Strongylus vulgaris associated with liver damage and neurologic symptoms in a gelding
By Barbara C. Lewis, DVM, MS, DACVP
A six-year-old mixed breed gelding experienced a sudden onset of neurologic signs including a stiff jaw, body tremors, dilated pupils, photophobia, and ataxia. Symptomatic treatment was unsuccessful, and the horse was euthanized after becoming intractable. The attending veterinarian performed a field necropsy and noted icterus, subcutaneous hemorrhages (attributed to trauma), acute pleural hemorrhage, agonal endocardial hemorrhages, hemomelasma ilei and chronic cranial mesenteric arteritis. Selected tissues were submitted in formalin for histopathologic examination at the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL). Ancillary laboratory testing included rabies and Equine Herpes Virus-1, both of which were negative.
Histologically the major changes included severe, diffuse, acute periacinar to centrilobular hepatocellular necrosis and lipidosis and severe, focally extensive, chronic ulcerative and necrotizing mesenteric arteritis with intralesional nematode parasites identified as Strongylus vulgaris. The liver lesion was attributed to either a toxic or ischemic hepatopathy of undetermined etiology. The cause of the clinical signs was attributed to hepatic encephalopathy. Since only fixed tissues were submitted, further toxicologic testing could not be pursued. Of ancillary interest in this case was the finding of cranial mesenteric arteritis with the nematode Strongylus vulgaris embedded in the lesion.
According to the AAEP parasite control guidelines, the commonly used strategies for control of gastrointestinal parasites in adult horses are based on concepts that are more than 40 years old, when S. vulgaris was the most important equine parasitic pathogen. Treatment was aimed at preventing the passage of strongyle eggs that would contaminate the environment. A two-month cycle of treatment proved to be effective in controlling S. vulgaris infections, and the incidence of disease associated with this parasite is now reportedly rare. This is especially due to the introduction in the 1980s of macrolide lactone anthelmintics (ivermectin and its derivatives). The cyathostomins (small strongyles) are now considered to be the primary equine parasite.
In addition, Parascaris equorum is a major parasitic pathogen of foals and weanlings, and Anaplocephephala perfoliata has been implicated as a possible cause of colic. Compared to S. vulgaris, the other named parasites have different life cycles and host–parasite dynamics which complicate parasite control. Anthelmintic resistance is widespread, forcing the development of new treatment strategies to keep horses healthy. The Fecal Egg Count Reduction Test (FECRT) is used to detect high eggs shedders in populations of horses which are then treated and retested in a selective manner. Some studies have suggested that this strategy may result in the reemergence of S. vulgaris as a primary parasite pathogen. The life cycle of S. vulgaris is characterized by a long prepatent period (6-7 months) in which the migrating larvae are undetectable. The conventional method for identification of S. vulgaris in vivo is larval culture from fecal samples, a laborious and time consuming process. A real-time PCR assay has been developed and validated for the detection and semi-quantification of S. vulgaris eggs in fecal samples. Even more recently reported is the isolation of a S. vulgaris antigen, SvSXP, which may be used to detect targeted IgG antibodies against the parasite during the prepatent period.
While generally viewed as a “historical” parasite, veterinarians should be aware of the potential reemergence of S. vulgaris as a cause of equine gastrointestinal disease. Examination of the cranial mesenteric artery for the presence of S. vulgaris especially in cases of colic, should remain as a part of the routine equine necropsy.
For more information about this case, contact Dr. Barb Lewis, veterinary pathologist at the College Station laboratory. To learn more about TVMDL’s test offerings, visit tvmdl.tamu.edu or call 1.888.646.5623.