11/23/2023
Tapeworms (Part One)
We recently took the time to discuss stomach bots. And while completing a different necropsy this past week, I saw tapeworms for the first time and thought it would be a great idea to dive into these parasites next! This is a rather long read so I decided to put it in two parts and I will link them together when they are both up!
The Life Cycle of the Tapeworm
Tapeworms have a unique life cycle where they have not one but two hosts: an intermediate and definitive (final) host. The intermediate host is the oribatid mite that lives in the soil and herbage. This mite which will ingest tapeworm eggs while feeding on manure. Upon ingestion, the outer shell of the egg is mechanically destroyed and the larval development begins until the parasite reaches the final infective stage known as the cysticercoid. Eventually, the oribatid mite is accidentally ingested by the horse during grazing and the tapeworm larvae can be released within the gastrointestinal tract of the horse.
While there are three different species of tapeworm, the most common one is the Anoplocephala perfoliata. This tapeworm makes its home at the ileocecal junction in the gastrointestinal tract, which is the connection between the final portion of the small intestine (ileum) and the initial part of the hindgut, the cecum. Here, the tapeworms can mature to adults and lay eggs, which ultimately leave the body in the manure to perpetuate this life cycle.
How does it impact my horse?
Horses do not develop immunity to tapeworms and can harbor them at any age. And while tapeworms are common and widespread, their prevalence is dependent on housing and climatic conditions. Horses grazing green, lush pasture are at the greatest risk of exposure (this is where mites live!) and are most likely to harbor tapeworms in the fall (at the end of the grazing season). In comparison, horses without pasture access or living in dry and arid regions are unlikely to be exposed to these parasites (Nielsen/AAEP). It is important to note that just because a horse is exposed to tapeworms, it does not mean they harbor the parasite. Rather, the number of horses within a herd infected by tapeworms can vary significantly. But the bottom line is: if your horse has access to pasture, it would not be surprising to learn they have tapeworms.
Considering this and that the most common species of tapeworm persists at the ileocecal junction, a small opening connecting the foregut and hindgut, horses who are diagnosed with an ileocecal colic or ileal impaction are statistically more likely to also have an A. perfoliata infection (Proudman et al., 1998). HOWEVER, the risk of having these types of colic if a horse has tapeworms is unknown as many horses harbor tapeworm infections and do not display any clinical signs or discomfort (Nielsen, 2016). So the takeaway here is: if your horse has an ileocecal colic or ileal impaction, you are more likely to find they also harbor an A. perfoliata infection, but just because your horse has tapeworms, does not mean they will experience this colic.
Part two will focus on how to test your horse for tapeworms and how to combat them so stay tuned!
Dr. DeBoer
Nielsen, M. American Association of Equine Practitioners. https://aaep.org/horsehealth/tapeworms-horses
Proudman, C.J., N.P. French, and A.J. Trees. 1998. Tapeworm infection is a significant risk factor for spasmodic colic and ileal impaction colic in the horse. Equine. Vet. J. 30: 194-199. https://doi.org/10.1111/j.2042-3306.1998.tb04487.x
Nielsen, M.K. 2016. Equine tapeworm infections: Disease, diagnosis and control. Equine Vet. Educ. 28: 388-395. https://doi.org/10.1111/eve.12394