Rhodococcus Equi

Rhodococcus Equi

Rhodococcus equi (R. equi) is a bacteria responsible for causing severe pneumonia in foals, leading to significant losses to the industry on a yearly basis.

Pneumonia is quite common in foals and a chief cause of illness and death in this population. Casualty rates of horses affected with R. equi are high - nearly 28%. In addition to economic loss from mortality, future athletic performances from recovered horses may be in peril as well.

R. equi bacterium live in the soil and feces of hervibores. The bacteria relies on warmth and nutrients found in manure to grow and it is unknown how long it can survive in this environment. Survival in the soil can last as long as one year. Some farms appear to be predisposed to this bacterium and more frequent occurrences, up to 20% of their foal population, are seen. 

Most foals are exposed but only some develop symptoms. It is unclear why, but likely this is due to immune status, environmental factors and farm management.

Foals become infected when they breathe bacteria in the soil, dust or feces. These bacteria multiply inside macrophages (white blood cells that normally kill bacteria) and can cause lung abscesses, which lead to pneumonia. Other conditions can also develop including osteomylitis (bone infection), neonatal diarrhea, lymph node inflammation, spinal cord abscesses, immune mediated diseases (purpura hemmorhagica, for example) and sudden death in an otherwise healthy foal.

Foals are believed to become infected when they are less than two weeks old but clinical signs are not obvious until they are one to three months old. It is an accepted belief that R. equi can cause pneumonia in foals up to six months of age.

Clinical signs develop rapidly. Horses will exhibit a thick, greenish white mucopurulent discharge from the nostrils, fever, and lethargy. Respiratory issues such as an increased breathing rate, crackles, and wheezes will be present as well.

The veterinarian will have to work to differentiate R. equi from other causes of pneumonia, including bacteria or fungal respiratory infections. This is not an easy disease to diagnose, especially early on. Diagnosis is usually obtained through thoracic radiographs, ultrasounding the lungs and also via trans tracheal wash. Routine bloodwork alone is not specific enough to confirm a diagnosis. Elevated white cell and fibrinogen counts will only indicate general infection or inflammation.

Treatment is expensive, time consuming (the horse will need to be treated several times daily) and can come with serious side effects. Early detection is vital. Antibiotic therapy utilizing erythromycin, rifampin, and clarithromycin work to penetrate the lung abscesses and macrophages where bacteria multiply. Complementary therapy of yogurt and/or Amaferm Recovery Paste should be added to treatment to ward off potentially fatal diarrhea. Watch for any signs of foal hyperthermia as well.

Treatment should be at minimum four weeks, and oftentimes needs to continue up to eight weeks. It should continue until lung scans and fibrinogen return to normal.

BVEH recommends an initial lung scan for at-risk foals beginning at 30 days of age, then continuing every 30 days until the foal is six months old.

R. Equi is difficult to control but good farm management and stringent hygiene protocols can minimize infection. Keeping foals on grassy pastures and frequently removing manure can significantly reduce the risk.

The only prevention for R. equi is to administer a specially prepared plasma, which is costly and unfortunately, not 100% effective. Treatment should begin shortly after birth, with a second treatment to follow 30 days later.

Horse to human transmission can occur, however the disease can only occur if the person’s immune system is severely compromised.

In a study conducted by Texas A & M University, researchers discovered that increasing the amount of foals per acre seems to correlate with an increased risk factor of R. equi. In addition, it was found farms with large populations of mares and foals, or large numbers of transient mares/foals had increased incidence of R. equi. It has been thought prophylactic azithromycin for the first two weeks of life may prevent bouts of R. equi, but unnecessary use of antibiotics is frowned upon and further research is needed to investigate the accuracy of this claim.