Principles of Biosecurity of Animals

Understanding disease transmission is central to designing proper biosecurity protocols. Diseases can be transmitted in many ways, and direct animal-to-animal contact as well as contact with contaminated fomites are some of the most common transmission routes. Contaminated semen and natural mating can be sources of sexually transmitted diseases.

Many fomites (inanimate objects) act as carriers of disease-causing agents. Survival of agents on fomites may depend on the composition of the particular fomite and how easily it can be disinfected. Examples of fomites considered high risk include trailers, vehicles, maintenance and repair tools, boxes, materials used to remove dead animals, loading chutes, etc.

Vectors are also capable of transmitting diseases; among the most significant vectors are birds, bats, rodents, feral and wild animals, stray and domestic animals, and insects.

Air can be a source of diseases, particularly in areas of high animal density. Contaminated water and feed and consumption of contaminated, raw, untreated animal products have also been implicated in disease transmission. Manure, bedding, and carcasses can also be a source of agents unless disposed of properly. Lastly, people can act as both mechanical and biologic vectors, and training and awareness of personnel working with animals is necessary for proper implementation of biosecurity programs.

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 way to detect infection; containing the infection through slaughter or other means, 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. Elimination of diseases without a regulatory framework is common in food animals if these diseases are economically significant and their elimination is advantageous for the producers.

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-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 because of 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 (ID), which is defined as the dose required to infect 50% of the 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.

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).