Did you know?
Only 20% of horses harbour 80% of worms.
The common practice of deworming all horses at regular intervals using a rotational dewormer without performing diagnostic tests such as faecal egg counts is poor practice and could lead to devastating health consequences for your horse.
Understanding Resistance
Resistance is the ability of the worms inside your horse to survive deworming treatments that were previously effective in killing them. The reason this has come about is because, just like other living organisms, worms are susceptible to selection pressure. The indiscriminate use of dewormers over the past fifty years means that we have been very successful at killing the susceptible worms, but what’s left behind are the resistant worms. These resistant worms then reproduce and the more we worm, the more resistance we get.
We now have resistance to every class of dewormer, but for the first time, we no longer have a new class of dewormer to turn to. This is a scary prospect, and unless we take steps to reduce the frequency of deworming, we will almost certainly see a rise in parasite related diseases such as colic.
Strategic Deworming
The aim of a strategic parasite control program is not to eliminate parasites completely, but rather to reduce them to a safe level to minimize the risk of parasite-related disease. What we are trying to achieve here, is to maintain ‘refugia’. These are worms that have not been exposed to a dewormer in order to dilute the resistant population.
Faecal egg counts are the cornerstone to a strategic deworming program and allow us to identify the low, moderate and high parasite egg shedders. Remember, faecal egg counts are not a correlation of your horse’s worm burden, but rather the shedding status of your horse.
The high shedders are those horses who are responsible for most of the parasite eggs that are shed onto pasture. These horses are typically dewormed more frequently to reduce their egg shedding, whilst those in the low to moderate group are dewormed at less frequent intervals. By targeting deworming use for high shedding horses, the level of infective parasites on pasture is lowered.
When and how?
The more faecal egg counts you perform on your herd, the better. This will allow your veterinarian to characterise the nature of infection within your horses. At a minimum, 3-4 faecal egg counts should be performed per horse, per year. These samples should be collected before you deworm your horse and should be spread out over the calendar year to determine the egg shedding status of individual horses.
Collecting a sample for a faecal egg count is very straightforward:
- Collect samples from each horse within 12hrs of excretion.
- Take multiple samples from at least three faecal balls to generate a sample the size of a tennis ball (40-50g)
- Place in a zip-lock bag, write your horse’s name on the outside, remove all the air and seal.
- Keep sample refrigerated prior to dropping off to your vet.
- The sample should be processed within 5 days of collection.
Limitations of Faecal Egg Count
Faecal egg counts aim to detect strongyle and ascarid eggs. They are not useful for detecting tapeworm, pinworms, bots or encysted cyathostomes. As a result, horses with consistently low or negative faecal egg counts may still benefit from an annual treatment targeting important parasites such as tapeworms and encysted cyathostomes.
Despite the importance of faecal egg counts in a strategic parasite management program, they cannot be used in isolation, whilst ignoring environmental management. Careful pasture management is important to break the lifecycle of parasites and further reduce the reliance on dewormers.
The following tips should be considered on all horse properties:
- Manure should be regularly removed and composted away from grazing areas.
- Cross grazing with cattle of sheep should be used where possible.
- Rest pastures.
- Do not overstock paddocks.
Faecal Egg Count Reduction Test
Another benefit of performing a faecal egg count, is the Faecal Egg Reduction Test (FECRT). FECRTs are currently the only method available for determining whether or not the dewormer you are administering is effective in your horse. To perform this test, a faecal egg count is performed prior to and 14 days after deworming. If there is an insufficient reduction in egg count then treatment failure or resistance is suspected and should be investigated further. Experts now recommend FECRT be performed annually to monitor the resistance status of your herd.
What’s the point?
Millions of tubes of dewormers are being sold worldwide which are killing very few parasites. This is because there are either very few parasites in your horse to begin with, or the dewormer is ineffective due to resistance. As with anything to do with your horse’s health, there is no one-size-fits-all program. The rise in resistant parasites is an alarming trend to veterinarians. Couple this with the lack of new classes of drugs available to treat these parasites and we are potentially facing a catastrophic health crisis. Faecal egg counts offer an evidence-based approach to deworming and should be embraced by all horse owners if they want to avoid the devastating consequences of worm resistance in their horses.
Dr Tania Sundra BSc.(Hons) BVMS MANZCVS (Equine Medicine)
Avon Ridge Equine Veterinary Services, Western Australia
