Disease control and prevention relies on the interrelated processes of bioexclusion, surveillance, and biocontainment. Prevention of disease is costly, difficult, and time consuming and is primarily directed at preventing epidemic or exotic diseases. It invariably involves eradication of disease-causing agents from a population of animals or geographic area. In contrast, control programs are less demanding and primarily focus on limiting endemic diseases to tolerable levels within a population of animals or geographic area. Although preventing exposure to disease-causing agents remains an objective of control strategies, strategies are primarily focused on limiting the consequence of disease.
Disease prevention depends on 1) stringent bioexclusion to avoid contact between the disease-causing agent and the host, 2) early detection of a breach in biosecurity through vigilant surveillance, and 3) rapid implementation of a ruthless biocontainment policy. This is feasible only if there is an effective means of detecting infection; containing the infection through slaughter, clean-out, and disinfection; and preventing dissemination of the disease-causing agent. Eradication is reserved for those diseases that pose a dire public health threat, that have a devastating effect on animal performance, or that severely compromise end-product quality.
In disease control strategies, the emphasis shifts from preventing disease to reducing its consequence or economic impact. Prevalence data are now used primarily to assess the level of protection and challenge, not merely the presence or absence of disease. Although biosecurity still relies on principles of prevention, disease-control programs focus more on limiting the extent and consequence of exposure. Many biosecurity measures aimed at preventing or eradicating epidemic disease also produce beneficial by-products, such as establishment of a firm foundation for control of erosive/endemic diseases and enhancement of host resistance through immunization.
Disease Management and Disease Determinants
Disease-risk management must be an integral part of any animal management program. Economic analysis is a critical step in biosecurity plan design, because resource allocation must be aligned with risk. The success of a disease control program depends on the ability to identify and subsequently address risk of infection. Disease risk in a population is characterized by the probability of point infection and subsequent spread. Aggregate risk is the sum of each individual risk of adverse health effects in an exposed population. The spread and consequence of point infection is influenced by several factors referred to as disease determinants.
Because infectious disease is the result of a complex interaction between several factors, any factor that influences the risk and consequence of disease challenge is a disease determinant. Disease determinants have traditionally been classified as primary or secondary; intrinsic or extrinsic; and host-, agent-, or environment associated. In intensive production units, the housing environment, agent, and host determinants are largely under the control of the manager/caretaker, who thus has the greatest influence on disease determinants.
Risk assessment is used to estimate the probability of exposure to an agent, the probability that exposure will cause infection and disease, the probability that disease will spread, and the consequence of such spread. Statistical techniques such as the chi-square test can be used to assess whether a specific factor/process is correlated with disease, but these provide no estimate of the degree of disease risk. The measure of association most frequently used to assess risk magnitude is the risk ratio (probability of disease given exposure divided by the probability of disease given no exposure).
Disease control begins with evaluating each part of the production process as a risk factor for infection. On a basic level, risk of infection equals the probability of each event causing infection multiplied by the number of times each event occurs. But estimating the degree of risk requires analysis of many other factors, including host resistance and the dose/virulence of the organism.
As a first step to limit health risk, a biosecurity program should critically assess the necessity of all events or processes that potentially carry risk, and only crucial ones should be allowed. Limiting the potential for infection within crucial events is focused on improving host resistance or reducing the challenge dose or virulence of the infecting organism.
Immune efficiency is the key factor governing host resistance. A healthy animal produces an appropriate immune response sufficient to combat infection and its impact on productivity. Conversely, a response that is either excessive or insufficient will adversely affect well-being and performance. Resistance varies among individuals primarily due to genetic differences that typically follow a Poisson distribution.
Immunosuppression reduces both individual and herd/flock immunity. The weakest members of a population will be most negatively impacted by individual stressors (eg, disease or poor nutrition), which have a cumulative impact on immunity. Such highly susceptible animals tend to skew the resistance curve to the right, resulting in a dramatic decline in flock/herd immunity.
Host resistance, challenge dose, and organism virulence determine whether an infectious challenge results in disease. Dose is the number of organisms to which an individual animal is exposed, and virulence is the inherent capability of the agent to infect (infectivity) and cause disease (pathogenicity) within the host. The traditional measure of infectivity is the infective dose 50 (ID50), which is defined as the dose required to infect 50% of animals in a specific population.
When designing a health program, it is important to remember that the ID50 represents more than a measure of disease virulence. It is more factually the dose required to infect the least resistant animal within the population. This is because the risk of a challenge exposure increases once one animal in a population becomes infected or diseased. Agent replication increases the risk of exposure by increasing the dose and (possibly) virulence of the agent, with each new infection escalating the risk further until even relatively resistant animals become at risk.
See also Basic Principles of Epidemiology. Effective biosecurity requires an understanding of the causal relations between exposure and disease, because the prevalence and consequence of any infectious disease involves a complex interaction between several disease determinants. The epidemiology and relative risk for each disease should be assessed to determine the best way to allocate resources for effective control procedures. Furthermore, epidemiologic statistics can be used to determine the best way to limit current and future financial risk. Several important epidemiologic factors need to be considered (below).
Source of Infection:
In addition to identifying those animals that have become infected, other variables, which are associated with the relative importance of a particular source, should be assessed, including the shedding pattern, host range, mode of transmission, and farming practices.
Disease transmission within a population is influenced by direct contact with infected animals, as well as indirect exposure from contaminated objects (fomites). These forms of horizontal transmission influence the rate and extent of transmission within and among groups. This is in contrast to vertical transmission between parent and offspring. Vertical transmission may result from direct contamination or (in some cases) from transovarial transmission of disease-causing agents within the embryo itself.
Disease spread is influenced by factors such as incubation period, replication rate, resilience, and virulence of the disease agent. These factors influence spread within the population as well as disease course (acute, subacute, chronic) within individuals. For example, disease spread is accelerated by a resilient organism with a short incubation period and high replication/shed rate.
Susceptibility and Predisposition:
For an effective control program, it is important to know the range (species, breed, type) of susceptible hosts. Furthermore, host, agent, and environmental factors can predispose animals to infection or disease. For example, environmental stress can compromise immune function.
Disease prevalence is directly proportional to the risk of challenge. Endemic diseases are difficult to prevent, whereas diseases that are exotic (ie, not normally present) or epidemic in nature are easier to track, contain, and eradicate.
Morbidity and Mortality:
Morbidity refers to the animals in a population showing signs of disease at particular times. Diseases that spread rapidly typically have a high morbidity rate (percent). Mortality rate represents the percentage of animals in a population expected to die during a particular disease outbreak.
Disease recovery is influenced by many factors (ie, disease determinants).
Last full review/revision August 2015 by Stephen R. Collett, BSc, BVSc, MMedVet