• require a blood sucking arthopod to complete their life cycle
• require a minimum of two hosts (a vertebrate and arthopod)
• must produce a level of viremia in the vertebrate host for the arthopod to become infected with a blood meal
• with few exceptions, zoonoses depend on animal species other than humans for maintenance in nature
• humans are "incidental hosts" who usually do not contribute to transmission cycle

• togaviridae
• flaviviridae
• bunyaviridae

• tropic areas
• cold weather disrupts life cycle

• denge fever viruses
• yellow fever viruses
• japanese encephalitis
  • west nile virus

• vector: Culex pipens mosquitoes as enzootic and epizootic vectors
• reservoir: birds (remember death of crows in US corresponded with disease prevalance across US)
  • correlates very well with flight patterns
• human syndrome
  • febrile illness
  • encephalitis

Can infect humans and domestic animals

• elderly and immunocompromised
  • affecting most of US
  • increase human fatalities

• severe neurological and fatal disease rare in humans and equines
• hypotheses
  • birds infected with more virulent strains too sick to migrate
  • cross protective immunity from heterotypic flavivirus Ab
  • numerous endemic and enzootic flaviviruses
  • cross reactive limits transmission
  • heterotypic favivirus Ab immumodulates clinical illness or viral load
  • intrinsic host factors select for less virulent strains
  • more severe are misdiagnosed as dengue infections

• epidemic vector: Aedes aegypti (female mosquitos)
  • primarily a daytime feeder
  • lives around human habitation
  • lays eggs and produces larvae preferentially in artificial containers
• reservoir hosts: humans
• human disease syndrome
  • undifferentiated fever
  • classic dengue fever
  • dengue hemorrhagic fever
  • dengue shock syndrome

1. virus trasmitted to human in mosquito saliva
2. virus replicates in target organs
3. virus infects WBC's and lymphatic tissues
4. virus released and circulates in blood
5. second mosquito ingests virus with blood
6. virus replicates in mosquito midgut and other organs, infect salivary glands
7. virus replicates in salivary gland

• previous infection
• maternal Ab's in infants
• higher risk in secondary infections
• higher risk in location with two or more serotypes circulating simultaneously at high levels (hyperendemic transmission)

1. primary dengue infection- develop Ab that can neutralize virus of homologous serotype
2. subsequent infection: pre-existing heterologous Ab's form complexes with new infecting virus serotype
   • do not neutralize the new virus
   • Ab dependent enhancement- new dengue virus, complexed with non- neutralizing Ab's can enter a greater proportion of monocytes, in creasing virus production
3. infected monocytes release vasoative mediators, resulting in increased vascular permeability and hemorrhagic manifestations characteristic of DHF/DSS

• pathogen endemic in nonhumans

• live organism that serves to communicate disease (must deliver disease to the host)

• location or object that serves as a continuing source of disease

occurence in a community or region of a group of illnesses of similar nature clearly in excess of normal expectations and derived from a common source

• all components of system, pathogen, vector and host must occur together in time and space for epizootics or epidemics to occur
• different arrangements of components results in patchwork of suitable and unsuitable places
• barriers (ex: water bodies, mountains, deserts) may prevent the occurence of a pathogen in an otherwise suitable location