What is Rhodococcus Equi?
Rhodococcus equi is a type of bacteria most commonly found in animals, especially horses and young foals. Since 1967, there have been important cases of this bacteria infecting humans. This bacteria was first discovered by a young man who was conducting research on drugs that lower the body’s ability to fight disease and infection in a stockyard. Other related types of Rhodococcus bacteria, including R fascias, R rhodochrous, and R erythropoiesis, can also cause diseases in humans. Rhodococcus is a part of the Nocardiaceae family of bacteria, which also includes Nocardia, Mycobacterium, Corynebacterium, and Gordonia. These all share some similar characteristics.
R equi is recognized as a bacterium found in soil and in the digestive tract of many herbivorous animals. It is common in animal feces, manure, field soils, and other farming areas. Even though R equi is considered rare, it is a specific danger to people with conditions that weaken their immune system. Defects in cell-mediated immunity, another term for how our bodies fight off diseases, significantly contribute to the incidence of this disease and is present in most reported cases. People with weakened immune systems or compromised immune systems from things like chemotherapy, HIV, leukemia, lymphoma, or lung cancer, long-term steroid use, or those receiving treatments like monoclonal antibodies or organ or stem cell transplants, are more likely to get infected.
R equi is a bacteria that unusually causes a slow-developing form of pneumonia that kills lung tissue, resulting in pneumonia with cavities and lung abscesses. The disease mostly leads to infections of the lungs, which can range from simple pneumonia to more severe conditions that destroy lung tissue and lead to lung abscesses.
What Causes Rhodococcus Equi?
The bacteria R equi, a type of coccobacillus that doesn’t move, was initially classified as Corynebacterium equi but was then placed in the genus Rhodococcus in the 1980s. This rod-shaped bacterium is distinctive for its red pigment, particularly in older cultures, while younger cultures show a light salmon-pink color when grown on a solid substance.
This bacteria is classified as a “nocardioform” actinomycete — a kind of bacterium often found in soil. It’s a bit unusual because it can survive inside our cells and resist efforts by our immune system to destroy it. This resistance can result in tissue damage and infection, potentially causing a severe type of pneumonia.
One protein, Vesicle-associated membrane protein (VAPA), can help R equi survive and multiply inside cells. While VAPA has been found in R equi that cause human infections, it is important to know that not all disease-causing R equi bacteria carry this protein.
The bacteria is found all over the place in soil tainted with animal manure. More than half of the known cases involve contact with farm animals like horses, exposure to farm dust, and living with people infected with this disease. The bacteria are mainly spread via dust particles in the air. Studies show that the amount of R equi borne by dust can impact the number of pneumonia cases in farm animals.
While some healthy people have been infected with R equi, it generally affects people with weakened immune systems. HIV infections, cancer, the use of certain medications like corticosteroids, and people who’ve had organ transplants are all risk factors identified.
Reports of R equi in people with a healthy immune system often involve exposure to horses. Out of 72 cases studied, about a third of the patients had come into contact with horses or other farm animals, regardless of their immune status.
Risk Factors and Frequency for Rhodococcus Equi
R equi is a type of organism that lives in the soil and is often found in the digestive systems of many herbivores, and in manure and grazing fields. The number of these organisms in the air goes up when it’s warm, reaching its highest on dry and windy days.
This organism is everywhere in solid form and grows best in the manure of plant-eating animals. People tend to get exposed to it from soil that’s been contaminated by this manure, either by breathing it in or consuming contaminated material. But there are also reports of it getting into animal hosts through wounds. Though horses and foals are the most usual hosts, other animals like sheep, cattle, cats, dogs, wild birds, and humans can also get infected.
The first time a human case was reported was in 1967, and only 12 more cases were reported for many years after that. But more recently, there’s been a noticeable increase in human R equi infections, which seems to be tied to the spread of HIV infections. Nowadays, it’s considered an opportunistic infection, meaning it tends to affect individuals with weakened immune systems. There have been cases showing serious illness in people with suppressed immune systems.
The occurrence of R equi in humans is pretty low overall. This is mostly because it usually affects people with underactive immune systems and requires certain exposure to infect people. Some studies hint that R equi might not be reported as often as the cause of pneumonia because it looks similar to a “diphtheroid” when gram staining is done. Also, when cultured, it may be disregarded as a pathogen as it might seem like part of the normal lung microbe population.
Signs and Symptoms of Rhodococcus Equi
R. equi is a bacteria that commonly causes lung infections in people. In a study, pneumonia was found in 76% of the patients who had this infection, and in 82% of these cases, the lungs were the only organ affected.
People with weak immune systems are especially vulnerable to this infection. When infection is subacute (develops slowly), the first symptoms they notice usually include fever, cough, chest pain, fatigue, and weight loss. Some might also experience pulmonary malakoplakia — a condition common in R equi infections — and bleeding from the lungs (hemoptysis), though its occurrence can vary. Bubble-like cavities (cavitary lesions) and fluid collections (pulmonary effusions) often form in the lungs as well.
Even without having pre-existing lung disease, people can get localized infections from R. equi which can lead to various complications.
- Septic arthritis (a painful, damaging joint condition)
- Cellulitis (skin infection)
- Meningitis (inflammation of the protective membranes of the brain and spinal cord)
Unfortunately, the existing knowledge about these complications is limited, but from what has been reported, the most common findings on x-rays include vague areas of lung consolidation and uneven cavities, which mainly appear in the upper part of the lungs.
Testing for Rhodococcus Equi
Rhodococcus equi, or R equi, can easily grow in lab cultures for microbiological tests. However, it’s important for doctors and lab workers to be on their toes because R equi can look very similar to tuberculosis, Nocardia, and certain Corynebacteria under a microscope. This can lead to R equi being incorrectly identified as “normal respiratory flora” in initial lab results.
Various samples can be used for R equi testing. These include blood, sputum (phlegm), pleural fluid (liquid from around the lungs), fluid from abscesses (infected areas), bronchoalveolar lavage fluid (liquid from a lung wash), peritoneal fluid (liquid from the abdomen), cerebrospinal fluid (liquid from around the brain and spine), wound samples, lymph nodes, and tissue samples from any organ thought to be affected.
Most people with R equi infection are immunosuppressed, meaning their body’s defense system is weakened. If lab tests find a type of bacteria known as gram-positive coccobacillus or acid-fast organisms in an immunosuppressed patient with cavitary lung disease (a condition where air-filled spaces called cavities form in the lungs), it should raise suspicion for R equi infection.
Blood testing is particularly important, especially in immunosuppressed individuals. This is because blood tests often come back positive when these individuals are infected with R equi.
Treatment Options for Rhodococcus Equi
Treating infections caused by Rhodococcus, a type of bacteria, can be challenging due to a lack of guidelines and comprehensive data. As a result, doctors tend to rely on veterinary data, individual case reports, and lab-based studies to help determine the best approach. Let’s summarize some popular strategies used when it comes to treatment:
- Frequently, multiple medications are used, especially in patients with a weaker immune system.
- If the bacteria show some vulnerability, a class of antibiotics called ‘beta-lactams’ are usually avoided because the bacteria can become resistant during treatment. An exception is a group of antibiotics called ‘carbapenems’.
- Long-term therapy is often required to fully treat the infection.
- Boosting the patient’s immune system is a key element of successful treatment.
Antibiotics that have proven to be effective against this bacteria include a range of medications such as extended-spectrum macrolides, rifampin, fluoroquinolones, aminoglycosides, along with others such as vancomycin, linezolid, and imipenem. It’s important to note that almost all types of R. equi are susceptible to vancomycin, and more than 90% of the isolates are also susceptible to macrolides, carbapenems, ciprofloxacin, and rifampin. However, they usually resist a few other types of antibiotics like trimethoprim-sulfamethoxazole, ampicillin-sulbactam, and clindamycin.
For healthy individuals with a strong immune system, a single agent such as an extended-spectrum macrolide or fluoroquinolone may be sufficient. However, for those with a weaker immune system, a combination of two or more medications is desirable, particularly ones that can penetrate macrophage cells, which are an essential part of the immune system. If the infection spreads to the brain and spinal cord, it’s crucial to select multiple medications that can reach these parts of the body.
In severe cases, patients with a weak immune system may need to receive intravenous therapy for 2 to 3 weeks, followed by oral administration of the medication for a minimum of 2 to 6 months. After undergoing the main treatment, patients who are immunocompromised are advised to follow a long-term course of therapy, termed as suppressive therapy, mainly composed of medications such as rifampin along with a macrolide, quinolone, or doxycycline.
What else can Rhodococcus Equi be?
When doctors see hollow areas (cavitary lesions) in your lung on an imaging test, they consider several conditions that might be the cause. These include:
- Tuberculosis
- Granulomatosis with polyangiitis (GPA, formerly known as Wegener Granulomatosis)
- Squamous cell lung cancer
- Mycobacterium avium complex (MAC) – a type of bacterial infection
- Nocardiosis – another kind of bacterial infection
- Several types of fungal infections, including histoplasmosis, blastomyces, coccidioides, paracoccidioides, and aspergillus.
What to expect with Rhodococcus Equi
The outcome and effectiveness of treatment for an infection is mainly determined by the patient’s immune system. For individuals with compromised immune responses due to conditions like HIV, lack of appropriate treatment, such as antiretroviral therapy, can cause higher death rates. Untreated HIV patients are 53 times more likely to die from the infection compared to those who are receiving proper antiretroviral treatment.
On the other hand, individuals who have a well-functioning immune system tend to have a very good chance of recovery. These patients, often referred to as “immunocompetent,” can recover successfully even with a brief period of antibiotic treatment.
Possible Complications When Diagnosed with Rhodococcus Equi
Information on the complications of this infection is mostly based on individual cases and case groups, many of which are quite old. The most common complications include bacteria in the blood (bacteremia) which can lead to a body-wide infection (sepsis). The infection caused by R equi has been found to cause brain abscesses, swelling of the lymph nodes in the chest and abdomen, bone infection (osteomyelitis), and infection in the spine (spondylodiscitis).
- Bacteremia
- Sepsis
- Brain Abscesses caused by R equi infection
- Swelling of lymph nodes in the chest and abdomen
- Bone infection
- Spinal infection
Preventing Rhodococcus Equi
For people living with HIV, starting antiretroviral therapy early is the best way to prevent further health problems, because it helps boost your immune system’s function. Additionally, people who have a compromised immune system are also typically advised to start suppressive therapy. This helps prevent conditions from worsening or recurring, as mentioned earlier.
