R. equi
Breeders should be on the lookout for R. equi in foals, which can quickly sicken the youngsters. can strike foals. • QHN File Photo.

Health Matters: The Ubiquitous R. equi

As the weather heats up and the land dries out from whatever spring rain you did or did not get, be careful to stop and listen when you go visit the broodmare pasture. Have an ear tuned to pick up the presence of subtle coughs while the foals are trying to sleep in the shade of their momma’s bellies.

By now, most foals are a few months old and getting stronger by the day. But while they grow, they can also be growing Rhodococcus equi (R. equi ) bacteria within their lung tissue. It’s easily the most common cause of severe pneumonia in foals from 1 to 3 months of age; however, the bigger, stronger foals can be affected just as easily.

R. equi is an intracellular bacterium found the world over, with a heavy presence in the soil. Being an intracellular bacterium makes it harder to treat, just as its ubiquitous presence in the ground makes it easy to pick up. This makes it an endemic pathogen on many breeding farms — meaning the bacteria is widespread in the ground at the facility — but if you raise even one foal, you are not exempt from the likelihood of infection.

R. equi has a significant economic influence on the horse industry, as it can impact growth rates and the viability of producing strong, healthy foals. The upfront costs to the mare/foal owner are felt on day one as the nature and severity of the pneumoniae it produces requires out-of-the-ordinary commitment in time and veterinary care to clear the lung tissue of infection.

Initially, R. equi foals do not look much different than other snotty-nosed, coughing foals.  They may have a tad higher fever — say 104-ish F — but at first sight, they cannot be distinguished from a foal with pneumoniae caused by Streptococcus spp., Klebsiella pneumoniae, Actinobacillus, or E. coli.

A thorough physical exam of the foal with a complete history of their short life is the cornerstone to building our diagnostics and treatment plan. Progressing off that, we evaluate the total and relative percentages of the foal’s white blood cell count and serum amyloid A (SAA) value. These objective numbers point you toward severity, not necessarily specificity.

Ultrasound examination of the foal’s lungs give you a good, internal look for abscesses forming in the lung tissue, which bacteria not limited to R. equi can generate. Preferentially, your vet will likely want to perform a trans-tracheal wash (TTW) to obtain a fluid sample from the foal’s internal airways. That can then be cultured to specifically identify the organism driving the disease. Now, we can narrow down our list of possible offenders.

Reaching for the biggest gun out of the cabinet to empirically treat foal pneumoniae is a matter of preference among vets. The standard of care in R. equi cases is to treat with an oral drug like azithromycin (like in a Z-pack) or clarithromycin, which are both macrolide antibiotics. The macrolide selected is combined with rifampin to improve lung tissue penetration of the antibiotic. Ideally, the culture result from the TTW will yield the identity of the bacteria, as well as which antibiotic is most effective in treating the infection.

On the plus side, these are solid choices to treat pneumoniae. On the other hand, they possess the potential to induce a merciless diarrhea and hyperthermia. These complications do not derail therapy completely, but they must be addressed and worked around with other medication choices.

Skipping ahead, I will tell you that prevention and screening are the lynch pins to successfully raising healthy foals, as well as retaining your sanity and financial solvency. Dr. Noah Cohen is the leading equine veterinary expert on R. equi pneumoniae in foals. His team at Texas A&M University College of Veterinary Medicine collaborated with Harvard Medical School to recently publish a research study proving the efficacy of vaccinating broodmares for R. equi in the prevention of pneumoniae in foals. Foals do not get antibodies through the placenta in utero, so the colostral antibody level in the mare’s first milk must be sufficient for protection.

Plasma transfusions to foals from horses hyper-immunized to R. equi have been historically used with great success in mitigating infections later in life. Many breeders with endemic problems begin screening their foals’ lungs with ultrasound exams at 4, 6 and 8 weeks of age, along with monitoring rectal temperatures and serial white blood cell counts to auspiciously identify subclinical or mild infections prior to the bacteria potentially seeding lung tissue, resulting in large, well encapsulated abscesses.

The single mare owner cannot escape by limiting numbers, either. Horses have the unremarkable ability to carry R. equi in the intestinal tract and routinely pass it in their manure. So, the very normal (although disgusting) habit of foals eating the mare’s manure to establish healthy gut bacterial flora can be yet another inescapable source of infection to the foal that can and should be identified through a fecal bacterial culture of the mare.

Ultimately, the reason screening and prevention is crucial to everyone’s success is due to the extended length of time that is required to clear a foal’s lungs of infection. The intracellular bacteria does an outstanding job of walling itself off from the body’s immune system, thereby making it formidable to penetrate with the antibiotics needed to kill off the infection. Even foals progressing well on therapy require numerous follow-up ultrasound exams to scan and measure the regressing abscesses. Treatment beyond clinical resolution is the norm to ensure your foal grows to its full potential and has the lung capacity to be the athlete it was intended.

This Insights & Opinions column was published in the May 1 issue of Quarter Horse News. To purchase this issue, click here.