Table 12-15.

Differential diagnosis of respiratory diseases of older (not newborn) foals

Disease	Epidemiology	Clinical findings	Clinical pathology	Necropsy findings	Treatment and response
Rhodococcus equi infection	Enzootic to a farm. Foals up to 6 months. Infection by inhalation. Case-fatality rate ≈30%.	Pneumonia in 1- to 6-month-olds. Occasional diarrhea. Aseptic synovitis and uveitis in affected foal. Septic osteomyelitis.	Inflammatory cells in tracheal aspirate. Culture or PCR detection of R. equi from tracheal fluid. Serum tests not useful in individual animals.	Suppurative bronchopneumonia. May be mesenteric and other lymph node abscess. Rarely septicemia.	Erythromycin estolate, or clarithromycin, plus rifampicin. Advanced cases may be refractory.
Interstitial pneumonia	Sporadic occurrence in foals to 6 months of age. Cause not identified.	Respiratory distress with minimal cough, slight nasal discharge and low grade to non-existent fever. Lungs sounds not remarkable.	None diagnostic. Rule out other diseases. Radiography useful.	Interstitial pneumonia.	Corticosteroids. Broad-spectrum antibiotics (e.g., penicillin and gentamicin). Supportive care.
Viral respiratory disease (see Table 12-16)	Foals usually over 2 months. Rhinitis virus, herpesvirus, and influenza virus infection.	Fever, cough, nasal discharge.	Viral isolation. Serology.	Usually survive although fatal influenza infection reported.	Supportive. Antibiotics for secondary bacterial (Streptococcus zooepidemicus) infection
Combined immunodeficiency of Arabian foals	Inherited as autosomal recessive trait. Affected animals are homozygous.	Poor condition, tire easily, cough, ocular and nasal discharge, diarrhea in some.	Severe lymphopenia. Hypogammaglobulinemic as passive immunity declines.	Lymphocytes absent from lymphoid tissue. Adenoviral pneumonia.	None.
Respiratory tract infection with S. zooepidemicus	Outbreaks in foals up to weaning. Likely secondary to viral infection.	Fever, nasal discharge, cough, inappetence. Minimal lymphadenopathy.	S. zooepidemicus in tracheal aspirates.	Usually survive.	Penicillin. Good recovery rate
Parasitic pneumonia	Migrating stages of Parascaris equorum. Foals > 6 weeks old.	Cough, slight nasal discharge. Rarely fever.	Eosinophils in tracheal aspirate.	Death rare.	Anthelmintics, e.g., fenbendazole
Pneumocystis jirovici (formerly P. carinii) pneumonia	Immunodeficient foals or foals administered corticosteroids.	Cough, mucopurulent nasal discharge, fever, lethargy, tachypnea.	Neutrophils and macrophages and P. jirovici cysts in tracheal aspirate or bronchoalveolar lavage fluid.	Pneumonia, diffuse with neutrophilic or lymphocytic/plasmacytic infiltration and alveolar edema. P. jirovici evident in silver-stained lung sections.	Sulfonamide/trimethoprim 30 mg/kg q12 h recommended but often not effective.

PCR, polymerase chain reaction.