Rhodococcus equi in foals

1. Rhodococcus equi in foals James Montgomery, DVM September 8, 2008

2. Rhodococcus equi • Gram-positive aerobic rod • Facultative intracellular pathogen • Helps it hide from the immune system • Foals have an immature immune system • Opportunistic pathogen in immunocompromised humans - HIV http://www.cvm.tamu.edu/acvp/Rhodococcus/Rhodococcus.htm

3. Rhodococcus equi • Saprophytic inhabitant of soil • Widespread in the environment of most horse farms • Clinical disease ranges from unrecognized or sporadic on some farms to enzootic and devastating on others • Morbidity rates as high as 40%

4. Rhodococcus equi Methods for control and prevention not well understood No effective vaccines available presently Administration of R. equi hyperimmune plasma has been shown to be effective prophylactically

5. Signalment • Foals between 3 weeks and 5 months of age http://www.ca.uky.edu/gluck/ResearchDVS.asp

6. Clinical manifestation • Most common: chronic suppurative bronchopneumonia with extensive abscessation • Early clinical diagnosis difficult – slow progression and foals able to compensate well • Early clinical signs: • Mild fever • Slight increase in respiratory rate • With progression: • Decreased appetite, lethargy, fever, tachypnea, and labored breathing (nostril flaring, and increased abdominal effort)

7. Infection • Inhalation is major route of pneumonic infection • Incubation period • Experimentally: 9 days with heavy inoculum to approximately 2 to 4 weeks when lower inoculum administered

8. Diagnostics • CBC, fibrinogen • Used for monitoring • Transtracheal wash • Isolate R. equi • Radiographs • Ultrasound

9. Radiographs • Useful in evaluating the severity of pneumonia and in assessing response to therapy • Should not be used as sole criterion for prognostication and euthanasia • Prominent alveolar pattern characterized by ill-defined regional consolidation • Consolidated lesions are often more discrete nodular and cavitary • Consistent with pulmonary abscessation Lamb, 1990

10. Radiographs • In foals < 4 months: nodular lung lesions and tracheobronchial lymphadenopathy is highly suggestive of R. equi infections • In foals 4 months or older, Streptococcus zooepidemicus is another common cause of lung abscesses http://www.ca.uky.edu/gluck/ResearchDVS.asp

11. Ultrasound • Early lesions may be nonspecific irregularities of the pleural surface • In more chronic cases may see well-circumscribed, encapsulated abscesses • Very useful for evaluating severity of pneumonia and assessing response to therapy – especially if no access to thoracic radiography Ramirez, 2004

12. Ultrasound • Pulmonary consolidation • Pulmonary bronchi give lung a liver-like appearance • Hyperechoic margin indicative of air in adjacent lung

13. Ultrasound • Slovis, 2005 • Thoracic ultrasonography performed on foals on R. equi endemic farms beginning at 30 days of age and repeated at 2 week intervals until weaned (16-20 weeks) • Grading scale • 0 (no evidence of pulmonary consolidation) to 10 (entire lung affected) • All foals with a graded pulmonary lesion were placed on antibiotics: • Azithromycin (10 mg/kg, q 24 h, PO for 7 days and then every other day) • Rifampin (5 mg/kg, q 12 h, PO) • Treatments continued until ultrsonographic resolution was determined • Farms that implemented thoracic ultrasonographic monitoring had no mortalities, and a marked reduction of clinical disease

14. References • Giguère S. Rhodococcus equi infections. In Smith BP, ed. Large Animal Internal Medicine, 4thed (St. Louis, MO: Mosby Elsevier, 2009) pp. 510-20. • Lamb CR, O’Callaghan MW, Paradis MR. Thoracic radiography in the neonatal foal: A preliminary report. Vet Radiol Ultrasound 1990; 31: 11-6. • Ramirez S, Lester GD, Roberts GR. Diagnostic contribution of thoracic ultrasonography in 17 foals with Rhodococcus equi pneumonia. Vet Radiol Ultrasound 2004; 45: 172-6. • Slovis NM, McCracken JL, Mundy G. How to use thoracic ultrasound to screen foals for Rhodococcus equi at affected farms. AAEP Proceedings 2005; 51: 274-8.