Detecting Leptospira spp. infection in cows by PCR: What is the best sample to test?

Bovine leptospirosis is a major reproductive disease. As cows can be leptospiral carriers both on the renal and genital tract, diagnosis can be challenging, with an underlying risk of misdiagnosis. Traditionally, the infection has been diagnosed by culturing or PCR from urine samples. Nevertheless, recent studies have suggested testing genital samples rather than urine, particularly for the diagnosis of genital colonization and reproductive disorders. The present study aimed to compare urine versus genital samples to detect leptospiral carriers in naturally infected cows with poor reproductive performance under field conditions. Five herds presenting >20 % of seroreactive animals against the Sejroe serogroup were selected. Of these, 106 cows with poor reproductive performance were studied, and urine, uterine fragment (UF), and cervicovaginal mucus (CVM) were obtained and tested by lipL32-PCR. A total of 73 (68.9 %) cows were infected; 64 of which (87.7 %) were diagnosed via positive genital samples (UF and/or CVM), while only 14 (19.2 %) by urine (p ≤ 0.001). Therefore, if the study had been limited to urine samples, as largely recommended, less than 20 % of the infected cows would have been detected, representing a huge misdiagnosis of the disease that could undermine the efficacy of control programs. In this context, the present study reinforces prior findings that testing genital samples, particularly CVM, is crucial to effectively diagnosing infected subfertile cows.