Pinworms historically were seen mostly in young horses; however, in recent years cases in adult horses appear to be becoming increasingly more common (Reinemeyer and Nielsen, 2014). Pinworm infections tend to be sporadic, and usually only one or a few horses are affected out of a group with outward signs including tail rubbing and hindquarter and/or perineal self-mutilation.
Some adult horses may have pinworm infections without showing any specific outward signs. Pinworm eggs can sometimes be found in Fecal Egg Counts, but as most of the eggs are laid on and around the anus, are often missed as they are not in the manure as are other worm eggs. A pin worm test involves sticking a piece of clear tape to the area around the horse's anus, remove it and then look for pinworm eggs under a microscope.
As a consequence of rubbing, horses can spread pinworm eggs throughout the horse’s environment; transmission can occur in stalls and from contact with grooming materials, tail wraps, fence posts, etc. Washing contact points in hot water with soap and/or disinfectants is recommended. (AAEP Parasite Control Guidelines 2019)
Whilst wormers containing ivermectin/modixdectin (Equimax, Equimec, Evolve etc) are labelled effective against pinworms (O. equi) in horses, apparent resistance to ivermectin has been described in several reports and appears relatively common in documented cases in the US, New Zealand and Germany (Rock et al., 2013; Wolf et al., 2014; Scott et al., 2015; Sallé et al., 2016).
Treatment not effective?So, if you’ve found a pinworm infestation to persist after initial treatment with an ivermectin or moxidectin wormer – it could be due to resistance. The following plan of action may be useful:
- Use a Benzimidasole (BZ’s or ‘asoles’) drug class wormer (Oxfendasole, Oxibendasole, Fenbendasole ie Kelato ‘Revolve’ (Oxfendasole) (Rock et al., 2013; Wolf et al., 2014 ; Reinemeyer C R., 2012; European Scientific Counsel for Companion animal Parasites, March 2019)
- Use a Tetrahydropyrimidine (THPs) drug class wormer (Morantel or Pyrantel) ie Kelato ‘Revolve’ (Morantel Tartrate) (Rock et al., 2013; Wolf et al., 2014 ; Reinemeyer C R., 2012)
- Benzimidasoles should be given priority over Tetrahydropyrimidines due to better historic efficacy levels (Reinemeyer and Nielsen, 2014).
As Kelato Revolve contains both drug classes recommended for persistent pinworms it makes a suitable alternative when a mectin-base wormer proves ineffective.
In persistent, severely affected cases, Mr Edd Knowles, (MA VetMB MVetMed DipECEIM MRCVS, RCVS and European Specialist in Equine Internal Medicine), speaking at the British Equine Veterinary Association (BEVA)’s congress in 2015, suggests to use three doses of Pyrantel Embonate (a Tetrahydropyrimidine) or Benzimidasole base wormer at two 2-3 week intervals.
Kelato Revolve is suitable for the above treatment.