Great job on submitting a fecal egg count for your horse!
The following information should be used as a guide only. Please contact your regular veterinarian for advice about your specific case.
If your horse has returned a worm egg count of zero, there is no need to deworm at this time and we recommend submitting another sample in three months.
BUT REMEMBER, all adult horses should be dewormed every 12mths with moxidectin or ivermectin, regardless of their worm egg count.
Cutoffs greater than 200-300epg on fecal egg counts were previously used to indicate deworming is required. However, a recent consensus paper indicated that in low risk horses, a cutoff greater than 500epg may be appropriate (Rendle et.al. 2019).
Use our FREE online parasite risk-assessment calculator* to help determine whether your horse requires deworming at this time (for horses between 5-15yo). See below for foal and young horse recommendations.
*This calculator is for adult horses only. It is not validated and should be used as a guide only.
If your horse has returned an egg count greater than 500 we recommend contacting us to perform a fecal egg count reduction test (FECRT) to determine if the dewormer you are using is effective against the worms in your horse.
Remember, the egg count is an indication of the shedding status of your horse. It is not an indication of your horse’s worm burden.
High shedding horses may need deworming more frequently (2-3x per year) whereas the low shedding horses (the majority of horses!) only need deworming once a year. High shedders are responsible for contaminating your pasture with worm larvae that could be ingested by other horses. Fecal egg counts offer important information but should only be used in conjunction with a “risk analysis” of your property/management.
What to do now ?
What you need to be focussing on is doing everything you can to minimise the number of times you deworm your horse. Remember, the emphasis is not to eliminate all worms but rather to prevent them from becoming a threat to your horse’s health. A recent 2020 study also found that reducing the frequency of deworming is not associated with any negative health risks to the horse (Nielsen et.al 2020).
The overuse of dewormers over the past 60yrs has meant that we have been very successful at killing susceptible worms, but what’s left behind are worms that are resistant to the drugs we now have available and cannot be killed by modern dewormers. These resistant worms go on to reproduce which leads to more resistant worms developing in your horse. The more frequently you deworm, the quicker resistance will develop. In the past we have always had new drugs to turn to, but we don’t have any new dewormers available for horses. If we choose to ignore the warning signs and continue to worm horses at regular intervals, we will begin to see more and more horses die from parasite-related colic.
When it comes to deworming, experts now recommend is a diagnostic-led approach which centres around performing fecal egg counts (FEC). FECs are a tool which helps us identify which horses are shedding the most amount of parasite eggs onto pasture. It is important to remember that FEC are not an indication of the worm burden of the horse being tested. They are however, an important tool to limit the exposure of parasites in a given horse population and should always be used in conjunction with a risk analysis.
Pasture management is critical to any parasite management program to minimise exposure to the herd. If undertaken correctly, this will markedly reduce the need for wormer use. Exposure to infective larvae can be limited by regular poo-picking, co-grazing with other species and reducing stocking density. To be effective, poo-picking should be performed as often as possible (ideally a minimum of twice per week). Dose-and-move strategies used to be recommended to remove exposure to parasites, however, this is had inadvertently increased the spread of resistant parasites. Dose and hold strategies are now advised to prevent infection of paddocks with resistant larvae.
Foals and young horses
Anthelmintic resistance has now been reported across all drug classes in Australia and poses a significant concern for owners of youngstock. A diagnostic-led approach to deworming is easily implemented in adult horses but may not be as straightforward for owners of foals and youngstock. Foals and youngstock should be treated very differently to adult horses. Whilst their immature immune system makes them more susceptible to parasite-related disease, they are also the ones who stand to pay the biggest price if resistance develops. As a rule of thumb, fecal egg counts should be performed every 2-3 months in young horses.
Small strongyles (cyathostomins) are considered the most important parasite in horses due to their high prevalence and potential to cause serious disease. When a parasite management program is insufficient, large numbers of encysted, immature larvae may accumulate in the walls of the intestinal tract and emerge in mass numbers resulting in severe illness. Signs of disease include rapid and severe weight loss and diarrhoea. This disease is most common in horses under 6yrs old but it can be seen in any age group. Mortality occurs in approximately 50% of cases. Moxidectin is usually considered effective against encysted larvae and should be administered yearly in young horses, regardless of the fecal egg count result. Adult horses (over 6yo) develop robust immunity to cyathostomins and can tolerate large burdens without developing any clinical disease. It is important to note that moxidectin resistance has recently been reported in the USA, UK and Australia and careful use of this wormer is important to ensure it is effective in high-risk horses.
Ascarids are an important cause of colic in foals, but ascarid impactions can be seen in older horses. Adult ascarids may be up to 25cm long and 4mm in diameter and when present in large numbers, can obstruct the small intestine. This can result in colic symptoms and are usually seen in horses who have recently been dewormed. Some cases may respond to medical treatment, however others require surgery. Resistance of ascarids to dewormers such as ivermectin and moxidectin is common. If ascarids are present on a fecal egg count then a benzimidazole dewormer should be administered.
It is clear that when dealing with young horses, a balance needs to be struck between preventing colic and other parasite-related diseases while minimising the development of resistance. With a diagnostic-driven deworming program and good pasture management, it should be possible to reduce the reliance on dewormers without compromising the health of individual horses.
Whilst the intricate details about parasite management in foals should be discussed with your vet, here are a few general guidelines.
- Foals should receive their first dewormer no earlier than 2-3mths of age – treating before this time is not recommended as there will be very few adult worms present before 2mths. The dewormer should be from the BZ class (eg. Ammo Green).
- Ascarids which commonly affect foals are resistant to ML’s (eg. ivermectin/moxidectin).
- From 4-6mths fecal egg counts should be performed to determine whether you need to target your worming towards ascarids or strongyles.
- Young animals should be placed on the cleanest pasture.
- Foals and youngstock will secrete the highest amount of eggs onto pasture and the paddocks should be suitably rested prior to turning out other horses.
- Blanket deworming of foals every couple of months is irresponsible and should no longer be considered acceptable.
- Regular treatment of foals with macrocyclic lactones (ivermectin/moxidectin) should be avoided. Ascarids are likely to be resistant to these dewormers and overuse will only hasten the development of resistance to cyathostomins.
Implementing sustainable parasite control measures on properties with foals does present some challenges. Working with your veterinarian who has specific knowledge of your property and management practices can help remove some of these complexities. Reducing dewormer use to the minimum amount to prevent disease should always be the goal as owners of young horses have the most to lose if resistance develops on your property.