Rhodococcus equi is a very well recognized pathogen in horses – it is a common cause of pneumonia in foals between the ages of 1-6 months, and infection is also sometimes associated with other problems such as diarrhea, swollen joints and abscesses in other parts of the body. The infection can be very difficult to treat because the bacteria are able to live inside white blood cells, which helps protect them from the body’s immune system, and because they often cause abscesses to form, which are difficult for antibiotics to penetrate. Rhodococcus equi infection in foals has been studied extensively, but there’s still a lot we don’t know how the body defends itself against this organism.  These are a few things we do know:

  • Almost all foals are exposed to R. equi as neonates, but most of them never develop signs of infection.
  • Giving newborn foals hyperimmune plasma (plasma with extra antibodies against R. equi) may have some beneficial effects on farms where the infection is a recurrent problem, but this practice is still controversial.
  • Adult horses are essentially immune to the infection.
  • In almost all cases if clinical disease in foals, the R. equi strain involved carries a special gene called vapA.
  • Mortality rates in foals vary considerably from 0% to 30%.
  • So far, efforts to develop a vaccine to help protect foals have been unsuccessful, but research in this area is ongoing.

People can also be infected with R. equi, and as in foals, pyogranulomatous pneumonia (infection of the lungs which results in the formation of many abscesses) is one of the most common conditions caused by this organism. However, there are a few important differences between infection in people and infection in horses:

  • 85% to 90% of people with R. equi infection are immunocompromised, meaning their immune system is weakened or suppressed for some reason, e.g. HIV infection, or immunosuppressive drugs taken by organ transplant or cancer patients.
  • Among people infected with R. equi who have normal immune systems (i.e. immunocompetent), about half of the infections are localized, meaning they only affect one small part of the body. Many of these are associated with wound infections.
  • Only 20% to 25% of the R. equi isolates in people carry the vapA gene.
  • Infection in immunocompetent people can be fatal in approximately 11% of cases, but among HIV-infected patients the mortality rate from R. equi infection can be as high as 50% to 55%.

Rhodococcus equi is actually a soil organism, and this is likely the most common source of the organism for both horses and people. Only approximately 1/3 of humans infected with R. equi report that they have had contact with horses or pigs (pigs can also carry the bacterium). So we don’t know how much of a risk an infected foal is to a person.  However, it is prudent for people, particularly those with weakened immune systems, to take precautions to avoid potential transmission of R. equi from horses.

  • Try to reduce dust levels on the farm. Because R. equi most often lives in the soil, it can get stirred up into the air in dusty areas, which can then lead to inhalation by animals and people. Doing things like planting grass or other vegetation, installing windbreaks in high-traffic areas, or wetting down dusty stalls or paddocks can help reduce dust levels in the air.
  • Keep open wounds and other broken skin covered when working around animals.
  • Always wash your hands after handling a foal (or any horse)
  • If you have a foal that develops signs of R. equi infection, make sure you have your veterinarian examine it as soon as possible so the diagnosis can be determined and the foal can be treated properly as soon as possible. Some foals with R. equi may develop severe pneumonia very quickly, so it’s important that they are examined right away.