60 MARCH 2013 • WORLD AQUACULTURE • WWW.WAS.ORG During the last decade, the term biosecurity has been used frequently to describe measures for prevention and control of diseases (FAO 2011, OIE 2011). However, biosecurity remains a term without broad acceptance in the international community. Biosecurity is used in human as well as veterinary medicine, including in relation to bioterrorism. Biosecurity has even been used when communicating about prevention of harmful environmental consequences associated with the spread of diseases and other invasive alien species. A commonly used definition of biosecurity in veterinary medicine refers to a set of measures that reduces the risk of introduction and spread of disease agents. Biosecurity can be applied to different population levels: from the farm to the region to the national level. Biosecurity measures are used to prevent the entry of infectious agents into an animal population, called external biosecurity or bioexclusion. However, the prevention of spread and control of pathogens within a population, called internal biosecurity or biocontainment, is also an important part of biosecurity. In populations of terrestrial animals, segregation, cleaning and disinfection are important elements of biosecurity, both external and internal. Even in aquaculture these measures are important, but they may have significant limitations. In landbased food production, biosecurity programs do not always include vaccination. Farming in the aquatic realm makes effective segregation from the environment more difficult or even impossible. Consequently, in aquaculture, vaccination has become an integral part of biosecurity. Vaccination may not completely prevent introduction of pathogens into a population of farmed fish. However, vaccination may reduce the risk of disease agents becoming established and spreading within a population. Furthermore, vaccination may reduce the risk of clinical disease outbreaks occurring in the farm if an infectious agent is introduced, and thereby reducing the risk of spread to neighboring farms. Consequently vaccination is an important tool in internal biosecurity. Disease prevention by stimulation of immunity has advantages in aquaculture where numbers of individuals are very high. Some fish may escape vaccination and a certain percentage of the fish may respond poorly. Because of herd immunity, which occurs when a major proportion of a population is vaccinated, protection against diseases and reduced mortality may be achieved even in non-vaccinated fish and poor responders. Herd Immunity Herd immunity describes a form of immunity that occurs when vaccination of a major proportion of a population provides protection for individuals without immunity. The level of herd immunity is dependent on different general factors, including the proportion of individuals with protective immunity and the infection pressure. Herd immunity is expressed as the proportion of the population that must be immune or resistant to prevent the spread of an infection. The effect of herd immunity is also determined by factors related to the pathogen causing the disease and the vaccine that is used for prevention. These variables are the efficacy of the vaccine (?) and the basic reproduction number of the agent (R0). Based on these two parameters, the herd immunity can be estimated mathematically: where ? is the vaccine efficacy, and R0 is the basic reproduction number. Vaccine efficacy is defined as the proportional relationship between mortalities in vaccinated and unvaccinated groups during challenge or a disease outbreak (Kirkwood 1988). Here the term vaccine efficacy is used interchangeably with Relative Percent Survival (RPS), which describes the efficacy of a vaccine and represents the percentage of the population that would have died from infection if not vaccinated (Amend 1981). The RPS can be calculated as: The basic reproduction number (R0) is the average number of secondarily infected individuals arising from a single primary infected individual during its entire infectious period in a naïve, entirely susceptible population (Anderson 1991). When R0 is greater than 1, each primary case will, on the average, produce more than one secondary case, leading to an epidemic. On the other hand, when R0 is less than 1, a disease outbreak is not likely to occur. R0 has been used to predict the magnitude of epidemics of infectious diseases, including foot-and-mouth disease, avian influenza, rabies, tuberculosis and others (Ferguson et al. 2001, Goodschild and Clifton-Hadley 2001, Kitala et al. 2002, Ward et al. 2009). Theoretically R0 is specific to a disease but not to a population (Ward et al. 2009). Epidemics of the same disease may be different in morbidity, mortality and duration in different populations. Therefore, the herd immunity can be variable among different populations. The R0 in aquaculture, similar to the R0 of diseases of terrestrial animals, depends on specific characteristics of diseases, such as transmissibility and infectious period. R0 of some diseases also depends on host density, which has a decisive influence on the frequency of contacts between individuals with Immunoprophylaxis in Biosecurity Programs Roar Gudding, 1 Atle Lillehaug*2 and Saraya Tavornpanich3
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