the accumulation of actinospore levels resulting in morbidity, severe outbreaks can be avoided, and significant losses associated with PGD can be deterred.