Vector Biology

-chitinous exoskeleton
-segmented appendages and body
-3 or more pair of jointed legs
-internal muscle attachment
Arthropods
-insecta or hexapoda
-chilopoda
-diplopoda
-arachnida
-crustacea
Classes of Phylum Arthropoda
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-approximately 1 million species
-found in every environment except oceanic
Insecta
-hematophagy / vector competency
-behavior
-biology
Complexity of Arbopathogen transmission (vector)
1) ametabolous – molt from immature to adult
2) hemimetabolous – molt and metamorphosis
3) holometabolous – complete metamorphosis
3 kinds of physical development
-virulence
-immune avoidance
Complexity of Arbopathogen transmission (pathogen)
-immune responses
-exposure (host preference/behavior)
Complexity of Arbopathogen transmission (host)
-temporal
-spatial
Ecological factors
-obligate blood feeders
* ticks, fleas, lice, tsetse fly
-reproductively obligate blood feeders
*nutrient rich blood for egg production
*mosquitoes, sand flies
Why blood for transmission?
-finding a host
-locating blood
-digesting blood
-fighting microorganisms (acquired in blood)
-converting blood to yolk/eggs
Hematophagy
-mechanics
*ice pick vs ax
-chemistry of saliva
*antiplatelet
*anticoagulant
*vasodilator
*antigenic polymorphism
Bloodmeal acquisition
-arthropod vector
-pathogen
-vertebrate host(s)
3 components of arthropod borne disease cycle
-mechanical
-biological
*propagative
*cyclopropagative
*cyclodevelopmental
Mechanisms of Arbo-pathogen transmission
pathogen transported by vector and perhaps introduced into wound, but does not have biological dependency on the vector (for multiplication or development)
Mechanical transmission
pathogen multiplies without developmental cycles
Propogative transmission
pathogen undergoes essential development and multiplies in vector
Cyclo-propogative transmission
pathogen undergoes essential development in vector but does not multiply
Cyclo-developmental transmission
-transovarial (vertical)
-transstadial
-horizontal
Types of pathogen transmission
specialized transmission from infected ovaries to the resulting progeny
Transovarial (vertical) transmission
maintaining infection from one stage to next developmental stage of the arthropod
Transstadial transmission
transmission of a pathogen from one vector to another through host sharing, mating etc..
Horizontal transmission
-intrinsic permissiveness of an arthropod vector to infection, replication and transmission
-evaluation of the vector’s capability to transmit pathogen
Vector competence
-midgut infection barrier (establish infection, replicate)
-midgut escape barrier (pass through midgut, replicate in other tissues)
-salivary gland infection barrier (infect salivary glands, shed in saliva)
Arbovirus transmission barriers
virus will attach and penetrate midgut epithelial cells. virus will replicate in these cells and penetrate basal epithelium to establish disseminated infection.
Permissive / Susceptible vector
-virus may fail to attach or penetrate midgut epithelial cells
-attach and penetrate midgut cells, can replicate but can’t penetrate the basal epithelium to establish disseminated infection
Refractory vector
measurement of efficiency of vector-borne disease transmission
Vectorial capacity
the period of time from when a mosquito infects the vertebrate host until that host is able to transmit pathogen to another vector
Intrinsic incubation period
the period of time from when a mosquito takes an infectious blood-meal until it is capable of transmitting the pathogen
Extrinsic incubation period
-M (can be mechanically transmitted)
-E (Eclipse period; titer decreases because only a fraction of ingested pathogens will successfully invade)
-R (replicate)
-T (transmission)
EIP titers
primary viremia, secondary viremia
IIP
infected males transfer infection to females during copulation through accessory gland fluids (has proteins)
Venereal transmission
epidemic (higher than expected incidence) in animals
Epizootic
higher than expected incidence (in humans)
Epidemic
– vector that maintains two different cycles – usually prefers one host, but will feed on both
Bridging vector
disease maintained in animals at a low level
Enzootic
disease found in animals
Zoonotic
C = (m)(a^2)(V)(P^n)/-logP
MacDonald’s equation
how many mosquitoes will be alive at the end of EIP
P^n
1) a^2 (squared bc vector must feed twice to transmit)
2) n (small changes have a large effect bc exponent to P)
3) P (MOST IMPORTANT)
Most important factors in MacDonalds equation
-entomological inoculation rate
-measure of risk (rathern than vectorial capacity)
-the product of the mosquito biting rate times the proportion of infected mosquitoes
EIR
the study of how vectors interact with other organisms and their environment
Vector ecology
-breeding habits
-environ. factors
-pop dynamics
-predator/prey interactions
Ecological factors that affect ‘m’
-feeding behavior
-length of gonodo. cycle (varies with temperature)
Ecological factors that affect ‘a’
-host preference/availability
-temp
-food availability and quality
Ecological factors that affect ‘v’
-temp
-food availability
-predator/prey interactions
-humidity
Ecological factors that affect ‘p’
-egg
-larva (aquatic)
-pupae (aquatic, motile)
-adult
Mosquito life cycle
-egg: single eggs
-larvae: rest parallel to water surface; no air tube
-most of body touches surface
-adult: palps are clubbed; resting posture at an angle
Anopheles
-egg: single eggs on dry surface
-larvae: rests at an angle; air tube
-pupae: md area of body touches surface
-adult: feathery palp; resting posture horizontal
Aedes
-egg: floating egg raft
-larvae: rests at an angle; air tube
-pupae: sm area of body touches surface
-adult: feathery palp; resting posture horizontal
Culex
-found in class arachnida
-fusion of head and thorax
Cephalothorax
-class: arachnida
-subclass: acari (fusion of cephalothorax and abdomen)
-families: Ixodidae (hard tick)
Argasidae (soft tick)
Tick taxonomy
-hard tick
-hard, waxy cuticle
-capitulum (feeding part) is anterior
Ixodidae
-soft tick
-knobby surface that can swell
-capitulum (feeding part) is ventral
Argasidae
-spiracular plate
-hole attached to tubes that innervates body cavity
-behind 4th leg on Ixodidae
-between 3rd and 4th legs on Argasidae
-spiracular plate is used as a morphological sign to differentiate between species
Ticks breathing
-egg
-larva (hexapod)
-nymph (4 pairs of legs, lack genital aperture; single molt for hard, multiple molts for soft)
-adult (4 pairs of legs, genital aperture)
(hemimetabolous development)
Tick life stages
-males are smaller, more ornate and have a large scutum
-females are larger, have a small scutum (will not stretch)
Tick sexual dimophism
single blood meal, tick drops from host, tissue degeneration, deposition of large clutch of eggs (up to 18,000), female dies
Ixodidae fecundity
female may consume multiple blood meals, each results in small clutch of eggs (10-200)
Argasidae fecundity
no mating required
Parthogenesis
no blood meal required by females
Autogeny
-questing (active in host habitats, put themselves in a position to find host)
-ambush/wait (nest or burrow parasite)
-hunter (actively seeking hosts/responding to stimuli)
Tick host acquisition strategies
-CO2
-NH3
-heat
-vibration
-aggregation pheromones
Feeding cues
-have many hosts
Argasidae feeding strategies
-1 host (completes entire cycle on one host; rare)
-2 hosts (immature on 1 host, adult on another host)
-3 hosts (most common; diff host for diff life cycle)
Ixodidae feeding strategies
-has 2 parts
-teeth
-imbeds in host tissue
Hypostome
base where other parts are mounted
Basis Capituli
-“tongue”
-sensory organ
-associated with smell and taste
Palp
-important for hydration (Ixodidae)
-involved in production and secretion of proteins necessary for blood meal acquisition
Tick salivary glands
-dermatitis and tissue damage
-exsanguination (loss of blood)
-otoacariasis (hearing loss due to swelling)
-has vector competence for a wide array of pathogens
Damage caused by ticks
-persistent blood feeders
-slow blood feeders
-wide host range
-extreme longevity
-transovarial transmission
-few natural enemies
-heavily sclerotized
-enormous reproductive potential
Tick high vector competence (8)
Borrelia burgdorferi (bacteria)
Lyme disease etiologic agent
Ixodidae scapularis (E N America) I pacificus (California) I ricinus (W Europe) I persulcatus ( E Europe)
Lyme disease vectors
-eggs in spring #1
-uninfected larva bites infected host, becomes infected in summer #1
-infected nymph in following spring #2 bites hosts
-nymph develops to adult in fall #2
(no trans-ovarial transmission)
Lyme disease lifecycle
-southern tick-associated rash illness
-transmitted by Amblyomma Americanum
-AKA Lone star tick
-hunter tick
-pathogen unknown
STARI
-rocky mountain spotted fever
-caused by Rickettsia ricketsii (intracellular pathogen)
-primary vector: Dermacentor variabilis
-secondary vector: amblyomma americanum
-sudden onset and rapid progression
-distribution: throughout US
-rash begins at periphery and moves inward
RMSF
-zoonotic circulation
-transovarial and transstadial transmission
-larva can be infected
-rabbit tick is a bridge vector
RMSF transmission cycle
Ovaries cannot become super-infected, only the strain of rickettsia that tick became infected with first will be transmitted transovarially
Competitive displacement
-caused by Borrelia spirochete
-boy scout fever
-pattern of fever remission and relapse (due to genetic shift of surface proteins)
-transmitted by soft ticks found in caves and decaying woodpiles
Tick borne relapsing fever
soft tick has an extra gland that excretes excess water through pores, water can contain bacteria and gets rubbed into wound
Soft tick hydration
-larva is only stage that blood feeds
-larva also called chiggers
-stylostome ‘cement straw’ sucks blood out
-nymphs and adults are predacious
Mites
‘cement straw’ mouth part tat chigger thrusts through the epidermis
Stylosome
predaceous on small arthropods
Mite nymphs and adults
-zoonotic rickettsial infection
-between mites and rodents
-transmitted transstadially and transovarially
Scrub Typhus
-body, head and crab (only body is a vector)
-eggs called ‘nits’ (attach to hair)
-obligate blood feeders
-transfer by contact/proximity
Human associated lice
-Epidemic typhus; agent: rickettsia
-Louse-borne relapsing fever; agent: borrelia
-Trench fever; agent: bartonella
Body Louse diseases
– caused by borrelia
-bacteria multiply in hemolymph, not expelled in feces
-mechanical transmission
Louse borne relapsing fever
bacteria replicate in lumen, expelled in feces, infective for months
Trench fever
-antenna are tucked in
-where legs attached = thorax
-dark chitin
-spiracles
-temporary obligate parasites
-holometabolous
Fleas
living in host bedding
Nidicolous
-eggs: few deposited at a time, oviposition on host or in nest
-larvae: active feeders, have chewing mouth-parts, adult frass necessary as larval food source
-pupae: develops from 3rd instar, very durable
-adult
Flea life cycle
1) free living – most important in terms of disease transmission (P Irritans)
2) nidicolous – only visit host for blood meal; most common type in nature (C Felis and C canis)
Flea living environments
-flea borne
-reservoir: rats
-rickettsia
Endemic typhus
-sylvatic cycle (wild rodents and fleas)
-urban cycle (infective flea and domestic rodents)
-bubonic plague (infective flea from either cycle or infected domestic rodent and human)
-pneumonic plague (secondary)
Plague cycles
1) bubonic plague – fever, chills etc
2) septicemic shock – shock and bleeding into skin and organs
3) pneumonic plague – difficulty breathing, rapid shock and death
Stages of plague infection
-togaviridae
-flaviviridae
-bunyaviridae
Families of arboviruses
-nucleocapsid: virions have a genome surrounded by protein capsid
-most are enveloped
-contain structural and nonstructural proteins
-can be mechanically or biologically transmitted
Arbovirus characteristics
-most have non-human vertebrate hosts
-some have adapted to amplify in domestic animals (epizootic)
-a few have evolved to replicate in humans (these are medically relevant)
Arbovirus transmission cycles
-togaviridae family
-encephalitis viruses (EEE, WEE, VEE, Ross river)
non-encephalitis (chikugunya, onyongnyong)
Alphaviruses
-mosquito vector
-reservoir host: song birds
-epizootic/epidemic events due to bridge vectors (mosquitoes)
Eastern equine encephalitis
-primary enzootic cycle: culex vector and wild bird hosts
-secondary enzootic cycle: aedes vector and small mammal hosts
-epizootic/epidemic events: horses, humans are dead end hosts
Western equine encephalitis
-enzootic cycle: culex vector and small mammal hosts
-epizootic cycle: caused by enzootic strains that have adapted to replicate in horses
Venezuelan equine encephalitis
-enzootic cycle: aedes vector, non-human primate hosts
-epidemic cycle: if chik spreads to urban areas; aedes vector and human hosts
Chikugunya transmission cycles
-thrives in human habitats
-breeds in any water
-eggs are very durable
Chikugunya aedes aegypti
-historically sylvatic cycles in africa with occasional epidemic events
-recently epidemics in E and W hemispheres
Chikugunya incidence
-causes: climate change, loss of herd immunity, globalization, virus evolution and global spread of aedes albopictus
-mutation makes it more transmissible to a albopictus
Chikugunya E hemisphere epidemics
-Asian lineage -cause of concern: albopictus invading W hemisphere
Chikugunya W hemisphere epidemics
density of vectors in relation to the density of hosts
m
proportion of vectors feeding on a host divided by the length of gonotrophic cycle
a
vector competence
V
daily survival of vectors
P
extrinsic incubation period
n
feeding behavior and flight activity is during the day
diurnal
feeding behavior and flight activity is during dawn and dusk
crepuscular
resting indoors or outdoors
endophilic/exophilic
feeding indoors or outdoors
endophagic/exophagic
feeds on humans or animals
anthropophilic/zoophilic
-variants of pathogen (ability to stick)
-nature of alimentary canal / teeth
-temperature
Factors that influence plague transmission
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