The Modern Equine Vet
June 2024
Vol 14 Issue 6 2024
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Cover Story | Respiratory

S. Equi Appears to Be Increasing Slightly

By Marie Rosenthal MS

A recent study of the epidemiology of Streptococcus equi subspecies equi found the disease occurs throughout the United States, affects all ages and is prevalent among all breeds. Unfortunately, its prevalence does appear to be increasing, too, according to Camilo Jaramillo-Morales, DVM, MS, DACVIM, a staff veterinarian at the Veterinary Teaching Hospital of the University of California Davis.

“However, we saw decreased likelihood for a horse to test positive to S. equi when the horses were vaccinated,”
Dr. Jaramillo-Morales said at the 69th AAEP Annual Convention held in San Diego.

S. equi is a highly contagious, mucopurulent inflammatory disease of the upper respiratory tract. The pathogen travels through the nasal passages and resides in the guttural pouches and lymph nodes for some time, maybe up to 18 months or longer before causing disease. In addition, there are a “lot of silent carriers,” he said, “which makes it very hard to control and to eradicate this disease.”

Several epidemiological studies have shown the disease is widespread throughout the world. In 2022, Dr. Jaramillo-Morales reported the prevalence was as high as 13.5% in Colombia, while the prevalence in the United States was about 6.4%, according to an earlier study done by Nicola Pusterla, DVM, PhD, DACVIM, DAVCS-Equine, and his colleagues in 2011.

The transportation and mobilization of horses around the world is probably behind its international spread, explained Dr. Jaramillo-Morales, who called S. equi “the most important respiratory diseases in horses worldwide.”

Dr. Jaramillo-Morales and his colleagues, including Dr. Pusterla, wanted to see if S. equi in the United States had changed since the 2011 study. They looked at nasal swabs from 9,409 horses from around the United States with acute onset of fever and respiratory signs to describe the prevalence factors and to determine the effect of vaccination on the detection of S. equi. The samples are part of the ongoing Equine Respiratory Biosurveillance Program between UC Davis School of Veterinary Medicine and Merck Animal Health.

They analyzed respiratory samples provided between March 2008 and December 2020. Veterinarians from 261 practices from across the country provided samples, which were tested by quantitative polymerase chain reaction (qPCR). Most of the practices were in the West (97), but the rest of the country was also represented with 82 practices from the South, 45 from the Midwest and 37 from the East providing samples, he explained.
The veterinarians filled out a questionnaire that asked about the characteristics of the horses, including signalment, and the intended use of the horse and breed. There was a wide variety of uses, including competition, ranch, farm and breeding. Many had been transported within the 14 days before the sampling.

The veterinarians reported clinical signs, which included lethargy, nasal discharge, coughing, ocular discharge, hypoxia or anorexia, as well as mandibular lymphadenopathy and fever.

There were samples from 3,305 male horses, 4,597 female horses, and no sex was reported for 1,507 horses. The samples were from several breeds with 3,341 being Quarterhorses, 1,498 being Warmbloods, 1,015 were Thoroughbreds, and 645 were Arabians. Almost 3,500 were competition horses.

“The attending veterinarian performed a physical examination and obtained a nasal swab wearing disposable gloves, and the samples were stored in a refrigerator and then shipped overnight to our laboratory. After that we did DNA extraction and amplification,” he explained. In addition to looking for S. equi bacteria, they looked for several common viral pathogens that could cause similar symptoms.

Most of the samples tested positive for equine herpes virus-4 (EHV-4) with 10.5% of samples being positive, followed by equine influenza virus (EIV) with 9.7%, and 7.6% were positive for S. equi.

Of the 715 horses positive for S. equi, 32% had a co-infection (226 horses). The most common co-infections were EHV-4 (4.8%), equine rhinovirus B (7%) and EIV (4%), he said.

“The prevalence of S. equi is comparable to what was reported previously, as were the demographic characteristics. And horses vaccinated against S. equi were less likely to test qPCR-positive for S. equi,” he said.

There was no significant difference in the results by sex or breed. One thing that did stick out for them was the age of the horses, however. The horses most likely to test positive to S. equi were middle-aged horses between 8 and 12 years old, he said.

“This makes sense, right?” he asked. “The middle-aged horses are going to be more likely to compete and are going to be in contact with other horses.

“We did look at clinical signs, and there was significant difference for most of them having higher incidences of these signs in the horses that tested qPCR positive: fever, lethargy, anorexia, nasal discharge, cough and ocular discharge. There was no significant difference between the horses testing qPCR negative and positive for distal limb edema,” Dr. Jaramillo-Morales said.

Among the horses that were vaccinated, 91% tested qPCR negative to S. equi; just 0.9% of vaccinated horses tested positive to S. equi.

Earlier studies found that young horses were more likely to test positive for S. equi than older horses, but now it appears to be affecting middle-aged horses. Dr. Jaramillo-Morales speculated that vaccination might be playing a role or perhaps, it’s just more likely that middle-aged horses are traveling more and competing more often, so they are more likely to be exposed.

“In the study that we did in Colombia, we found that every year of age decreased the risk for S. equi, which was a bit contradictory with what we found in the study here in the U.S.,” he said.

Although the disease did occur throughout the year, there was a seasonality with more cases occurring during the winter and the spring.

Dr. Jaramillo-Morales admitted the study had limitations; most importantly, it was not a randomized case selection, rather samples were voluntarily submitted. In addition, the samples were from nasal swabs rather than guttural pouch lavages, which are more specific. In addition, they did not have information about specific time of vaccination and the type of vaccination, so it would be difficult to draw any conclusions about vaccination and infection.

S. equi remains an important infectious disease in horses in the United States, and is probably endemic for this country as well. This pathogen affects all ages, breeds and is more prevalent in middle-aged and competing horses, and that the vaccination against S. equi appears to be protective in this specific horse population.

“The prevalence that we found is similar to the prevalence that had been previously reported. However, it went from 6.4% to 7.6%,” he said, so the prevalence appears to be increasing.

“We are quite far away of controlling or containing this disease, and it’s probably due to the silent carriers,” he said. MeV

For more information:
Jaramillo-Morales C, Barnum JK, Chappel W, et al. Voluntary biosureveillance of Streptococcus equi Supsp. equi in nasal secretions of 9409 equids with upper airway infection in the USA. N Vet Sci. 2023;10(2):78.

Jaramillo-Morales C, Gomez DE, Renaud D, et al. Streptococcus equi culture prevalence, associated risk factors and antimicrobial susceptibility in a horse population from Colombia. Equine Vet Sci. 2022 Apr;111:103890 doi: 10.1016/j/jevs.2022.103890 E pub 2022 Feb 3