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Picture: The Donkey Sanctuary
Veterinary researchers from the University of Liverpool carried out the research, which was funded by the Donkey Sanctuary in Britain, which has provided homes for more than 11,000 donkeys on its farms since 1968.
The registered charity aims to prevent the suffering of donkeys worldwide, through advice, training and welfare support. Most of the world's estimated 59 million donkeys are found in developing countries, and are mostly used as draught or pack animals.
Like horses, colic is a serious condition in donkeys, with cases arising from impacted food (impaction colic) being the most common form.
The researchers investigated 807 colic cases in a population of 4596 donkeys between January 1, 2000, and March 31, 2005. All lived on Donkey Sanctuary farms. Most cases (54.8%) were due to a suspected or confirmed diagnosis of impaction of the gastrointestinal tract.
The death rate for all colics (51.1%) was higher than reported in other equids (93% of affected donkeys were euthanized). The incidence rate of all colics (5.9 episodes per 100 donkeys a year) and of impaction colic (3.2 episodes) was similar to that in horses. It is believed to be the first study to have ever quantified the incidence rate of colic in a large population of donkeys.
The researchers found that older donkeys, those fed extra rations, and those that had previously suffered colic were at increased risk of impaction. Lighter body weight, musculo-skeletal problems, and dental disease were also significantly associated with cases of impaction colic.
Results of their investigations support the hypothesis that dental disease is associated with impaction colic.
"Musculo-skeletal problems may be associated with colic for various reasons.
"We hypothesize that donkeys with musculo-skeletal problems may have reduced their amount of exercise, may have more difficulty walking to water sources and may be confined or are less able to access pasture. These changes could contribute to the development of impaction colic."
It was possible that adequate exercise may be important in maintaining normal large-intestine function.
Identification of risk factors for impaction colic should allow owners to highlight high-risk donkeys and may allow intervention strategies to be introduced to reduce the incidence of the disease.
The researchers pointed out that colic is significant and complex problem in all breeds, and in some equine populations it is the most common cause of death.
Some studies have produced conflicting results about the importance of individual risk factors. However, many potential causes have been identified, such as feed type, changes in management practices, lack of access to pasture or to water.
"While research has aimed to identify risk factors for colic in horses, little is known about the incidence of the disease, or specific risk factors in donkeys which differ in many respects to other equids."
Autopsies identified the pelvic flexure of the colon as being the most common site of the impaction (39.6%), but researchers found that the site of impactions could not be identified in 37.1% of investigations.
The incidence rate of colic in the donkeys was similar to horses, which range from 3.5 to 10.6 episodes per 100 horses a year.
"The proportion of impactions diagnosed in this donkey population is much higher than the 5 to 12% that has been reported for horses."
However, they pointed out that the study involved an aging population of donkey (the average age was 25), which may have had a major bearing on this figure.
The fatality rate, at 51%, is much higher than that reported in horses, which varies from 6.7% to 15.6%, depending on the population and colic type.
However, the researchers point out that the mortality rate may be an overestimate, as some donkeys may have experienced mild colic and recovered without the case being identified or treated. Diagnosis of colic in donkeys can be more difficult than in horses because donkeys show few overt signs of abdominal pain and colic may not be identified until the donkey is in the terminal stages of the disease.
Secondly, this mortality risk included cases only confirmed at post mortem. If these cases were excluded, the fatality rate would be 39.0% (253 deaths/648 colic cases) - a rate still much greater than that recorded in other equine populations.
Also, age was likely to have a strong influence "In our study the majority of donkeys that did not survive were euthanized, and it is likely that the decision for euthanasia also considered the health of other body systems."
Researchers found that the duration of colic in donkeys was much longer than in horses, in which single episodes are more likely to last for hours rather than days.
The researchers found that cases of impaction colic among the donkeys peaked in autumn, while numbers were low in the spring and early summer months.
"The peak in impaction colics in the autumn coincides with the change to winter housing, which is associated with a change in diet and a decrease in exercise. These have been identified as risk factors for impaction colic."
A link between dental problems and impaction colics was also strong.
"It has been suggested," they wrote, "that dental disease can adversely affect the ability to adequately masticate feed for subsequent digestion and may result in long fibres entering the large colon, predisposing animals to intestinal tract obstruction.
"These observations highlight the need for regular dental examinations," they wrote.