Foal pneumonia (Proceedings)

Foals between 1 and 6 months of age frequently present with lower airway infection and is the primary cause of disease and death in foals aged between 1 and 6 months of age. Foal pneumonia is primarily caused by bacterial infection and among all isolates, Streptococcus zooepidemicus and Rhodococcus equi are the most important. Several other aerobic bacterial species may also be involved including, Actinobacillus spp, Bordetella bronchiseptica, Escherichia coli, Klebsiella pneumoniae, Pasteurella spp, Pseudomonas spp, Salmonella spp and Staphylococcus spp.

Etiologies

Streptococcus zooepidemicus - Streptococcus zooepidemicus equi subsp. zooepidemicus is a beta hemolytic, gram-positive coccus that is a commensal organism colonizing the tonsil and nasopharyngeal mucosa of healthy horses. However, it is also a recognized pathogen of the horse's respiratory tract. The exact mechanisms implicated in the pathogenesis of S. zooepidemicus pneumonia in horses are not known. Opportunistic infection with S. zooepidemicus possibly occurs after the host defense mechanisms have been overwhelmed.

Rhodococcus equi - Rhodococcus equi is a pleomorphic gram-positive coccobacillus and a facultative intracellular pathogen of macrophages. It is a major cause of pneumonia in foals aged between 1 and 6. Rhodococcus equi causes chronic pyogranulomatous pneumonia; however, foals may less commonly develop intestinal or abdominal lymph node abscessation, osteomyelitis, uveitis, pahophthalmitis, cellulitis, subcutaneous abscesses, and hepatic and renal abscesses.

Although R. equi is found in the soil of most farms, pneumonia caused by this organism can be endemic, sporadic or unrecognized, depending on the farm studied. Several factors may influence the incidence of the disease including the degree of contamination on the farm, density of horses, climatic conditions and virulence of the isolate. The infection is acquired through inhalation. Environmental conditions appear to play a role in the pathogenesis of the disease with warm, dry and windy conditions favoring multiplication and aerosolizing of the organism. The key to the pathogenesis of R. equi pneumonia is related to the ability of the organism to survive and replicate within alveolar macrophages by inhibiting phagosome-lysosome fusion after phagocytosis. Immunity to R. equi infection in foals, although not fully understood, probably involves both humoral and cell-mediated immunity. The role of antibodies has been shown by the protective effect of prophylactic administration of R. equi-specific hyperimmune plasma [36-38] or purified immunoglobulins against VapA and VapC. Studies in mice have shown that CD4+ T lymphocytes are the most important cells involved in immunity against R. equi with a T helper-1 response allowing clearance of the infection and a T helper-2 response being detrimental.

Parascaris equorum - In foals, Parascaris equorum is a common parasite and because its life cycle involves migration through the lung, which can potentially cause signs of respiratory disease. Parasite-free foals infected experimentally with P. equorum developed a mild to severe cough at the time of migration of the parasite through the lung.

Clinical Presentation

Early clinical signs include, coughing especially during eating or exercise, respiratory rate above 30 breaths per minute, an increased respiratory effort, depression or failure to nurse/eat, growth retardation, mucopurulent nasal discharge (inconsistently), or elevated rectal temperature. Later in disease, wheezes or crackles may be present with thoracic auscultation. Labored breathing and cyanosis can occur in the most severe cases with extensive pulmonary involvement. Lymph node enlargement may be noted. The intestinal form of the disease may manifest itself by fever, depression, anorexia, weight loss, colic or diarrhea. Foals with immune-mediated polysynovitis will present with multiple joint distension accompanied by mild or no apparent lameness. Heat, pain and severe lameness are characteristics of R. equi septic arthritis or osteomyelitis.

Diagnosis

The diagnosis of pneumonia in foals is based on clinical signs and physical examination. Diagnosis of etiology and assessment of severity and prognosis is made by use of transtracheal aspirate, thoracic ultrasound, and thoracic radiography. A complete blood count (CBC) may reveal neutrophilic leucocytosis and hyperfibrinogenemia and can be used to monitor the response to treatment. Thoracic radiographs may reveal caudal ventral opacification with alveolarization or consolidation in foals with Streptococcus zooepidemicus pneumonia.

In contrast, diffuse abscess formation is more consistent with Rhodococcus pneumonia. Thoracic ultrasonography may reveal comet tails and/or cavitary lesions ventrally in foals with Streptococcus pneumonia and extensive diffuse cavitary lesions in foals with Rhodoccocus. Ultrasound is less sensitive when the surface of the lung is normal; however this occurs in a minority of cases. Transtracheal aspirate culture and cytology (most often Rhodococcus PCR) to identify the etiology and determine whether Rhodococcus therapy is necessary.

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. Penicillin, ceftiofur, and chloramphenicol are some of the antimicrobial drugs most commonly used in treatment of Streptococcal pneumonia. Foals with Streptococcal pneumonia that present with respiratory dyspnea commonly require intravenous penicillin (potassium penicillin 20,000 iµ/kg four times daily) for resolution. Concurrent infection with gram negative microbes necessitates broad spectrum choices such as ceftiofur, chloramphenicol, or aminoglycoside combination with penicillin. Although Rhodococcus pneumonia can be successfully treated with a variety of antimicrobials, macrolide therapy results in more rapid and reliable responses. Erythromycin (25 mg/kg q 6 - 8h) and rifampin (5 to 10 mg/kg q 12 - 24h), administered orally, usually provide effective therapy for R. equi infection. Erythromycin administration can be associated with side effects such as severe diarrhea in foals and in their accompanying mares and hyperthermia. Clarithromycin (7.5 mg/kg every 12 h PO) or azythromycin (10 mg/kg daily for 7 days and then every other day) each combined with rifampin is more commonly used for treatment of R. equi than is erythromycin. Antimicrobial-induced diarrhea sometimes occurs with clarithromycin or azithromycin but less frequently. Adequate hydration for mucociliary clearance and maintenance of a cool, shady environment are vital for foal recovery.

The duration of therapy is dependent on the severity of disease and extent of pulmonary damage. The minimal duration required with Streptococcus pneumonia with minimal lung damage is usually 3 weeks. Early withdrawal of therapy leads to recurrence. Severe Rhodococcus pneumonia may require therapy for 3 months. In severe cases, monitoring of lung sounds, CBC, fibrinogen, and ultrasound and radiography are advised to document improvement. Then, monthly reevaluation of these parameters can be used to identify when resolution has occurred. Treatment one week past radiographic and/or ultrasonographic resolution is advised.

The prognosis is generally good when an appropriate antimicrobial therapy is initiated early in the course of the disease. The future athletic performance of horses that have a history of pneumonia as foals has not been thoroughly investigated. The preponderance of evidence suggests that horses that had pneumonia as a foal do race successfully. Horses that had R. equi infection as a foal have a slightly decreased chance of racing; however, those that make it to the track race comparably to horses that did not experience R. equi pneumonia.

Prevention

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 commonly used to prevent R. equi pneumonia. Efficacy of this practice is highly controversial. Effective vaccinations are currently not available. Preventative administration of antimicrobial drugs in for the first two weeks often only delays development of pneumonia or leads to resistance.