Rhodococcus equi Pneumonia in Foals: An Update on Epidemiology, Diagnosis, Treatment, and Prevention
By Noah D. Cohen, VMD, MPH, PhD, DACVIM-LA
Rhodococcus equi is a bacterium that inhabits the soil and can cause pneumonia in foals aged 1 to 6 months. Disease in adult horses, people, and other animals is rare and often associated with a deficiency of the immune system, such as people infected with HIV or undergoing cancer chemotherapy. Infection with R. equi also can cause disease in other sites in the body, including the eyes, bone, joints, and the gastrointestinal tract and intestinal lymph nodes. Signs of disease in these other organs may occur in foals with or without concurrent pneumonia..
Rhodococcus equi is ubiquitous in the environment of horse farms: it is found in soil samples of equine farms and horse feces in all continents other than Antarctica. The vast majority – if not all – mares have R. equi in their feces. The bacteria thrives in soil that is contaminated with feces from horses or other herbivores. It is considered impossible to eliminate R. equi from the environment of mares and foals. The frequency of pneumonia caused by R. equi varies among farms: some horse breeding farms have affected foals each year whereas some farms only occasionally or even never have affected foals. The incidence of R. equi pneumonia also varies among years, making it difficult to predict the burden of disease and whether a given control measure or management practice is truly responsible for change in the occurrence of disease. Although it remains unknown why some farms are affected and others are not, it appears that greater density of mares and foals (i.e., mares or foals per acre) and higher concentrations of R. equi in the air (but not the soil) increase the likelihood of disease. Notably, the disease is not associated with poor hygiene or management: it occurs at large breeding farms that implement excellent practices for managing control of infectious diseases and endoparasites.
Infection is thought to occur as a result of inhalation of airborne R. equi of foals during early life. The disease progresses very slowly over weeks or months, and foals have usually developed large abscesses in their lungs by the time clinical signs of pneumonia such as coughing or difficulty breathing are detected. Because of this insidious onset, many farms have implemented programs for screening to detect evidence of pneumonia before clinical signs develop (please see Prevention below).
The diagnosis of R. equi pneumonia should be based on isolating the bacterium from an aspirate of fluid from the trachea of a foal with clinical signs suggestive of pneumonia, including depressed attitude (not suckling well, spending more time laying down than expected), coughing, fever, labored breathing, and a rattling sound heard in the trachea as a result of pus in the airways.. It is also important to document evidence of lung abscesses with ultrasound or radiography (“X rays”). As mentioned above, the disease progresses insidiously such that foals do not show clinical signs until the disease is well advanced. For this reason, many farms now perform ultrasonographic examination of the lungs in foals to identify evidence of lung damage before it has progressed too far. Because the probability that these lesions are caused by R. equi is very high in a foal at a farm that has a long history of recurrent cases of R. equi pneumonia, many veterinarians eschew performing the procedure to collect tracheal fluid because it is labor-intensive, can be hard for very sick foals to endure, and adds considerable expense for owners. It is important to note that not all such foals will progress to develop clinical signs, and some may have pneumonia caused by other bacteria such as Streptococcus.
Currently, the treatment of choice for R. equi pneumonia is a member of the class of antibiotics known as macrolides combined with another antibiotic known as rifampin. Examples of macrolide antibiotics are azithromycin, clarithromycin, and erythromycin. This combination has been used to treat foals for about 40 years, and more widespread use of these antibiotics has led to the emergence of strains of R. equi that are resistant to these drugs. Unfortunately, there are few good alternatives to the macrolides for treating R. equi. Treatment is generally prolonged, typically requiring at least a few weeks of medication. Veterinarians use a variety of factors, including evaluation of the lungs with ultrasound or radiography, results of analysis of complete blood counts, and physical examination of the foal to determine when treatment can be discontinued. Earlier recognition of the disease by using serial ultrasound examinations of the lungs is thought to lead to reduced durations of treatment.
Medication with macrolides is not without risk. They can cause diarrhea in treated foals, and this diarrhea can sometimes be severe. Rarely, mares of foals that are being treated will develop diarrhea because they ingest small amounts of these antibiotics to which they are exposed from their foals. Although these cases are infrequent, the diarrhea in these cases can be very severe (even fatal). To avoid this problem, some veterinarians recommend wiping off and/or rinsing the mouth of foals after treatment, and trying to avoid treating foals adjacent to feed or water that mares will ingest. Many veterinarians will administer a probiotic to foals in an effort to reduce the likelihood of diarrhea developing with antibiotic treatment. The efficacy of this practice is unknown, but many consider that the benefits are likely to outweigh any risks.
Macrolides can also cause foals to stop sweating. Because horses and foals are dependent on sweating to dissipate heat, when they stop sweating their body temperatures can soar in hot and humid conditions. This elevation of body temperature is termed hyperthermia, and is different from the the rise in body temperature that occurs with infections that we call fever. It is therefore very important to monitor closely foals being treated for R. equi pneumonia, especially if they are turned out in paddocks when ambient temperatures or humidity are high. Foals that have trouble with sweating will have very high respiratory rates (because they are trying – unsuccessfully – to dissipate heat by panting like a dog), will not show signs of sweating, and will have body temperatures that are very high (over 105oF). These foals need to be cooled down as quickly as possible. An air-conditioned environment is ideal but often unavailable. Foals will respond to ice baths or alcohol baths, and even hosing off with cold water will help. After applying ice, ice-water, alcohol, or cold water, it is important to allow the liquid to warm as it draws heat from the foal, then to scrape off the liquid and repeat the process. Typically, only a few applications are necessary, and consulting with a veterinarian about the procedure is recommended.
Because of the aforementioned risks associated with macrolides and the emergence of R. equi resistant to macrolide antibiotics and rifampin, there is great need to identify effective alternatives to the macrolide-rifampin combination for therapy. Unfortunately, traditional antibiotic agents do not appear to offer hope, other than a few drugs whose use is recommended to be restricted for people. Researchers are investigating alter
Prevention of disease is always preferable to curing it. Unfortunately, there is not a vaccine against R. equi pneumonia in foals. This is largely because foals are considered to become infected very soon after birth, at a time when they are highly susceptible to infection and when their immune systems are not fully mature. Moreover, immune responses after infection can take weeks to achieve effective levels, and infection can gain the upper hand during this period. The only products licensed to reduce the incidence of pneumonia at farms is transfusion of foals with plasma from horse donors immunized against R. equi. These plasma products, however, can vary in quality and do not completely eliminate the disease. They can be expensive, and there are some risks to handling foals and reactions to the plasma can occur on rare occasion. Licensed commercial plasma producers screen their donor horses for infection with viruses that can cause disease in transfusion recipients. Although it has been suggested to treat all newborn foals for a period of time with macrolide antibiotics to prevent pneumonia, this approach should not be used because it can promote antibiotic resistance and evidence that the practice is effective is conflicting.
Because there is not a vaccine to prevent R. equi foal pneumonia and because plasma transfusion is only partially effective, veterinarians for many large breeding farms have implemented the practice of examining all foals using serial ultrasound of the chest (i.e., repeated exams at intervals typically ranging from 2 to 4 weeks) to screen for foals that have lung abnormalities suggestive of R. equi pneumonia. The advantages of this practice are that it enables identification of foals in the early stages of pneumonia; treating foals in the earlier stages of treatment leads to better chances for full recovery and likely a shorter duration of treatment. Disadvantages of this approach include the additional labor and veterinary medical expenses for ultrasound examinations. Moreover, many foals that have ultrasound findings suggestive of pneumonia will resolve without treatment; however, because veterinarians cannot tell which foals would ultimately require treatment and which do not, more foals than necessary get treated with antibiotics. This increased use of antibiotics as a result of treating foals based on ultrasound findings has been linked to an alarming rise in R. equi resistant to the antibiotics of choice, viz., macrolides and rifampin.
Pneumonia caused by the bacterium R. equi remains a health problem for foals on a global basis. There is great need to better understand the epidemiology of R. equi pneumonia to identify causal factors for why some foals or some farms are affected, and if management practices such as lowering airborne concentrations of R. equi can reduce the incidence of disease. Research to identify diagnostic testing that could accurately identify R. equi without the need to collect tracheal fluid is much needed. The emergence of resistance to the antibiotics used to treat R. equi creates an urgent need for more research to identify effective alternatives. Finally, development of an effective vaccine to prevent R. equi is much needed by the equine breeding industry on a worldwide basis.