Biodefense Reference Library
Foreign Animal and Zoonotic Disease Center
One Medicine: One Health (Zoonotic Disease) Online Course



(Text Modified from Document Created by Michael S. Rand, DVM, ACLAM)


The World Health Organization defines Zoonoses (Zoonosis, sing.) as "Those diseases and infections which are naturally transmitted between vertebrate animals and man". 

I. Impact of zoonotic disease

Delay or terminate 
Monetary loss
Adverse effect on morale of personnel
Unfavorable publicity
Medicolegal implications
Man-hours lost

II. Epidemiologic Concepts

Incidental host - not required for the perpetuation of the organism.
Link host - bridges the gap between the maintenance host and man.
Amplifier host - increases the number of the infective agents (viruses and bacteria) to which man may be exposed.
A laboratory animal can be both a link host and an amplifier host.

III. Mode of transmission

via aerosol, oral, contact with bedding or animals, etc. 

IV. The probability of disease transmission from animals to man is influenced by several factors:

  • Length of time the animal is infective. 
  • Length of the incubation period in animals (this is important in some diseases with long incubation periods, because the animals may be studied and euthanatized before they become infective for humans). 
  • The stability of the agent. Most important in direct transmission, where the agent is exposed to environmental changes. 
  • Population density of the animals in the colony. 
  • Husbandry practices. 
  • Maintenance procedures and control of wild rodents and insects. 
  • Virulence of the agent. 
  • Route of transmission. 

V. Classification of zoonoses

A classification system based on the type of life cycle of the infective organism seems the most useful in planning a preventive medicine program. The following categories are recommended by the World Health Organization Expert Committee on Zoonoses: 
  • Direct Zoonoses. Transmitted from infected vertebrate host to a susceptible vertebrate host by direct contact, fomite, or by a mechanical vector. No developmental change or propagation of the organism occurs during the transmission. Examples: Rabies, trichinosis, and brucellosis. 
  • Cyclozoonoses. Requires more than one vertebrate host, but no invertebrate host. Examples: Human taeniasis, echinococcosis, and Pentastoma infections. 
  • Metazoonoses. Agent multiplies, develops, or both in an invertebrate host before transmission to a vertebrate host is possible. (This means that a definite prepatent or incubation period must be completed before transmission.) Examples: arboviruses, plague, and schistosomiasis. 
  • Saprozoonoses. To transmit these infections a non-animal development site or reservoir is required, such as food plants, soil, or other organic material. Examples: larva migrans and some of the mycotic diseases. 

VI. Direction of transmission

Anthropozoonoses - Infections transmitted to man from lower vertebrates.
Zooanthropozoonoses - Infections transmitted from man to animals.
Amphixenoses - Infections maintained in both man and lower vertebrates, and may be naturally transmitted in either direction.

VII. "Emerging zoonoses"

are defined as zoonotic diseases caused either by apparently new agents, or by previously known microorganisms, appearing in places or in species in which the disease was previously unknown. New animal diseases with an unknown host spectrum are also included in this definition. Natural animal reservoirs represent a more frequent source of new agents of human disease than the sudden appearance of a completely new agent. Factors explaining the emergence of a zoonotic or potentially zoonotic disease are usually complex, involving mechanisms at the molecular level, such as genetic drift and shift, and modification of the immunological status of individuals and populations. Social and ecological conditions influencing population growth and movement, food habits, the environment and many other factors may play a more important role than changes at the molecular level. 


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