Equine epidemiology: The search for all contributors to disease

This field of science tries to encompass all potential causes, not just one principal one
Nov 01, 2011

During the 57th annual American Association of Equine Practitioners (AAEP) conference in November, Noah D. Cohen, VMD, MPH, PhD, Dipl. ACVIM, professor in the Department of Large Animal Clinical Sciences at Texas A&M University's College of Veterinary Medicine, will present the Frank J. Milne State-of-the-Art Lecture titled "Equine epidemiology: Counting for something in equine practice." Cohen completed his MPH and PhD degrees in epidemiology at the John Hopkins University School of Hygiene and Public Health.

In prelude to his lecture, I had a chance to discuss epidemiology of equine diseases and injuries with him. Cohen outlined some basic premises of epidemiology.

It is common to think about causes of events (including disease or other health-related events), simplistically—this batch of grain caused my horse to colic, this racing surface causes racing injuries, this bacterium caused my horse to have diarrhea, etc.

Single causes that are both necessary and sufficient to cause an event (e.g., disease, injury) are almost never encountered. For example, not just diet but exercise level, access to water, anthelmintic administration and many other factors identified or unknown contribute to a given case of colic. In addition to a racing surface, the caliber of race, preexisting pathology, age, sex, training regimen and other factors known or unknown can contribute to a racing injury. And besides a given organism, factors such as antibiotic administration, diet changes, transport, amount and composition of diet and other known and unknown factors contribute to the development of diarrhea.

Experimental science is largely concerned with helping us determine and understand the mechanism by which a given component of cause (e.g., racing surface) contributes to the effect (e.g., racing injury), while epidemiological (observational) science is concerned with identifying the role of components in the face of other known and undetermined components of cause. Put another way, it is a science equipped to deal with the multifactorial nature of cause.

The fact that epidemiology is observational is the root of its strengths and limitations. Its principal strength is that observations about horses in the real world are directly relevant to the real world. The field of epidemiology is based on methods to account for multiple contributing factors, some of which may be unknown or unmeasured. The principal disadvantage is that the extraneous factors that aren't controlled by the scientists making the observations can lead to noise that may obscure or alter our ability to recognize the signal.

These basic premises are illustrated in several articles on Cohen's research on a variety of equine diseases. In each case, one or more causes are examined as possible risk factors of disease.

The causes of Rhodococcus equi pneumonia in foals

One article looked at the association of equine breeding farm characteristics and management practices as risk factors for the development of Rhodococcus equi pneumonia in foals1 —a variety of causes are suspected to be associated with the disease. The results of the study showed that there were specific farm characteristics associated with increased risk for development of R. equi pneumonia in foals, notably farms with large acreage and large numbers of horses, mares and foals; high foal density; and the presence of transient mares and their foals. Briefly, this is a disease of large, well-managed farms.

Another article looked at the association of soil concentrations of R. equi and the incidence of pneumonia attributable to R. equi in foals on farms in central Kentucky.2 The article stated, "Since only virulent organisms are believed to cause disease, it is therefore important to determine the virulence status of isolates in epidemiological studies of Rhodococcus equi. The epidemiologic aspects of pneumonia in foals caused by this bacterium are not clearly characterized."2

Various factors relate to occurrence of disease in populations of foals on various farms—R. equi is endemic on some farms, may develop intermittently on others and is not found on most farms. There may be year-to-year variability in foals with pneumonia attributable to R. equi at farms where the disease is endemic. Variability in occurrence among and within farms is common, though the reason is unknown.

"Molecular epidemiological data indicate that recurrence of the problem at a farm does not appear to be attributable to a particular strain of R. equi at that farm, and various genetic strains may be isolated from affected foals or their environment at a particular farm. Although the disease appears to be most common at larger breeding farms that use practices deemed desirable for management of optimum foal health, these criteria do not differentiate farms with foals that have pneumonia attributable to R. equi from farms with unaffected foals."2

Management practices other than the density of mares and foals at the farm do not seem to differentiate occurrence of disease, though larger, well-managed breeding farms are more commonly affected. It seems plausible that environmental burden of virulent R. equi in soil, feces or the air and contamination of stalls, paddocks or pastures and feed and feed troughs could play a role in explaining the epidemiology of R. equi. Data from a small number of farms in Japan had suggested that concentrations in soil might be higher at farms affected with R. equi than at those that are unaffected.

The breeding farm management study described earlier tested this hypothesis. The study of 37 horse breeding farms in central Kentucky investigated soil samples and cumulative incidence of R. equi pneumonia for their association.1 The authors reported that "neither the concentration nor proportion of virulent isolates of R. equi in soil explained the reason that some farms have foals affected with pneumonia attributable to R. equi during a given year, whereas others are not affected." Exposure to virulent R. equi in soil samples was widespread and common at breeding farms, irrespective of their status, with respect to the incidence of pneumonia attributable to R. equi among foals. The study concluded that "although virulent R. equi is necessary to cause pneumonia attributable to R. equi, it is clearly not a sufficient cause, at least with regard to its concentration in the soil."