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Equine influenza - Using epidemiology to contain an outbreak
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Summary
Equine influenza, as a highly infectious disease, can be prevented and controlled through an effective surveillance and outbreak response system, based on sound understanding of epidemiological principles. In this context, structured risk assessment and management as outlined in the OIE risk analysis framework provide stakeholders with a transparent mechanism of integrating state-of-the art scientific knowledge into the decision-making processes. Prevention and control of EIV outbreaks needs to acknowledge the complexity of the eco-social system within which the spread of the virus occurs, which means that epidemiological as well as social science drivers of virus transmission need to be understood and, where feasible, changed. As with many other infectious diseases in animals and humans, vaccination against EIV infection plays a key role in prevention of spread in at risk populations, but its efficacy at the population level is subject to a complex mix of percent vaccinated, proportion effectively protected and circulating strain - vaccine match.
Body Text
Spread of infectious diseases amongst susceptible populations, whether they are humans or animals, follows the same epidemiological principles. If the pathogen, as in the case of equine influenza virus (EIV), is spread through direct or indirect contact, prevention and control of any outbreaks primarily relies on preventing opportunities for direct or indirect contact between infectious and susceptible individuals. A key parameter is the infectiousness of the pathogen, which can be expressed mathematically as the basic reproduction number. This is calculated as the average number of new cases generated by each infected individual during its infectious period in a susceptible population. The goal of prevention and control measures is to reduce the basic reproduction number to below unity. To put it into perspective, the basic reproduction number is estimated as 12 - 18 for measles, as 2 - 3 for the 1918 pandemic influenza strain, as 1.5 – 2.5 for Ebola virus. The corresponding numbers for EIV vary substantially, with 1.4 - 5 at one end and 10 at the other end of the spectrum [1, 2]. This indicates that EIV is highly infectious, and prevention or control of outbreaks requires quick and effective implementation of appropriate strategies that will reduce the basic reproduction number to below 1. Effective prevention rests on preventing exposure of susceptible horses to direct or indirect contact with the virus, which is translated into preventing introduction of potentially infected horses into EIV free populations. Transport of horses within and between countries represents the main factor for spread of the virus. Where there is a risk of exposure to EIV, vaccination allows prevention or at least slowing down of the speed of transmission through a population of equines. For this to work, the aim needs to be to achieve as high a vaccination coverage as possible. In the UK a cross-sectional study reported 82% vaccination coverage in a mixed horse and pony population, for which it is important to note that it was not representative of the national population [3]. For a highly infectious virus such as EIV, a higher coverage would be desirable to achieve herd immunity that minimizes the risk of outbreaks [4]. Also, the vaccine needs to match the circulating EIV and the vaccination protocol needs to be optimised [5, 6]. Antigenic drift and shift of the viruses means that currently used vaccines need to be replaced regularly. As in other areas of animal health, for most effective decision-making equine infectious disease prevention and control should be based on the OIE risk analysis framework’s scientifically based risk assessment and management [7, 8]. Often the implementation of surveillance is considered only from the perspective of monitoring virus characteristics [9, 10], but a cost-effective disease surveillance programme should be designed on the basis of epidemiological principles so that it can provide key information for effective risk assessment and management [11, 12]. As part of this, EIV surveillance should aim for early detection of outbreaks, which requires willingness of owners and veterinarians to report to the surveillance programme. This can be achieved, if confirmed detection results in actions that are considered appropriate by the stakeholder community [13, 14]. It needs to be recognised that these acceptable actions will vary between and within countries and depending on the local EIV situation. Similar to other influenza viruses, the global situation of widespread EIV occurrence requires continuous monitoring of infection prevalence and molecular diversity, continued investment into vaccine development and constructive engagement with the different types of stakeholders in the equine stakeholder communities around the world to minimise the risk of EIV spread.
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1. Daly, J.M. et al. (2013) What can mathematical models bring to the control of equine influenza? Equine Vet J 45 (6), 784-8.
2. Hughes, J. et al. (2012) Transmission of equine influenza virus during an outbreak is characterized by frequent mixed infections and loose transmission bottlenecks. PLoS Pathog 8 (12), e1003081.
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