Small vs Large Strongyles in Horses

On the back of dewormer boxes, you may see the list of worms that the drench targets are divided into ‘large strongyles’ and ‘small strongyles.  Is there a difference between them, and do you need to be worried?

There are three species of large strongyle, Strongylus vulgaris, Strongylus equinus and Strongylus edentatus, and many species of small strongyle (Genera Triodontophorus, Trichonema, Cyathostomum, Cylicocyclus, Cylicostephanus, Cylicodontophorus, Gyalocephalus, Oesophagodontus, Craterostomum). And as their names suggest, large strongyles are larger than small strongyles. Large strongyle adults are between 2cm-5cm in length, while small strongyles are between 1.5cm-2.5cm in length.

L - R: S. vulgaris, S. equinus, and S. edentatus. Image from Studzińska et al 2012

I’d like to preface the rest of this piece by saying that for the most part, large strongyles have been almost eradicated from horses in Australia due to the heavy use of broad spectrum dewormers over the past ~30 years. Small strongyles are now the most problematic worm infection in horses, and it is generally assumed that all strongyle infections in horses are small strongyles.

Prior to the use of broad spectrum anthelmintics, large strongyles were the main cause of colic in horses. This was due the migratory phase of their lifecycle, which is not present in small strongyle lifecycles. Apart from the migratory phase, both small and large strongyles have similar lifecycles: adults live in the intestines, lay eggs that pass out in the manure, eggs hatch into larvae, larvae are ingested along with pasture and mature into adults and the cycle starts again.

Small strongyles generally mature straight from larvae into adults within the lumen of the intestine (some may encyst in the walls of the intestine). Large strongyles have a slightly more complex maturation with larvae migrating outside of the intestine throughout the horse’s abdominal cavity before returning back to the intestine as adults.

S. vulgaris is the most pathogenic of the three large strongyles. Larvae penetrate through the intestinal walls and migrate into nearby blood vessels. They travel through the arteries for two weeks before making it to the superior mesenteric artery (the artery responsible for most of the blood supply to the gut) where they stay for four months. They then travel via the arteries back to the intestines to finish their lifecycle as adults. As is the case with most strongyles, it is the larvae that cause the most damage, rather than the adults. While the larvae migrate through the blood vessels, they can cause damage to lining of the vessels, which can stimulate the production of blood clots. It is these clots that can lead to blockages in smaller vessels, leading to thromboembolic colic (colic caused by the lack of blood flow to the gut).

Life cycle of S. vulgaris from https://horses.extension.org/strongyles-in-horses/

S. edentatus and S. equinus are similar in their migratory phases, with larvae of both species migrating from the intestines to the liver and back to the gut as adults. Unlike S. vulgaris, it is the adults that cause more problems due to the feeding habits of adult worms. Adult S. edentatus feed on the mucosal lining of the intestines which leads to intestinal damage and protein and blood loss (leading to symptoms of diarrhoea, fever, weight loss, anorexia, depression). Adult S. equinus suck blood, up to 10ml per worm per day with heavy infections are associated with anaemia.

In terms of management of large vs small strongyles, the biggest difference in in their lifecycle spans. The prepatent period of large strongyles in between 6 to 9 months; that is, it takes that long for eggs to be present in the manure following infection with larvae. Small strongyles, in comparison, can complete their lifecycle in six weeks. The difference in lifecycle length is likely the contributing factor as to why large strongyles have mostly been eradicated from domestic horses with twice plus yearly anthelmintic treatment, while small strongyles remain ubiquitous.

As with all strongyles, differentiation of large and small strongyle infection is not possible via FEC alone due to the similarities between eggs. If it is required to determine if large strongyles are present, further diagnostics, such as larval cultures or DNA based tests are needed. However, at the moment in Australia, no such tests are needed due to the rarity of large strongyles (likely due to our penchant for over-worming). Interestingly, large strongyles are making a return in some parts of Europe where the use of anthelmintics is strictly controlled to limit further development of drug resistance.

Because the prepatent period for large strongyles is so long, and diagnostics require the need for eggs to be present in the manure, diagnosing colic caused by large strongyles is usually based on symptoms and the known prevalence of large strongyles. As mentioned previously, large strongyles are now extremely rare in domestic horses in Australia, so the risk factor for them causing colic is very low.

Apart from their differences in pathology and lifecycle lengths, the overall management and control of large and small strongyles remains the same: FECs, pasture management, diet, use of the same anthelmintics when needed.  However, is it wise to keep in mind that trends occurring the Europe may eventually make their way to Australia as we hopefully move towards strategic worming across the entire Australian herd. As with all worms, the presence of them is not of great concern as long as we know that they are there and can manage them appropriately (hopefully with anthelmintics that are still effective).

References

Tydén, E., Enemark, H. L., Franko, M. A., Höglund, J., & Osterman-Lind, E. (2019). Prevalence of Strongylus vulgaris in horses after ten years of prescription usage of anthelmintics in Sweden. Veterinary parasitology, 276, 100013.

 Nielsen, M. K., Vidyashankar, A. N., Olsen, S. N., Monrad, J., & Thamsborg, S. M. (2012). Strongylus vulgaris associated with usage of selective therapy on Danish horse farms—Is it reemerging?. Veterinary Parasitology, 189(2-4), 260-266.

Kaufmann, J. (2013). Parasitic infections of domestic animals: a diagnostic manual. Birkhäuser.

Studzińska, Maria & Tomczuk, Krzysztof & Demkowska-Kutrzepa, Marta & Szczepaniak, Klaudiusz. (2012). The Strongylidae belonging to Strongylus genus in horses from southeastern Poland. Parasitology research. 111. 1417-21. 10.1007/s00436-012-3087-3.