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Equine epidemiology: The search for all contributors to disease
This field of science tries to encompass all potential causes, not just one principal one


DVM360 MAGAZINE


Contributing factors of colic

The association between various management practices and the development of colic was studied in 821 horses treated for colic and 821 control horses treated for noncolic emergencies by practicing veterinarians in Texas.3 The study shed light on factors associated with colic in the general population of horses (as opposed to studies from teaching or private referral hospitals) and illustrated the value of private practitioners contributing data from their practices to study possible contributing factors to colic. The findings helped document that changes in diet play an important role in colic and suggested the importance of reminding horse owners that horses with a prior history of colic are more likely to experience the condition again.

Previously, sex, age and climate were deemed inconclusive risk factors to the incidence of colic. Data collected for each case of colic included horse age, breed, sex, number of horses on the farm, stabling conditions, type of stall, history of change of housing/stabling, feed offered, feeding practices, history of any change in diet, frequency of dental care, parasite control program, deworming, immunizations, vaccinations, performance level, recent change in activity level, history of recent transport, history of previous colic, previous surgery, colic and colic outcome.3 Study results showed that history of previous colic surgery, along with history of previous abdominal surgery, and history of recent change in diet were found to be significantly associated with the occurrence of colic.

Farm management factors associated with disease and death in foals

The aforementioned studies were analytical epidemiological studies: Their aims were to identify risk factors for disease using inferential statistical methods. A study conducted on associations related to death in foals illustrated the value of descriptive (rather than analytical) epidemiology.4 The results of this study "provide a rational basis for establishing priorities for research and development in the areas of diagnostic, therapeutic and preventive medicine in Texas, and possibly in other states," says Cohen. Diarrhea and septicemia were the principal concerns for foals 1 month of age or less, with both conditions the principal cause of death for foals 7 days of age or less. The most common cause of death for foals 32 to 80 days of age was pneumonia or respiratory tract disease.

In the population of foals studied, parturition in pasture was associated with decreased morbidity attributed to diarrhea.4 Also, the crude incident morbidity from septicemia and pneumonia was significantly lower on farms where passive immunity was assessed. This association does not mean that checking passive immunity leads to reduced morbidity and mortality, but rather suggests that farms and veterinarians using good management practices such as testing for passive immunity had lower incidence of disease.

None of the management practices evaluated in the study were associated with crude mortality.4 These practices included the type of vaccination and age of mares and foals when vaccine was administered, the type of deworming agent and age of mares and foals when the deworming agent was administered, whether passive immunity was assessed for foals, whether parturition took place in stalls or on pasture, type of stall bedding, feeding practices and the number of mares on the farm at the beginning of the year.

Summary

In human medicine, epidemiology identifies risk factors for disease and targets for preventive medicine. In equine medicine, epidemiology can be defined as the study of causes, distribution and control of diseases or injuries in populations of horses. Risk factors should be considered where a change in outcome can be accomplished, such as the risk factors managed or the circumstances changed to reduce the incidence of the disease or injury. The data, including risk factors, should be quantitative and descriptive.

Epidemiology is also the basic science of evidence-based medicine. Our best clinical evidence comes from studies involving groups of patients similar to the ones we treat. These patient-based studies are epidemiological ones. They inform our interpretation of diagnostic findings, selection of treatments and estimation of prognosis. Thus, epidemiology is used by equine veterinarians every day: Each time we evaluate a patient, we think about other horses with similar presentations that we have seen ourselves or learned about from presentations or publications. Those experiences with other horses, including understanding risk factors derived from analytical epidemiologic studies, guide our clinical decisions in each and every case. No science is more fundamental to what a practitioner does each day.

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.

References

1. Chaffin MK, Cohen ND, Martens RJ. Evaluation of equine breeding farm characteristics as risk factors for development of Rhodococcus equi pneumonia development in foals. J Am Vet Med Assoc 2003;222(4):467-475.

2. Cohen ND, Carter CN, Scott HM, et al. Association of soil concentrations of Rhodococcus equi and incidence of pneumonia attributable to Rhodococcus equi in foals on farms in central Kentucky. Am J Vet Res 2008;69(3):385-395.

3. Cohen ND, Matejka PL, Honnas CM, et al. Case-control study of the association between various management factors and development of colic in horses. J Am Vet Med Assoc 1995;206(5):667-673.

4. Cohen ND. Causes of and farm management factors associated with disease and death in foals. J Am Vet Med Assoc 1994;204(10):1644-1651.


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