Barely a month goes by without another report of anthelmintic resistance. The usual culprits are the cyathostomins - the small red worms.Now it seems that the pinworm, Oxyuris equi, might also be developing resistance. Or at least, doubts are being raised about whether anthelmintics are beginning to lose their effectiveness against the parasite. Compared with the cyathostomins, Oxyuris equi is less important as a cause of disease. Generally, the parasite is not considered pathogenic. However, the female worms cause irritation by depositing their eggs on the skin around the anus of the host. This leads to tail rubbing - and may be mistaken for "sweet itch" (insect bite hypersensitivity.) Thus far, it has not been necessary to formulate specific control measures against Oxyuris, as it has always been assumed that routine treatment with modern anthelmintics would be sufficient to control it.However, in a letter to the Veterinary Record, Andy Durham of the Liphook Equine Hospital and Gerald Coles of Bristol Veterinary School mention several anecdotal reports of Oxyuris infection despite recent anthelmintic treatment. In contrast, earlier studies have shown Oxyuris to be susceptible to commonly used anthelmintics.Durham and Coles have received enquiries from clients about horses rubbing their tails - despite having been recently dewormed with macrocyclic lactones. They would like to hear from veterinary surgeons who have encountered similar cases, to assess if this is a widespread problem.They offer to examine, free of charge, sticky tape preparations from suspected cases. Veterinary surgeons wishing to submit a sample are asked to contact Dr Gerald Coles, at the Bristol Vet School.