Leptospirosis occurs worldwide and is caused by pathogenicLeptospira. There are 64 Leptospira genomospecies classified phylogenetically in two major clades: “Pathogens” (37 genomospecies), containing all the species responsible for infections in humans and/or animals, and “Saprophytes” (27 genomospecies), which include environmental species for which the virulence status has not been proven. Seroepidemiologically, leptospires are categorized into about 300 serovars, with antigenically related serovars being clustered into serogroups for convenience. Pathogenic and nonpathogenic serovars may belong to the same genomospecies, and, given that genetic heterogeneity within the same serovar has previously been demonstrated, the use of both classifications for the determination of a strain is thus well established.
Etiology of Leptospirosis in Ruminants
Two major groups of agents are classically defined. The first is represented by strains adapted to the affected host, which are carried by them and are influenced by, but not dependent on, environmental conditions for transmission. In this group, members of serogroup Sejroe, as serovar Guaricura in the Americas and serovar Hardjo, predominate worldwide. The other group consists of incidental infections caused by strains carried by either free-living or domestic animals (from other species) Transmission of these strains is more dependent on poor management practices and environmental conditions. These primarily include serovars Pomona, Grippotyphosa, and Icterohaemorrhagiae. In addition, wildlife may carry additional Leptospira strains, which can increase the exposure of livestock to an environment with a high and diverse contamination load.
Clinical Findings of Leptospirosis in Ruminants
Bovine leptospirosis is a well-recognized reproductive disease. Acute leptospirosis, strongly associated with incidental strains, is seen less frequently and usually presents as an outbreak. This syndrome is mainly characterized by abortions at any time of a pregnancy as well as birth of weak offspring. In lactating cows, blood-tinged milk has been reported. In contrast, the most frequent presentation is a chronic, silent infection associated with reproductive failure as embryo losses and estrus repetition. Serovars Hardjo and Guaricura are able to colonize and persist in the genital tract of infected cows and bulls; thus, these strains are more related to the chronic reproductive form of infection. This genital presentation of leptospirosis has been increasingly recognized as the major leptospirosis syndrome affecting cattle.
Diagnosis of Leptospirosis in Ruminants
Bacterial culture, serology, or PCR assay
The gold-standard diagnostic method is bacterial culture, but it is not often conducted due to technical and cost limitations. However, isolation and molecular characterization of isolates is important for epidemiologic studies of strains infecting animals from a given region.
Diagnosis of incidental or clinical infections in cattle is relatively straightforward. In the acute disease, infected animals develop high titers to the infecting serovar; an antibody titer ≥800 detected by a microagglutination test (MAT) is considered evidence of leptospirosis. In those cases, leptospires can also be demonstrated in the placenta and fetus by immunofluorescence, PCR assay, and immunohistochemistry.
In contrast, diagnosis of subclinical or chronic infection caused by Sejroe strains is more difficult. Serology alone often fails to identify infected animals because seronegative shedders are common in infected cattle herds. It has been repeatedly shown that at an individual level serology does not agree with shedding status. In those cases, screening on a herd level with serology (MAT) may identify suspect herds, but culturing, immunofluorescence or, preferentially, PCR assay is required for an individual diagnosis. Although urine has been extensively used as a primary sample, recent studies have demonstrated the usefulness of cervicovaginal mucus for the diagnosis of genital infection.
Treatment, Control, and Prevention of Leptospirosis in Ruminants
An integrated program of antimicrobials, management changes, and vaccination is often recommended
Due to its complex and dynamic epidemiology, control of bovine leptospirosis is still controversial and frequently frustrating. An integrated program based on antimicrobials, specific management alterations, and vaccination is often recommended. Antimicrobial treatment reportedly prevents abortions and other reproductive problems. Streptomycin (25 mg/kg, IM, single dose) is typically used and usually eliminates the renal carrier status. Oxytetracycline, tulathromycin, and ceftiofur are also reportedly effective.
Considering the correction of environmental/management factors, subdivision of animals into smaller batches, soil sealing, and regular cleaning of milking sectors have been demonstrated to significantly reduce the exposure of animals to leptospires. Access by cattle to collected water, puddles, or swampy areas, as well as cograzing (particularly with pigs), should be avoided.
Vaccination is the cheapest control method and represents an essential measure for the control of leptospirosis; its adoption is strongly recommended. Its efficacy varies, and the failure of commercial vaccines in the prevention of kidney colonization has been demonstrated. Usually, vaccination against leptospirosis occurs every six months, with one preferentially before the breeding service season, coinciding with the beginning of the spring when environmental contamination is more intense.
Efficacy of the control measures varies depending on the infecting strain. Although incidental infections are more easily controlled, infections by Sejroe strains are impossible to eradicate, requiring constant vigilance and a program based on reduction of reproductive problems and consequent economic hazards.
Leptospira strains may be adapted specifically to cattle (eg, members of serogroup Sejroe) or incidental pathogens (eg, Pomona, Grippotyphosa, and Icterohaemorrhagiae).
Incidental strains typically lead to outbreaks and acute disease with clinical signs, whereas adapted ones generally lead to asymptomatic reproductive infections.
Control is challenging but feasible and relies on vaccines, management alterations, and antimicrobials.
For More Information
Loureiro AP, Lilenbaum W. Genital bovine leptospirosis: A new look for an old disease. Theriogenology. 2020 Jan 1;141:41-47.