Despite the constant development in housing and management measures in poultry breeding, chickens kept on deep litter often must be treated for parasitic infections. The present study deals with a field trial to investigate the efficacy of two commonly used anthelmintics as fenbendazole and levamisole against GINs of domestic chickens. Our results indicated there is a reduction efficacy in fenbendazole and levamisole in three Iranian domestic chicken flocks. The result of this study revealed that fenbendazole was a better effective dewormer than levamisole on the three Iranian domestic chicken flocks but it is not significant. Capillaria spp. were the most generally resistant nematodes followed by Trichostrongylus spp., and A.anseris.
Previously, AR in GINs of sheep[8] and horses[14] had been reported from Iran against benzimidazoles compound, but no previous data has been published on the field efficacy of benzimidazole compound in poultry.
The FECRT results of this study should be interpreted carefully as we tested two anthelmintics in domestic chickens the dose rate recommended in articles[15, 16] because various information is available on the therapeutic doses of these two anthelmintics in chickens.
Fenbendazole and levamisole are widely used broad-spectrum anthelminthic drugs in sheep and horse and cattle in Iran. Anthelmintic resistance to BZs and Levamisole was reported in GINs of sheep from Iran [12]. Recently, Mohseni et al. [13] conducted a regional survey to evaluate the prevalence of AR in GINs of sheep in the northeast of Iran and they found an average efficacy of 46% and 44% for albendazole, and levamisole, respectively. Similarly, we found that fenbendazole was more effective (83.7%) than levamisole (71.8%). The reason for the reduction in sensitivity of levamisole in poultry flocks may be due to frequent routine treatments which impose strong selection pressure on worm populations and encourage the development of resistant strains. As only three major chemical groups are currently available for the treatment of gastrointestinal and pulmonary nematodes, it is imperative that their usefulness is conserved for as long as possible.
There are three main helminths that usually affect chickens - these are gapeworms, roundworms, and tapeworms. Fenbendazole and levamisole are generally used for control three species of helminth parasites of chickens: A. galli, Heterakis gallinarum, and Capillaria spp. ,which has no adverse effect on egg -laying or hatching[2]. Of species found in domestic chickens, A. galli is by far the most common that poultry maintained under free-range conditions may be heavily parasitized [20, 21]; hence, control measures such as preventing infections or chemotherapy can improve weight gain and egg production. The results of our study revealed that fenbendazole and levamisole were most successful in eliminating A. galli. It should be noted that evidence of fenbendazole resistance in A. dissimilis, a closely related ascarid of chickens, has been recently reported in turkey with a history of the heavy administration of frequent intervals of fenbendazole [22].
Moreover, H. gallinarum was another most frequent species of helminths in domestic chicken in Iran[20], which fenbendazole and levamisole are highly effective in this species. Additionally, the Syngamus trachea, which is a parasiticnematode worm that infects the tracheas of individual birds, is also common in young and domesticated chickens[23]. It was concluded that fenbendazole and levamisole are suitable for treatingthe important intestinal and tracheal worms of poultry, a dose of 5 mg/kg for three consecutive days, and 16 mg/kg being recommended for use under field conditions, respectively.
On the other hand, among other nematodes, fenbendazole and levamisole do not affect Capillaria spp., which affects many species of wild birds and domesticated poultry, turkeys and ducks[23]. The signs of infection with Capillaria spp. can be challenging to identify due to the chronic nature of infection, and some non-specific signs include intermittent diarrhea, reduced appetite, ill thrift, and weight loss[19]. When Capillaria spp. are present in high enough numbers, it can be fatal to the chicken[23]. Given the ubiquity of Capillaria spp. and its possible economic impact, parasite control is an essential issue for bird health and productivity. Furthermore, Trichostrongylus tenuis, which is a gut nematode found in Iranian domestic chicken has not sensitive to fenbendazole and levamisole. This endoparasite can cause poor condition and reproduction, leading to a condition often called strongylosis or grouse disease[23].
The finding of resistance to fenbendazole and levamisole in Capillaria spp. and Trichostrongylus spp. may have far-reaching consequences for the poultry industry, both in terms of animal welfare and economic impact. Given the results of our study along with the former finding of the suboptimal efficacy of fenbendazole in A. dissimilis [22], it seems likely that resistance may be expected in the helminth parasite of poultry. Subsequently, more extensive scale surveys for resistance are needed to determine the prevalence of anthelminthic resistance in the poultry industry. Furthermore, studies addressing the production costs of drug-resistant Capillaria spp. are required to determine the financial impression on the industry.