201312 BS Understanding Inf Dis V2.0

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Understanding infectious disease

Introduction Infectious agents include many different types of bacteria, viruses, fungi and parasites that can cause a wide range of clinical diseases. Individual infectious diseases often require quite different treatment and control strategies. Trying to put separate strategies in place for each one can be a difficult task. However, there are many similarities between infectious diseases that can help our understanding of these diseases and their control. Control strategies are often effective against more than one infectious agent. Figure 1 illustrates the interactions common to infectious diseases. This highlights that there are three different components which all interact to influence the occurrence and severity of infectious disease. Thinking about each component when infectious disease is confirmed on your farm is a good starting point to understanding infectious disease and preventing future disease outbreaks.

The farm environment / management


The infectious agent

The animal

Figure 1: The most important influences on infectious diseases.

The infectious agent Infectious agents or pathogens are the small organisms (mainly bacteria, viruses and parasites) that are capable of causing an animal to become sick. However, infectious agents don’t necessarily cause an animal to become obviously ill, and they can often be found in and around healthy animals. Primary infectious agents in cattle Many of the disease challenges faced on farms are from infectious agents that have adapted to be able to infect cattle and to spread from animal to animal. Animals with little or no immunity to these pathogens will often show severe signs of disease. This happens most often when a pathogen enters the herd for the first time and, when many animals are affected, this is often referred to as an epidemic or an outbreak. To prevent outbreaks of disease requires good farm biosecurity management. When a pathogen has been circulating in the herd for some time, animals will have been infected and develop adequate immunity. However, the disease may remain present in the herd usually by persisting in ‘apparently healthy’ animals called carriers. Pathogens can cause illness if they spread from carriers to animals with little or no

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