A bull can carry Leptospira in his reproductive tract without obvious clinical signs and without a positive antibody test. In work from the Laboratory of Veterinary Bacteriology at Fluminense Federal University in Brazil, PCR detected genital infection in 80.5% of asymptomatic bulls, compared with just 13.5% that were seropositive.
The study evaluated 37 unvaccinated bulls from a single herd with poor reproductive performance in their cows. Researchers compared microscopic agglutination testing (MAT) with PCR testing of semen, preputial cytology and urine to determine which approaches were most useful for detecting bovine genital leptospirosis (BGL).
Serology May Miss Infected Bulls
Bovine leptospirosis is associated with reproductive problems including early embryonic death, repeat estrus, stillbirth and infertility. Cattle-adapted Leptospira, particularly organisms in the Sejroe serogroup, can cause chronic infections that remain clinically unapparent.
Evidence also indicates that Leptospira can persist in the male reproductive tract. The organism has been detected in bull semen, epididymis and vas deferens, supporting the reproductive tract as a potential site of colonization outside the kidneys.
That creates a limitation for serology. Asymptomatic carriers may have low antibody titers or be seronegative, making antibody testing less useful for identifying individual carrier bulls.
Which Samples Detect Bovine Genital Leptospirosis in Bulls?
Blood was collected from all 37 bulls for MAT. Urine samples were collected from 32 bulls, semen from 34 and preputial cytology samples from 36. The genital and urine samples were tested using the molecular diagnostic test lipL32-PCR.
A bull was classified as genitally infected if at least one genital sample, semen or preputial cytology, was PCR-positive.
Diagnostic Results for Asymptomatic Bulls
Only five bulls were seroreactive, and all reacted to the Sejroe serogroup. PCR identified genital infection in 29 of 36 bulls. The difference between PCR and MAT was statistically significant, as was the difference between genital and urine PCR results.
It’s important to consider these results in context. Researchers tested just 37 bulls from a single herd with poor reproductive performance, so the result cannot be used to estimate BGL prevalence among bulls more broadly.
Genital samples produced more positive PCR results than urine, supporting the study’s focus on the reproductive tract when diagnosing BGL. Therefore, urine is not an adequate sample for detecting BGL in bulls; testing genital samples is preferred. The preputial cytology findings support the idea that the reproductive tract can serve as a site of extrarenal Leptospira colonization.
Semen and Preputial Cytology Provide Different Results
Although semen had the highest detection rate, there wasn’t a significant difference between semen and preputial cytology.
The two sample types also rarely identified the same bulls as positive. Their Cohen’s kappa was 0.039, a measure used to assess how much two testing methods agree beyond what would be expected by chance. This may reflect uneven distribution of Leptospira within the reproductive tract, with colonization varying among the preputial lumen, accessory glands and semen.
For now, the findings support both semen and preputial cytology as useful genital samples. However, larger studies are needed before replacing preputial sampling with semen collection can be recommended.
Testing Individual Bulls for Bovine Genital Leptospirosis
The findings support different roles for serology and PCR. Serology may help with herd-level screening, while PCR on genital samples can help identify individual bulls with evidence of genital Leptospira. For BGL specifically, the findings favor genital sampling over urine testing. A bull with no clinical signs and a negative serologic result may still test positive for Leptospira DNA in a genital sample.
The high rate of genital PCR positivity adds to evidence that bulls deserve consideration when investigating bovine genital leptospirosis. The authors conclude that PCR using genital samples, including semen and preputial cytology, is the preferred approach for diagnosing BGL in asymptomatic bulls under field conditions. Larger studies are needed to determine how consistently different genital samples identify carriers and to better define the role of males in BGL epidemiology.
