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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
Symptoms
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]
Diagnosis
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 thr