Veterinary Epidemiology and Zoonosis

ASSOCIATED WITH HOST, AGENT AND ENVIRONMENT

• These three groups of factors are some­times called the "epidemiological triad".
• In some diseases an infectious agent is the main determinant and host and environmental factors are of relatively minor importance. Such diseases are 'simple’. E.g. foot-and mouth disease­ and rinderpest.
• In other disease termed 'complex', their multifactorial nature predominates and a clear interaction between host, agent and environment can be identified.
• Thus, 'environmental' mastitis involves an interaction between Escherichia coli (the agent), milking machine faults, and poor hygiene resulting from inadequate bedding, drainage and cleaning of passage ways (the environment). In addition, cows (the hosts) are most susceptible in early lactation.