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Dear Editor

We would like to thank Dr. Hamond and colleagues for their interest and for sharing their experience with leptospirosis in foals.[1] Leptospira spp. infections appear to be relatively common in horses, and increasing age has been associated with increasing seroprevalence and antibody titer, even in healthy horses.[2] With regard to the difference in treatment success between our study in foals and the study of Dr. Hamond et al, published in Pesquisa Veterinaria Brasileira,[3] we believe that it is difficult to compare these studies. First, our study reports on foals between 3 weeks and 5 months of age. The study of Dr. Hamond et al reports on 2 to 5 year old racing Thoroughbreds. They found that exercise associated pulmonary hemorrhage (PH) was more commonly found in horses that were seropositive than in those that were seronegative. A 15-day course of procaine penicillin G and streptomycin in 7 seropositive horses resulted in decreased severity of PH after exercise. However, in none of the horses were attempts made to isolate leptospires or identify their DNA. As such, it is difficult to establish whether the improvement in clinical signs was due to resolution of active Leptospira spp. infection or due to other beneficial effects on pulmonary function arising from a 15-day course of treatment with a broad spectrum antibiotic, or both. No seronegative horses with exercise induced PH were treated with antibiotics as a control group. Secondly, the clinical condition of our foals (ie, suffering from severe renal failure, acute respiratory distress, and severe systemic infection) is in contrast to the good general condition in the racing Thoroughbreds. Antibiotic treatment in the latter group would have been unlikely to be associated with aggravation of clinical signs or increased mortality. Finally, streptomycin was not a suitable treatment option in the foals in our study because of their compromised renal function, but we expect that a different antibiotic treatment regimen would not have altered the outcome in our foals. Also, both treatment approaches have proven to be effective against leptospirosis. In conclusion, the 2 studies were very different and cannot be compared regarding treatment outcome. We agree with Dr. Hamond and colleagues that leptospirosis in horses is probably underdiagnosed and that additional research should be undertaken to elucidate the clinical importance and pathophysiology of this pathogen in young as well as in adult horses.

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