Healthwatch: Research Links Cushing's to Laminitis

Cushing's disease (CD) has been identified as the most common cause of laminitis among horses seen at the University of Pennsylvania's New Bolton Center. Dr. Mark Donaldson, an assistant professor at the university's School of Veterinary Medicine, led the research effort.

The results of the study appeared in the April 1 issue of the Journal of the American Veterinary Medical Association.

In CD, the pituitary and adrenal glands produce abnormal amounts of hormones that play a vital role in the regulation of metabolism and inflammatory and immune responses. All horses that developed laminitis during a six-year period (1996-2002) were tested for CD by evaluation of plasma ACTH concentrations. ACTH is one of many hormones that are secreted in excess by the dysfunctional pituitary gland. Twenty-eight of the 40 horses with laminitis were diagnosed with CD. Although CD is considered a disease of older horses with an average age of 20 years, the study showed that CD is common in horses in their teens.

One of the most common clinical signs of CD seen in the study's horses was abnormal fat distribution, including accumulation of fat in the neck, top of the back, and over the tail head, in a horse with visible outlines of the ribs. Only a third of the group had a long hair coat, another conspicuous sign of CD. A fifth of the group with CD did not have any other clinical signs.

In 21% of the horses with CD, onset of laminitis occurred during September. Excess consumption of lush grass was commonly blamed for laminitis in many horses that also had CD. Dietary modification of horses with CD may be helpful in preventing this serious complication.

Several studies have shown that the most effective treatment for CD is pergolide, a medication that suppresses secretions of hormones from the abnormal pituitary gland. In addition to dietary modifications and corrective shoeing, treatment with pergolide results in an improvement in laminitis. A large dose range exists and the dose must be titrated based on endocrine function tests such as plasma ACTH concentration.

Grass sickness is strongly associated with low antibody levels to the bacterium Clostridium botulinum, according to a study completed at the University of Liverpool in England and funded by The Home of Rest For Horses. The finding might lead to routine vaccination against the bacterium in horses. A story about the research appears on the Web site for The Horse:

Dubai Millennium, a champion in England and Dubai, died of grass sickness in 2001. The disease was first identified around 100 years ago, but scientists have struggled to understand grass sickness and identify its cause. Few or no cases have been found in the United States.

The disease usually is fatal and presents itself in two different ways--either as severe colic or weight loss and difficulty eating. Both manifestations of grass sickness are as a result of nerve damage to the intestine. Grass sickness is a seasonal disease, with the majority of cases occurring in the spring.

"This research is important because it confirms the link between grass sickness and Clostridium botulinum," said Dr. Chris Proudman of the University of Liverpool. "This link, first proposed in the 1920s, was recently re-evaluated by researchers at the University of Edinburgh. Our study builds on this previous work and demonstrates protection against the disease in horses with high antibody levels against the bacteria. This strongly suggests that vaccination may be effective in preventing the disease."

The new research, published in the Equine Veterinary Journal recently, involved the study of 66 horses with grass sickness in 58 different premises. Blood samples were taken to evaluate antibody levels against C. botulinum, and compared to antibody levels in unaffected horses from the same premises. Information on management practices was analyzed along with samples of soil and pasture in order to identify factors that altered the risk of grass sickness occurring.

A number of new risk factors were identified including change of feed type or quantity and the use of the deworming agent ivermectin--all of which make horses more susceptible to grass sickness.

The research team that collaborated on the project is now exploring potential Clostridial vaccines for evaluation in horses.

Excerpted from The Horse: Your Guide to Equine Health Care. Free weekly newsletters at