Rhodococcusequi Pneumonia in Foals

Rhodococcal pneumonia is the most important life-threatening lower respiratory disease in young foals.  The causative agent is Rhodococcusequi, a facultative intracellular pathogen with virulent and avirulent strains, is a common soil organism, and is also present in the intestinal tract of (healthy) horses.  The major route of transmission is aerosolization via dust particles, occasionally followed by secondary intestinal infection due to swallowing of sputum containing the organism.  However, a minor route of infection is primary intestinal infection which may lead to secondary hematogenous pulmonary infection.  Concurrent helminthic infection of the intestine predisposes to intestinal infection with Rhodococcusequi. R. equi preferentially targets macrophages where it is able to survive and multiply, presumably by inhibition of phagosome-lysosome fusion and triggering the premature release of lysosomal contents.

  R. equi infection is primarily observed in 2-4 month old foals.  At this age, maternal antibodies wane, making foals susceptible to infection.  In addition, foals with coprophagic behavior are predisposed to infection since R. equi depends on volatile fatty acids in herbivore manure for optimal multiplication.  In addition, R. equi is capable of multiplication in the intestinal tracts of foals up to 12 weeks of age which allows the organisms to concentrate on farms where foals are raised.  Although most farms are infected with R..equi, disease occurrence ranges from nonexistent on most farms to sporadic or endemic.  Variable infection rates are associated with several factors, including environmental temperature, soil pH, strain virulence, and management factors.

Clinical findings Most cases of rhodococcal pneumonia occur in foals between two and four months of age. 

Disease onset is insidious, and foals are generally able to compensate for the progressive loss of pulmonary function, making early clinical diagnosis difficult.  Despite the chronicity of lung lesions, foals usually present with an acute onset of respiratory distress and/or sudden death.  Observed clinical signs include anorexia, lethargy, slight mucopurulent nasal discharge, fever (101.5⁰ -104⁰F), tachypnea, increased respiratory effort, and lack of response to common antibiotics.  On auscultation, wheezes and crackles may be present and percussion may reveal areas of dull resonance associated with chronic abcessation.  Coughing, if present, varies from moist to nonproductive, and may be intermittent or frequent.

  Approximately 50% of pneumonic foals also present with ulcerative colitis.  Clinical signs in foals with severe gastrointestinal involvement include diarrhea, weight loss and ascites.  Immune-mediated polysynovitis is seen in approximately 1/3 of foals with lung lesions.  In these foals, effusion is most common in the tarsal and stifle joints, and is not associated with lameness.  An agent is rarely cultured from synovial fluid.  Other uncommon manifestations of R. equi include septic arthritis and osteomyelitis, subcutaneous abcesses, and uveitis.

Clinical diagnosis:  Clinical signs suggestive of R. equi infection include age of the foal, presence of fever, and lack of nasal discharge.  Hyperfibrinogenemia is the most consistent laboratory finding.  Thoracic radiographic findings include a prominent alveolar pattern characterized by discrete nodular or cavitary lesions consistent with abscesses.

  Serologic tests that have been used in diagnosis include agar gel immunodiffusion (AGID), ELISA, and synergistic hemolysis inhibition.  A major problem with serologic diagnosis is the widespread exposure of foals to R. equi in the environment, resulting in antibody production which is not necessarily associated with infection.  The presence of maternal antibodies may further confound interpretation, resulting in additional false positive tests.

  Definitive diagnosis of Rhodococcal pneumonia is based on PCR, bacterial culture, and cytology of transtracheal wash (TTW) fluid.  Presence of large gram positive pleomorphic organisms in TTW cytology is consistent with R. equi  infection.  Organisms are usually present in low numbers.  R. equi can be isolated by aerobic culture, generally within 48 hours.  RCR is a new method which is more rapid than bacterial culture and can differentiate between virulent and avirulent strains.

Postmortem diagnosis: Gross pulmonary lesions are consistent with multifocal to coalescing pulmonary abcesses and/or granulomas with consolidation and congestion of lung parenchyma; airways in affected regions may be clogged with mucopurulent exudates.  Similar abcesses and/or granulomas may be found within tracheal lymph nodes.  Histologically, lesions are composed of a necrotic central core with a collar of degenerate neutrophils.  An outer zone of macrophages, occasional giant cells, and lymphocytes surrounds the core.  The amounts of intralesionalneutrophils is variable and is increased in older lesions.  Lesions within the adjacent parenchyma include infiltration of bronchi, bronchioli, and alveolar spaces with macrophages and neutrophils in varying amounts.

  Almost ½ of necropsied foals will show multifocal ulcerative typhlocolitis, often associated with abcesses and/or granulomas within mesenteric lymph nodes.  On gross examination, the mucosa is thickened by infiltration with (numerous) macrophages, giant cells and neutrophils.  There are sharply demarcated foci of coagulation necrosis with ulceration of the mucosal surface.

  Gram-positive and partial acid-fast small rod-shaped bacteria consistent with R. equi can be detected within the cytoplasm of macrophages and giant cells by use of the special stains Brown and Brenn and Ziehl Nelson.

Prevention and treatment:  Screening methods to detect early disease on farms with a history of rhodococcal pneumonia include frequent physical examination, twice daily rectal temperature measurement, thoracic auscultation, and diagnostic imaging to detect early pulmonary lesions.  Frequent removal of manure from foaling stalls and paddocks may help decrease environmental contamination and exposure to foals.  Efforts to reduce population density and dust in the environment should be considered on large breeding farms with endemic R. equi pneumonia.

  Transfusion of hyperimmune plasma, preferably in the first few days after birth and again in the third week of life, is the only method proven, though not failsafe, to prevent R. equi pneumonia.  Oral administration of colostrums from immunized mares is not protective and effective vaccinations are currently not available.

  Standard treatment of rhodococcal pneumonia is the combination of erythromycin and rifampin.  The combination has synergistic activity and excellent penetration of pulmonary alveolar macrophages.  Dosages range from 5-10 mg/kg of oral rifampin twice daily, and 10-37.5 mg/kg of oral erythromycin 3 times daily.  Length of treatment may last from 30-60 days, depending on clinical response, return to normal laboratory values, and resolution of radiographic changes.  Additional supportive therapy includes intravenous fluids, nasal oxygen for foals with respiratory distress, NSAIDS, and a cool environment.

-by Kim, Maratea, Class of 2003

-edited by Dr. Sandra Schoeniger, ADDL Graduate student

References

1.  Bertone JJ, 1998.  Pneumonia and other disorders associated with Rhodococcusequi. Equine Internal Medicine, 970-980.

2.  CohenND, Chaffin MK, and RJ Martens, 2000.  Control and prevention of Rhodococcusequi pneumonia in foals.  CompendContinEducPract Vet, 22: 1062-1069

3.  Giguere, S. and JF Prescott, 1997.  Clinical manifestations, diagnosis, treatment, and prevention of Rhodococcusequiinfection in foals.  Vet Microbiol, 56: 313-334.

4.  Prescott, JF,             1991.  Rhodococcusequi: an animal and human pathogen.  ClinMicrobiolRev 4: 20-34.

5.  Prescott JF and AM Hoffman, 1993.  Rhodococcusequi, Vet Clin North Am Equine Pract. 375-384