Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

Rhodococcus equi is a bacterium that is an important cause of respiratory disease in foals. It typically causes numerous abscesses in the lungs, and it is an important cause of illness and death in foals from a few weeks to about six months of age. Infections of other parts of the body can occur, and these can cause serious problems because they can be hard to diagnose and hard to treat.

A recent study in the Journal of the American Veterinary Medical Association (Reuss et al 2009) described extra-pulmonary disorders (EPDs, disorders in parts of the body besides the respiratory tract). They studied 150 foals, and 74% had a least on EPD. These included:

• Immune-mediated synovitis (25%): This is a well-recognized problem where R. equi-infected foals get enlarged joints because of accumulation of excess joint (synovial) fluid. Fluid-distended joints can occur with joint infections, but with immune-mediated synovitis it's caused by the immune system alone. This typically does not result in long-term joint problems, although it's important to determine whether the foal has an infection which needs aggressive care, or just inflammation which is best left alone.
• Abdominal abscesses (17%): This is a very serious problem and abscesses in the abdomen can be very difficult to treat. These foals often die.
• Uveitis (11%): Inflammation of the eye, which can occur as a result of the immune response or infection. 
• Septic synovitis (9%): As opposed to immune-mediated synovitis, this is an infection in the joint. This is very serious and can cause permanent joint damage.
• Pyogranulomatous hepatitis (11%): This is inflammation of the liver with abscesses and pus.
• Pleural effusion (3%): Accumulation of fluid between the lungs and body wall. This can be the result of severe lung disease.
• Meningitis (3%): Infection of the layer covering the brain, which is never a good thing.

Foals that had EPDs were more likely to die than foals with infections only involving the lungs, which isn't particularly surprising. In particular, the presence of uveitis, bloodstream infection, joint infection (not immune-mediated synovitis) and abdominal abscessation were associated with failure to survive.

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Erythromycin is an interesting drug in horses, having saved the lives of countless numbers of foals but able to readily kill an adult horse. Erythromycin is an antibiotic of the macrolide class. It is effective against many Gram-positive bacteria (e.g. Staphylococcus, Streptococcus, Rhodococcus), but not Gram-negative bacteria (e.g. E. coli, Salmonella).

There are several very good aspects of erthyromycin, including its ability to kill bacteria that have invaded cells (intracellular bacteria) and the ability to treat abscesses (many antibiotics can't do that very well at all). Erythromycin is also available in an oral form, which makes it much easier to give. It is most commonly used for the treatment of Rhodococcus equi pneumonia in foals, almost always in combination with the antibiotic rifampin, and it's a highly effective treatment.

However, the news isn't all good.  Erythromycin is far from a benign drug, and should only be used in certain situations. Some of the problems associated with its use are:

Diarrhea: Any antibiotic can cause intestinal upset, ranging from mild diarrhea to fatal colitis, but erythromycin is a particularly high-risk drug. Diarrhea can occur in foals being treated with erythromycin, but this is not very common and when it occurs diarrhea is not usually very severe. However, giving oral erythromycin to an adult horse is a very high-risk proposition that can easily result in fatal colitis. In Sweden, there have been reports of fatal colitis in mares whose foals were treated with erythromycin - the hypothetical cause of this phenomenon is that mares were exposed to extremely low amounts of erythromycin from the foals' mouths or feces. Erythromycin should be avoided in adult horses at all costs.

Hyperthermia: This is a strange but rare side effect that is most often seen in foals being treated in sunny areas without access to shade. Some treated foals lose the ability to properly control their body temperature, and can develop extremely high fevers. The classic situation is an otherwise healthy foal that is responding nicely to treatment suddenly found down in the field one day with a life-threateningly high body temperature. This is uncommon, but it is a concern and treated foals should always have access to shade and be closely monitored.

Erythromycin can be a very useful drug is some situations. It's use is declining somewhat as related drugs like azithromycin are sometimes being used in its place, but it can still be a good treatment option in some cases. Because of the risks of adverse effects (and because of basic principles of antibiotic use), you should only use erythromycin under the direction of a veterinarian. Never use it in an adult horse unless you have no other choice, and realize there is a reasonable chance the horse will develop potentially fatal diarrhea.

Image: 3D model of erythromycin molecular structure (source: www.3dchem.com)

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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.

This equIDblog entry was originally posted on the Worms & Germs blog on 09-May-09.

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Rhodococcus equi is a common pathogen in foals between the ages of 1 and 6 months of age that is most infamous for its ability to cause pneumonia. Classic R. equi infection results in the formation of large abscesses throughout the lungs of young foals (see picture left), which can be especially difficult to treat because the bacteria are able to hide from the body’s immune system by living within white blood cells. However, this organism’s bag of tricks doesn’t end with lung abscesses – it can also travel to other parts of the body and cause all sorts of trouble. These kinds of infections may occur with or without the classic lung infection, and are referred to as extrapulmonary disorders (EPDs).

At the 2008 Forum of the American Association of Equine Practitioners (AAEP), researchers at Texas A&M presented the results of a study they recently completed looking at EPDs in 150 foals with R. equi infection over a 20 year period. Here are some of the highlights:

• 74% of the foals had an EPD associated with their R. equi infection. On average foals had two EPDs each and up to as many as nine EPDs in a single animal.
• Many of the EPDs did not cause separate, detectable clinical signs. These were therefore most often found on necropsy in foals that died.
• The most common EPD was diarrhea, which occurred in 50 foals (33%).
• Immune-mediated polysynovitis (inflammation and swelling of the joints without infection of the joints themselves) was the second most common EPD
• 31 foals (21%) had ulcers and inflammation somewhere in their intestine (ulcerative enterotyphlocolitis), all of which were diagnosed at necropsy
• 25 foals (17%) had abscesses in the abdomen, 71% of which could be detected by ultrasonography

Thirty-nine different EPDs were identified in the group.  Other EPDs included uveitis (inflammation of the eye), hepatitis (inflammation of the liver), septic arthritis (joint infection), lymphadenopathy (enlarged lymph nodes), peritonitis (inflammation of the lining of the abdomen) and septicaemia (bloodstream infection).

• Among the foals with EPDs, 43% survived hospitalization, whereas 82% of foals without EPDs survived.  However it is very important to remember that many EPDs were only detected at necropsy, therefore they were more often found in foals that died, but it is unknown if they actually occurred more commonly in one group or the other.
• Risk factors for foals developing EPDs included longer time from onset of clinical illness to referral (e.g. foals that were sick at home for longer before being sent to the hospital were more likely to have an EPD), higher heart rate on admission and a higher white blood cell count.

It’s important to remember that even though an infectious pathogen may usually affect an animal in a certain way, “the bugs don’t read the textbooks” (as we often say), and they can cause problems in other ways. That’s just one of the reasons it’s so important to have your veterinarian perform a full physical exam of your animal if it is sick - even if it looks similar to something you’ve seen before - in case the pathogen causing the problem starts to affect other parts of the body, which may require more or different kinds of treatment.

Photo credit: M. Anderson

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