Exposure of New Zealand Thoroughbreds to Leptospira common, study findings suggest

A scanning electron micrograph of Leptospira bacteria. Image: CDC/Rob Weyant, public domain, via Wikimedia Commons

A quarter of New Zealand Thoroughbred racing and breeding horses tested in a study showed evidence of previous exposure to the bacteria that causes leptosporosis.

“Given the level of exposure found, horses may play a role in the epidemiology of leptospirosis in New Zealand,” Dr Charlotte Bolwell and her Massey University colleagues reported in the open-access journal, Animals.

Leptospirosis frequently occurs in horses worldwide. It has been linked to abortions in mares and can cause a painful eye condition.

It can also infect humans and has been of increasing concern to public health authorities in the country.

The researchers noted that the incidence of human cases notified in New Zealand increased from 1.2 cases per 100,000 people in 2014 to 2.3 cases per 100,000 in 2018, peaking at 2.9 cases per 100,000 in 2017.

The variants reported in human cases are consistent with those reported in livestock and rodents in New Zealand, with a strong occupational association (meat workers and farmers) in human cases.

Leptospira — the bacterium behind the infection — is endemic in livestock in New Zealand, with the evidence of exposure, based on serum testing, ranging from 3% to 90% depending on the animal species and the variant of the bacteria.

Bolwell and her fellow researchers noted there was a lack of data on the occurrence and exposure to leptospirosis among horses in New Zealand.

For their research, blood samples were collected from Thoroughbreds in the Manawatu, Waikato and Auckland regions of the North Island for testing to determine the frequency of five different types of Leptospira known to cause leptospirosis in humans and livestock in New Zealand.

In all, 499 horses were sampled across 25 properties, of which 80% were female, and 67% were broodmares.

Horse owners were also surveyed about exposure to potential horse and property‐level risk factors for infection.

The results showed that 25% of the horses sampled had previously been exposed to Leptospira.

Several management factors, such as grazing horses alternately with cattle or sheep, increasing horse age, and breeding horses, were linked to exposure to Leptospira in this group of horses.

A higher prevalence of Leptospira exposure was found among broodmares compared to racehorses in the study.
A higher prevalence of Leptospira exposure was found among broodmares compared to racehorses in the study.

The authors noted that, based on the questionnaire results, none of the horses enrolled in the study had previously been diagnosed with leptospirosis, nor any other livestock on the 25 properties involved.

On one property, it was reported that an owner previously tested positive to Leptospira, and on a further property, the respondent reported “my husband had leptospirosis”. No further details were provided about these potential cases.

Just under half of the broodmares that previously had an abortion were positive to at least one of the leptospira variants, with 32% of horses that did not have an abortion being positive to at least one variant.

“As horses are not vaccinated for Leptospira in New Zealand, our results showed widespread natural exposure to the five serovars tested in this cohort of apparently healthy horses,” the researchers said.

The overall seroprevalence rate of 25% is the same as that reported in Korea, and slightly lower than a study in Australia (29%). Studies in Brazil, Italy, and North America have reported much higher rates.

They noted a higher prevalence among broodmares compared to racehorses.

“The pasture‐based management of broodmares on stud farms frequently involves co or cross‐grazing with other livestock,” the study team noted.

“In this study, alternating grazing horses with sheep or cattle and grazing horses at the same time as sheep were associated with a higher exposure.”

They concluded: “Given these findings, horses may be a potential risk to other animals, particularly livestock that they may be co‐ or cross‐grazed with, and to those working in the racing and breeding industries in New Zealand.

“Further work is needed to identify if horses in New Zealand actively shed Leptospira and the extent of exposure in the wider population of horses in New Zealand.”

The seroprevalence identified in the 25% of Thoroughbreds in the study points to previous exposure to Leptospira, and no inference can be made as to whether any had an active infection at the time of sampling, or may have been shedding the bacteria.

The full study team comprised Bolwell, Chris Rogers, Jackie Benschop, Julie Collins‐Emerson, Brooke Adams, Katherine Scarfe and Erica Gee.

Bolwell, C.F.; Rogers, C.W.; Benschop, J.; Collins-Emerson, J.M.; Adams, B.; Scarfe, K.R.; Gee, E.K. Seroprevalence of Leptospira in Racehorses and Broodmares in New Zealand. Animals 2020, 10, 1952.

The study, published under a Creative Commons License, can be read here

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