Identification of equine mares as reservoir hosts for pathogenic species of Leptospira

Authors: HAMOND, CAMILA - Animal And Plant Health Inspection Service (APHIS); ADAM, EMMA - University Of Kentucky; STONE, NATHAN - Northern Arizona University; LECOUNT, KAREN - Animal And Plant Health Inspection Service (APHIS); ANDERSON, TAMMY - Animal And Plant Health Inspection Service (APHIS); Putz, Ellie; CAMP, PATRICK - Animal And Plant Health Inspection Service (APHIS); HICKS, JESSICA - Animal And Plant Health Inspection Service (APHIS); STUBER, TOD - Animal And Plant Health Inspection Service (APHIS); VAN DER LINDEN, HANS - University Of Amsterdam; BAYLES, DARRELL - US Department Of Agriculture (USDA); SAHL, JASON - Northern Arizona University; SCHLATER, LINDA - Animal And Plant Health Inspection Service (APHIS); WAGNER, DAVID - Northern Arizona University; Nally, Jarlath

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2024
Publication Date: 5/9/2024
Citation: Hamond, C., Adam, E., Stone, N.E., LeCount, K., Anderson, T., Putz, E.J., Camp, P., Hicks, J., Stuber, T., Van Der Linden, H., Bayles, D.O., Sahl, J.W., Schlater, L.K., Wagner, D.M., Nally, J.E. 2024. Identification of equine mares as reservoir hosts for pathogenic species of Leptospira. Frontiers in Veterinary Science. 11. https://doi.org/10.3389/fvets.2024.1346713.
DOI: https://doi.org/10.3389/fvets.2024.1346713

Interpretive Summary: Animal and human leptospirosis, a global zoonotic disease, is caused by an unusual and atypical bacterium in the genus Leptospira. Domestic and wild animals can act as reservoir hosts of infection and excrete Leptospira from kidneys via urine into the environment where they can survive in suitable moist conditions. Transmission of disease to animals and humans is facilitated by direct contact with urine from infected hosts or with contaminated environmental sources. Horses are known reservoir hosts of leptospirosis in Europe, but no evidence has been provided to determine if horses in the U.S. can act as reservoir hosts of leptospirosis. In this study, urinary shedding of pathogenic Leptospira from thirty-seven asymptomatic mares was assessed using culture and molecular assays. Results determined that three asymptomatic mares were shedding pathogenic species of Leptospira. One of these isolates was classified as L. kirschneri serogroup Australis serovar Rushan, a serovar never identified in the U.S. or in horses, and not included in current vaccines. It is essential to know what species and serovars of Leptospira are circulating in equine populations to design efficacious vaccines and diagnostic tests. Our results demonstrate that horses in the U.S. can act as reservoir hosts of leptospirosis and shed diverse pathogenic Leptospira species via urine. This report also details the detection of a species and serotype of Leptospira not previously reported in the U.S.

Technical Abstract: Equine leptospirosis can result in abortion, stillbirth, neonatal death, placentitis, and uveitis. Horses can also act as subclinical reservoir hosts of infection which are characterized as asymptomatic carriers that persistently excrete leptospires and transmit disease. In this study, PCR and culture were used to assess urinary shedding of pathogenic Leptospira from thirty-seven asymptomatic mares. Three asymptomatic mares, designated H2, H8 and H9, were PCR positive for lipL32, a gene specific for pathogenic species of Leptospira. One asymptomatic mare, H9, was culture positive and the recovered isolate was classified as L. kirschneri serogroup Australis serovar Rushan. DNA capture and enrichment of Leptospira genomic DNA from PCR-positive, but culture negative, samples determined that asymptomatic mare H8 was also shedding L. kirschneri serogroup Australis whereas asymptomatic mare H2 was shedding L. interrogans serogroup Icterohaemorrhagiae. Sera from all asymptomatic mares were tested by the microscopic agglutination test (MAT) and 35/37 (94.6%) were seropositive with titers ranging from 1:100 to 1:3200. In contrast to asymptomatic mares, serum from a mare designated H44 that presented with acute spontaneous abortion caused by L. interrogans serogroup Pomona serovar Pomona had an MAT titer of 1:102,400. Comparison of L. kirschneri serogroup Australis strain H9 with that of L. interrogans serogroup Pomona strain H44 in the hamster model of leptospirosis corroborated differences in virulence of strains. Since lipopolysaccharide (LPS) is a protective antigen in bacterin vaccines, the LPS of strain H9 (associated with subclinical carriage) was compared with strain H44 (associated with spontaneous abortion). This revealed different LPS profiles and immunoreactivity with reference antisera. It is essential to know what species and serovars of Leptospira are circulating in equine populations to design efficacious vaccines and diagnostic tests. Our results demonstrate that horses in the U.S. can act as reservoir hosts of leptospirosis and shed diverse pathogenic Leptospira species via urine. This report also details the detection of a species and serotype of Leptospira not previously reported in the U.S.