How to Manage Equine Rhodococcosis on an Endemic Breeding Farm

Rhodococcus equi (R. equi) pneumonia and diarrhoea can cause high losses in foals and consequently economic losses in breeding farms because of high morbidity and mortality. R. equi is a gram positive bacterium with the ability to survive and proliferate in the macrophages of foals up to the age of 5-6 month thereby causing pneumonia and pulmonary abscesses (Martens et al. 1982). Older foals and adult horses with an intact immune system do not develop pulmonary disorders due to R. equi. This has lead clinicians and researchers to evaluate the efficacy of medical prophylactic methods in foals of endemic breeding farms such as vaccinating mares, vaccinating foals, repeated administrations of hyperimmune serum to neonates and older foals, immunostimmulatory treatment in foals and metaphylaxis through administration of macrolid antibiotics within the first weeks of live. None of these measures have shown to provide protection to the foals and reduce the morbidity on endemic breeding farms (Hurley and Begg 1995, Becú et al. 1997, Chaffin 2003, Schulte 2005, Baumann 2006, Hullmann 2006, Venner et al. 2007c).
Considering the biology of R. equi in the environment on a horse breeding farm, a stunning ability of this bacterium to survive in soil and dust under dry and sunny conditions was already described in the early 20th century (Magnusson 1923, Takai et al 1987, Muscatello et al. 2006). Several farm management measures have been evaluated whether they influence the disease rates on endemic breeding farms. The effect of separation of sick foals, high hygiene and medical level in the perinatal period, reduction of dust and reduce animal density on the farm was investigated (Chaffin 2008). So far no specific farm management measures have been identified, which could account to a reduced morbidity of foals on R. equi endemic farms.
According to our investigations on a large warmblood breeding farm consisting of about 800 broodmares, the only reliable methods of reducing morbidity and mortality amongst foals are diagnosing the disease in an very early state and initiating the appropriate therapy.
Diagnoses are obtained through clinical findings associated with WBC, isolation of R. equi and diagnostic imaging. Rhodococcus equi can be isolated from faeces, tracheobronchial secretions, nasal swabs or in air samples. It was shown recently that diagnostic imaging techniques such as sonography and radiography are more sensitive in diagnosing R. equi pneumonia in foals than culture or PCR (Walter 2006). Furthermore these imaging methods can help to detect abcsessing pneumonia in an very early stage of the disease. In daily practice, careful evaluation of the foals through the owner and thorough clinical examination along with sonography of the lungs at first signs of the disease will help to detect foals in an early state of R. equi pneumonia. Treatment should be started early in order to reduce shedding of R. equi into the environment. Adequate treatment is considered to be the combination of a macrolid antibiotic (azithromycin, clarithromycin or tulathromycin) and rifampicin over a period of 4 to 8 weeks. [...]