Low doses of a common anticoagulant drug may help prevent a life-threatening neurological complication of equine herpesvirus type 1 (EHV-1) infection.

Typically, EHV-1 causes only mild clinical signs, but in outbreak situations, an estimated 33 percent of infected horses may develop equine herpesvirus myeloencephalopathy (EHM), which is characterized by damage of the blood vessels in the brain and spinal cord and ensuing neurological deficits.

Vaccines against EHV-1 infections are available, but they do not fully protect against the neurological form of the disease. “Some data are available on antiviral drugs, but mostly limited to the bioavailability of the compounds or on their effectiveness in cell culture,” says Jasmin Walter, DVM, DECAR, of the University of Zurich in Switzerland. “Proper randomized clinical studies are almost impossible to perform under natural outbreak conditions, which makes it hard to obtain scientific evidence for EHM prevention options.” Treatment of EHM is limited to anti-inflammatory medication and supportive care.

Faced with these challenges during an outbreak of EHV-1 at a large breeding facility in southern Germany in the spring of 2009, researchers and clinicians there decided to see whether the drug heparin would prevent EHM.

“The idea of using heparin was brought up early during the outbreak by Kerstin Fey, DVM, DECEIM, an equine internal medicine specialist, and Klaus Osterrieder, DVM, PhD, who specializes on the virology side,” says Walter, who was the veterinarian in charge at the stud at the time.

Heparin is an anticoagulant used to treat diseases associated with abnormal or excessive blood-clotting, such as thrombosis, laminitis or endotoxic0 shock. “EHM has a thromboischemic pathomechanism—meaning it causes blood to clot—which may be inhibited by anti-coagulant drugs like heparin,” explains Walter. Furthermore, in cell culture experiments, heparin and related substances are able to prevent virus entry into cells.

A total of 79 horses were kept in seven different barns at the German farm where the EHV-1 outbreak occurred, and 61 developed at least one clinical sign of infection—most often fever. In the early phase of the outbreak, 30 of the affected horses did not receive heparin, and these horses served as the control group. Thirty-one horses were given heparin as soon as signs of illness appeared.

“I decided to use a very tentative treatment regime with low doses for a short, defined period to avoid possible side effects,” says Walter. “Regular bloodwork in all the horses was not possible to check for critical conditions like anemia0 or thrombocytopenia0, which can arise under heparin medication.” Treated horses received subcutaneous injections of 25,000 IU of heparin twice daily for three days. Horses with only a fever were given no other treatment; those exhibiting neurological signs received anti-inflammatory drugs, as well as antibiotics when necessary to prevent bladder infection due to paralysis.

When the outbreak ended nearly a month later, the researchers reviewed the records of all the horses and found a distinct trend: Only one of the horses treated with heparin developed EHM, while seven of the horses in the control group came down with neurologi-cal disease. The researchers describe these results as “promising” and hope other clinicians try this approach when faced with an EHV-1 outbreak to collect more data.

“To my knowledge no one else has tried this, but I hope our paper will encourage equine clinicians and researchers to work on this topic,” says Walter.

Reference: “Prevention of equine herpesvirus myeloencephalopathy—Is heparin a novel option? A case report,” Tierärztliche Praxis Großtiere, October 2016

This article first appeared in the May 2017 issue (#476) of EQUUS magazine