Lecture 5: Transmission of Diseases Flashcards
Modes of infectious disease transmission
- Direct contact with pathogen (vector borne diseases)
- Ingestion of pathogen
- Inhalations of pathogen from air
Vector
Invertebrate serves as a mechanism to transport the pathogen to a host
Transmission efficiency
The rate at which pathogens are transmitted from invertebrate host
-Influenced by: environmental factors, weather, infection rates, insect life history
Mechanical transmission
Pathogen does not biologically interact with the arthropod
“hitching a ride” often on the mouth
Biological transmission
- Pathogens depend on arthropod as part of life cycle and for transmission
- Propagative, cyclo-developmental, cyclo-propagative
Propagative transmission
When the pathogen simply propagates (reproduces) within the arthropod
Cyclo-developmental transmission
Seen only in parasites, where the parasite must be in the arthropod host to complete part of its life cycle
Cyclo-propagative transmission
Pathogen both completes a developmental stage and can propagate within the arthropod
Trans-ovarial transmission
- Vertical transmission occurs when pathogens are passed from a parent arthropod to offspring (TOT)
- Females infect eggs during any part of egg development
- Males infect offspring with infected sperm
Trans-stadial transmission
- Transmission of a disease within the same organism from one developmental stage to the next
- Molting, Metamorphosis, Pupation
Horizontal transmission
Disease travels from one arthropod to another via a vertebrate host
(ex: cofeeding)
Pathogen in the vector
-Often ingested by vector, multiplies in midgut, infects salivary glands
Barriers
- Limit transmission
- Physical: lining of midgut, body cavity
- Chemical: chemical or immune responses that may limit or prevent virus from disseminating
How do diseases enter the host
- Passive transfer
- Active transfer
Passive transfer
Pathogen is transmitted from the vector to the host via some mechanism involving the vector
-commonly through saliva, sometimes feces or regurgitation
Active transfer
Some pathogens rely on the host destroying/crushing the body of the vector, releasing the pathogen
Incubation period
Time from when an organism ingests the pathogen to when it is capable of transmitting it
Intrinsic incubation period
From when the host becomes infected to when it can transmit the pathogen
Extrinsic incubation period
From when the vector ingests the pathogen to when it can transmit
Primary transmission cycle
Transmission that occurs under natural settings which is essential to the survival and continued transmission of the pathogen
Enzootic transmission
- The vertebrate host in primary transmission is an animal
- May serve as “reservoir” for long term survival of the pathogen
Dead end hosts
Transmission cycle will stop
Epidemic transmission
Can refer to any transmission that involves a human host
Disease amplification
Rapid increase in prevalence of a pathogen over a short period of time under specific conditions
Arboviruses
- Febrile, hemorrhagic, or neuropathic effects (many share common “flu-like” symptoms)
- Mild infection to severe encephalitis to death
Rickettsiae
- Genus of ancient bacteria
- Almost always involves hematophagous arthropods
- Frequently transmitted via TOT
- Often lethal; all (except typhus) involved in enzootic transmission cycle
Bacteria
- Only a few involved in arthropod transmission cycles (mostly mechanical)
- Biologically by hematophagous arthopods
- Antibiotics
Protozoa
- Sporozoans and flagellates may utilize arthropods for transmission
- Produce serious, widespread epidemics
- Complex life cycles and transmission patterns
- Most transmitted in the salivary glands
- Most cycles are enzootic, but some use humans
Helminths
- Develop in arthropod, but do not reproduce there
- Usually not fatal
