Other Names
Rattles, rattles disease, Corynebacterium equi infection (historical), rhodococcal pneumonia, foal pneumonia.
What is Rhodococcus equi Infection?
Rhodococcus equi is a Gram-positive, soil-dwelling bacterium that is a significant cause of pneumonia and other disease in foals, typically between one and six months of age. Infection causes an insidious, progressive suppurative (pus-forming) pneumonia that can be fatal if not identified and treated early. The condition is colloquially known as “rattles” — a reference to the characteristic rattling respiratory sounds heard in severely affected foals.
R. equi is found in soil worldwide, particularly in soil enriched with horse manure where the bacteria can replicate in enormous numbers. On endemic farms, where the bacteria have been accumulating in the soil for years, R. equi pneumonia can affect a high proportion of foals in a given year. The disease is almost exclusively a problem of young foals — adult horses have effective immunity, though immunocompromised adults can rarely be affected.
What Causes Rhodococcus equi Infection?
Rhodococcus equi possesses a virulence-associated plasmid (VapA-positive strains) that enables it to survive and replicate inside macrophages — the very cells designed to kill bacteria. This intracellular survival strategy is what makes the bacterium so pathogenic and what makes treatment prolonged and challenging.
Foals inhale R. equi-laden dust particles from contaminated soil and become infected via the respiratory route. The bacteria are ingested by alveolar macrophages but are not killed; instead they replicate within the macrophages, forming granulomatous abscesses in the lung tissue. The infection is typically subacute — it progresses over weeks before the foal becomes visibly unwell, by which time lung pathology may already be extensive.
Factors that increase farm-level risk include high stocking density, dry and dusty conditions, and warm temperatures that favour R. equi multiplication in soil. Historically endemic farms present the highest risk.
Symptoms and Signs
The insidious onset is a hallmark of R. equi pneumonia — foals often appear normal until lung pathology is already advanced. Signs include:
- Subtle reduction in activity and willingness to follow the mare
- Mild fever — may be intermittent and initially missed
- Increased respiratory rate (tachypnoea) — an important early warning sign on high-risk farms
- Progressively worsening cough — dry initially, later more productive
- Mucopurulent nasal discharge
- Laboured breathing and flared nostrils as disease advances
- The characteristic “rattling” respiratory sound from which the disease gets its name — abnormal breath sounds on auscultation
- Weight loss and failure to thrive
- Rapid deterioration once clinical signs become apparent
R. equi can also cause extrapulmonary disease — particularly abdominal abscesses, ulcerative enteritis with diarrhoea, bone and joint infections (septic arthritis, osteomyelitis), and lymphadenopathy. These may occur alongside or independently of pulmonary disease.
Diagnosis
Early diagnosis is critical but challenging given the insidious onset. On high-risk farms, regular screening of foals from 3–4 weeks of age is standard practice.
Thoracic ultrasonography: The most practical screening tool. Lung ultrasound can detect peripheral lung abscesses before clinical signs develop. Regular ultrasound screening of all foals on endemic farms (every 2 weeks from 3 weeks to 4 months of age) has become the standard of care for early detection.
Clinical signs and auscultation: Abnormal lung sounds — crackles, wheezes, muffled ventral lung fields (indicating consolidation or abscess), and the characteristic “rattles” — support the diagnosis in clinically affected foals.
Haematology: White blood cell count (leukocytosis with neutrophilia) and elevated serum amyloid A (SAA) indicate significant bacterial infection. Serial monitoring can track disease progression.
Tracheal aspirate (transtracheal wash): Samples tracheal secretions for cytology (septic suppurative inflammation) and bacterial culture to confirm R. equi and determine antibiotic sensitivity.
Thoracic radiography: Shows the extent of lung pathology — nodular or patchy consolidation and abscess formation. Less sensitive than ultrasonography for early lesions.
See also our entries on equine asthma for comparison of other respiratory conditions in horses.
Treatment
Treatment requires prolonged antibiotic courses because of R. equi‘s intracellular location. The combination of a macrolide antibiotic plus rifampicin has been the gold standard of treatment for decades, as both drugs penetrate cells and achieve high intracellular concentrations.
Azithromycin + rifampicin: Currently the most widely used combination in the USA and increasingly in Europe. Azithromycin is given on a 3-day-on, 3-day-off protocol to reduce the risk of macrolide-associated hyperthermia. Rifampicin is given daily.
Clarithromycin + rifampicin: An alternative combination; sometimes used when azithromycin resistance is suspected.
Erythromycin + rifampicin: The original combination; still used but associated with higher rates of antibiotic-associated diarrhoea in foals and mares (who ingest drug residues from the foal’s environment) and hyperthermia.
Treatment duration typically ranges from 4–12 weeks depending on clinical response and serial ultrasonographic assessment of lung lesion resolution. Premature discontinuation risks relapse.
Important cautions: Macrolide antibiotics can cause severe, potentially fatal diarrhoea in mares and other adult horses that inadvertently ingest drug residues (through the foal’s faeces or salivary contact). Treat foals in designated areas and prevent adult horse contact with treated foals’ manure. Monitor nursing mares’ faecal consistency closely.
Supportive care: NSAIDs for fever and inflammation, appropriate nutrition, and rest. Severely affected foals may need oxygen supplementation or bronchodilators.
Prevention
- On endemic farms, institute regular thoracic ultrasound screening of all foals from 3 weeks of age
- Prophylactic hyperimmune plasma (containing high titres of anti-R. equi antibodies) given to high-risk foals at birth or within 24 hours significantly reduces severity of disease on endemic premises — discuss with your vet
- Reduce dust exposure on the farm — water down dry paddocks and yards, reduce stocking density
- Maximise paddock space per foal to reduce R. equi concentration in the environment
- Rotate paddocks — resting heavily contaminated paddocks reduces environmental bacterial load over time
- Keep foaling paddocks away from heavily used, manure-rich areas
When to Call the Vet
- Any foal under 6 months of age with increased respiratory rate (over 40 breaths/min at rest), cough, or nasal discharge
- A foal that appears dull, less active, or is not nursing as eagerly as normal
- Any foal on a farm with a history of R. equi disease — these farms require proactive screening, not reactive treatment
- Foals showing swollen joints, lameness, or diarrhoea alongside respiratory signs — may indicate extrapulmonary disease
Prognosis
The prognosis is good to excellent for foals diagnosed early (before extensive lung abscesses develop) and treated appropriately. Studies from well-managed screening programmes report survival rates of 90%+ when foals are identified ultrasonographically before becoming clinically ill. Foals that are severely affected by the time of diagnosis — with extensive lung pathology, severe respiratory distress, or systemic disease — have a more guarded prognosis and may have reduced athletic capacity as adults due to lung scarring.
Frequently Asked Questions
What is the “rattles” sound in foals?
The nickname “rattles” refers to the abnormal lung sounds — particularly moist crackles and wheezes — heard when auscultating the chest of a foal with severe R. equi pneumonia. The sounds result from air passing through fluid, pus, and consolidated lung tissue. By the time these sounds are audible, disease is typically advanced. On endemic farms, relying on audible rattling as the trigger for investigation is too late — ultrasonographic screening at an earlier stage is far preferable.
Can adult horses get Rhodococcus equi?
Adult horses typically have well-developed immunity to R. equi and rarely develop clinical disease. Exceptions include severely immunosuppressed adults — horses on long-term corticosteroids, horses with equine infectious anaemia, or horses with other conditions causing immune compromise. When adult horses do develop R. equi infection, it typically causes pulmonary or lymph node abscesses and carries a poor prognosis due to delayed diagnosis.
Is Rhodococcus equi present on all horse farms?
R. equi is ubiquitous in soil worldwide wherever horses have been kept. The key variable is the concentration of virulent VapA-positive strains in the soil, which builds up over years on farms that routinely keep young foals. A farm that has not had a clinical case of rhodococcal pneumonia for many years likely has lower environmental bacterial loads, but any foal farm carries some risk. Farms with a history of R. equi disease are at substantially higher risk and should implement proactive screening programmes.