Redworm
Small and Large Redworm – Cyathastomins and Strongylus spp
There are two groups of redworm commonly found in the horse- often referred to as small and large strongyles and we’ll discuss them below:
Ask us what treatment we recommend for your horse
The Parasite:
These are the most common parasitic threat to horse health and can be life-changing or threatening if left untreated. Figures suggest that 95% of parasite burdens in horses can be attributed to small redworms.
This parasite’s life-cycle can be as short as 6 weeks, and when combined with their ability to reproduce in large numbers, we can see why worm burdens can increase to very significant levels.
Physically the small redworm is up to 2.5cm long, thin, and usually reddish/reddish brown in colour. Like many parasites when unfed it will appear white in colour. Although classed as small these will be visible to the naked eye in their adult stage.
The unique and worrying attribute of small redworms is their encysted and inhibited stage (see below) where a high risk of damage exists if large numbers of encysted larva emerge. A mass emergence, often in spring time, can result in a massive inflammatory response in the horse that is frequently fatal. If you are dealing with a small redworm infection, especially a persistent one, then recommendation is to seek advice.
Although these actually have the potential to be more damaging to your horse, their numbers have been significantly reduced since the introduction of modern chemical wormers and regimes.
Physically the large redworm is not surprisingly somewhat bigger than the small redworm coming in at up to 5cm long. It’s often a darker red in colour. Again, these are visible to the naked eye.
Details and Treatment
Symptoms of Redworm Infection
Infection with species from either redworm group has some shared clinical signs which include:
- Loss of condition
- Weight loss and anaemia
- Stomach distention
- Staring or dull coat
- Diarrhoea and colic
As you can see, many of these symptoms are generic in nature and it’s almost impossible to diagnose infection on that basis alone.
Long term, small redworm infections can seriously damage the intestinal wall due to their life-cycle stage where larvae will burrow into the gut wall and encyst (encysted redworm). This process can significantly affect the horse’s ability to absorb nutrients from its diet. A severe infestation can be fatal although conversely, it is possible for a horse to appear perfectly normal while carrying a significant burden.
Encysted Redworm
Most small redworm species have a life-cycle stage that causes particular concern for horse health and welfare. Namely when they become encysted within the gut wall of the large intestine. To understand this we need to know a little about the parasite’s life cycle and larval stages.
A simplified life-cycle for the small redworm looks like:
- Adult females lay eggs within the horses intestinal tract which are then passed out via the faeces.
- Whilst on the pasture the eggs hatch into their larval stages and progress through larval stages L1, L2 and finally reach the infective L3 stage
- The tiny L3 larval stages are motile and will move away from the dung and spread within the pasture.
- These larvae are then subsequently coincidentally ingested when the horse eats its pasture forage
- The larva moves through the horse’s digestive system until they reach the intestine where they attach and burrow into the mucosa of the colon and caecum
- At this point the larvae are encysted in the gut and will stay in this state for ca 10-14 days before routinely re-emerging into the intestine.
- However, there is a change in life-cycle in the winter where the encysted larva do not re-emerge but instead become inhibited in this encysted stage. They can remain encysted and inhibited for up to 3 years but generally emerge the following spring
- On emergence, the larvae develop into adult worms within the horse’s large intestine
- There is a high risk of damage if large numbers emerge at once, as can happen with a mass emergence in the spring of encysted and inhibited larval stages.
- Once sexually mature the now adult female worms begin the cycle again.
Using Worm Counts
Faecal worm egg counts are a highly reliable tool in detecting the adult egg laying stages of both small and large redworm. Good news!
So a FEWC/FEC is recommended to be conducted regularly, roughly every 8 –12 weeks across the year. We recommend you record and track all egg counts as they will provide useful information on the overall program and identify particularly susceptible horses or those who carry high burdens.
The faecal count will not distinguish between small and large redworm but present an overall Eggs Per Gram (EPG). Broadly speaking an EPG of less than(<) 200 will not require treatment.
The worming treatment for both small and large adult redworms is the same.
The red flag here is that a routine worm egg count will not show encysted stages of the small redworm life cycle as the larva are not sexually mature so do not lay eggs. The encysted stage occurs most commonly in the autumn/winter months so that when they do emerge it’s onto springtime pasture where they have a higher chance of survival and finding another equine host.
Consequently, horses can have a high redworm burden (encysted) over the winter months which will not be detected by a FEWC. For these reasons, until testing is developed that allows a fully evidence based treatment, then a suitable wormer is recommended to be given once per year in the winter months to guard against the possibility of encysted redworm.
Blood or Antibody tests for small redworm
Due to the shortcomings of the FEWC in identifying encysted stages of the small redworm life-cycle there is an alternative test available although this will require your veterinarians involvement.
With the blood test your vet will take a blood sample and send it to a specialist laboratory for analysis. The test utilises an enzyme-linked immunosorbent assay (ELISA) which detects IgG(T) antibodies specific to three carefully selected cyathostomin antigens, representing the common species as well as all intra-horse stages of the life cycle including the clinically important encysted larval stage.
It’s important to note that not all horses are suitable to test. The Horse Trust have a really useful guide which can found here https://horsetrust.org.uk/wp-content/uploads/2019/12/AustinDavisSmallRedwormGuidelinesFinal2.pdf
Differing FEWC/Egg counts within the same herd ?
Absolutely, and one of the reasons that counts should be conducted on a per horse basis rather than taking an assumed representative sample. Regular counts will help you build up an overall picture of both individual and herd health.
Within a single herd it’s not uncommon to note that 80% of the worms are accounted for by only 20% of the horses. Generally speaking this is down to individual susceptibility due to age (young or old), compromised health status or grazing habits that are favorable for the parasites for example. These animals are known as “high egg shedders” and identifying and managing them accordingly is very beneficial.
Treatment
A large number of wormers are licensed and effective at treating the adult stages of mixed redworm infection. The active ingredient found in these products includes:
- Ivermectin
- Moxidectin
- Pyrantel
- Fenbendazole
Of these active ingredients only moxidectin, and a 5-day or elevated dosage course of fenbendazole, are effective against the encysted stages of small redworm. Always read and adhere to the product label.
Resistance to Wormers
Documented resistance of cyathastome’s to the older chemical classes has been reported- namely Fenbendazole and pyrantel. So wormers containing these actives should only be used after a resistance test has been carried out otherwise efficacy will be severely limited or not at all.
One of the classic early signs of resistance is the egg reappearance time after worming. A reducing time between worming and egg reappearance can point to developing resistance in the parasite population.
For all these reasons treatments should be targeted and strategic.
In order to keep an effective treatment for encysted redworm the use of Moxidectin is recommended in the winter only, with other chemical classes used as necessary across the year.