No Evidence of Levamisole Resistance in Ascaridia Galli on a Freerange Egg Farm in Australia

With the growing popularity of free range production systems, the incidence of helminth infections has increased in commercial poultry farms in Australia. Adequate control can only be maintained by regular application of commercial anthelmintics. Until very recently, levamisole (LEV) and piperazine (PIP) were the only registered chemicals to treat nematode infections in chickens with no published appraisal of their efficacy status since registration. We report the first formal investigation into the efficacy of commercial anthelmintics against chicken ascaridiasis in Australia.

A controlled experiment following international guidelines (Yazwinski et al., 2003) was conducted to evaluate the efficacy of LEV, PIP and fenbendazole (FBZ) plus levamisolepiperazine combination (LEV-PIP), administered via drinking water or as a single oral drench, against an experimental A. galli infection in chickens. The A. galli isolate used in this study was isolated from naturally infected free-range laying hens from a private commercial farm with a history of regular application of LEV. Adult female worms were recovered from hens killed at the time of scheduled depopulation three weeks post flock deworming with LEV and used as source of infective eggs for artificial infection. We therefore defined this isolate as ‘a suspected case of resistance to LEV’. A total of 108 A. galli artificially infected cockerels were randomized into nine experimental groups of 12 birds each, with each treatment administered as a single individual oral dose or in drinking water delivered via nipple drinkers with drip trays over 8 hours each day for 1, 2 and 5 days respectively for LEV, PIP and FBZ (Panacur 25® Sheep drench). Chickens received label-recommended doses of LEV (28 mg/kg), PIP (100 mg/kg) or LEV-PIP co-administered at their full individual dose rates while FBZ was tested off-label at two dose rates (10 mg/kg as a single oral drench or 5 mg/kg in drinking water). Anthelmintic efficacies were estimated by both worm count reduction (WCR %), and excreta egg count reduction (EECR %). Values below 90 and 95% respectively were considered as indicative of loss of efficacy for WCR % and EECR %. Ten days post treatment, the untreated control birds harboured significantly higher (P < 0.0001) worm burdens (7.67±0.91) and excreta egg counts (440±122.2) than all treatment groups with individual oral treatment causing a greater (P = 0.03) reduction than medication via drinking water. The WCR efficacies for the single oral drenches were 97, 89, 94 and 100% respectively for LEV, PIP, FBZ and LEV-PIP, whereas efficacy values of 92, 82, 72 and 93% respectively were recorded for the corresponding treatments applied in drinking water. The EECR values were largely consistent with WCR %.

An overall lower efficacy was observed when the drugs were administered via drinking water. Put together, contrary to our suspicion, this study revealed no evidence of LEV resistance against the test A. galli isolate but a depressed efficacy of PIP which may signal the onset of resistance. The LEV-PIP combination, however, exhibited superior efficacy and may provide an option for control of chicken ascaridiasis. FBZ provided optimum efficacy (94%) as a single oral drench but this was an offusage for target species, dose rate and route of administration for the product used.