What is the red bloodworm?
The red bloodworm is a parasite found in pastures where horses graze. Infection occurs when a horse ingests infected larvae from the grass. After ingestion, the larvae end up in the cecum and colon. There, they settle in the intestinal lining and develop into adult worms.
These adult worms then produce eggs that are excreted in the manure, contaminating the pasture again. This keeps the infection cycle going, and the pasture can become reinfected repeatedly.
Bloodworm life cycle outside the horse
The life cycle of small and large bloodworms outside the horse is the same. Bloodworm eggs end up on the pasture through manure, hatch within a few days, and develop into infectious larvae. This development only occurs on pasture and is optimal in warm, moist summer weather. Horses kept in stables or on sand paddocks are not at risk of infection, as the conditions are unsuitable for larval development.
While grazing, the horse ingests infectious larvae. Under cool and moist conditions, these larvae can survive for up to 6 months on pasture! In dry and hot weather, the lifespan of a larva is much shorter. To increase their chances of being ingested, larvae climb up the grass blades in wet conditions. After ingestion, the larvae continue to develop into adult worms inside the horse. This development differs between small and large bloodworms!
Manure testing looks for bloodworm eggs. Since eggs of small and large strongyles cannot be distinguished from one another, the total number of strongyle eggs is reported. Based on this egg count, you will receive advice on whether treatment is necessary.
Life cycle of small bloodworms
There are 50 species described within the group of small bloodworms (small strongyles or Cyathostominae), but only 10–12 are commonly found. Adult small bloodworms live in the cecum and colon of horses of all ages, except newborn foals. After ingestion, larvae immediately penetrate the intestinal wall. After a minimum development period of 6 weeks (the pre-patent period), adult egg-laying worms can be found in the intestines. Larvae may also wander in the intestinal wall for months. Because they keep moving, the immune system struggles to eliminate them, often resulting in an inflamed intestinal wall—especially in young horses.
Small bloodworm larvae have the unique ability to enter a temporary dormant state, encapsulated in the intestinal wall. Whether a larva enters dormancy or develops immediately depends on many factors: horse age, season, immunity, etc. Larvae can remain encysted for up to 3 years, waiting for the chance to emerge. When a signal is received from the gut that adult worms are absent or insufficient, some larvae emerge from the wall into the gut lumen. There, they rapidly develop into adult worms and start laying eggs.
Symptoms and risks: Especially young horses are vulnerable
Symptoms of red bloodworm infection are mainly seen in young horses, as their immune system is not yet fully developed to fight off these parasites. Typical signs of infection include:
– Weight loss and lethargy
– Anemia and protein deficiency
– Loose manure or diarrhea
– Severe cases may lead to shock due to intestinal inflammation
Adult horses can also become infected but usually show fewer visible symptoms. That's why manure testing is important, even in the absence of clear complaints.
The importance of manure testing and deworming in autumn
In autumn, manure testing is an essential part of horse management. It focuses primarily on counting worm eggs, which gives insight into the level of adult worm infection. The test not only reflects the horse’s current worm status, but also the effectiveness of deworming and pasture management over the past season.
It is often advised to perform a preventive broad-spectrum deworming once a year to target encysted bloodworms, bot larvae and tapeworms. Depending on the horse’s age, test results, health, season, housing and filled-in questionnaire, a tailored treatment plan will be provided.
Why broad-spectrum deworming is too early now? Read more in our other blog.
Prevention: How to reduce worm infection in the pasture?
For effective worm management—especially in autumn—the following steps are important:
If you're unsure what to do, please contact us for free, expert advice to ensure you’re doing the right thing for your horse’s health.
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