The life cycle of both small and large red bloodworms is the same outside the horse. Red bloodworm eggs are excreted in the manure and end up on the pasture. Within a few days, a larva hatches from the egg and develops on the pasture into an infectious larva. This development happens only on grass and is most efficient in warm, moist summer weather. Horses kept on sand paddocks or stabled are unlikely to contract red bloodworm infections, as the conditions there are unfavorable for larval development.
While grazing, horses ingest these infectious larvae. Under cool, damp conditions, the larvae can survive for up to six months on pasture. In dry and warm conditions, their lifespan is much shorter. To increase their chances of being ingested, the larvae crawl up grass blades during wet weather and are then eaten by the horse. Once inside the horse’s body, the larva develops into an adult worm. This internal development differs between large and small red bloodworms.
During manure research, we look for red bloodworm eggs. Since the eggs of large and small strongyles cannot be distinguished microscopically, the total number of strongyle eggs is listed on the results report. Based on this worm egg count, we advise you whether treatment is necessary for your horse.
Small red bloodworms are also known as small strongyles or Cyathostomins. In total, about 50 species of small red bloodworms have been described, of which 10 to 12 are commonly found.
Adult small red bloodworms are found in the large intestine and cecum of horses of all ages, except newborn foals. When infectious larvae are ingested, they immediately penetrate the intestinal lining upon arrival in the large intestine. After a development period of at least 6 weeks (the pre-patent period) in both the intestinal wall and contents, adult worms can be found in the gut. These worms lay eggs, which are then excreted via the manure.
Larvae may also migrate for several months within the intestinal wall. Because they constantly move, the horse's immune system often fails to deal with them effectively. The result is an inflamed intestinal wall, especially in young horses.
Small red bloodworm larvae have the unique ability to enter a type of temporary hibernation by encysting in the intestinal wall. Whether a larva enters this dormant state or immediately develops into an adult worm depends on various factors, including age, season, and the horse’s immune resistance.
The larvae can remain encysted for up to 3 years, waiting for an opportunity to develop into adult worms. When a signal from the gut indicates that few or no adult worms are present, some larvae migrate from the intestinal wall into the lumen, where they rapidly mature into adult worms and begin laying eggs.
Sometimes the migration from the intestinal wall is so massive—often after deworming—that large parts of the large intestine’s mucosal lining become damaged. This damage is caused by larvae pushing their way from the intestinal wall into the gut lumen. Horses affected in this way become ill and develop severe diarrhoea, which often has a fatal outcome.
In these cases, no eggs will be found in the manure, as the adult worms were killed by the previous treatment, while the young larvae are not yet capable of laying eggs. Needless to say, such a situation must be avoided.
If acute diarrhoea caused by worms occurs, we strongly advise you to contact your veterinarian immediately.
Dewormers containing ivermectin are the first choice for treating red bloodworm infections. Since horses up to the age of 5–6 years are especially sensitive to encysted larvae, we recommend that animals in this age group who have grazed on contaminated pastures in autumn be treated preventively with moxidectin in November–December. Other dewormers are not effective against the encysted stages. It's practical to combine this with treatment against tapeworms and bot larvae.
If manure testing shows your horse is infected with red bloodworms, we will provide you with a personalised treatment plan. This ensures your horse’s health remains the priority, without unnecessary deworming. Feel free to contact our team if you have any questions. Our enthusiastic team is happy to help!
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