ClinQuiz: Equine parasites


The answers, with complete explanations, are provided.

Figure 1


1) A training facility for Thoroughbreds has experienced an increasing incidence of colic over a four-year period. In the last two months, ileal impaction has been diagnosed in three horses at surgery. Centrifugal flotation of fecal samples collected from 25 horses at this facility revealed these eggs (see Figure 1) in 14 of the horses. What parasite is this?

A. Anoplocephala perfoliata

B. Strongylus vulgaris

C. Eimeria leukarti

2) Numerous ova of Parascaris equorum, the equine ascarid, were identified on centrifugal flotation of a fecal sample collected from a 4-month-old Standardbred foal. The farm manager reports that this foal has been dewormed with ivermectin or moxidectin every month since the first month of life; the most recent deworming was performed 21 days before this fecal sample was collected. Which explanation may account for the presence of these eggs, given the deworming history provided?

A. The Parascaris equorum nemotodes in this horse are resistant to macrocyclic lactones.

B. In the three weeks since its last deworming, the foal has been reinfected with Parascaris equorum and adult worms have developed.

C. Parascaris equorum larvae, acquired transplacentally from the mare, have matured.

Figure 2

3) A 4-year-old American Quarter Horse gelding in Arizona developed acute otic irritation accompanied by head shaking. Examination revealed several of these ticks (see Figure 2) in the external ear canal of both ears. What are these ticks?

A. Dermacentor nitens, the tropical horse tick

B. Otobius megnini, the spinose ear tick

C. Amblyomma maculatum, the Gulf Coast tick

4) Three horses at a stable that boards a total of 16 Arabians and Thoroughbreds developed acute diarrhea, fever, depression, and anorexia. PCR testing of whole blood and fecal samples from each horse confirmed a diagnosis of Potomac horse fever caused by Neorickettsia (Ehrlichia) risticii. How did the horses most likely become infected with this pathogen?

A. The horses were bitten by Dermacentor species ticks.

B. Infection occurred through mechanical transmission from Tabanus species flies.

C. The horses ingested infected trematodes within aquatic insects such as addisflies or damselflies.

Figure 3

5) Four of 16 well-cared-for adult Percheron carriage horses from a commercial stable in Charleston, S.C., developed apparent anal pruritus. On examination, the affected horses had broken tail hairs and minor excoriations from rubbing against support posts in their stalls. A centrifugal fecal flotation revealed these parasite eggs (see Figure 3). What is your diagnosis?

A. Small Strongyle infection

B. Oxyuris equi infection

C. Parascaris equorum infection

Susan E. Little, DVM, PhD, DEVPC

This quiz was provided by Susan E. Little, DVM, PhD, DEVPC, Department of Veterinary Pathobiology, Center for Veterinary Health Sciences, Oklahoma State University, Stillwater, OK 74078.

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