MICROBIOLOGIC CHANGES DURING ANTIMICROBIAL TREATMENT AND RATE OF RELAPSE OF DISTAL RESPIRATORY-TRACT INFECTIONS IN FOALS

Despite the high incidence of distal respiratory tract infection of un determined cause on farms, to our knowledge, the microbiologic effects of conventional antimicrobial treatment for this condition have not b een studied. We evaluated the possible pathogenic role of bacterial is olates from the distal airways of foals with clinical respiratory trac t disease, by correlating changes in their numbers (increase or decrea se) with clinical, endoscopic, and pulmonary cytologic signs of diseas e resolution during treatment with antimicrobial drugs. We also determ ined qualitative changes in in vitro antimicrobial susceptibility of b acterial isolates after 7 days of treatment and relapse rate of foals. Significant (P < 0.05) decrease in the numbers of an isolate in the a irways was considered strong evidence of a pathogenic role in this dis ease syndrome. Foals with endoscopically confirmed distal respiratory tract infection (DRTI; n = 65) were selected at random for treatment ( n = 56) or nontreatment (n = 9), and bronchial lavage specimens were c ultured and evaluated cytologically before and after 7 days of treatme nt with trimethoprim-sulfamethoxazole (TMS) and a beta-lactam drug (pe nicillin, ampicillin, or sulbactam-ampicillin), the standard treatment in all foals. The effect of treatment was to abruptly reduce the clin ical (nasal discharge, cough, adventitious lung sounds) and cytologic signs of airway infection. Severity of disease in nontreated foals, ho wever, did not change or did worsen over time. Reduction in the freque ncy and numbers of Streptococcus zooepidemicus isolated during treatme nt supported a causal role for this organism in the clinical syndrome observed. On the other hand, the frequency of non-Str zooepidemicus is olates (eg, Staphylococcus epidermidis, Streptomyces spp, alpha-hemoly tic streptococci) actually increased during treatment, compatible with a commensal or competitive role for these organisms. Significantly (P < 0.001) more pretreatment isolates were susceptible in vitro to eith er TMS or beta-lactam drugs than to beta-lactam drugs alone; more post treatment isolates were susceptible to either TMS or beta-lactam than to either drug alone. These data indicate that there may be some benef it to combined use of TMS plus beta-lactam drugs in foals with DRTI. M ean +/- SEM relapse rate was 31 +/- 6% (range 0 to 57%); risk factors (clinical signs of disease, laboratory variables) for relapse could no t be identified. In conclusion, treatment resulted in significant (P < 0.001) reduction in airway inflammation in foals with clinical DRTI. The high reinfection rate indicates that a predisposing factor, possib ly age-related immunodeficiency, may predispose foals to illness and p ersists after treatment.