Pericarditis Study Results Released

Pericarditis is a disease involving the sac around the heart and the heart itself. The disease is uncommon. During the spring of 2001, however, an unexpectedly large number of cases were observed in Central Kentucky. These cases coincided with the mare reproductive loss (MRLS) syndrome. A recent study provided interesting findings about this outbreak.

First, the research showed scientifically that MRLS (which included early fetal loss and late-term abortions) and the cases of pericarditis (inflammation of the sac surrounding the heart) seen in Kentucky in the spring of 2001 are related.

Drs. Nathan Slovis, of Hagyard-Davidson-McGee, and Johanna Reimer, of Rood and Riddle Equine Hospital, both in Lexington, Ky., collected data for the study that was analyzed and interpreted by a group headed by Dr. Noah Cohen of Texas A&M University. This study was created by a task force set up by Kentucky Gov. Paul Patton and funded by the commonwealth and the Grayson-Jockey Club Research Foundation. Other members of the team included Drs. Vincent Carey, James Donahue, Janyce Seahorn, Chris Smith, and Shelly Lenz.

“This study (of pericarditis cases) points out things that are interesting and significant, but we can’t say what the cause is,” said Slovis. “We can say MRLS and pericarditis in Kentucky this spring was related; they might not be brother and sister, but they certainly are cousins.”

Slovis explained that most researchers and veterinarians think the problems seen in the spring of 2001 were caused by a number of different factors. For example, if it is found that toxins A and C cause pericarditis, it might be found that toxins B and C cause MRLS.

The study looked at 38 affected horses and 30 control horses. Cohen said that factors significantly associated with pericarditis may be markers for other factors that are truly associated with the disease. For example, the study found that the proportion of pericarditis cases fed hay grown outside Kentucky was significantly greater (78%) than controls fed hay grown in Kentucky (53%). However, farms which fed hay grown outside Kentucky might have used management practices or have been located in environments that predisposed horses to the development of pericarditis and MRLS, rather than the hay source being the factor.

Cohen explained that the small sample size limits the statistical power to detect differences and to account for effects of multiple exposures simultaneously. “However,” he added, “the small sample size should not be overemphasized. For example, even if we had studied 3,000 pericarditis cases, we might not have been able to determine the cause of the pericarditis problem.

“Similarly, small sample size is not the only--or even most likely--reason why statistically significant findings might actually point to or be associated with other causative factors. A factor may be significantly associated with a given exposure (eg., smokers tend to be heavier coffee drinkers than non-smokers) and if the exposure is significantly associated with disease (eg., smokers are at increased risk of lung cancer), one can observe an association of coffee drinking with lung cancer that is spurious. After one accounts for effects of smoking, coffee drinking would have no role in risk of lung cancer. This is an example of what epidemiologists call confounding.

“Confounding can occur in studies of populations of any size--large or small. In our pericarditis study, after accounting for effects of caterpillars, many of the associations (pond water, cattle) disappeared; this may have been due to confounding, but also might have been attributable in part to small sample size.”

 Cohen added that like other findings, caterpillars might be merely a marker for some other environmental exposure and/or condition that caused the problem.

Statistically significant points in the study:

  • MRLS and pericarditis were related. The proportion of cases of pericarditis that were from farms where there were mares or foals affected by MRLS (84%) was significantly greater than that of the controls (56%).
  • There was a significant association of pericarditis with reported exposure to caterpillars.
  • The proportion of pericarditis cases from farms where there were mares or foals affected by MRLS (84%) was significantly greater than for controls (56%).
  • The cases of pericarditis appeared to result from a point-source, meaning the exposure to the causal factor(s) likely occurred during a brief, focal period in time.
  • Caterpillars, but not cherry trees, were associated with the development of pericarditis. (In this study, it was asked if cherry trees were in fields, in close vicinity of the fields, or were accessible over the fencelines.) It is known that Eastern tent caterpillars travel from cherry trees and can infest other types of trees.
  • Pericarditis cases were less likely to have access to pond water (5%) than controls (23%), showing some association with management or water source that was potentially protective.
  • Pericarditis cases had access to orchard grass to graze (87%) compared to controls (65%).
  • Contact with cattle appeared to be protective; that is, control horses were significantly more likely to have had direct contact with cattle.

A larger study by the same group of epidemiologists is underway to look at early fetal loss and late-term abortions to see if there are any correlations with the pericarditis study.

Researchers said, “The results of the study do not provide a basis for making recommendations for the prevention of the problem. Results of this study indicate that several proposed practices (offering mineralized sat blocks, mowing the grass, removing cherry trees, etc.) would not be expected to be effective for preventing pericarditis.

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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