SECONDARY BACTERIAL PNEUMONIA

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Causative Agents

This condition is caused by multiple bacterial organisms though, Streptococcus equi subspecies zooepidemicus are the most common bacterial pathogens isolated from adult horses with pneumonia. Non-enteric Gram-negative bacteria organisms like Pasteurella spp. and Actinobacillus spp. are also frequently isolated, with or without Streptococcus zooepidemicus. Enteric Gram-negative bacteria such as Klebsiella spp, Escherichia coli, Enterobacter spp., and Salmonella enterica may also be isolated. Other aerobic Gram-positives such as Staphylococcus spp. and Rhodococcus equi or Gram-negatives such as Pseudomonas spp. and Bordetella bronchiseptica are occasionally isolated from clinical samples.

 

Clinical Presentations and Epidemiology

Consistent clinical presentations in equine pneumonia are generally fever, cough, and nasal discharges of varying consistencies. Although these clinical signs are rare, they cannot be ruled out in viral pneumonia.

Diagnostic Considerations

The use of antibacterial drugs in this clinical condition exclusively based on clinical signs constitutes non-judicious antibacterial use. The common guiding principle is that treatment must be targeted at a specific bacterial pathogen wherever practicable. Thus, antibacterial susceptibility tests of bacterial isolates obtained from culture of the trans-tracheal wash, tracheal aspirates, broncho-alveolar lavage, or lung biopsy is highly recommended as the basis for the selection of the most effective antibacterial drug. Also, chest X-ray (to detect lung inflammation), complete blood count (to assess immune status), and Pulse Oximetry (to measure blood oxygen level) are additional diagnostic tools that could enhance effective therapeutic intervention in this disease in horses.

Management and Treatment

Treatment should be based on the result from bacterial culture, isolation, and susceptibility test of the required samples. Empirically, potentiated sulphonamide, i.e., a fixed dose formulation of trimethoprim with any of the sulphonamides (e.g., sulphadiazine, sulphamethoxazole, or sulphadoxine, etc.) is required at the early stage of this clinical condition. Erythromycin alone is an empirical antibacterial treatment in moderate bacterial pneumonia. But when it becomes severe, a combination of erythromycin and rifampin is recommended.

Erythromycin estolate: 25mg/kg, q24h x 5 days, PO. Rifampin: 10mg/kg, q24h x 5 days, PO. Trimethoprim/Sulphonamide: 30mg/kg x q24h, PO.

It is important to note that oral bioavailability of trimethoprim in horses is reduced if food is given concurrently with it. Consequently, any fixed-dose formulation containing trimethoprim, or its single formulation should be administered 2 hours prior to or 2 hours after feeding the horse to circumvent this.