Discussion

Rhodoccocus equi is widespread in soil and faeces on horse farms. Virulent strains are characterised by their ability to survive and replicate within macrophages. The most important virulent antigen in the horse, although not the only one, appears to be VapA. R. equi organisms are highly dependent upon iron for survival and VapA expression may controlled by iron. R. equi can be found in the manure of adult horses and in foals that are only a week old. Faecal shedding in foals is caused by either coloniSation, pass-through from the environment, or in slightly older foals swallowing infected sputum. Inhalation of dust particles with R. equi is the route of pulmonary infection in foals. Most foals that develop severe respiratory disease are probably infected in the first 14 days of life, before enteric R. equi stimulates an appropriate and possibly protective immune response. Older foals may develop pulmonary infections, but most have self-recovery and clinically silent disease. R. equi is an intracellular pathogen and its pathogenicity is related to its ability to replicate in and eventually destroy macrophages. Large numbers of inflammatory cells migrate to the infection and granulomas/abscess develops.
Diagnosis can often be made based upon farm history, clinical signs, age CBC, and ultrasound findings. Most R. equi pneumonia foals are 1 - 4 months of age, most have minimal nasal discharge, and frequently have either swollen joints without lameness or uveitis. R. equi may also cause septic arthritis/physitis or diarrhoea in this similar age foal. Acute onset of septic physitis/arthritis without history of injury is strongly suggestive of either R. equi or Salmonella. R. equi diarrhoea is often insidious and persistent until proper treatment is provided. I have not seen the high morbidity diarrhoea outbreaks with R. equi that I sometimes see with Salmonellosis in nursing foals. Most foals with septic arthritis/physitis and/or enteritis caused by R. equi have lung pathology, but pathology in the lung may be clinically silent in approximately half of the cases. Likewise, many foals with R. equi pneumonia have colitis, but may not have diarrhoea. Abdominal abscess and peritonitis may occur in some foals and unless the peritonitis is septic, the diagnosis can be difficult. Faecal cultures, abdominal ultrasound and finding of significant weight loss are all helpful in the diagnosis. Other clinical syndromes include; pleuritis, severe immune mediated anaemia, endocarditis, subcutaneous or skin abscessation, and abscesses in other organs, e.g. liver.
A substantial number of field trials and experimental studies on Rhodococcus equi pneumonia provide valuable information which permits a strong evidence-based approach to the management of this disease. There are several scientific publications on early identification of R. equi pneumonia cases. Ultrasound screening on a weekly basis beginning at 2 - 3 weeks on endemically affected farms would appear to be the most useful and practical method of detecting early cases of R. equi pneumonia. Those foals with lesions >2 cm, should be treated in my opinion, especially if the foals are <2 months of age. Thoracic radiographs and culture, PCR for R. equi or VapA gene from a transtracheal wash and faecal sample can be used to confirm the diagnosis, but one or more of these may be unnecessary. Due to occasional antimicrobial resistance against macrolides and rifampin, culture and sensitivity is recommended in some situations.
Treatment is based upon both clinical trials and the combination of: in vitro sensitivity of the organism, in addition to knowledge of bioavailability, and pharmacodynamics of the selected antibiotics. Clarithromycin combined with rifampicin was the preferred treatment in one study (Giguere 2004). Other macrolides, i.e. azithromycin in combination with rifampin, are still used by some veterinarians, depending upon cost and treatment practicality. The macrolide and rifampicin treatments might be best separated by 2 h based upon competition for absorption (Peters 2011). The macrolides have a relatively high incidence of diarrhoea in older foals and this author would caution their use in equines >4 months unless the diagnosis of R. equi is confirmed. If a gram negative organism, e.g. pasteurella is also cultured from the tracheal wash sample, ceftiofur or gentamycin can be added to the above treatment depending on in vitro sensitivity. Some farms may have concurrent R. equi and Salmonella problems, in which case, treatments may vary depending upon the in vitro sensitivity of the organisms. We have successfully used chloramphenicol for some of the concurrent Salmonella/Rhodococcus infections. If the chest radiographs have a ground glass appearance around the abscesses or pneumocystis carinii is seen on TTW or BAL, then TMP/S is added to the treatment regime. Doxycycline is also used in some foals when small abscesses (<1 cm) are observed on ultrasound examination. Non-antibiotic peptide treatments have been recently reported (Schlusselhuber 2012). There are likely a large number of foals with R. equi abscesses that recover with no treatments making efficacy studies more difficult. Supportive treatment for R. equi pneumonia is indicated and may include, omeprazole, fluid therapy if diarrhoea is present and topically applied ophthalmic corticosteroids for uveitis/corneal oedema. Intranasal oxygen and nebulisation with bronchodilators are indicated in severely hypoxaemic cases. If the foal is lame then septic physitis or arthritis should be presumed and joint lavage, regional perfusion and/or physeal injections with erythromycin lactobionate administered. Aspirates of the physis or joints can be used to confirm the aetiological diagnosis. Duration of treatment for R. equi pneumonia is a minimum of 3 weeks and generally is continued for 2 weeks after the WBC count, fibrinogen and serum iron have returned to normal range and/or abscesses are no longer cavitated on ultrasound examination.
Prevention of the disease can be difficult since many risk factors (number of foals on the farm, introduction of new foals, environmental conditions) may be unavoidable (Chaffin 2003). Regardless, efforts should be made to decrease environmental risk factors. Prophylactic administration of hyperimmune plasma has been shown in a few studies to decrease the incidence of clinical pneumonia on previously endemic farms and is recommended by this author for farms with endemic R. equi problems or high risk factors for R. equi including high numbers of virulent R. equi in the air samples (Chicken 2012).