Fenbendazole resistance in Heterakis gallinarum, the vector of Histomonas meleagridis, on a broiler breeder farm in South Carolina
Authors: Collins James , Jordan Brian , Vidyashankar Anand , Bishop Andrew , Kaplan Ray M. 
Published: March 2022

Abstract
Background: Due to their ubiquity, management of parasites is a common and important factor for profitable production of poultry. Heterakis gallinarum, a cecal nematode, is the most common nematode parasite of poultry. While typically causing no direct pathology, H. gallinarum is the vector of Histomonas meleagridis, a protozoan parasite that causes blackhead disease in poultry. Histomonas meleagridis is highly pathogenic in turkeys, often leading to high mortality within flocks. In contrast, disease caused by H. meleagridis is much less severe in chickens, where it primarily reduces productivity without manifestations of clinical disease. There are no approved treatments for H. meleagridis, making control reliant on control of the helminth vector. In the United State, the benzimidazole anthelmintic fenbendazole (FBZ) is the only drug labeled for treatment of H. gallinarum, whereas flubendazole is approved in several other countries. We were contacted by an industry veterinarian regarding concerns in a broiler-breeder house due to histomoniasis, despite frequent anthelmintic treatments. Since we had recently diagnosed resistance to FBZ in Ascaridia dissimilis, a closely related nematode of turkeys, we were interested to determine if H. gallinarum had also evolved resistance to FBZ.Methods: Heterakis gallinarum eggs were isolated from litter collected from the breeder house and used to infect 108 Cobb 500 chicks. Treatment groups included a non-treated control, a label-dose, and a 2X-label dose of FBZ, with 36 birds per group divided into two replicate pens of 18 birds each. Birds were placed at 1-day post hatch, and at 3 weeks of age were infected with 150 embryonated eggs via oral gavage. Two weeks post infection treated birds were administered a minimum of either a label- or 2X label-dose of FBZ in water for 5 days (SafeGuard® Aquasol, 1mg/kg BW). To increase the likelihood that all birds consumed the full intended dose at a minimum, the dosage was calculated using 1.25 times the average body weight. One-week post treatment, birds were euthanized, ceca removed, and parasites enumerated. Efficacy was calculated by comparing the total numbers of worms recovered from each treatment group to the numbers recovered in the non-treated control group. Results: There were no significant differences in worm numbers recovered from any of the three groups (p-value=0.3426), indicating that both dosage levels of FBZ failed to provide expected levels of efficacy. Conclusions: These data provide strong evidence that H. gallinarum has developed resistance to FBZ on this farm. Consequently, on this farm and any others with FBZ-resistant H. gallinarum, H. meleagridis will be able to cycle through the birds in an unrestricted manner. Further investigation is needed to determine the prevalence of resistance in H. gallinarum on chicken farms, but it is clear this has the potential to have a large-scale economic impact on the poultry industry. These data when viewed together with our recent findings of FBZ resistance in A. dissimilis, suggest that drug resistance in ascarid nematodes may be an important emerging problem on poultry operations. Additionally, drug-resistant poultry ascarids can serve as an important resource for studying drug resistance in the important ascarid of humans, Ascaris lumbricoides.