Rhodococcus equi is a bacterium that inhabits the soil and causes pneumonia in 1 to 4 month old foals. Disease does not occur in adult horses unless there is a severe immunodeficiency. R. equi has also rarely caused infections in immunodeficient humans (such as those people with HIV. It is often confused with Rotavirus, a highly contagious virus that causes diarrhea in foals. R. equi may occasionally also cause disease in other body systems. Inflammation of the eye (uveitis), bone infection, inflammation of the joints, diarrhea and abdominal abscess formation are noted infections outside of the lungs.
Epidemiology: R. equi is detectable in most horse farms globally. However, disease prevalence varies region-by-region, farm-to-farm and year-to-year. The mechanisms for this variation of disease incidence are largely poorly understood. Stocking densities, low soil moisture and low pasture heights do appear to contribute to the disease but other factors have not been consistently identified.
The lifecycle of the organism is quite simple. R. equi inhabits the soil and obtains entry into the foal’s respiratory tract primarily by inhalation of airborne bacteria. Entry may also occur through ingestion of soil where the bacteria gains entry to the blood stream and it seeds the lungs. The disease progresses very slowly as large abscesses begin to grow throughout the lung fields. As the infection matures the foals begin to show signs of pneumonia: fever, cough, depression. However, many of the symptoms are quite insidious until the disease has progressed to a life threatening state.
The organism replicates within the respiratory tract and then is coughed up and swallowed by the foal. Replication continues as the bacteria travel though the intestinal tract after which time large amounts of R. equi are defecated and subsequently regain entry into the soil. Although adults are only very rarely infected, it is known that mares will also contaminate the soil by ingesting bacteria and passing larger numbers in their feces. Studies have not shown there to be a relationship between which mares are shedding the bacteria in their feces and which foals become infected with R. equi pneumonia.
Recent research out of Australia has indicated that foals do have significant numbers of R. equi in their breath. Specialized air filters were used to measure bacteria populations exhaled from foals with R. equi pneumonia. Although these findings are very interesting, it has yet to be determined if foals can transmit the disease foal to foal by way of contaminated breath.
Diagnosis: Clinical signs can be characteristic of the disease and include fever, cough, depression, labored breathing and tracheal rattle (noise in the windpipe due to mucus accumulation). Unfortunately, many of these foals do not begin to show symptoms of the infection until the disease has progressed to a severe state. Confirmation of the disease is dependant on accumulating information regarding previous disease on the farm (how likely is this case also to be caused byR. equi), clinical signs, ultrasound examination of the lungs, x-rays of the lungs, culture of the airways and blood work. In some cases, the disease requires less diagnostic evaluation on the part of the veterinarian. This would be true at a farm that has historically had a large percentage of foals develop R. equi pneumonia and where an appropriate aged foal of 1 to 3 months is evaluated and found to have clinical signs or ultrasound findings consistent with R. equi infection. Cases where the prior incidence of the disease is unknown, in small populations of horses or in older foals (>3 months) requires more diagnostic testing to confirm the cause of the pneumonia. Additionally, severely affected foals may also require additional testing as other bacteria may begin an opportunistic infection in the lungs and these may require a supplementary antibiotic approach.
Treatment: For many years R. equi pneumonia carried a very poor to grave prognosis due to the lack of a good treatment protocol. Currently, we have identified antibiotics that are very good at treating the infection. Erythromycin, clarithromycin and azithromycin are individually paired with rifampin to treat foals. While potent and effective, these drug combinations do carry the risk of serious side effects to the mare and foal. Erythromycin, clarithromycin and azithromycin have the tendency to cause diarrhea and overheating in foals, with this especially being true in older foals in warmer climates or seasons. For this reason, we are very careful to only treat foals that have been diagnosed by a veterinarian with R. equi with these medications. I have had the unfortunate experience of trying to save foals that have presented with life threatening diarrhea because they were treated with these drugs by laypersons that had made the diagnosis themselves. Many of these were inappropriately aged foals for R. equi pneumonia (5 to 6 months old) and turned out to not have this disease once properly evaluated. This serves as a word of caution in self-diagnosis and inappropriate use of these antibiotics. These medications can also cause life threatening diarrhea in mares that are mistakenly administered the foal’s medications or ingest the medications that end up in the foal’s environment. Rifampin, however, is generally a safe antibiotic for both foals and adult horses.
Care should always be taken to avoid turnout during especially hot periods to prevent overheating. I recommend to my clients to keep foals up during the day in a fan-cooled stall and then to turn them out at night. Caretakers should closely monitor all foal’s temperatures and stools for presence of fever or diarrhea. Often we will use probiotics, such as Saccharomyces boulardii, to offset the chances of the foal developing diarrhea. Other medications, such as anti-inflammatory agents and stomach protectants, are also administered during the course of treatment.
Treatment durations vary dependant on the stage of the pneumonia but generally run 2 to 8 weeks. Your veterinarian will use clinical signs, blood work and ultrasound or x-ray imaging to determine when the foal has completely cleared the infection.
Prevention: R. equi has proven itself quite resistant to our approaches of preventing the disease. Due to many complicated immunological reasons, vaccine technology has not allowed us to prevent the disease by immunization. Researchers have identified the most likely time of infection to be during the first 2 weeks of life by using statistical analysis. This finding has lead to several approaches that are currently being evaluated in clinical trials. The first trial has involved the use of one of the antibiotics we use to treat the disease. Foals are administered this antibiotic during the first 2 weeks of life. Initial research conclusions indicate this may reduce, but not completely eliminate, infection in foals. However, a study in Europe found that the treated foals acquired the disease at a similar rate to the control group; they just developed the disease at an older age. Another approach to this idea is currently being studied and involves the use of a drug that affects iron metabolism in the bacteria itself. This drug, gallium, has been shown to be very safe in foals. This approach, if it works, may also be the best because it will not lead to bacterial resistance, something that we fear when we use a treatment drug to prevent an infection.
Another approach that has been used for sometime is the administration of hyperimmune plasma, a blood product high in antibodies, to foals during the first few days of life and then again at 3 weeks of age. Studies evaluating the effects of plasma administration show that it may reduce incidence of the disease by up to one third and that it may also lead to a reduction in severity of the pneumonia if the foals are infected.
The last method in preventing the disease is early detection. Through these methods we are not able to prevent the infection but the aim is to detect it before the bacteria cause clinical signs of infection. Different tools have been utilized for early detection but the two most common are blood-work and ultrasound screening. While the two have not been compared head-to-head, it is perceived that ultrasound screening may provide a more specific assessment of R. equi infection, whereas blood work may be abnormal for reasons not induced by R. equi pneumonia. My practice commonly utilizes ultrasound screenings at large breeding farms. Foals are evaluated biweekly from 30 to 90 days of age and are started on antibiotic treatment if typical R. equi abscesses are detected. This method has almost entirely eliminated death loss from R. equi and has significantly reduced the number of sick foals and treatment times.
In conclusion, pneumonia caused by R. equi is a troublesome disease in most parts of the country. Through research we have begun to further understand the epidemiology of the disease, acquired an arsenal of suitable treatment options and identified prevention strategies that are actively being studied. This topic is of considerable importance and funding/participation for research and clinical trials should be encouraged amongst the horse owning population.
Contributed By: R.P. Franklin, DVM, Dip.ACVIM