DISEASE E&E (Transmission 2) Flashcards
Indirect horizontal transmission:
-occurs whenever horizontal transmission of the pathogen between two hosts of the same species requires another organism
Vector-borne life cycles:
-require an arthropod vector and a vertebrate host
Complex life cycles of helminth parasites:
-require an intermediate host and final host
Vector-borne diseases:
-caused by vector-borne pathogens that are transmitted among vertebrate hosts by hematophagous (blood-feeding) arthropod vectors
Examples of important arthropod vectors:
-mosquitoes
-ticks
-sandflies
-assassin bugs
When do arthropod vectors transmit the vector-borne pathogens?
-during the blood meal
Vector-borne pathogen (VBP) examples:
-viruses
-bacteria
-protozoan parasites
-nematode worms
Vector-born pathogens lifecycle 2 critical steps:
- Uninfected vectors must acquire VBP from an infected vertebrate host
- Infected vectors must transmit the VBP to other uninfected hosts
Condition of vector-borne transmission:
-vector must take at least 2 blood meals on different vertebrate hosts
Dengue virus life cycle example:
-mosquitos=vector
-humans=host
1. Human-to-mosquito transmission: uninfected mosquito feeds on infected human and acquires Dengue
2. Mosquito-to-human transmission: infectious mosquitos bites an uninfected human and human becomes infected
Vector becomes infectious:
*vector is not immediately infected after feeding on an infected host
-need to migrate the pathogen from the midgut to the salivary glands (can take days or weeks)
-in salivary glands=infectious
EIP:
-time for a vector to become infectious after feeding on infected host
Temperature and EIP:
-influences the rate at which vectors become infectious
High temperature on EIP:
-increase pathogen replication and migration
-higher abundance of infectious vectors
*shorter EIP
Pros of VB life cycle:
-motivated vector: hematophagous arthropods are motivated to seek out new hosts
-dispersal: winged arthropods can travel distances and contact new hosts
-one infected host can produce many infected arthropod vectors
Cons of VB life cycle:
-evolve ability to live in both arthropod vector and vertebrate host
-infected vector may feed on ‘wrong’ vertebrate host
West Nile fever:
-mosquito-borne disease
-an RNA virus
-birds: reservoir hosts
-mammals: dead-end hosts
Reservoir hosts:
-competent hosts
Ex. birds with high viremia (WNV) have high host-to-mosquito transmission
Dead-end hosts:
-incompetent hosts
Ex. mammals have low levels of WNV in their blood (ex. low viremia)
>WN doesn’t want to be in a mammal
Low vs. high viremia when mosquitoes feed on infected birds:
-low: fewer mosquitoes acquire WNV
-high: most mosquitoes acquire WNV
*high viremia in birds, increases chance of WNV transmission to feeding mosquitoes
Infection with WNV mortality:
*can kill both reservoir and dead-end hosts and also be asymptomatic
-high in birds
-outbreaks recognized in crows
-40% mortality in horses
-humans are asymptomatic
*mortality and morbidity are not good predictors of host competence
“bridge” vector:
-connects reservoir host (birds) and the dead-end hosts of interest (humans and humans)
Ex. have 2 species that prefer bird blood but will do 20% of their blood from mammals
Vector competence:
-the ability of a vector to transmit a disease
-occurs because VBP have evolved to become adapted to certain species of arthropod vectors
Vector competence comprises of:
- Capacity of vector to be infected
- Maintain and transmit an infectious agent
Vector competence ticks:
-few spirochetes: unable to transmit Lyme disease (D. variabillis, A. Americanum)
-I. scapularis: high
Complex life cycle (macroparasites):
-carried by one or more intermediate hosts (invertebrate or vertebrate) and one final vertebrate host
-sexual reproduction often takes place in final vertebrate host
-results in shedding of an infectious stage into the environment
-eggs hatch and resultant life stages infect the intermediate host
Food chains (complex life cycle):
-facilitate tropic transmission of parasites form intermediate hosts to predators
Type of hosts (complex life cycle):
-final or definitive host
-intermediate host
Final or definitive host:
-organism in which parasite reaches maturity and reproduces sexually
Intermediate host:
-organism that harbors the immature stages of the parasite
-required by the parasite to undergo development and compete its life cycle
Types of helminths (parasitic worms):
-roundworms
-flukes
-tapeworms
-thorny-headed worms
Parasitic roundworms and direct life cycles:
-infective L3 larvae eaten by vertebrate host: develop into adult worms
-adult worms produce fertilized eggs expelled into fecal matter
-eggs hatch in dung
-L3 move onto pasture and cycle continues
*direct=one-species
*roundworm larvae spend lots of time in the environment
Fasciolosis in livestock:
-common liver fluke
-complex lifecycle: final vertebrate host and intermediate snail host
-adult worms in liver produce eggs
-miracidia and cercariae
Miracidia (fasciolosis):
-infect snails and develop into free-swimming cercariae
Cercariae (fasciolosis):
-encyst on water plants and are ingested by vertebrate hosts
Is FMDB a complex life cycle?
-no
-it is NOT REQUIRED to have a second host
Life cycle of tapeworm: Echinococcus multilocularis
-complex
-final and intermediate hosts are canids and rodents
-adult host lives in small intestine of the definitive host and produces eggs
-intermediate host (rodent) ingests eggs
-eggs hatch in small intestine and release oncosphere that targets liver
Oncosphere (echinococcus)
-develops into cyst, which grows over time and produces protoscolices
-dog eats mouse and protoscolices develop into adult worms
Cysts replace tissues in liver (echinococcus):
-organ will lose it’s function
If dog or human ingests tapeworm egg (echinococcus):
-it will treat the dog or human as an intermediate host
Alveolar echinococcosis:
-if dog or humans consumes eggs, tapeworm will treat them as intermediate host
-eggs hatch into oncosphere that develops cysts in liver
-growing cysts in liver will kill patient because organ stops functioning
*tapeworm cannot skip steps in its complex cycle
How could cysts in dog complete life cycle (echinococcus);
-dog would need to die and be consumed
Cons of a complex life cycle:
-evolve ability to make a living in many different hosts
-expect probability of completing a life cycle to decrease with number of hosts
Host size and trophic level vs. helminth life-cycle length:
-host mass increased along the links of the parasite life cycle
-host tropic level increased along the links of the parasite life cycle
Size of final host and adult parasite vs. life-cycle length:
-final host was larger in helminth life cycles with more hosts
-adult parasites were larger in helminth lifecycle with more hosts
Advantages of a helminth life cycle with more hosts:
- First hosts are small and numerous and easy to find
- Final hosts are larger, longer-lived endotherms that allow bigger adult body size
>Bigger female worms produce more offspring
Advantages of adding a predator to the lifecycle:
-bigger host=more resources
-longer-lived=more time to reproduce
-parasite can grow larger=produce more offspring
*adding the predator offsets losses due to predation
Advantages of adding a prey host to the lifecycle:
-predators are unlikely to ingest immature stages (ex. eggs) in the environment
-prey more common=easier to encounter
-prey more likely to feed on immature stages
-high predation rates concentrate parasite into original predator host
Trophic transmission:
-highly efficient because food webs are ubiquitous in nature
-predators encounter prey frequently
-infecting prey is a great way to meet a predator
-delay sexual maturation until you reach your final host: have space and a long life
