Introduction
Managing and dealing with disease outbreaks in cattle herds can be a challenging proposition for the practising veterinarian. The demands of practice often make it difficult for the local practitioner to devote significant time to an outbreak when it occurs. The herd manager may be frustrated and may also be unwilling to invest more finances in a situation that has already had significant losses. The practitioner needs an efficient, systematic approach to outbreaks, in order to maximise the value of their services to the producer and to provide an effective method of dealing with and preventing future losses.
An outbreak is defined as an increase – often sudden – in the observed number of cases of a disease, compared with the expected number for a given place or among a specific group of animals over a particular period of time (MacDonald, 2011). General strategies for managing outbreaks have been presented by several authors (Schwabe et al., 1977; Kahrs, 1978; Blood, 1982; Lessard, 1988; Ribble et al., 1998; Waldner, 2001a; MacDonald, 2011). Others have developed guidelines for investigating specific disease problems and productivity shortfalls (Wikse, 1988; Mickelsen, 1990; Wikse et al., 1992; Ruegg, 1993; Wikse et al., 1992; Sanderson & Christmas, 1997; Waldner, 2001b).
The immediate goal of the practitioner is to take steps to minimise additional herd losses (Ruegg, 1996). After any necessary actions have been taken to address the immediate threat, the next step is to identify the factors that contributed to the development of disease or change in productivity. The practitioner must then determine which of these factors can be controlled by management to reduce further losses and the risk of future outbreaks. These objectives can be achieved by addressing the following questions for each outbreak (W5: what, who, when, where, and why) (Schwabe et al., 1977):
- define the problem – what and how much?
- identify groups for comparison – who, when, and where?
- evaluate potential risk factors and identify key determinants – why ?
- develop recommendations for treatment, control and prevention where appropriate with follow up to assess the effectiveness of the intervention
Define and describe the problem: What? Is it worth investigating? How much?
The first and most critical task early in the investigation is to clearly define the problem (Ribble et al., 1998). The initial complaint could include unexplained mortality; clinical disease (Waldner et al., 1999; Gow et al., 2005); sub-clinical disease; impaired performance and negative trends in productivity (Waldner et al., 2001a, 2001b; Waldner, 2001b); or potential public health concerns related to food safety (Ruegg, 1996; Pritchard et al., 2000) and environmental issues (Waldner et al., 1998).
The development of a case definition is an important step in describing the problem. This case definition is critical when comparing cases with non-cases, to determine the importance of potential risk factors for disease. This definition will be re-evaluated and refined as more information become available. The case definition can be very sensitive (identifying all possible cases), such as ‘calves with diarrhoea that require treatment’, or it can be more specific and detailed (identifying cases only associated with the outbreak), such as ‘calves less than one week old with watery diarrhoea and clinical evidence of dehydration’. Case definitions can also include laboratory verification. A simple, easily recognised and applied definition facilitates the consistent reporting of cases by the herd owner, other farm workers, and cooperating veterinarians....
