Why Has It Been So Hard To Develop A Vaccine For Foal Rhodococcus?

For a broodmare manager, one of the trickiest foes has got to be Rhodococcus equi, an insidious bacteria that packs a serious punch against the youngest of foals. R. equi can cause pneumonia and also may cause abscesses in the abdomen, central nervous system, and bones. Estimates of fatality rates range from 20 to 40 percent.

So why don’t we have a vaccine for it yet?

Dr. Bill Gilsenan answered this common question at Rood & Riddle Equine Hospital’s annual client education seminar last month. The trouble with this particular bacteria, Gilsenan said, is the type of immunity that’s needed to fight it isn’t so easy to replicate.

Most people think about the immune system as a network of antibodies that fight off disease, but Gilsenan said it’s a little more complicated than that. Humans and animals have a variety of antibodies circulating in their bloodstream, each one specially engineered to take down a particular pathogen. The antibody binds to this pathogen and removes it if it’s detected.

Vaccines work by introducing a modified version of the pathogen to the body to prepare it for an invasion by a real pathogen.

“What we’re really trying to do is train the horse’s immune system to recognize that pathogen as foreign,” said Gilsenan. “The next time they see it, they can recruit the rest of the immune system to come in and remove it from the body as quickly as possible.”

A vaccine will help teach the body to build antibodies to a specific pathogen, but a foal may also acquire antibodies from its dam, which is why broodmares are vaccinated for some diseases while pregnant. An adult animal may have antibodies after encountering the pathogen before — sometimes from a past illness, and sometimes without showing any symptoms. The response of antibodies is referred to as humoral immunity.

There’s another important way foals can build their immune systems, of course.

“Colostrum is full of antibodies,” said Gilsenan. “That’s the whole purpose of colostrum, is to provide the foal that’s born with none, some kind of immune system to protect it. That said, colostrum does not protect against R. equi.

That’s because R. equi has the ability to invade a cell. Scientists believe a foal catches R. equi by inhaling it, and the bacteria travels down into the lungs and into the alveoli, which are tiny air sacs in the lungs that take in oxygen and help filter out dust and other particles. But R. equi is able to survive inside the cells of the alveoli, replicating there and beginning to cause disease before an antibody can reach it.

For this type of pathogen, Gilsenan said the body has to use cell-mediated immunity, where the body mobilizes T cells, which attack the pathogen inside the cell or on its surface. An antibody can’t behave that way — which is why a traditional vaccine isn’t much help.

Gilsenan said he is “hopeful” a vaccine may someday reach the market. Recent research from Texas A&M University seemed to uncover a plasma-based product that could provide immunity when given to the pregnant mare. A continuation of this research has been funded for 2020 by the Grayson-Jockey Club Research Foundation. An additional study also funded by Grayson will test whether the delivery of genetic code for an R. equi antibody may help protect foals from infection.

The post Why Has It Been So Hard To Develop A Vaccine For Foal Rhodococcus? appeared first on Horse Racing News | Paulick Report.

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