Rattles, clinically known as Rhodococcus equi pneumonia, is a serious respiratory condition that affects growing foals. This disease is a leading cause of severe illness in young foals, and is characterized by coughing, nasal discharge, and rapid breathing.

The rattle infection is caused by the bacterium Rhodococcus equi, which is ubiquitous in the environment, particularly on breeding farms. The disease is not contagious, but the high concentration of bacteria in a contaminated environment can lead to multiple cases on the same farm.

Treatment for rattles typically involves an extended course of antibiotics. The overall prognosis for affected foals is fair to good with appropriate treatment. No vaccines are available for the bacteria, so many farms use screening protocols to identify at-risk foals.

This article discusses the disease progression, clinical signs, diagnosis, treatment, and prevention of rattles in foals.

Rattles in Foals

Rhodococcus equi is a bacterium that is the most common cause of pneumonia in foals between 3 weeks and 5 months of age. [1][2] Foals acquire the bacterium from their environment, usually from fecal matter. [2]

This infectious disease affects the lower respiratory tract of young horses. The clinical presentation of rattles can vary from mild to severe, potentially leading to the formation of abscesses within the lungs.

Pneumonia caused by R. equi infections (colloquially known as “rattles”) are particularly challenging for veterinarians and horse breeders. The bacterium can become endemic on horse farms, meaning it cannot be removed from the environment.

Farms that are impacted may have multiple affected foals per year, resulting in significant veterinary expenses, increased personnel time for long treatment protocols, and even death of afflicted foals. [1][2]

How the Disease Develops

Foals acquire the R. equi bacteria by inhaling contaminated material during the first two weeks of life. [2] The bacteria settle in the lungs and are engulfed by macrophages, the main immune system cells that help clear infections. [2]

However, R. equi bacteria is able to evade the macrophages’ normal destruction mechanisms in young foals, allowing this pathogen to replicate within the cell. [2] As the bacteria replicate, they eventually cause the death of the cell. This leads to the release of the bacteria, which can then go on to infect other macrophages. [2]

If the cycle of infection persists, it can lead to the formation of abscesses in the lungs, which are accumulations of pus containing dead cells. Abscesses interfere with normal respiratory function, resulting in the characteristic symptoms of R. equi pneumonia. [2]

Typically, it takes weeks to months for foals to start showing symptoms of the disease after the initial infection. [2]

Extra-Pulmonary Infections

An extra-pulmonary infection refers to the spread of the rattles infection beyond the lungs to other parts of the foal’s body. While R. equi most commonly affects the lungs, it can affect other organ systems if the bacteria enters the bloodstream. In severe cases, multiple organ systems may be affected simultaneously.

Other organs that may be affected by R. equi include: [2]

  • Intestinal tract – resulting in diarrhea and poor growth
  • Joints – leading to joint swelling and lameness
  • Eyes – causing accumulation of pus within the eye
  • Vertebrae of the spine – resulting in difficulty walking and poor coordination

Risk Factors

Investigations into outbreaks of R. equi have identified several factors that increase a foal’s risk of exposure to the bacteria.

Risk factors for rattles include: [3][4][5][6][7]

  • Dry climate
  • High number of mares and foals in the paddock or field
  • Poor manure management in stalls and paddocks containing foals
  • Foaling in stalls rather than paddocks or pastures
  • Previous occurrences of R. equi pneumonia on the farm
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R. equi is predominantly associated with equine pneumonia, resulting in symptoms that are common of respiratory diseases. Clinical signs include: [2]

  • Fever
  • Reduced appetite
  • Lethargy
  • Rapid breathing
  • Difficulty breathing
  • Coughing
  • Nasal discharge

Not all foals infected with R. equi show symptoms. Research indicates that approximately 70 – 85% of affected foals remain asymptomatic, and any abscesses that do occur in these foals tend to naturally resolve over time. [2]


Confirming a diagnosis of R. equi is critical for effective treatment, because this bacterium may not respond to antibiotics typically used for other forms of bacterial pneumonia. [2] Veterinarians cannot rely solely on symptoms to make a diagnosis, since all types of bacterial pneumonia cause similar symptoms. [2]

To diagnose rattles in a young foal, your veterinarian may use one or a combination of the following techniques:

  • Ultrasound or X-ray of the lungs to identify abscesses
  • Bloodwork
  • Tracheobronchial aspirate
  • Fecal testing for the bacteria

Bacterial Culture

Definitive diagnosis of an R. equi infection requires a tracheobronchial aspirate (TBA). In this procedure, the veterinarian introduces sterile saline into the trachea, then retrieves a fluid sample for further testing.

If the laboratory finds that R. equi has grown in the bacterial culture from the TBA sample, it confirms the presence of this bacterium in the foal’s respiratory system, indicating an R. equi infection. [2]

Since bacterial culture can take several days, many veterinarians begin treatment based on the medical history, symptoms, and identifying abscess-like structures on ultrasound or X-ray. However, confirming the diagnosis through bacterial culture is still important.

Some strains of R. equi are resistant to the antibiotics commonly used to treat this disease. [2][8] TBAs allow for development of a sensitivity panel to determine which antibiotics will be most effective against the particular strain of R. equi responsible for the foal’s infection. [2]

Diagnosing Extra-Pulmonary Infections

Diagnosing extra-pulmonary infections of R. equi can be challenging, particularly when abscesses occur in the abdomen.

Diagnosis often involves a combination of clinical signs, imaging studies like X-rays or ultrasound, and sometimes even biopsies to confirm the presence of R. equi in the affected tissues.

Unfortunately, many cases of extra-pulmonary R. equi infections are only diagnosed at necropsy, after the death of the affected foal. [2]


Treating R. equi infections in foals is difficult because these bacteria reside within immune cells. Common antibiotics used for bacterial pneumonia are often ineffective against R. equi because they cannot reach effective concentrations within macrophages. [2]

To treat R. equi, veterinarians use a combination of targeted antibiotics. Common medication protocols include: [1][2]

  • Clarithromycin and rifampin
  • Azithromycin and rifampin
  • Erythromycin and rifampin

Treatment of R. equi typically takes between 4 to 12 weeks, depending on the severity of disease. [3]

Side Effects of Treatment

Studies show between 17 and 36% of foals treated with these antibiotic combinations develop diarrhea. [1] In severe cases, these foals may require supportive care such as intravenous fluids to prevent dehydration.

Foals are also prone to hyperthermia because antibiotics used to treat them may interfere with their ability to sweat. [2] Foals undergoing treatment should be kept in a cool area and monitored closely for signs of rapid breathing or lethargy.

On hot days, keeping foals indoors and periodically cold hosing them may help prevent them from overheating. [1]


The prognosis for foals with R. equi pneumonia is currently considered fair to good. With appropriate antibiotic treatment, most foals make a full recovery from the infections.

Studies from referral centers treating severe cases of rattles report survival rates between 59 to 72%. [1][9]

Research on racing Thoroughbreds shows that affected foals are slightly less likely to race as adults. However, those that do race have similar performance metrics as those that were unaffected. [1]

Foals that develop abscesses in locations other than the lungs have a guarded prognosis. [10] Foals with extra-pulmonary infections typically respond poorly to treatment, and have a higher mortality rate compared to foals with abscesses in the lungs only. [10]

Endemic Disease

R. equi can become endemic on farms, meaning that it is present in the environment and difficult to remove. Foals living on endemic farms have a higher risk of infection than foals on farms who have never experienced the disease. [4]

Studies show that endemic farms not only have higher quantities of bacteria in the soil, but also commonly have more aggressive strains of R. equi. [4]

Once a single foal becomes infected on a farm, it sheds R. equi bacteria in its feces. This shedding results in contamination of the environment, which can potentially expose other foals. [4]

Over time, repeated cases of R. equi in foals, perhaps spanning numerous years, results in significant environmental contamination and an “endemic” status for the farm. [4]

Resolving Endemic Disease

Unfortunately, there is no effective method for removing R. equi from the soil once it is contaminated. Thus far, no effective disinfectants have been identified for neutralizing R. equi on farms.

Possible management strategies include changing the location on the farm where foals are born and raised to reduce exposure to contaminated soil. Adding new topsoil to an existing location may also be beneficial, although this has not been studied. [4]

Current research recommends foal-centered prevention methods for endemic farms, such as health screening, rather than treating the environment, to maximize efficacy and minimize cost.


Prevention is particularly important for farms where R. equi is endemic, as virtually all foals born on the property will be exposed. Current preventative measures focus on limiting exposure, boosting immunity, and screening foals for early treatment.

Unfortunately, vaccine development against R. equi has been unsuccessful thus far. [3] The main reason is that vaccination requires a robust immune response against the bacteria before the foal becomes infected. [2] Foals have weak immune systems that typically cannot produce antibodies early in life, limiting the effectiveness of vaccines. [2]

Attempts at developing a vaccine for mares, so they can pass antibodies to the foal in their colostrum, have also been unsuccessful. [2]

Limiting Exposure

Environmental management strategies to prevent exposure to R. equi have not been thoroughly studied. [3] However, based on what is known about the disease, strategies that may reduce exposure risk include: [3][6][7][9]

  • Increasing grass cover in paddocks
  • Reducing the number of mares and foals in paddocks
  • Routine manure removal from stalls or paddocks
  • Foaling in pastures instead of dry lots or stalls
  • Cleaning stalls thoroughly before housing foals

Boosting Immunity

Although there are no vaccines available, there are other ways to boost the foal’s immunity to R. equi.

The main method is plasma transfusion, which transfers antibodies from another horse into the foal. The most common type of plasma used is hyperimmune plasma, which comes from a horse previously exposed to R. equi. [11]

The use of hyperimmune plasma is controversial, with some studies showing protective benefits and others showing no benefits at all. [3][9][11] One study found that hyperimmune plasma transfusion decreased shedding of the bacteria in the foal’s feces, which could reduce exposure of other foals. [3][9][11]

If using hyperimmune plasma, most veterinarians recommend performing the transfusion within 1-2 days of birth, so the foal has antibodies when they are most susceptible to disease.  Some veterinarians follow up with another hyperimmune plasma transfusion at 3-4 weeks of age to act as a “booster”. [3]

Disadvantages of Plasma Transfusions

The main downside of plasma transfusions is the cost. Plasma is very expensive, as it must be purchased from manufacturers who maintain a disease-free donor herd that undergoes regular testing.

Since the product comes from donor animals, there can also be significant variability in plasma quality, as measured by the concentration of antibodies against R. equi. [2]

Additionally, foals commonly have adverse reactions to plasma transfusions. Studies show that 6% of horses have a negative response to plasma transfusion, with side effects more common in foals. [11]

Symptoms of a negative plasma transfusion reaction include:

  • Fever
  • Rapid heart rate
  • Rapid breathing
  • Colic
  • Muscle tremors

When adverse reactions to plasma transfusions occur, it becomes necessary to slow down the rate of plasma administration to the foal. This prolongs the treatment process and increases the labor involved. In severe cases, plasma transfusion may need to be completely halted. [2]

Foal Screening

The main method of prevention for endemic farms is routine screening of all foals to identify disease early.

Regular lung ultrasounds are the most popular screening method, allowing the veterinarian to identify abscesses and measure their size. [3] Most protocols begin at three weeks of age. [1]

Recommended Screening Protocols

Recommendations for screening have changed significantly in the last 15-20 years. Initially, all foals with identifiable abscesses on ultrasound were treated with antibiotics. [2] Now, only abscesses of a certain size are treated.

Studies performed on farms routinely treating all foals with identifiable abscesses showed that these farms had a higher incidence of antibiotic-resistant strains of R. equi bacteria. [2][8]

The number of antibiotic-resistant strains diagnosed on farms increased from 0.7% to 13% between 2010 and 2017. [2] Based on these findings, new screening protocols were developed to address concerns about antibiotic resistance.

The initial screening protocols recommended treatment of foals containing abscesses greater than 10 mm (1 cm) in diameter. [9]

From 2004 to 2014, the minimum threshold for treatment increased up to a 100 mm (10 cm) diameter. [9][12] Foals with smaller abscesses frequently recover completely without treatment and without developing any symptoms. [3][12]

Altering the minimum threshold for treatment has significantly reduced antibiotic use. One study showed an approximately 20% decrease in antibiotic use between 2008 and 2016. [12]

The hope is that continued adjustment of treatment protocols will reduce the number of antibiotic-resistant R. equi strains affecting farms, while not adversely affecting foal health. [3]


  • Rattles or Rhodococcus equi infection is the most common cause of pneumonia in foals
  • R. equi can be endemic on farms, making it a challenge for breeding operations
  • Treatment involves long-term antibiotic therapy targeted to R. equi bacteria
  • There are no vaccines available, so many breeders and veterinarians use screening protocols to identify at-risk foals

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  1. McKinnon, A. O. et al. Equine Reproduction. Wiley-Blackwell, 2010.
  2. Sanz, M. G. Rhodococcus Equi–What Is New This Decade?. Veterinary Clinics of North America: Equine Practice. 2023. View Summary
  3. Bordin, A. I. et al. Rhodococcus Equi Foal Pneumonia: Update on Epidemiology, Immunity, Treatment and Prevention. Equine Veterinary Journal.View Summary
  4. Takai, S. Epidemiology of Rhodococcus Equi Infections: A Review. Veterinary Microbiology. 1997. View Summary
  5. Chaffin, M. K. et al. Foal-Related Risk Factors Associated with Development of Rhodococcus Equi Pneumonia on Farms with Endemic Infection. Journal of the American Veterinary Medical Association. 2003.. View Summary
  6. Muscatello, G. Rhodococcus Equi Pneumonia in the Foal – Part 1: Pathogenesis and Epidemiology. The Veterinary Journal. 2012. View Summary
  7. Coleman, M. C. et al. Foal-Level Risk Factors Associated With Development of Rhodococcus Equi Pneumonia at a Quarter Horse Breeding Farm. Journal of Equine Veterinary Science. 2019.View Summary
  8. Higgins, C. and Huber, L. Rhodococcus Equi: Challenges to Treat Infections and to Mitigate Antimicrobial Resistance. Journal of Equine Veterinary Science. 2023. View Summary
  9. Rakowska, A. et al. Current Trends in Understanding and Managing Equine Rhodococcosis. Animals. 2020. View Summary
  10. Huber, L. Medical Management of Rhodococcus Equi Infections: A Clinical Epidemiology Perspective. Equine Veterinary Education. 2021.
  11. Kahn, S. K. et al. Transfusion of Hyperimmune Plasma for Protecting Foals against Rhodococcus Equi Pneumonia. Equine Veterinary Journal. 2023. View Summary
  12. Arnold-Lehna, D. et al. Changing Policy to Treat Foals with Rhodococcus Equi Pneumonia in the Later Course of Disease Decreases Antimicrobial Usage without Increasing Mortality Rate. Equine Veterinary Journal. 2020. View Summary