Rhodococcus equi

Over the last few years vets at Pool House Equine Clinic have been seeing more and more cases of Rhodococcus equi (R. equi). This bacterium is usually associated with pneumonia in older foals (usually 2-5 months of age) but in some cases it causes diarrhoea, uveitis, bone infections, abscesses and joint ill. It is a potentially fatal disease but in the majority of cases can be treated successfully.

What causes it?

R. equi has been found all over the world in soil on premises where horses are kept but the occurrence of disease varies enormously between farms. Risk factors are high stock levels, dry soil and short grazing; this seems to be related to the ease with which loaded dust is flung up into the air to be breathed in by susceptible animals. It is very important to note that the infection is contracted very early in life – probably in the first couple of weeks. It is NOT spread directly foal to foal, but the faeces of affected foals can spread the R equi bacteria into the soil so clinical cases may be best stabled or put on grazing only normally used for adult horses. Adult horses do not develop disease following exposure to the bacterium as they develop a solid immunity. They can however carry and spread the bacterium via their droppings.

Clinical signs

Signs progress very slowly over a long period of time as abscesses are formed throughout the lungs. —Eventually an elevated temperature, coughing, increased depth and frequency of breathing and abnormal respiratory sounds (rattles) develop. On auscultation of affected foals’ lungs adventitious sounds like wheezes and rattles are often heard which can be quite severe. Many foals do not show obvious signs of disease until it has progressed to the point where they are severely ill.

Diagnoses

Diagnosing a foal with R. equi infection can be very difficult. Clinical signs can be very typical but to get further evidence supporting a suspicion can be disappointing. —There is no blood test specific for R equi infection (although blood fibrinogen and platelet levels are usually markedly increased). Nasal swabs are not usually helpful. In some cases the bacterium can be cultured from a tracheal wash; a sample of fluid is taken from the trachea going directly through the skin (not via a scope through the nose) to avoid contaminating the sample with other bacteria present in the upper airways. One of the easier ways to diagnose the condition is by thoracic ultrasound where classic ‘comet tails’ can be seen in case micro-abscesses are present. Sometimes evaluating the response to treatment is the only way to get further information. Depending on the yard history with relation to this disease, the presentation of the foal and other individual requirements a diagnostic plan will be made.

Treatment

Various antibiotics have been used in the treatment of Rhodococcus. The most effective courses involve the combination of Rifampicin with either Erythromycin, Clarithromycin or Azithromycin. These usually have to be given for a minimum of 4 weeks depending on improvement seen in clinical signs or other diagnostics used. If started early enough and treatment has been administered correctly most foals will make a full recovery.

Prevention

There are currently no effective ways to prevent the disease from developing. If recognised and treated early the recovery chances of the affected foals are much better and a shorter course of treatment is required. This currently has to be the main focus and on high risk yards regular screening of foals (ultrasound examination of the lungs) has been advised. Vaccination against the disease is unfortunately not possible at the moment. Administration of antibiotics in the first weeks of life has been tried but not found to be very effective, it also promotes antibiotic resistance and is therefore advised against. Giving foals hyperimmune plasma with specific Rhododoccus antibodies during the first days of life and at 3 weeks of age has shown to reduce the number of foals that get the disease and the severity of the pneumonia. This could be an option for high risk yards.

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