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Biodefense Reference Library
Foreign Animal and Zoonotic Disease Center
One Medicine: One Health (Zoonotic Disease)
Online Course
ZOONOTIC DISEASES
ZOON |
NOSES = ANIMALS | DISEASES
(Text
Modified from Document Created by Michael S. Rand, DVM, ACLAM)
INTRODUCTION
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
Illness
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
Feces
Urine
Saliva
Blood
Milk
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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