Each year during the Kester News Hour at the annual American Association of Equine Practitioners Convention, three esteemed veterinarians offer their top picks from current published studies in their respective specialty areas of surgery, reproduction, or internal medicine. This fast-paced research roundup showcases new and interesting results reported by equine scientists from around the world. At the 2017 meeting, held Nov. 17-21 in San Antonio, Texas, Robert MacKay, BVSc, PhD, Dipl. ACVIM, a professor at the University of Florida, in Gainesville, presented his selection of research topics in equine medicine, ranging from respiratory infections to internal parasites. Here’s a closer look at the studies that caught MacKay’s attention in 2017.

SAA, Equine Asthma, and Respiratory Infection

Serum amyloid A (SAA), a protein the body produces in response to inflammation, is undetectable in healthy horses but increases within six to 12 hours of inflammation onset. That means an SAA blood test could potentially indicate a horse is sick even before he shows significant clinical signs of disease. Veterinarians might also use SAA to differentiate horses with infectious respiratory diseases from those with allergy-related equine asthma (which is not contagious). This could help veterinarians monitor equine populations at shows, rodeos, races, or other events to help prevent or track disease outbreaks.

To test this theory, a University of California, Davis (UC Davis), research team set out to measure SAA in both healthy horses and horses with equine asthma, equine herpesvirus-4 (EHV-4), equine influenza (EI), and strangles.

The scientists evaluated 207 horses using a commercial horse-side assay with a recommended cutoff of 50 μg/mL to distinguish between infectious and noninfectious inflammatory conditions.

The researchers found:

  • Only one of 40 healthy horses in the control group had SAA levels above 0 μg/mL, but that horse was well below the cutoff (used to separate healthy from unhealthy horses) at 3.2 μg/mL, MacKay noted;
  • Only two of 30 equine asthma cases had SAA levels above the cut-off;
  • In contrast, 117 of the 129 horses with respiratory infections had levels above the cutoff point.

The SAA levels of horses with viral diseases (such as EHV-4 and EI) and those with bacterial infections (strangles) didn’t differ. “So, SAA isn’t useful in distinguishing between bacterial and viral infections,” MacKay said.

Serum amyloid A levels do, however, distinguish between infectious and noninfectious respiratory disease, he said.

“This is useful for the veterinarian to figure out if the coughing horse is also an infectious horse,” MacKay said.

For more information read The Horse’s full coverage of the study in our article “Comparing SAA Levels in Horses With Respiratory Diseases.”

Viner, M., Mazan, M., Bedenice, D., Mapes, S. and Pusterla, N. (2017) Comparison of Serum Amyloid A in Horses With Infectious and Noninfectious Respiratory Diseases. Journal of Equine Veterinary Science 49, 11-13.

THO Treatment Outcome Review

Temporohyoid osteoarthropathy (THO) is a relatively uncommon cause of equine neurologic disease characterized by facial and vestibulocochlear (related to hearing and balance) nerve deficits. The condition causes head-tilting and ear, face, and muzzle paralysis.

Because THO is rare, research on the disease and patient outcomes is limited. To address that issue a UC Davis-led research team performed a retrospective study looking at the outcomes of 77 horses diagnosed with and treated for THO from 1990 through 2014.

Treatment for THO typically includes either medical management (antibiotics to address infection, non-steroidal anti-inflammatory drugs to fight inflammation, and eye protection to prevent corneal injury) or surgery. Historically, surgical intervention included one of two procedures:

  • Ceratohyoid ostectomy (CHO), involving the ceratohyoid bone, which is closer to the skin than the stylohyoid bone and therefore requires less dissection for a surgeon to reach and remove it.
  • Partial stylohyoid ostectomy (PSHO), which involves removing a section of the stylohyoid bone. For this procedure the surgeon needs to make a larger incision than is required for CHO and perform a deeper dissection to reach the stylohyoid bone.

Because the PSHO is a more invasive procedure, the CHO has become the surgical treatment of choice for THO.

In their study the researchers looked at how many horses in each treatment group showed improvement, as well as how many horses in each group died because of THO:

  • CHO-treated—25 horses (all showed some improvement, one died);
  • PSHO treatment—8 horses (one died; and • Medical treatment—20 horses (55% showed no improvement, 13 [15%] died).

Compared with CHO, study results showed that medical treatment was significantly associated with horses not surviving. The researchers found no significant difference in survival between medically and PSHO-treated horses.

The researchers also found that all CHO-treated horses showed some improvement after surgery, compared to only 45% of medically treated horses. Additionally, 65% of CHO horses returned to previous function, compared to 12.5% of medically treated horses.

“The study validates CHO as the standard of care,” MacKay said.

Espinosa, P., Nieto, J.E., Estell, K.E., Kass, P.H. and Aleman, M. (2017) Outcomes after medical and surgical interventions in horses with temporohyoid osteoarthropathy. Equine Veterinary Journal 49, 770-775.

A New EPM-Related Bug?

Equine protozoal myeloencephalitis (EPM) is a serious neurologic disease that has two widely accepted protozoal causes: Sarcocystis neurona and Neospora hughesi. However, recently Toxoplasma gondii, which causes toxoplasmosis (a disease that’s commonly a concern for pregnant women exposed to litter boxes of infected cats), has caught EPM researchers’ attention.

Scientists at UC Davis hypothesized that T. gondii titers are related to horses showing clinical signs of neurologic disease because, as they noted in the study, “While toxoplasmosis is not commonly considered a clinical disease of equines, previous seroprevalence studies have reported differing background rates of Toxoplasma gondii infection in horses globally.”

For the study, the researchers took blood serum samples during the fall from horses showing neurologic signs related to EPM, as well as control (healthy) horses located in California. In doing so, they found the horses showing neurologic signs were more likely to:

  • Have high T. gondii titers;
  • Be Warmbloods, ponies, or Quarter Horses;
  • Be 1 to 5 years old; and
  • Have lower S. neurona or N. hughesi titers.

The last point, MacKay said, is a surprising finding, and he encouraged study replication.

“However, this could be the first indication of a third EPM disease,” he said.

James, K.E., Smith, W.A., Packham, A.E., Conrad, P.A. and Pusterla, N. (2017) Toxoplasma gondii seroprevalence and association with equine protozoal myeloencephalitis: A case-control study of Californian horses. Veterinary Journal 224, 38-43.

Early EHV-1 Intervention

Equine herpesvirus-1 (EHV-1) is a highly infectious respiratory disease that can spread quickly through equine populations. This is especially true at horse shows and events where unfamiliar horses comingle in close quarters while under stress from travel and competition, which makes them more vulnerable to disease. The virus can also cause a deadly neurologic condition, EHV-1 myeloencephalopathy (EHM), which makes it even more concerning for veterinarians and horse owners.

To prevent EHV-1 disease and deaths, researchers are looking for early intervention approaches that might prevent development of neurologic disease in infected horses, suppress viral shedding, and limit disease spread during outbreaks. In one study out of Oklahoma State University, researchers investigated if a preventive course of valacyclovir hydrochloride (an antiviral drug known by the brand name Valtrex) would be more effective than starting treatment after horses showed clinical signs of the disease.

The study included 18 aged mares divided into treatment groups prior to exposing them to a neuropathogenic strain of EHV-1:

  • Group 1: The controls, which received a placebo treatment;
  • Group 2: Received prophylactic (preventive) treatment with valacyclovir hydrochloride 24 hours prior to EHV-1 exposure; and
  • Group 3: Did not receive valacyclovir hydrochloride treatment until onset of EHV-1 clinical signs (specifically, fever).

The horses in Groups 2 and 3 exhibited significantly reduced levels of nasal shedding of the virus compared to the control group, researchers found. The horses in Groups 2 and 3 also had lesser viral loads And, most significantly, said MacKay, they suffered from less severe ataxia (a lack of coordination typical of EHM). That means treating a horse with valacyclovir hydrochloride either prophylactically in cases of disease threat or known exposure or at onset of fever during an outbreak could potentially prevent the neurologic version of the disease.

“It’s a very promising result that needs to be tested during outbreaks,” MacKay said.

He noted, however, that prophylactic treatment or treatment at first fever didn’t reduce non-neurologic EHV-1 clinical signs.

Maxwell, L.K., Bentz, B.G., Gilliam, L.L., Ritchey, J.W., Pusterla, N., Eberle, R., Holbrook, T.C., McFarlane, D., Rezabek, G.B., Meinkoth, J., Whitfield, C., Goad, C.L. and Allen, G.P. (2017) Efficacy of the early administration of valacyclovir hydrochloride for the treatment of neuropathogenic equine herpesvirus type-1 infection in horses. Am J Vet Res 78, 1126-1139.

Pelleted Fungal Spores for Internal Parasite Control

Researchers in Spain recently looked at feeding parasite-killing fungi spores to reduce fecal egg counts in horses also treated with anthelmintic (deworming) drugs. The randomized-control trial included the following three groups of seven horses:

  • An untreated control group;
  • Treated with ivermectin; and
  • Treated with ivermectin and commercially produced pellets containing spores of parasite-killing fungi.

Researchers followed the horses for 64 weeks, checking fecal egg counts weekly. All group fecal egg count averages were above 500 prior to the study. The control group fecal egg counts stayed above 300 for the study’s duration. Both groups treated with ivermectin had fecal egg counts of 0 for Weeks 2 and 4 of the study. But by Week 4 the ivermectin-only group had returned to above 300, while the ivermectin and spores remained below 300 throughout the study.

The results show that manufacturers can successfully include parasite-killing fungi spores in commercially produced pelleted feeds, and that these can biologically extend the effect of ivermectin.

“(Fungal parasite predators) deserve more research in this country,” MacKay said.

Hernandez, J.A., Arroyo, F.L., Suarez, J., Cazapal-Monteiro, C.F., Romasanta, A., Lopez-Arellano, M.E., Pedreira, J., de Carvalho, L.M.M., Sanchez-Andrade, R., Arias, M.S., de Gives, P.M. and Paz-Silva, A. (2016) Feeding horses with industrially manufactured pellets with fungal spores to promote nematode integrated control. Veterinary Parasitology 229, 37-44.