Zoonoses

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Zoonoses

• Diseases and infections which are naturally
  transmitted between vertebrate animals and man
  (WHO)

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Zoonoses Historical aspects

• Conditions dating back for centuries
• encephalitis in dog with Rabies
• ringworm in humans and animals
• Glanders and tetanus in horses and humans
• Epidemic of urban plague
• Late 1600s Leeuwenhoek invented the microscope,
  investigation of etiological agents soon followed

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Zoonoses Historical aspects

• Mycotic diseases
• ringworm first described in 1840s cat as
  reservoir of human M. canis in 1902

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Zoonoses Historical aspects

• Bacterial diseases
• tick-borne relapsing fever (spirochete)- 1873
• B. anthracis (Koch) 1877
• by 1890, most bacterial zoonoses known to occur
  at that time had been described

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Zoonoses Historical aspects

• Rickettsial diseases
• Rocky Mountain spotted fever (R. rickettsii)
  1909
• most rickettsial agents described by 1930s

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Zoonoses Historical aspects

• Viral diseases
• 1798 Jenner reported protection from human
  smallpox conferred by innoculation of cowpox
• first confirmed viral zoonosis (rabies) 1903
• most initial reports in 1930s after advent of EM

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Zoonoses Historical aspects

• Parasitic diseases
• most cestodes described by 1800
• most trematodes by the 1870s
• most nematodes by late 1800s, filarids by 1930s
• most zoonotic protozoa1885 to 1915
• plasmodia of primates 1930 to 1960s
• clarification of coccidia and their role more
  recent to present

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Definitions

• communicable disease - illness that results from
  transmission of agent from infected host to
  susceptible host
• host - animal (vertebrate/invertebrate) that
  harbors agent (primary, secondary, incidental)

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Definitions

• reservoir - host in which agent normally lives
  and multiplies and depends on for survival and
  transmission

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Definitions

• infection - entry and development of agent
• infectious disease - clinically apparent disease
  that results from infection
• infected individual - harbors infectious agent
  and either has either apparent or inapparent
  disease (carrier)

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Definitions

• carrier - harbors infectious agent in absence of
  clinically apparent disease
• healthy or asymptomatic carrier
• convalescent carrier
• can either be transient or chronic
• communicable period - period during which an
  infectious agent can be transmitted (direct or
  indirect) to a susceptible host

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Definitions

• incubation period - period between initial
  contact with infectious agent and onset of
  symptoms
• contamination - presence of an agent on the
  surface of the body or on the surface of
  inanimate objects such as clothing, bedding,
  food, water

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Definitions

• Index case first individual recognized as being
  affected with disease
• Proband (propositus) first (primary) case
  having disease
• Efficient disease control when the index case
  primary case with increased frequency

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Natural history of disease
Preclinical
Clinical
Outcomes Cure Control Disability Death
Healthy
Symptoms (clinical onset)
Biological onset
Diagnosis
Tertiary prevention
Primary prevention
Secondary prevention
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Classification of zoonoses

• Direct zoonoses
• transmitted from infected host to a susceptible
  host by direct contact, inanimate object, or a
  mechanical vector
• no developmental change or propagation of the
  organism occurs during the transmission
• Example rabies, trichinosis, brucellosis

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Classification of zoonoses

• Metazoonoses
• agent multiplies, develops, or both in an
  invertebrate host before transmission to a
  vertebrate host is possible
• a definite prepatent or incubation period must be
  completed before transmission
• Examples arboviruses, Lyme disease

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Classification of zoonoses

• Cyclozoonoses
• Requires more than one vertebrate host, but no
  invertebrate host
• Example human taeniasis, echinococcosis

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Classification of zoonoses

• Saprozoonoses
• Transmission requires a non-animal development
  site or reservoir, such as food plants, soil, or
  other organic material
• Example larva migrans, mycotic diseases

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Emerging zoonoses

• 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

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Emerging zoonoses

• most involve transmission to humans from an
  ongoing reservoir
• some are derived from a newly established life
  cycle in humans that no longer depends on the
  animal reservoir

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Emerging zoonoses

• Factors explaining emergence
• modification at the individual host level
• changes in immunological status
• physiological changes
• behavioral changes

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Emerging zoonoses

• Factors explaining emergence
• modification at the host population level
• immunological changes
• behavioral changes
• societal changes
• movement

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Emerging zoonoses

• Factors explaining emergence
• length of time the animal is infective
• length of the incubation period in animals
• the stability of the agent
• population density

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Emerging zoonoses

• Factors explaining emergence
• husbandry practices
• maintenance procedures and control of wild
  rodents and insects
• virulence of the agent
• route of transmission

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Emerging zoonoses

• Factors explaining emergence
• modifications at the environmental level
• ecological changes
• changing climate
• encroachment--urban sprawl
• bioterrorism

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Emerging zoonoses

• Factors explaining the emergence of a zoonotic or
  potentially zoonotic disease
• mechanisms at the molecular level (agent and
  host)
• mutation
• natural selection
• evolution

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Emerging zoonoses

• Since 1973, majority of newly recognized
  pathogens/infectious diseases are zoonoses
• Lyme disease
• Ebola viral hemorrhagic fever
• Human ehrlichiosis
• Hantaan viral disease
• E. coli O157H7
• Cyclosporiasis

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Emerging zoonoses

• In past decade
• Hendra virus
• Nipah virus
• Australian bat Lyssa fever
• Bartonella henselae
• new variant Creutzfeldt-Jakob disease

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Zoonoses

• Central precept of public health regarding threat
  of disease
• there is always a risk, and quantification of
  that risk is imperative in order to ensure
  appropriate prevention and control of disease

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Principals of zoonotic diseases

• Disease surveillance
• detection of disease
• measure extent of occurrence
• identify needed interventions
• evaluate impact of interventions
• Disease monitoring
• all of the above except evaluation of
  intervention aspect

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Principals of zoonotic diseases

• Detection (surveillance) dependent on knowledge
  of life cycle
• reservoir host
• definitive (primary)
• intermediate (secondary)
• carrier
• asymptomatic (healthy or subclinical)
• convalescednt (incubationary)

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Principals of zoonotic diseases

• Detection (surveillance) dependent on knowledge
  of life cycle
• habitat of host(s)
• sylvatic - wildlife
• domiciliated - rats, mice
• area of infected maintenance hosts called natural
  focus (nidus, niche)

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Principals of zoonotic diseases

• Epidemic unusual occurrence of disease
  exceeding the expected number (endemic)
• Outbreak a cluster of cases that may or may not
  be an epidemic
• Pandemic widespread occurrence of disease
  globally

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Principals of zoonotic diseases

• Outbreak or epidemic can be due to
• Common source indirect transmission due to
  exposure to a common vehicle (food, air, water,
  mechanical)
• Point source brief period of exposure
• Propagative secondary cases resulting from
  exposure to primary case which serve to extend
  period of infectivity

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Principals of zoonotic diseases

• Prevention
• inhibiting the introduction of a disease causing
  agent into a specific area, population, or
  individual
• Control
• steps taken to reduce and maintain at a tolerable
  level

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Principals of zoonotic diseases

• Eradication
• elimination of a disease causing agent from a
  specific area or population
• total versus practical eradication
• Smallpox only disease totally eradicated
  worldwide
• not practical due to economic and other
  constraints (geopolitical)

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Principals of zoonotic diseases

• Prevention and control involve
• reservoir neutralization
• removal
• test and slaughter
• mass therapy
• manipulation of environment or vehicle
• pasteurization
• disinfection
• sterilization

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Principals of zoonotic diseases

• Prevention and control involve
• reduction of contact potential
• isolation - keep the agent in
• quarantine - keep the agent out
• population control - benign such as leash laws or
  restriction of movement of livestock more
  drastic - reduction
• herd immunity

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Principals of zoonotic diseases

• Prevention and control involve
• increasing host resistance either to prevent or
  lessen severity
• genetic selection (manipulated or natural)
• chemoprophylaxis
• immunization

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Mechanisms of transmission

• direct - immediate transfer
• touch
• bite
• sexual transmission
• droplet projection

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Mechanisms of transmission

• indirect - transmission by intervening factor
• vector-borne (mechanical or biological)
• arthropod-borne
• tick-, flea-, mosquito-borne
• vehicle-borne
• food-, water-, soil-borne

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Mechanisms of transmission

• airborne - dissemination of infectious aerosols
  to a suitable port of entry
• suspension of agents may remain for long periods
  of time
• small particles (usually lt 5 ?) are easily drawn
  into alveoli
• droplet nuclei - result from evaporation of
  fluids or atomization
• dust - particles which arise from soil

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Mechanisms of transmission

• horizontal - direct or indirect transmission from
  infected individual to susceptible individual
• various modes of interspecies spread of zoonoses
• vertical transmission from one generation to
  another
• dam to offspring in utero or via colostrum
• important feature in many vector-borne zoonoses

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Epidemiology - who, when, where

• risk factors characteristics that increase or
  decrease likelihood of exposure, subsequent
  infection, and prognosis
• predicting risk comes from knowing
• Host age, sex, breed/race, etc.
• Place geographic features of occurrence
• Time time of day, seasonal
• Environment factors (time place)

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Emerging zoonoses
Zoonotic disease reports 1/2001 6/2001 (Promed)