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Background  The existence nematodes of veterinary importance such as  Haemonchus contortus  resistant to anthelmintic drugs, including the macrocyclic lactones, has become a major concern in animal health. Macrocyclic lactone resistance in  H. contortus  seems to be multigenic including the active efflux of these drugs by P-glycoproteins, members of the ABC transporter family, present in this parasite. The goals of the present work were to determine the activity of  H. contortus  P-glycoprotein 9.1 ( Hco -PGP-9.1) and its interaction with the avermectins, ivermectin, abamectin, and also the milbemycin, moxidectin. Additionally, the localisation of  Hco -PGP-9.1 was sought in adult worms.    Methods   Hco - Pgp-9.1  was cloned and expressed in mammalian cells and its expression profile was determined at the transcriptional and protein level by qRT-PCR and Western-blot, respectively. The nematode transport activity was assessed using the tracer dye Rhodamine 123. A ligand competition assay between different macrocyclic lactones and Rhodamine 123 was used to establish whether or not there was interaction between  Hco -PGP-9.1 and the avermectins (abamectin and ivermectin) or moxidectin. In addition, immunostaining was carried out to localise  Hco -PGP-9.1 expression in the transgenic cells and in adult female parasites.    Results   Hco -PGP-9.1 was expressed in the cell membrane of the transfected host cells and was able to extrude Rhodamine 123. Ivermectin and abamectin, but not moxidectin, had a pronounced inhibitory effect on the ability of  Hco -PGP-9.1 to transport Rhodamine 123 .  Antibodies raised against  Hco -PGP-9.1 epitopes localised to the uterus of adult female  H. contortus .    Conclusions  These results suggest a strong interaction of the avermectins with  Hco -PGP-9.1. However, possibly due to its physico-chemical properties, moxidectin had markedly less effect on  Hco -PGP-9.1. Because of the greater interaction of the avermectins than moxidectin with this transporter, it is more likely to contribute to avermectin resistance than to moxidectin resistance in  H. contortus . Possible over expression of  Hco -PGP-9.1 in the female reproductive system in resistant worms could reduce paralysis of the uterus by macrocyclic lactones, allowing continued egg release in drug challenged resistant worms.    Electronic supplementary material  The online version of this article (doi:10.1186/s13071-016-1317-8) contains supplementary material, which is available to authorized users.

Characterisation of P-glycoprotein-9.1 in Haemonchus contortus