Rhodococcus equi — “the silent killer”

Rhodococcus equi is a name that carries weight on any breeding farm not because it is untreatable, but because of how quietly and insidiously it develops, and how severe the consequences can become once the disease is established.

Often described by veterinarians as a “silent” disease, Rhodococcus equi does not follow the pattern breeders might expect from a respiratory infection. Foals can appear bright, active and clinically normal while significant lesions are already developing within the lungs. By the time more obvious signs such as a dry cough, fever or respiratory distress appear, the disease is often well advanced and considerably more difficult to manage.

This combination of environmental persistence, subclinical progression and delayed clinical expression makes Rhodococcus equi particularly challenging in practice. The bacterium becomes a recurring management issue rather than an isolated event, which is why long-term vigilance is essential.

To better understand how it presents itself and how it is managed under different conditions, we spoke with veterinarian Ilse Gerits, whose experience spans Belgium, Australia, the United States and the UAE, and veterinarian Marie Guiot of Vet’Argonne / Haras de Talma in France, where proactive, and farm-specific strategies have been implemented.

Understanding the disease

At its core, Rhodococcus equi is an environmental bacterium, commonly found in soil on horse farms, particularly under dry, dusty conditions. It is spread through manure and becomes airborne in dust particles, which are then inhaled by foals. Exposure occurs very early in life, within the first days to weeks after birth, making infection less about direct contact between animals and more about the environment in which they are raised.

“Once inhaled, the bacterium reaches the lungs, where it interacts with the foal’s immune system,” explains Marie Guiot. “Under normal circumstances, immune cells, particularly macrophages, eliminate pathogens. However, Rhodococcus equi can survive and multiply within these cells. If the foal’s immune response is insufficient, whether due to immaturity, stress or individual susceptibility, the bacteria can proliferate and form abscesses within the lung tissue.”

What makes the disease particularly challenging is its progression. Foals may remain clinically normal while significant pathology develops. Clinical signs, including fever, lethargy, tachypnoea, nasal discharge and respiratory distress, typically appear later, most commonly between two and six months of age. While Rhodococcus equi is most associated with pulmonary disease, it is important to recognize that the infection is not always limited to the lungs. In rare cases, the bacterium can spread beyond the respiratory system, leading to extrapulmonary manifestations. These may include abdominal abscessation, septic arthritis or polysynovitis, and, more rarely, uveitis affecting the eyes. By the time the most visible signs are evident, the disease may already be advanced, and in some cases, it can already be too late.

Because early clinical signs can be subtle, daily observation remains one of the most important tools on any breeding farm. The earlier changes are recognized, the better the chances of successful intervention.

“You have to look at your foals every day,” Ilse Gerits emphasises. “If you see something that is not normal, you should act immediately, not wait until the next day.”

Breeders should pay particular attention to:

•   Change in attitude - Slight lethargy, reduced interaction, or a foal that is less bright than usual

•   Nursing behaviour - Drinking less, slower nursing, or reduced appetite

•   Respiration at rest - Increased respiratory rate, shallow breathing, or subtle abdominal effort

•   Body temperature - Recurrent or persistent fever, even if mild

•   Exercise tolerance - Foals that tire more quickly or lie down sooner than expected

•   Nasal discharge or dry coughing - Often absent in early stages, but absence does not rule out disease

•   Growth and development - Foals that fall slightly behind or do not thrive as expected are in general more prone to infections.

“Many foals will have Rhodococcus equi present without showing any signs,” says Ilse Gerits. “The key question is not whether they are exposed, but whether they actually develop the disease.”

This distinction is critical. While the bacterium is widely present in breeding environments, only a proportion of foals will go on to develop clinical pneumonia. Recent studies have also pointed toward a possible genetic component. As Ilse Gerits notes, “some foals may be inherently more susceptible than others, meaning that exposure alone does not fully explain why one foal develops clinical disease while another remains subclinical.” Although this remains an emerging area rather than a practical diagnostic tool, it may help explain some of the variability seen in the field.

This ability to survive within immune cells is also what makes Rhodococcus equi an opportunistic pathogen beyond equine medicine. In rare cases, it has been reported in immunocompromised humans, such as patients undergoing chemotherapy or individuals with HIV, highlighting its dependence on host immunity.

Exposure, risk and environment

While Rhodococcus equi is commonly associated with large stud farms, where high horse density and constant movement create ideal conditions for the bacterium to circulate, the line is far less clear-cut. Recipient mares, breeding stations, foaling centres and clinic visits all create links between farms, meaning that even smaller breeders can be exposed.

A particularly challenging aspect of Rhodococcus equi is its inconsistency. On so-called endemic farms, estimated in some studies to represent roughly 13–25% of breeding operations, the overall level of environmental contamination is higher, and the likelihood of clinical cases increases. However, even within the same group, under seemingly similar conditions, one foal may develop severe disease while another remains unaffected or only shows mild, subclinical changes.

Importantly, Rhodococcus equi is not confined to pasture. The bacterium is spread through manure and transported via dust, equipment, footwear and airflow. Even well-managed stables are not immune to environmental contamination. “It’s not like a virus,” Ilse emphasizes. “It’s not coughing that spreads it; it’s the dust in the environment.”

Foals born later in the season may therefore face increased exposure as contamination builds and conditions become warmer and drier. Yet severe cases can still occur outside typical high-risk conditions.

Prevention and management strategies

Because eradication is unrealistic, the focus shifts to management. On breeding farms, risk is influenced not only by climate and stocking density, but also by how manure is handled, how often paddocks are cleaned, how stables are ventilated, and how much contaminated material is moved between areas.

“To avoid Rhodococcus equi completely, you would essentially have to eliminate dust, and that is not realistic”, says Marie Guiot.

Preventive strategies beyond basic management vary depending on the farm, the level of exposure, and what is practically feasible, and include:

 Regular manure removal

• Careful hygiene of stables and equipment
• Minimizing dust exposure as much as possible
• Avoiding manure spreading on fields
• Avoiding harrowing or dragging fields in dry conditions

For Ilse Gerits it has become particularly clear through her work across Belgium, Australia, the United States and the UAE, that warm and dry environmental conditions tend to favour the bacterium. “The bacterium is always there,” Ilse explains. “But the pressure is not the same everywhere.” In dry, warm and dust-prone environments such as Australia or the UAE, exposure pressure is significantly higher. “You don’t have the same grass coverage, and the dust carries the bacteria much more easily, while heat itself also affects the foal’s immune system.”

On larger stud farms, hyperimmune plasma is often used to support passive immunity in young foals. Ilse Gerits saw this approach used widely in the UAE, where environmental pressure was high and breeders aimed to protect foals during their most vulnerable early period. “It is effective,” she notes, “and hyperimmune plasma may reduce the risk or severity, but it is not a guarantee.”

In systems with increased environmental challenge, such as those encountered in Australia, structured screening protocols form a central part of management. “Regular thoracic ultrasound allows you to detect lesions at a much earlier stage,” Ilse explains. “In high-risk environments, foals are often scanned systematically from around three weeks of age, allowing close monitoring of lesion development and progression.”

Regular thoracic ultrasound, combined with clinical examination and blood parameters, allows earlier identification of lesions and supports a more selective, evidence-based treatment strategy.

At Haras de Talma in France, a farm-specific autogenous vaccine has been implemented, based on Rhodococcus equi strains identified on the farm itself.

“We have much land, but also many horses, and when you have many horses on the same fields over time, the contamination builds up,” says Dr. Marie Guiot. “In these conditions, environmental pressure increases, and the need to manage the environment properly and work proactively becomes very important.”

“Like many other large-scale European breeding farms, we used hyperimmune plasma in the past. It provides passive immunity and can be very effective, but it requires intravenous administration, which is not without risk and is not always practical in such young foals. There were also regulatory limitations in France, which made it difficult to continue.”

“Over the years, we have built a collection of the strains present here, and these are all included in the vaccine. Today, we vaccinate all foals, and we have not seen cases in vaccinated foals,” she explains. “The only concern would be if a new strain appears or mutates, but so far the system has been very effective.”

However, Marie is clear that this is not a universal solution. “This works for our farm because the vaccine is based on our strains. Each farm has its own bacterial population, and you never know exactly which strain you are dealing with.”

The protocol starts at 14 days of age and consists of three subcutaneous injections at increasing doses.

Autovaccine protocol: 

• Day 14: 0.5 ml SC
• Day 21: 1.5 ml SC
• Day 28: 2.5 ml SC