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and that many other places didn’t have the benefit of seeing the aborted fetuses like we did, or didn’t test enough horses. He devised the study to look at this across the nation, using serum left over from Coggins testing. In virtually every state they tested, he found prevalence of serum antibodies in those samples. Those horses had been exposed
and infected because they had mounted a response,” says Brown. This was a huge eye- opener regarding the prevalence of lepto in the national equine population.
“I sit on a committee here for the Kentucky Thoroughbred Association and
I’m the liaison for their Scientific Advisory Committee. We had money left over from
our research on Mare Reproductive Loss Syndrome (MRLS) that we periodically use as seed money for other issues that are important for the horse industry relative to reproductive problems. So, the Kentucky Thoroughbred Owners and Breeders (KTOB) group invested about $10,000 on the sero-prevalence study, which demonstrated that it’s not just a local problem,” he says.
This study got the attention of Zoetis and they recognized that the disease was a big problem nationally. “Through their repre- sentatives, Zoetis identified some hot spots in the U.S. where they were seeing a lot of cases, such as the Genesee River Valley area in New York, some of the areas in northern Mississippi, the Memphis area in Tennessee, and in the Carolinas. They were able to col- lect additional data from these locations, and got more sera from some of these places to do more testing,” says Brown.
“They looked for the predominant serovar, which was pomona. It had the big- gest number of clinical cases associated with it. I have also been curious as to whether the cases associated with grypotyphosa are on the rise, but these cases have been a distant second compared to the number of cases of pomona in national prevalence.”
“It’s interesting, watching the titers. I have three large herds of broodmares that have
been vaccinated since the vaccine became available. We are seeing some cross reactivity with the vaccine to some of the other serovars. Vaccine titers only go up to about the 100 to 800 range. Natural infection usually causes
“This is very exciting, to finally have a vaccine, and the efficacy data was very good.”
much higher titers, up to 100,000 or more. But, we are also seeing lower titers to some of the other serovars, and our assumption has been that there is some cross-reactivity between the parent organism and the various sub-types,” Brown says.
There have been questions regarding whether vaccination titers will cloud the abil- ity to diagnose a horse with active infection. “On a scale of gradation, however, there is quite a difference in the titer level we see in the vaccinated horse versus natural infec- tion. We may be able to differentiate the
two because of the magnitude of response to natural infection. A high titer would indicate natural infection,” he explains.
He has talked with the vaccine company, looking at some of the serum titers after vac- cination, pointing out the low level of some of the serovars besides pomona. “We are wonder- ing if it might actually be protective. It will
be interesting to see if we can recognize farms that vaccinated versus farms that didn’t in the prevalence of infection in a year that might be likely to cause exposure,” he says.
“One of the impressive things about this vaccine is that the model they used to chal- lenge horses with it (to determine efficacy) was an intraperitoneal model,” Brown says. An intraperitoneal injection is an injection into the body cavity. “This is a significant way to
expose a horse to this infection. In those stud- ies, 15 of the 16 control horses that received saline instead of vaccine shed leptospires in their urine for many days after they were infected. None of the horses that received the actual vaccine and were then experimentally infected shed it in their urine. In only one of those horses were they able to actually recover any pathogens (and at a very low level) from the renal tissue. The vaccine prevented shed- ding,” Brown says.
“This prevented a reservoir situation. One of the things that causes a reservoir of this disease in horses is that the leptospires get into the kidneys and are shed in urine. This potentially exposes herdmates to infection. The vaccine reduces the exposure rate of other horses if a mare gets infected and actually aborts with lepto. For a strategy of trying to protect the herd from the disease, the vaccine provides that benefit as well by reducing the shedding,” he explains.
“It’s difficult to ascertain how long infected horses might actually shed. Most
of us think that the vaccine is a proactive
way to prevent this disease. Up until now, in Kentucky, we were drawing tens of thou- sands of titers every year, trying to find the high titers. We do these horses multiple times in the herd. And when we find them, we’d start treating those horses. But, this is after the fact; they are already infected and shed- ding the pathogen; we were just constantly managing via the rearview mirror. The vaccine gives us the potential to be proactive, rather than reacting to the problem after it occurs,” Brown says.
Leptospirosis is a disease that has risk of exposure even for horses that never leave home or never have interaction with any other horses, however, if wildlife or other animals on the home farm are shedding leptospires. “It is also hard to know all the risk factors for recurrent uveitis (moon blindness). We don’t under- stand that part of it very well, regarding how
it occurs, and if stress factors into it to some degree,” Brown says.
The Dangers Of Lepto For Pregnant Mares
If a mare is exposed to leptospirosis dur- ing pregnancy she may not show signs of disease herself, but the foal may be lost. The outcome may depend on which stage of preg- nancy she became exposed. She may abort, or give birth to a compromised foal if exposed in late gestation.
“This is another thing we don’t under- stand very well. Here in Kentucky, if the exposure rate is high from late November through the first part of February, we see a lot of mares that are in their second or
third trimester by that time. We’ll see some mares abort prior to 9-months gestation, but also see some with compromised neo- nates if the mares were exposed closer to term,” says Brown.
“It may also be dose dependent on the amount of exposure. We don’t know whether the serovar type makes a difference in the outcome. Our learning progress has been slow. We are still using the same methodol- ogy for serum titer detection that has been around since the mid-1960’s. There hasn’t
been any dramatic improvement in our abil- ity to assess or diagnose leptospirosis other than using the micro-agglutination titers, which are highly technically challenging. A person needs a lot of experience to know how to read them and how to discern the signifi- cance of the findings.”
Another challenge, in determining whether a horse has lepto or whether an abortion was due to lepto, is that not very many regions have a laboratory that can do these tests.
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equine health