Animals and Mechanisms of Disease Transmission

More than 60% of human infectious diseases are shared with domestic or wild animals, with over a billion illnesses in the global population and millions of deaths each year. It is estimated that 75% of emerging infectious diseases are zoonoses. There is a great diversity of animals, terrestrial and marine vertebrates, which can directly or indirectly transmit a wide variety of microbial pathogens to humans. This chapter reviews the means of transmission by different groups of animals [pets, farm animals, and wildlife] that can result in zoonoses. The diverse range of infectious agents encompasses the gamut of microbes that can affect humans: bacteria [including rickettsiae and mycobacteria], viruses, fungi, parasites [protozoa, metazoan, and helminths], and prions. Infectious agents can be transmitted by animals by several different mechanisms: fecal-oral route with foodborne zoonoses or accidentally through contamination of drinking water or the unwashed hands, direct contact with or without bites or scratches, indirectly through various vectors [mosquitos, sandflies, fleas, and ticks], and incidentally by environmental contamination with animal pathogens, including aerosol of dried infected animal excrements.