Equine Worming Strategy.
Please understand when reading this article that our advice to clients on the subject of worming will vary depending on many factors including the horses age, health status, time of year, stocking density, knowledge of previous worm egg counts, both for an individual animal, and on a particular yard. Our advice is bespoke. We do not advocate a “one size fits all” approach.
Worm burdens cause problems in equine populations worldwide. Clinical problems can include colic,diarrhoea, weight loss, and loss of performance.
There are several types to consider.
1. Large Strongyles
2. Small Strongyles (cyathostomes)
3. Parascaris Equorum
When making decisions about worming your horse there are several things to consider. Firstly, are there things I can do to reduce the number of worm eggs my horse is likely to ingest?
Yes! Lifting droppings from the paddock, at least twice weekly is a very good place to start. The fewer eggs your horse ingests, the fewer eggs can mature inside the horse. Reducing stocking densities also means there is less faecal contamination.
Dessication of the dung will kill most worm eggs. This requires the use of grass harrows to break up the dung and allow the sun and air to dessicate it, again twice weekly.
Co grazing with another species, e.g. sheep can also be useful as the sheep destroy equine worm eggs as they ingest them. (the horses do the same for the sheep).
Not all these measures may be practical, but “poo picking” is the most beneficial, and should be practicable whatever the yard set up.
A pasture can only be regarded as free of worm larvae after a period of 5 months without horses on it.
Secondly, does my horse need treated? A worm egg count is the starting point for deciding this, particularly during the spring,summer and early autumn. A low worm egg count (less than 200epg for adults, less than 100epg for youngsters) should not need treated with a roundwormer.
This situation changes during the late Autumn and Winter months, as many Cyathostomes (small redworms) go into an “encysted” dormant stage within the lining of the caecum and colon. These worms do not produce eggs at this time, giving a negative worm egg count.
Cyathostomes are clinically the most significant of all the roundworms in the UK. In late Winter and Spring they can emerge from the lining of the gut in large numbers at the same time causing severe diarrhoea and sudden weight loss.
For this reason we may advise treatment with a product containing moxidectin or a 5 day course of fenbendazole product during late Autumn or early Winter. Again specific advice would depend on the age of the animal, (younger animals more susceptible), recent worming history, and worm egg counts taken during the summer months.
For example, young horses heavily stocked are far more likely to pick up Cyathostome eggs which subsequently encyst than an adult horse grazing the side of a hill on its own and where Summer worm egg counts have shown there to be a low worm burden.
Pinworms (Oxyuris equi) live inside the anus of the horse, and protrude through the anal ring in order to lay eggs on the skin around this area. This can cause intense signs of anal pruritis, causing the horse to rub its backside and tailhead. The hair is often rubbed out , and can be confused with sweet itch.
These worm eggs are not picked up on egg counts as the eggs are laid outside the body on the skin. A measure of control can be had by washing around the anus, and under the tailhead with warm water. This should remove most of the worm eggs.
Tapeworms are not readily identified on faecal testing. They are often suspected in colic cases, as they tend to congregate at a part of the gut known as the ileo-caecal junction, and can affect gut motility. Testing for this parasite is usually via a blood test, measuring antibody levels. High levels indicate exposure and infection with tapeworms. Generally worming once yearly either in spring or autumn with a product containing praziquantel will suffice, although sometimes twice yearly dosing is necessary.
Despite the plethora of trade names on the market, there are only 3 groups of roundwormers. Resistance to wormers is a huge problem, and unless these products are used correctly with caution, when evidence shows they are necessary, then levels of resistance will continue to rise. There is no sign of any new chemicals coming to market for equines. We need to preserve the drugs we have. Once resistance develops within a worm population it is effectively permanent.
Three main drug classes are licensed for roundworm control in the UK.
1) Benzimidazoles (BZ; eg fenbendazole found in products such as Panacur)
2) Tetrahydropyrimidines (THP; eg pyrantel salts found in products including Strongid P)
3) Macrocyclic Lactones (ML; eg ivermectin found in Panomec or Eqvalan; and moxidectin found in Equest)
All 3 classes have licensed efficacy against ADULT cyathostomes. The ML class also have licensed efficacy against all stages of large strongyles. All three classes have licensed efficacy against most stages of parascaris equorum, and pinworms. None are effective against tapeworms.
Moxidectin (single dose) and fenbendazole administered at the standard dose for 5 consecutive days have licensed efficacy against ENCYSTED cyathostomin larvae.
There are high levels of BZ resistance in cyathostomins in the UK.
Because of the efficacy of moxidectin against encysted larvae, this drug should be reserved for this purpose, because, should resistance develop together with BZ resistance, there are no other options available to control these pathogenic stages.
Anthelmintics effective against tapeworms are
1)Pyrantel (Strongid P) administered at twice the dose recommended for roundworms.
2)Praziquantel, found in products such as Equitape. Also found in combination products such as Equimax (ivermectin and praziquantel).
When worming it is vital to be accurate with weight. Weighing your horse is ideal but a good weigh tape is useful exp for thoroughbred types. Underdosing promotes anthelmintic resistance.
Avoid using the same class of anthelmintic year after year.
The use of “dose and move” (the practice of moving animals following worming to clean, or ungrazed pasture) has been shown to increase selection pressure for anthelmintic resistance.
Do not do this.
Taking a faecal worm egg count 14 days after dosing allows us to monitor the effectiveness of a worming regime. This is referred to as a faecal egg count reduction test. This is particularly relevant where resistance to wormers is suspected.
Remember that 20% of the horses on a yard will carry 80% of the worm burdens. The reasons for this are not well understood, but regular worm egg counts will help identify these animals (they tend to be the same ones), and plot trends. As a rule of thumb worm egg counts should be taken 5 times a year, for example in Feb, May, July, Oct, Dec. This is a moveable feast, and depending on the product previously used, the interval may be longer or shorter.
Special Considerations. New Arrivals
Treat new arrivals to a yard with moxidectin for roundworms incl. encysted cyathostomes, preferably combined with praziquantel for tapeworms. A 2 week post treatment egg count should be performed to ensure that the incoming horse is not introducing resistant worms to the resident population.
Special Considerations. Young stock, Pregnant mares and Stallions
The age at which a wormer can be used depends on the active chemical ingredient for example
Panacur BZ can be used at any age
Equimax ML+praziquantel can be used from 2 weeks of age
Strongid P THP can be used from 4 weeks of age
Eqvalan ML can be used from 4-6 weeks of age
Panomec ML can be used from 6-8 weeks old
Equest ML can be used from 4 months old
Ideally use the same product in the foals as you are using in the mares and slot the foal into your mares’ worm control strategy. All the products listed above can be safely used in pregnant mares and stallions.
At Forth Valley we can do worm egg counts in house. We aim for same day turnaround, and provide holistic impartial advice to you and your yard. We can also provide worm egg sample pots. Should you wish us to create a whole yard worming protocol for your yard, please contact the Surgery on 01786 430387 and one of our qualified team will be happy to discuss your requirements.