C H A P T E R
S e c t i o n 6
Prevention and Control of Infectious Diseases
61 Epidemiology of Equine Infectious DiseasePaulo C. Duarte, Ashley E. Hill, Paul S. Morley
Epidemiology is the study of the occurrence of disease in popu-lations1 and the application of this knowledge to control or prevent disease. The underlying tenet of epidemiology is that disease does not occur randomly in a population, which means that there are always reasons why some horses become sick and others stay healthy, even if we do not always understand those reasons. This principle has tremendous implications for veteri-narians: they can identify causes and risk factors for disease and take actions to prevent or decrease the impact of a disease. In this sense, it is critical to consider more than just disease agents and hosts; it is also critical to consider the environment and management factors that impact interactions among agents and hosts. Because of this broader implication, epidemiology is also sometimes defined as medical ecology, as will be discussed.
The mare reproductive loss syndrome (MRLS) outbreak in Kentucky in 2001 was an excellent example of epidemiology in action.2 Even before veterinarians and producers understood the etiology of this disease, veterinarians were able to use epi-demiology to identify risk factors for disease (exposure to eastern tent caterpillars2 or pasture3), which allowed farm man-agers to implement control measures and prevent some abor-tions that would have otherwise occurred.
A key epidemiologic approach to understanding and controlling disease involves looking for patterns of disease in the popula-tion of interest. Which horses are sick, and what do they have in common? Which horses are healthy, and what do they have in common? Which groups have been most affected? Great insight can be gained into causal mechanisms and control points that can be exploited in disease prevention efforts by (1) describing a population and identifying patterns, (2) making comparisons among different groups within a population, (3) comparing different populations, and (4) comparing the same population at different time points.
It is useful to consider the five Ws when trying to under-stand disease occurrence in a population: who, what, where, when, and why. Who is affected (and unaffected)? Include age, breed, gender, housing, water source, and vaccination status, as well as any other variables that may be relevant. What are the circumstances related to disease occurrence and has anything changed? Where are the affected and unaffected animals located? Use a map of the barn or farm with food, water, and ventilation sources marked and spatially locate the ill animals. When did each ill animal develop disease? Use this information to identify groups most affected (e.g., age groups, barns, breeds) using the tools described later in this chapter. All of this should
be interpreted with a focus on ultimately identifying why. Why did these animals develop disease, and why were others not affected? Understanding why disease occurs allows identi-fication of ways that disease can be prevented.
When trying to understand reasons why particular animals become diseased, it is clearly important to consider more than just an individual host and a particular agent as causes for a specific occurrence. The population to which an individual belongs must also be considered, in addition to the patterns of interactions and the environment that influences these interac-tions and impacts the likelihood of contagious transmission. Because of the importance of these broader considerations, epidemiology is sometimes referred to as medical ecology, or the interactions of all organisms and their environment as these pertain to health.
Mare reproductive loss syndrome, which was initially reported among broodmares in central Kentucky in 2001, is one example of disease arising from a combination of host, agent, and environmental factors. An epidemic of equine abortion, endophthalmitis, and pericarditis began in late April 2001 and lasted until June 20014-6; fetal losses occurred both early and late in pregnancy3,7 and affected more than 60% of mares on some farms.7 Multiple bacterial species were identified8 in tissue of aborted fetuses. The syndrome was subsequently found to be associated with ingestion of the eastern tent caterpillar,4 and it has been proposed that bacterially contaminated barbed cater-pillar hairs migrated out of the alimentary tract, spread hema-togenously, and were directly responsible for the observable signs of MRLS.9 Eastern tent caterpillars are ubiquitous in the eastern United States but were particularly abundant in Ken-tucky that spring because a rapid temperature increase in early spring was superimposed on an unusually dry winter and spring.10 These climatic conditions caused an explosion of bio-logic activity, including growth of black cherry trees on which eastern tent caterpillar eggs are laid and larvae develop.11 During that spring with its unusual climactic conditions, grazing on pasture4 with black cherry trees12 exposed horses to disease; fetuses were particularly vulnerable. The sensitivity of the fetus to disease, the environmental conditions that led to the over-growth of caterpillars, the bacteria themselves, and the manage-ment of the broodmares all contributed to the occurrence of MRLS.
Disease AgentCharacteristics of the disease agent, including infectivity, con-tagiousness, pathogenicity and virulence, immunogenicity, host range, life cycle, and antimicrobial susceptibility, influence the speed and scope of disease spread. Infectiousness (infectivity) refers to the ease with which an agent infects susceptible hosts,
516 Section 6 Prevention and Control of Infectious Diseases
transport, or mixing, are more likely to develop a disease than their unstressed counterparts. The risk of disease is not equal for similar horses when managed differently or housed in dif-ferent environments.
IndividualEnvironmental characteristics that affect risk of disease in indi-vidual horses include climate, landscape, flora and fauna, cleanli-ness, air quality, housing, diet, and events that affect stress levels. Some of these factors (e.g., cleanliness, ventilation, housing, diet, stress level) are directly related to management practices and may be changeable, thus affecting risk of disease. Some management strategies (e.g., housing in open pastures, using outdoor drinking-water sources) increase potential exposure to insect vectors and also increase the risk of other diseases such as Potomac horse fever (Neorickettsia risticii infection) and vesicular stomatitis. On the other hand, indoor housing, espe-cially if it is high density or poorly ventilated, can increase exposure to diseases transmitted by aerosol or oral-fecal routes, such as influenza virus and Salmonella. Some environments and climates support larger vector populations than others, thus increasing potential risks for diseases, such as equine infectious anemia (EIA) or the equine encephalitides, including western equine encephalitis (WEE), eastern equine encephalitis (EEE), and WNV encephalitis. Although the climate itself cannot be changed, management practices, such as using animal-safe insect repellents, treating open-water sources with larvicides, and housing horses indoors at dusk and dawn, can be used to reduce disease risk.
PopulationIn addition to characteristics of individuals that affect their disease risk, the aggregate characteristics of the population to which the individual belongs affects the disease risk for that individual. This aggregate of the populations susceptibility to disease is often called herd immunity, described as immunity of an individual that is conferred by the population to which it belongs, or the ability of a population of animals to withstand exposure without succumbing to disease because the immunity of a population is more than the sum of its parts.31 Herd immu-nity is created when the likelihood is small that an infected horse shedding a disease agent will encounter a susceptible horse (Fig. 61-1). If most horses are immune or if contact among horses is heavily restricted, it is unlikely that the few susceptible horses will have contact with the infected horse sufficient to allow transmission. For example, consider a barn in
which is sometimes quantified in relation to the amount of agent required to reliably infect an individual. Contagiousness relates to the likelihood that an agent will move between infected and susceptible hosts; it is sometimes quantified by the number of new infections that will likely result from exposure to an infected animal or as the speed with which a disease agent is transmitted through a susceptible population. Equine influ-enza virus and equine herpesvirus are both highly infectious, but influenza virus is more contagious. Although equine proto-zoal myeloencephalitis (EPM) is an infectious disease, it is not a contagious disease because the etiologic agent is not transmit-ted directly between horses. Pathogenicity describes the likeli-hood that an infected horse will develop clinical disease, and virulence describes the likelihood that disease will be severe. West Nile virus (WNV) is highly virulent in horses; more than 30% of horses with clinical disease die.13 In contrast, EPM is not highly pathogenic; most equids exposed to the disease agent do not develop clinical disease.14-16
Characteristics of the disease agent that enable it to survive and spread without detection are particularly important to con-sider when instituting preventive or control measures. Agents that can persist in the environment, such as Clostridium diffi-cile17 or Streptococcus equi subsp. equi, re