While it seems that the wheels of progress spin slowly, progress does get made. Sometimes without fanfare or accolades. Generally with the hard, dedicated work of a few people. Oftentimes because of a simple question.

In this case, 10 years ago a question was asked: What is causing foal diarrhea outbreaks in Kentucky? Many farms would have 70% or higher incidence of sick foals at a time when they were in the middle of the breeding and foaling season. A Thoroughbred farm manager once said the smells of spring in Kentucky included fresh-mown grass, sweet clean air, and Pepto-Bismol (given to foals with diarrhea). Lloyds of London brokers and Kentucky agents funded a three-year study to look into the outbreaks, as proposed by David Powell, FRCVS, an epidemiologist at the Gluck Equine Research Center at the University of Kentucky.

Several Thoroughbred farms in the Central Kentucky area allowed researchers from the Gluck Center to obtain blood and fecal samples throughout the foaling seasons of 1986, 1987, and 1988, explained Roberta Dwyer, DVM, MS, Diplomate American College of Veterinary Preventive Medicine, who also worked with the study. The first year, researchers found that the most common cause of diarrheal outbreaks was rotavirus.

The second year of the research, more farms were added and the criteria was expanded. Researchers looked at healthy and sick foals and also checked mares at foaling and two days after foaling to see if the mares were shedding the rotavirus into the environment where the foals could pick it up (see sidebar on rotavirus). Although none of the mares tested was found to be shedding rotavirus, it is known to happen in other species, so it was thought that the mares might have been shedding sporadically or the test might not have been sensitive enough.

The third year of the study, researchers traveled only to farms where there were outbreaks of diarrhea in foals, as reported by attending veterinarians. They found sick foals on these premises, as well as clinically healthy foals which were shedding rotavirus.

Over the three years of the study, multiple farms had outbreaks of diarrhea in foals. While campylobacter and salmonella were cultured in some individual cases, researchers determined that 90% of the outbreaks over the three years were attributable to rotavirus.

"Our conclusion was the rotavirus was the primary cause of foal diarrhea outbreaks in this area," said Dwyer, who pointed out that this followed the findings of research undertaken in other parts of North America and the world, including England, Ireland, France, and Japan.

The next step was deciding what to do about the outbreaks.

"Having decided rotavirus was the primary cause, discussions were begun with companies as to the viability of developing a vaccine," said Powell. "Because the market for equine vaccines is small, it took some time to convince a company to get involved."

Finally, Fort Dodge stepped up and joined the team.

"We got interested because this is a serious infectious disease and we thought we could make a vaccine," said David Hustead, DVM, Director of Professional Services at Fort Dodge Animal Health.

The company wanted to develop an inactivated equine vaccine that could be administered to mares in the last stage of pregnancy to enhance antibodies in the colostrum and offer foals protective immunity during the critical period when they are the most susceptible--the first 60 days of life.

It took about a year to draw up the legal agreement by which "some indemnity" was granted to the farms participating in the study and that would be satisfactory to Fort Dodge, the University of Kentucky, and those involved in researching the vaccine in the field.

Once the administrative aspects were resolved, the researchers were once again in the field, this time in 1993 on a university-owned farm testing on university-owned mares. This was done to make sure the vaccine was safe and that expectations were reasonable before a larger field trial was conducted. Everything went according to plan.

Field Trials

In 1994, nine Central Kentucky farms agreed to be included in the field trial.

"We're really appreciative of the fact that farms agreed to allow mares and foals to be used in this study," said Powell. "In terms of monitoring and data collection, it was a time-consuming exercise for them. But, it enabled a very comprehensive study to be undertaken. The veterinarians involved in the farms were enthusiastic in their support and greatly assisted us."

"Without that type of cooperation, this type of field study couldn't be done," concluded Dwyer about the farm participation.

Mares on the nine farms were listed in order of expected foaling date, then were randomly assigned to either the group that received the vaccine, or the control group that received a placebo resembling the vaccine. Farm managers, workers, and veterinarians did not know which mares were receiving the vaccination so that those animals were not observed or treated differently. All mares had blood samples taken before vaccination and at foaling.

Foals had blood samples taken for rotavirus antibody testing at 12-18 hours of age, when blood was drawn to check IgG levels. Then, the foals were blood tested every 30 days until 120 days of age.

The blood samples in the mares indicated that titers against rotavirus increased in the mares which were vaccinated. The foals from those mares had blood samples that showed they did receive rotavirus antibodies from the mares' colostrum. Both vaccinated and control mares and foals were blood sampled and checked.

Any foals which developed diarrhea had additional blood and fecal samples taken, and the level of sickness was scored from 1 to 4. One was a foal that had a "cowpile" type of manure, and 4 was a foal which needed IV fluids and intensive care on the farm or was hospitalized. In some cases, there were daily comments on the condition and reactions of the sick foals. In all cases information was kept on duration and severity of the diarrhea.

"After all that data was compiled, we found a significant difference in the antibody titers in vaccinated versus control mares," said Dwyer. "We also found the titers were transferred to the foals. In the foals, those antibody levels slowly declined, like all maternal antibodies, out to 120 days when we stopped testing."

Dwyer and Powell also found that there was a difference in the sickness rates of foals from vaccinated mares versus unvaccinated mares.

"In the first year, a number of foals got sick," said Dwyer. "There was a significant difference between control foals (from unvaccinated mares) having more severe diarrhea and longer sick times versus vaccinated mares' foals. But the numbers were small (in the trial) the first year."

In 1995, the same nine farms were again utilized in the field trial. Mares which were vaccinated in 1994 got a booster 30 days before their anticipated foaling date. Control mares from the previous year and any new mares on the farm were again divided into two groups--half of which were vaccinated and the other half serving as controls and receiving placebos. Multiple blood samples were taken and diarrhea incidents were investigated.

As expected, titers were higher in boostered and vaccinated mares than control mares. However, there was a low incidence of diarrhea in foals in 1995, so there wasn't as big a difference in the sickness and duration rates between foals from vaccinated and control mares. On some of the test farms, 70% of the mares were vaccinated or boostered for the 1995 season, so there might have been some herd immunity assisting in lowering the rotavirus rate, commented Dwyer. The farms weren't complaining, however. These were nine farms with a history of a rotavirus problem, and not to have a severe or devastating diarrhea outbreak was considered unique.

However, the low incidence of disease in foals from vaccinated versus control mares in the second year of the field trial meant further studies were needed, and resulted in the USDA granting a conditional license to Fort Dodge for the rotavirus vaccine.

"Data generally in terms of efficacy was not sufficient to give a full license, but they gave a conditional license, which means a reasonable expectation of efficacy," explained Powell. "There will be further studies undertaken in order to obtain a full license."

The conditional license means that the vaccine is released state-by-state, and the state veterinarian has to give approval for use of the vaccine in the state and monitor how much is being used. For 1996, only Kentucky has granted the conditional license, and the vaccine was released prior to the start of the breeding season. Any horse owners interested in obtaining equine rotavirus vaccine should contact their state veterinarian (http://www.TheHorse.com/ViewArticle.aspx?ID=3251) and request release of the vaccine in their state.

Testing and the USDA

"This is one of the first studies of an equine vaccine comparing non-vaccinated to vaccinated animals in a field situation," he said.

The acid test of any vaccine is to evaluate if it protects the animals when challenged. That is what Powell termed "clinical efficacy." He said in the two years of field studies, there was a small amount of clinical disease, which is the primary reason why more data is needed in regard to efficacy.

"This is one of the first (equine) vaccines to be evaluated in this way," he noted.

"The bottom line is the USDA is concerned with licensing vaccines that are safe and efficacious," said Powell. "Previously, emphasis was on safety, but consumer confidence requires that vaccines provide protection, and that's the reason the studies take such a long time and require a considerable amount of field and laboratory input."

Hustead of Fort Dodge stated: "We will continue with field trials to work for a full license."

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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