Jeanette L. McCracken
Screening for Rhodococcus equi Pneumonia
The most common cause of severe pneumonia in foals from the age of 4 weeks to 3 months is Rhodococcus equi. Older foals (up to 6 months of age) may also develop Rhodococcus pneumonia; however, other pathogens are frequently the cause of pneumonia in these animals, including Streptococcus spp, Klebsiella pneumoniae, Actinobacillus spp, and Escherichia coli. Rhodococcus equi is a facultative intracellular coccobacillus that is found around the world in soil and air samples. Only strains that carry the VapA plasmid are virulent and responsible for clinical disease. Foals become infected within the first few days of life but usually do not develop signs of clinical illness until they are 1 month of age or older. Infection by the organism results in the development of suppurative abscesses in the lungs and at extrapulmonary sites, and clinical signs may include high fevers, coughing, high respiratory rate and increased effort, depression, and weight loss. The most common site of R equi disease in foals is the lungs; however, the organism also causes extrapulmonary disease manifestations that include abdominal abscesses, enteritis or colitis, hepatitis, septic arthritis, polysynovitis, osteomyelitis, and uveitis.
A foal of the appropriate age with high fever, coughing, high respiratory rate, and labored breathing of any degree should be suspected of having R equi pneumonia. Although physical examination, ultrasound findings, and blood analyses (high white blood cell [WBC] count and fibrinogen concentration) are often used to diagnose R equi infection on endemic farms, bacteriologic culture or polymerase chain reaction (PCR), combined with cytologic examination of transtracheal aspirate fluid, is still the gold standard for definitively diagnosing R equi pneumonia. Treatment of foals with clinical disease typically involves weeks to months of antimicrobial treatment and management adjustments. The antimicrobials of choice for treatment of rhodococcal disease are macrolides (erythromycin, azithromycin, or clarithromycin) and rifampin. Fever is often difficult to control early in the disease, and some foals need oxygen supplementation. Management of ill foals may be intense and may necessitate close monitoring and changes in routine to prevent adverse effects of macrolide administration, including hyperthermia and enteritis. The prognosis for severely affected foals is guarded.
Prevention
Prevention of R equi pneumonia is an important goal of owners and veterinarians because of the extreme severity of clinical disease, prolonged treatment, possible death, and heavy economic burden associated with the disease. Several disease prevention strategies have been investigated, namely, transfusion with hyperimmune plasma, chemoprophylaxis during the neonatal period, and thoracic ultrasound screening. Studies examining the efficacy of administration of hyperimmune plasma have yielded conflicting data. Treating foals with azithromycin and rifampin during the neonatal period can reduce clinical pneumonia on endemic farms, but this cannot be recommended as a prevention method because of the high risk for development of bacterial resistance. Recently, thoracic ultrasound has been used to identify foals with subclinical disease. These foals can be treated before the onset of clinical signs or monitored more closely in an attempt to identify clinical problems as early as possible. Ultrasound screening programs lower the incidence of clinical pneumonia.
Screening
Recognition of foals with R equi pneumonia before development of clinical signs may reduce losses and limit costs associated with long-term treatment of clinically affected foals. Different methods of screening foals for early detection of R equi have been investigated. It is important to note that a “positive” result on a screening test is not definitively diagnostic for R equi pneumonia. Depending on the screening method used, a positive finding may mean only that the foal has evidence of some sort of infection, the foal is infected with R equi but the infection is subclinical, or the foal is in the early stages of clinical R equi pneumonia. Interpretation of a positive screening result should be done with consideration to the sensitivity and specificity of the method used. Any decision to treat an animal with positive findings should be made by balancing the benefits and risks of treating the animal and considering the reliability of the screening tests that have been employed.
Observation and Temperature Monitoring
Visual observation and temperature monitoring of foals has been advocated as an economic method of early detection of R equi pneumonia. The disease is notoriously insidious in onset, and clinical signs such as coughing, elevated respiratory rate and effort, depression, and fever are usually not observed until the disease becomes clinical. By definition, appearance of any of these signs indicates the early stages of clinical disease. Noticing signs of illness promptly may allow for rapid treatment initiation and may help prevent development of more severe disease. Unfortunately, by the time a clinical sign is present, the infection is usually firmly established in the foal. Any foal on an endemic farm that is noticed to have one or more such clinical signs warrants further examination. This may include thoracic imaging and complete blood counts. Only if additional screening methods yield positive findings should treatment for R equi pneumonia be considered.
Thoracic Auscultation
Biweekly thoracic auscultation of foals at high risk for R equi pneumonia has been proposed as a method of screening for early disease detection. This is not recommended as a solo screening method because of the expense of veterinary examinations and the lack of specificity of a “positive” result. Rhodococcus equi lesions are typically abscesses in the lung parenchyma, which alone do not cause abnormal lung sounds during examination. Only by careful auscultation and detection of areas in which respiratory sounds are absent (i.e., detection of functional dead space) are abscesses identifiable through auscultation. The small size of subclinical abscesses makes such detection nearly impossible. Identification of preclinical infection is therefore unlikely. After R equi infection progresses to clinical illness, lung sounds may reflect generalized lung inflammation. This may be appreciated on auscultation, but is not specific for R equi and is indicative of active clinical disease.
White Blood Cell Count and Fibrinogen Concentration
Monitoring WBC count and fibrinogen concentration has been evaluated as a method of early diagnosis. High values for these blood parameters are nonspecific indicators of infection and inflammation. Rhodococcus pneumonia is one of many disease processes that can result in high WBC counts and fibrinogen concentrations. If this is used as a screening method, further examination of an animal with positive findings is warranted to determine the source of infection. It has been proved that a high WBC count is a more reliable indicator of R equi infection than fibrinogen concentration, contrary to historical beliefs. One study has revealed that use of a cutoff value of 14,000 cells/µL is both reasonably sensitive (88.1%) and specific (80.6%). Sensitivity decreases with higher cutoff values, whereas specificity increases. Determination of what WBC count to use as a cutoff value should thereby be determined by the historical level of infection on endemic farms. Similarly, fibrinogen concentrations higher than 600 mg/dL have good specificity for R equi but poor sensitivity. As such, evaluation of WBC count is preferred over fibrinogen concentration for a screening method. It is not known what time frame exists between the increase in WBC count and onset of clinical disease. In the author’s experience, WBC counts and fibrinogen are very commonly normal on the days immediately preceding development of clinical signs. It is possible that serial evaluations may increase the diagnostic value of this screening method; however, no data on this issue have been published.
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