Pinworm can be difficult to eliminate. To succeed, treatment needs to target three different areas: the intestine, the skin and the environment.
Unfortunately it seems that normal doses of worming products are not dealing effectively with pinworm so you may need to consult your vet.
Your vet can prescribe careful overdosing with suitable wormers, so that all of the wormer is not absorbed in the foregut and a sufficient dose arrives in the rectum. (3 x ivermectin followed by Panacur guard, has been suggested).
The eggs adhering to the skin are removed by thorough daily cleansing of the tail, anus, hindquarters and any other affected areas with warm, diluted disinfectant.
This should be followed by painting on a mixture of Ivomec with propylene glycol, which adheres to the skin and kills the pinworm larvae.
At present there is no licensed veterinary medication for this purpose so it will need to be individually prescribed by a vet.
Remove all bedding from the stable, power wash the floor and walls. Use a heavy duty disinfectant, allow to lie for an hour before hosing the stable.
Pick up field droppings at least twice weekly.
Do not share grooming brushes. Keep them clean and disinfected.
Article kindly provided by Gil Riley (MRCVS)