Zoonoses

Human populations encounter animals with varying frequency depending on their occupation, geographical location and the prevailing culture of the country. Whether living in an urban or rural environment, animals are constantly present and humans may have close contact with animals on their farms (food-producing animals), in their homes (dogs, cats, caged birds), through leisure activities (horses, wildlife) or by virtue of their occupation as veterinarians or animal nurses.

Apart from their obvious benefits as a source of food, draught power, transportation and companionship, animals may occasionally have a negative impact on the human population through pollution of the environment, as a cause of traffic accidents, injury to humans through bite wounds and attacks on other susceptible species such as dogs attacking sheep. Health hazards associated with animals are related to communicable diseases. The term zoonoses is applied to those diseases and infections which are naturally transmitted between vertebrate animals and man.

Transmission of disease may be direct, simply by contact with an animal, or indirect, through food, non-edible products, secretions or excretions (Fig. 68.1). Apart from food-borne zoonoses the importance of particular zoonotic diseases often varies with a person’s occupation, the nature and type of animals present and the diseases prevalent in the animal population in a particular geographical region. The more frequent and direct the contact with animals, the greater the risk of acquiring a zoonotic infection. Farmers, owners of companion animals, workers in slaughterhouses or by-product processing plants, veterinarians and laboratory staff dealing with infectious material, workers in zoos and circuses and personnel engaged in servicing sanitary services are more likely to acquire zoonotic diseases than workers who have infrequent contact with animals.

Figure 68.1 Direct and indirect transmission of infectious agents from animals to the human population. Human diseases acquired in this manner are referred to as zoonoses.

An analysis of 335 emerging infectious disease (EID) events occurring between 1940 and 2004 revealed a number of interesting global trends (Jones et al. 2008). More than 60% were zoonoses with the majority of these (71.8%) originating in wildlife. The analysis also found that 54% of EIDs were due to bacteria (particularly drug-resistant bacteria) and the threat of emerging infectious diseases to global health has been increasing over time. As international travel and trade have proliferated, disease-causing agents now have the ability to move around the globe at faster rates.

Zoonoses may be classified according to their aetiology as being of bacterial, fungal, viral or parasitic origin (Tables 68.1, 68.2 and 68.3). Selected zoonotic diseases in each category are briefly reviewed.

Table 68.1a

Viral and prion zoonoses