Vet advice: Why worming your horse is crucial

Worming expert Blaise Scott-Morris BVSc MRCVS, Veterinary Advisor at Virbac, gives an overview of the parasites affecting horses and how we can assist owners throughout the year to keep their horses happy and healthy.

Advising on worming regimes can be complicated especially as there is not a ‘one size fits all’ solution. The challenges include ensuring an accurate estimation of weight trying to ensure the horse receives the full dose it is prescribed and combating increasing resistance to worming treatments.

Worms of importance

The importance of different equine gastrointestinal parasites varies with a horse’s age the time of year and the parasites migratory pathway within the horse. Cyathostomins commonly known as small redworms (figure 1) are considered one of the most important equine parasites worldwide and a particularly important parasite to consider in the spring.

After ingestion the larval stages of the parasite become encysted once they reach the large intestine. Up to 90% of encysted larval cyathostomins may become ‘inhibited’ in this encysted state and can live in the large intestinal wall for up to 2 years. In the spring these larval stages emerge en masse causing severe damage to the intestinal wall resulting in diarrhoea colic and possibly even death.

Worms with migratory pathways such as Parascaris equorum commonly known as roundworms (figure 2) can contribute to gastrointestinal and non-gastrointestinal signs.  P. equorum migrates via the liver and lungs causing signs such as coughing and nasal discharge.  The eggs are extremely resistant to external environmental conditions so can sustain the cycle from one year to the next through multiple batches of foals.  Infected foals can pass millions of eggs daily. As foals grow the volume of larvae that reach the small intestine and the number of eggs produced dramatically decreases conferring good age immunity after approximately the first year of life (2 6).  P equorum infection in young horses can be significant causing ill thrift poor growth weight loss colic and intestinal impaction or perforation leading to death (figure 3).

Intestinal parasites such as Anoplocephala perfoliata (tapeworms) and Strongyles (redworms) (figure 4) are commonly an underlying cause of colic in adult horses.  Parasites cause colic through damage at their site of attachment or migration either to blood vessels or intestinal mucosa and furthermore physical blockage of the intestine which may lead to an impaction or in the worst case intestinal rupture.

Oxyuris equi more commonly known as pinworm can be particularly challenging to control due to its location within the gastrointestinal tract.  This parasite has a unique lifecycle living in the caecum and large colon.  Female worms then migrate a short distance to the rectum stick their back ends out and lay eggs in clumps on the perianal region.  There is significant perianal irritation and the eggs can be seen as yellow white gelatinous streaks.  Most commonly owners initially see their horse persistently rubbing their back end on objects such as fences causing a ‘rat-tail’ appearance.

Risk factors and diagnosis

A horse’s lifestyle should be assessed for factors which would place it in a high or low risk category when it comes to exposure to worms and subsequent worming strategy.

High risk situations include horses on larger yards and sharing grazing especially if there is a high turnover of horses with unknown worming histories.  If all horses on the yard can be wormed at the same time with the same wormer strategic dosing can be used with any new animals on the yard wormed immediately and not turned out for 24 hours unless the worming history is unknown and full worming may induce further problems.

In this case a more targeted approach may be warranted which avoids killing all the worms at once as this can lead to further complications for the horse.  If cooperation on a yard is not possible then interval dosing or targeted strategic dosing are both options depending on owner commitment.  Most importantly ensure there is a strategy of some sort in place! Other risk factors would include horses that travel to shows a lot or compete if they have access to grass or different forage.

On smaller yards or those with a stable horse population targeted strategic dosing may be more easily achievable.  This involves determining the level of parasite infection a horse has.  There are several ways of performing this: a faecal worm egg count (FWEC) such as the FECPAK system (Techion Group Ltd) can be used to assess the roundworm burden during the summer grazing season.  The standard threshold for retreatment on a FEC is a minimum of 200 eggs/g; retreatment is necessary to keep a low level of pasture re-infectivity and continue to decrease parasite burden in the herd (1). Tapeworms can be diagnosed via a blood or saliva test.  The saliva test can be performed quickly and simply by the owner and then returned in the post to a lab who will analyse the sample and email the result.

The targeted strategic dosing strategy also supports the role of refugia in response to resistance.  By leaving some worms in refugia i.e. unexposed to wormer treatments you dilute the population of worms which will develop resistance.  This can be achieved by using FWEC to decide which horses are treated therefore limiting the number of horses treated using a wormer that only kills adult worms thereby leaving the young emerged larvae essentially in refugia (after assessing the risk of mass emergence on a case by case basis) or managing the larval numbers on pasture via poo picking.

Direction – Worming strategy options

There are several options to consider when advising on worming in different situations as outlined above.  Depending upon how and where the horse or horses are kept.

Dosage – Treatment Considerations

It is vital to ensure horses are receiving the correct amount of wormer for their weight however in reality this presents as problem.  When a horse’s weight is estimated on average the estimate is approximately 20% below the actual weight.  Also a horse’s weight will fluctuate considerably at different times of year depending upon its workload so the same weight should not be assumed all year around.

Delivery – Treatment considerations

A second factor to consider is the importance of a horse receiving the full dose of wormer.  In each syringe of wormer is approximately one or two teaspoons of paste containing the total dose of the active ingredient.  Any ‘spit-out’ can represent a significant volume of the overall dose being lost and horses not receiving the correct dose.  Under-dosing has two consequences firstly the product will not work as it should leaving customers dissatisfied; and secondly it can contribute to the rapid development of resistant worms.

There are now more options when considering how to dose a horse for worming. In addition to a traditional syringe of paste tablets are also available.  Tablet wormers such as Equimax Tabs® or Eraquell Tabs® (Virbac) allow the flexibility of wormers being given as a treat with a treat or mixed in with the food.  This presentation aims to address the issue of ‘spit-out’ giving flexibility to owners with horses who are more challenging to worm.

General pasture management such as poo picking is important in the control of all equine intestinal parasites alongside a well implemented and controlled worming strategy.  For other worms such as Oxyuris equi the most important measure is management not worming strategy.  Ensuring owner awareness and vigilance will ensure horses with a possible infection are spotted early.  This can then be addressed through hygiene measures in the environment and washing of the horses back end.

In conclusion a few simple steps can be taken to make advising on worming a stress free experience for both you and your client.  Ensure that the weight of the horse is up to date and accurate establish what worming strategy will best fit their lifestyle speak to the owner about how they think their horse would best take a wormer and advise on an appropriate preparation.

Biography

Blaise Scott-Morris BVSc MRCVS qualified at Bristol in 2011.  For three and a half years Blaise worked in Small Animal Practice for both independent and corporate practices developing a keen interest in client communication and practice profitability.  In October 2014 Blaise joined Virbac as a Veterinary Advisor providing technical support and training both internally and externally and participating in pharmacovigilance.