RHODOCOCCUS equi affects foals on many farms in Ireland each season, including those farms breeding smaller numbers. Early recognition and effective treatment with preventative screening protocols can reduce mortality rates, time and costs associated with lengthy treatment protocols.

Rhodococcus equi treatment is expensive, time-consuming and best avoided. This article aims to explain the important characteristics of Rhodococcus equi, how it affects foals and the principal strategies employed for its prevention, early recognition and effective treatment. Having in-depth information and understanding of the disease can greatly help to reduce its incidence with a vet/owner team approach.

CHARACTERISTICS & TRANSMISSION

Rhodococcus equi is a bacterium which causes disease in foals usually between one to four months of age. It most commonly causes pneumonia and has traditionally been associated with a drier and dustier climate, eg an Australian summer. However, it is not exclusive to this type of environment and still causes disease in wet conditions such as that of a typical Irish summer.

Rhodococcus equi lives and grows in soil. It is ingested during grazing and passes through the gastrointestinal tract where it amplifies and is then deposited in the horse’s faeces back into the soil. The bacteria survive well in horse faeces and so reduced stocking densities play a major role in reduction of spread of disease.

Adult horses and foals both deposit the disease in their faeces while foals deposit larger numbers than adults. Foals showing overt clinical signs of disease shed the largest amount of bacteria. This is why removal of faeces from fields in considered an important and effective preventative technique in the spread of the disease.

Rhodococcus equi is also airborne. Dust particles from soil and dry faeces are inhaled and infect namely the respiratory tract of susceptible foals. Warm, windy and dry conditions and low grass cover aid in an increase in disease prevalence. High stocking densities and a tight foaling window give birth to a large number of foals simultaneously which increases the risk of disease transmission. This describes the typical flat thoroughbred calendar, however the more extensive National Hunt breeding window is not without risk.

Like tuberculosis, Rhodococcus equi lives and replicates in cells within the foal’s body called “macrophages”. This makes the bacteria particularly difficult to penetrate by antibiotic therapy in comparison to other diseases. Antibiotic therapy must therefore be precise with respect to the animal’s individual weight and given until infection is no longer active; often a relatively long period of time.

CLINICAL SIGNS

RECOGNITION OF CLINICAL SIGNS AND EARLY DIAGNOSIS CAN GREATLY REDUCE MORTALITY RATES, SPREAD OF DISEASE AND COST OF TREATMENT.

RESPIRATORY DISEASE

The most common manifestation of Rhodococcus equi is pneumonia caused by abscesses in the lungs. Foals identified with milder clinical signs are more easily treated for shorter periods of time. Endemic farms in Australia have managed to almost eradicate the problem through early recognition of clinical signs.

Clinical signs of Rhodococcus equi pneumonia include:

  • Increased respiratory rate especially after exercise
  • Difficulty breathing
  • Coughing
  • Fever
  • Nasal discharge
  • Ill thriftiness, dull coat, “not thriving”.

    • Other signs include:

  • Diarrhoea
  • Colic
  • Weight loss
  • Lameness
  • Swollen joints
  • Sore eyes (uveitis)
  • Rarely sudden death

    • DIAGNOSIS & TREATMENT

    Your veterinary surgeon will employ a multimodal strategy for diagnosis of Rhodococcus equi. History and clinical signs of Rhodococcus equi will often suffice. Ultrasonography of the lungs and blood work are common diagnostic techniques. The clinician may obtain measurements and calculate the collective diameter of abscesses across the lung surface in order to monitor disease progression. A tracheal wash (fluid sample from the trachea) sent for bacterial culture is the most reliable method for obtaining a definitive diagnosis. It allows your veterinary surgeon to exclude other diseases with similar clinical signs.

    A “macrolide” (family of antibiotics ending in “mycin”) in combination with rifampin is presently the gold standard treatment of choice for Rhodococcus equi. Traditionally erythromycin – rifampin was the combination of choice however erythromycin has largely been replaced by much more effective macrolides which are reported to have less side effects eg clarithromycin and azithromycin. A once weekly injectable called Draxxin has been investigated but has been proven unreliable in the treatment of Rhodococcus equi and is therefore not licenced for use in horses.

    It is largely accepted today that the clarithromycin-rifampin combination is the most efficacious and requires a shorter course of treatment when compared to the other combinations. It is a twice daily treatment rather than three times daily and it is associated with less of a risk of antibiotic-induced diarrhoea and hyperthermia like that of other combinations.

    Doxycycline is also effective in the treatment of Rhodococcus equi but it takes longer to effectively clear infection. It is however a good option in the event of macrolide resistance and for foals showing adverse reactions to treatment.

    The macrolide – rifampin combination has historically been laborious and difficult to give requiring the opening and/or crushing of tablets etc. A drugs compounding company, BOVA Pharmaceuticals, has recently come from Australia to the UK (based in London) and now to Ireland. The company specialises in making drug combinations eg clarithromycin-rifampin in an easy to administer paste form with a molasses base which taste nice according to foals!

    All macrolides come with a risk of antibiotic-induced diarrhoea in adult horses. Precautions should be taken to ensure that the mare ingests none of these medications while treating the foal.

    ANTIBIOTIC RESISTANCE

    Antibiotic resistance is an alarming problem in human and veterinary medicine. Use and misuse of antibiotics are the cause of resistance. Drs Fenton and Buckley used data from the Irish Equine Centre to show that erythromycin and rifampin resistance increased in Ireland each year between 2007 and 2014. The worry is that soon we won’t have antibiotics that are effective against the disease!

    The horse owner plays an important role in reducing the development of resistance. Use the correct dose of antibiotics by obtaining an accurate weight for the foal. Under-dosing = resistance. Foals may gain weight over the course of the treatment and a dose increase maybe required.

  • DO NOT skip treatments. This will cause resistant strains of bacteria. Treat for the correct period of time to prevent residual infection and consequent resistance.

    • There is no set time period for treatment. It is not advised to depend solely on the absence of clinical signs to stop treatment. Residual infection can remain. Ask your veterinary surgeon to perform ultrasonography +/- blood work to investigate whether infection has been cleared from the body.

    CONTROL & PREVENTION

    Reduced stocking densities play an important role in minimising disease. Studies have shown that Rhodococcus equi is contagious in the aerosol form between foals particularly in large groups. Field rotation to prevent overgrazing helps prevent dust accumulation. Reseeding areas that have very little grass is advised. Removing faeces from paddocks and not spreading affected faeces/manure on land is helpful because virulent Rhodococcus equi can live in soil. Spreading lime on soil may help since Rhodococcus equi likes an acidic PH. This strategy is employed on many Australian stud farms although its effectiveness has not been proven. Removal of affected animals from the healthy herd and practicing good biosecurity techniques are obvious steps in preventing spread of disease.

    Preventative techniques on endemic farms include the administration of hyperimmunised plasma in the first 24 hours and at a later date. This does not guarantee prevention but softens the blow on endemic farms. Foals are born with no antibodies and acquire their immunity from colostrum.

    This antibody protection is finite leaving a short window of vulnerability until its own immunity has fully developed. The argument for two stage plasma administration is that Rhodococcus equi antibody levels will remain boosted throughout this vulnerable window.

    It is advantageous but not 100% effective and does not eliminate the need for screening protocols. The cost effectiveness of transfusion depends on the value of the foal and prevalence of disease.

    Screening protocols include scanning chests regularly until four months old when the immune system of the foal is mature. Daily temperature checks on foals is also good practice on endemic farms.

    Until the invention of an efficacious vaccine, the responsibility remains largely on farm management techniques to control and prevent Rhodococcus equi associated disease. Early recognition and diagnosis, effective treatment and protection of the reliable antibiotic combinations we have left as well as sensible biosecurity techniques create the framework for control and prevention of this economically significant disease.

    Susan Salter BSc (Hons), BVM&S, MRCVS, graduated from Edinburgh in 2011. She is a stud medicine focused veterinarian who performs stud work in Australia (Victorian Equine Group/Woodside Park Stud) and based in North Yorkshire working with leading UK experts at Equine Reproductive Services. For any questions about the content of this article, feel free to email Susan to suevet16@gmail.com.