By Maria Paz Zuñiga Barrera, DVM, Msc(c), Dipl. Equine Production, WEVA board member.

Rhodococcus equi, a Gram-positive facultative intracellular pathogen, is one of the most common causes of pneumonia in foals 1-6 month of age. While it can cause significant economic losses, researchers are still working to understand how and why it develops and how to prevent it.

Epidemiology

The pathogen is ubiquitous—it can be cultured from the environment of basically all horse farms. Despite that fact, not all farms see clinical cases; disease occurrence ranges from endemic, sporadic, or nonexistent.

R. equi pneumonia is known to develop on farms that have appropriate contagious disease control and prevention measures, as well as those with less stringent biosecurity practices. Thus, R. equi occurrence does not seem to be due to solely poor hygiene or suboptimal management.

Mare and foal density has been positively associated with the R. equi pneumonia development. And, although high soil concentration of virulent R. equi are not correlated with a disease incidence, there is evidence that airborne virulent R. equi concentrations are positively associated with disease development. Still, researchers know that, under identical circumstances, some foals develop disease while others do not. This indicates that host factors are important for clinical sign development.

Anecdotally, some researchers and veterinarians believe certain mares and stallions are more likely to produce foals that develop R. equi more often than others. This could suggest a genetic predisposition for developing R. equi infection.

The bottom line: All this information indicates that R. equi’s epidemiology is complex and more research is needed.

Diagnosis and Management

R. equi foal pneumonia leads to important economic losses due to treatment expenses and foal disease and death. On the farms with endemic disease, the cost associated with the morbidity and mortality can be very high.

The gold standard method for detecting the pathogen is based on culture from fluid obtained via tracheobronchial aspiration and polymerase chain reaction (PCR) to detect the most relevant virulence factor of the R. equi: the VapA gene. In some cases, however, aspirates cannot be collected due to the risk associated to the procedure or the cost.

Often, veterinarians make their diagnosis based on clinical sings of pneumonia, with or without the use of ultrasonographic screening to detect pulmonary abscesses or consolidation.

Another important aspect is the potential for extrapulmonary disorders (EPD, those outside the lungs) that might or might not be associated with the primary clinical pneumonia. Such EPD can include:

• Diarrhea;
• Ulcerative enterotyphlocolitis (small erosions and abscesses in the wall of the small or large intestines;
• Immune-mediated synovitis ( inflammation of the lining of a joint);
• Intra-abdominal lymphadenitis (inflammation in one or more lymph nodes) or abscesses; and
• Uveitis (inflammation within the eye).

For most part the presence of EPD is associated with a poor prognosis.

A variety of antimicrobials have been proven effective against R. equi in vitro (in the laboratory), but not all of them have been effective in vivo (in the live horse). This is because some drugs antimicrobials fail to reach intracellular concentrations high enough to kill the pathogen. A combination of rifampicin and a macrolide has been the treatment of choice since 1980s. Initially, veterinarians used macrolides such as erythromycin; more recently, they’ve turned to clarithromycin or azithromycin.

Macrolide use for subclinical cases, based on ultrasonographic screening, with the goal of preventing clinical cases is a common practice on some farms. However, this practice can lead to the emergence of resistant R. equi strains. At this point veterinarians don’t have therapeutic tools to consistently treat foals infected with resistant strains of R. equi, and there are no standard procedures that allow us to objectively differentiate foals with subclinical lesions that will progress to clinical disease from those that will not.

Prevention

Veterinarians are still working to find an early disease detection, but researchers are making promising steps toward developing a new vaccine to help protect foals against R. equi. Morris Animal Foundation-funded research at two major universities—Texas A&M University (led by Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM) and Harvard Medical School—has revealed that vaccinating pregnant mares at three and six weeks prior to birth protected their newborns from developing foal pneumonia.

In the first study Cohen’s research group found that antibodies against R. equi transferred to the foal via the mare`s colostrum. This is critical because antibodies can’t be transferred from the mare to the fetus through the placenta. In that same study researchers challenged the foals with R. equi and found that the antibodies protect almost all foals born to vaccinated mares from contracting pneumonia.

In a follow-up study, the team administered plasma with high levels of antibodies against R. equi to five foals and standard plasma to four. The hyperimmune plasma proved to be protective—none of the five treated foals developed pneumonia after a challenge with R. equi.

These studies provide promising resources to researchers and veterinarians working to control and ultimately prevent R. equi pneumonia in foals.