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INTRODUCTION
Because of the implementation of the EU directive 1999/74/EC, the occurrence of Ascaridia galli (A. galli) has dramatically increased within the past decade in commercial laying hens on farms in some European countries (Thapa et al. 2015). This situation has led to a significant increase in frequency of anthelmintic use in laying flocks in several EU countries. Only benzimidazole drugs (BZs), e.g., flubendazole (FLBZ) and fenbendazole (FBZ), have been approved for treatment of A. galli within the EU member states (EMA/42178/2014). In Sweden, the amount of BZs prescribed for poultry has been doubled between 2009 when the drug was first introduced, and 2015; from 19 to 46 kg BZ active substance, without a corresponding increase of the sector/population/industry (Girma, 2016).
A recent attempt to develop an efficient deworming control strategy against A. galli (Tarbiat et al. 2016b ), indicated that the build-up of parasite eggs in a poultry barn can be prevented by early detection of infection followed by repeated anthelmintic treatments at strategic time points. This led to an overall significant lower parasite burden and egg expulsion at flock level but at a cost of more frequent treatment occasions (op cit).
The extensive use of anthelmintics to control helminth infections in livestock is in general known to be a contributing factor to the selection for resistance alleles (Coles, 2005). From the parasitic-disease control standpoint, this poses a growing threat in the absence of other anthelmintic classes with different mode of actions. In nematode parasites of grazing livestock and horses resistance to all major classes of anthelmintics have been reported worldwide, especially for BZs in gastrointestinal nematodes (GIN) of ruminants (Kaplan and Vidyashankar, 2012). For example, it was recently shown that a combination of sub-therapeutic dosages with high treatment frequency lead to rapid selection for BZ resistance in Ostertagia ostertagi of cattle (Knapp-Lawitzke et al. 2015). In view of these results it is of utmost importance to screen for early signs of resistance in A. galli in laying hens.
It has been suggested that treatment of the parasite population, when the proportion of egg and larval stages in the environment (e.g. in refugia) is low, can increase the risk...