Triploid Grass Carp Susceptibility and Potential for Disease Transfer when used to Control Aquatic Vegetation in Reservoirs with Avian Vacuolar Myelinopathy

Abstract

Avian vacuolar myelinopathy (AVM) is an often-lethal neurologic disease that affects waterbirds and their avian predators (i.e., bald eagles Haliaeetus leucocephalus) in the southern United States. Feeding trials and field surveys provided evidence that AVM is caused by a toxin-producing, undescribed cyanobacterium (UCB), which grows as an epiphyte on the leaves of submerged aquatic vegetation (SAV). Reservoirs with documented AVM epornitics support dense growth of nonnative SAV. Waterbirds ingest the toxin when feeding on aquatic plants with the epiphytic UCB, and secondary intoxication occurs when raptors consume these birds. Vegetation management has been proposed as a means to reduce waterbird exposure to the putative toxin. We fed aquatic vegetation with and without the UCB to triploid Grass Carp Ctenopharyngodon idella in laboratory and field trials. Only Grass Carp that ingested aquatic vegetation with the UCB developed lesions in the central nervous system. The lesions (viewed using light microscopy) appeared similar to those in birds diagnosed with AVM. Grass Carp that received aquatic vegetation without the UCB were unaffected. Grass Carp tissues from each treatment were fed to domestic chickens Gallus domesticus (an appropriate laboratory model for AVM) in a laboratory trial; the chickens displayed no neurologic signs, and histology revealed a lack of the diagnostic lesions in brain tissues. Results from our trials suggest that (1) triploid Grass Carp are susceptible to the AVM toxin, although no fish mortalities were documented; and (2) the toxin was not accumulated in Grass Carp tissues, and the risk to piscivorous avifauna is likely low. However, a longer exposure time and analysis of sublethal effects may be prudent to further evaluate the efficacy and risk of using triploid Grass Carp to manage aquatic vegetation in a system with frequent AVM outbreaks.

ALTERNATE FOOD-CHAIN TRANSFER OF THE TOXIN LINKED TO AVIAN VACUOLAR MYELINOPATHY AND IMPLICATIONS FOR THE ENDANGERED FLORIDA SNAIL KITE (ROSTRHAMUS SOCIABILIS)

“…In AVM studies in Arkansas, Georgia, and South Carolina, mortality of Bald Eagles and American Coots occurred within 1-5 d of the animals being observed with clinical signs or as quickly as 5 d after being introduced to a disease site (Thomas et al 1998;Rocke et al 2002;Haynie et al 2013). There are also many cases of bald eagle mortalities where AVM was the suspected cause, but never confirmed, because the carcasses were not recovered in time for postmortem examination and diagnosis (Fischer et al 2006).…”

“…There are also many cases of bald eagle mortalities where AVM was the suspected cause, but never confirmed, because the carcasses were not recovered in time for postmortem examination and diagnosis (Fischer et al 2006). Wild animals are stoic when diseased and will often seek heavy cover, making carcass recovery difficult or impossible even when an active monitoring and recovery protocol is in place (Haynie et al 2013).…”

“…While feeding trials indicate that fish and reptiles fed hydrilla collected from AVM sites develop vacuolar lesions, it has not been demonstrated that they can transmit the AVM toxin to their predators (Haynie et al 2013;Mercurio et al 2014). In a field and laboratory study, triploid grass carp (Ctenopharyngodon idella), which are frequently used as a biological control agent for dense SAV, were fed hydrilla from an AVM site.…”

“…In a field and laboratory study, triploid grass carp (Ctenopharyngodon idella), which are frequently used as a biological control agent for dense SAV, were fed hydrilla from an AVM site. The grass carp developed brain lesions, but chickens that consumed those grass carp tissues in the lab did not develop AVM (Haynie et al 2013). Possible transfer of the AVM toxin to birds through herbivorous invertebrates has not been investigated.…”

Experimental Feeding of Hydrilla verticillata Colonized by Stigonematales Cyanobacteria Induces Vacuolar Myelinopathy in Painted Turtles (Chrysemys picta)

“…Previous work showed that grass carp ( Ctenopharyngodon idella ) experimentally fed Hydrilla colonized with the UCB developed vacuolations consistent with avian models, yet domestic pigs ( Sus domesticus ) and laboratory mice did not [5], [13], [14]. To date, there are no reports for any species representing the remaining major vertebrate lineages (amphibians or reptiles).…”