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BMED 225

EU 2021

EPIDEMIOLOGY AND INTEGRATED VECTOR MANAGEMENT OF VECTOR
BORNE DISEASES
Epidemiology of Vector Borne Diseases
Globally, outbreaks of many pathogens have been on the increase, and collectively these are known as
emerging infectious diseases
...

Epidemiology developed as a science through the investigation of outbreaks of infectious diseases
...
Vector-borne diseases result
minimally from a triad that includes an arthropod vector, a vertebrate host, and a parasite
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Definition of terms
Environmental factors such as temperature and rainfall affect these processes by affecting the rate of
parasite maturation within the arthropod host as well as arthropod and vertebrate host abundance in
time and space
...
Disease is the response of the host to infestation or
infection with a parasite outside or inside the host’s body, respectively
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Parasites may or may not cause disease
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A vector-borne disease,
therefore, is an illness caused by a pathogen that is transmitted by an arthropod
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Ectoparasites live on or outside the host, whereas
endoparasites live inside the host
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The manifestation of disease depends on the host-parasite interaction
after infection
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A host capable of parasite maintenance, particularly in infectious form, without
clinical symptoms is a carrier
...
Parous
females have completed one or more gonotrophic cycles and have a greater probability of being infected
with parasites than do nulliparous females that have not reproduced and are feeding for the first time
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Unlike parasites that are transmitted directly from host to host, parasites transmitted by arthropods
generally have replaced free-living or environmentally resistant stages with those that can multiply and
develop within the arthropod and be transmitted during the blood-feeding process
...

a vertebrate host (or hosts) that develops a level of infection with the parasite that is infectious
to a vector
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Vector-borne parasites have evolved mechanisms for tolerating high constant body temperatures and
evading the complex immune systems of the vertebrate hosts as well as for tolerating variable body
temperatures and avoiding the very different defensive mechanisms of the arthropod vectors
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Among sexually reproducing parasites, the host in which gametocyte union occurs is called the
definitive host, whereas the host in which asexual reproduction occurs is called the intermediate host
...

For example, humans are the definitive host for the filarial worm Wuchereria bancrofti because adult
male and female worms mate within the human lymphatic system, and the mosquito vector Culex
quinquefasciatus is the intermediate host where larval development of the worms occurs without
reproduction
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Host Immunity
A disease results from the response of the host to infection with the parasite and can occur in either
vertebrate or arthropod hosts
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Some species or individuals within a population have natural (or innate) immunity and are refractory to
infection
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For example, humans do not become infected with avian malaria parasites such
as Plasmodium relictum, even though infective Culex mosquito vectors feed frequently on humans
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Individuals may acquire immunity after becoming infected with parasites
...
Partial immunity may allow some parasite
development or reproduction within the host and may reduce the severity of disease, whereas
complete protection results in clearance of the initial infection and usually prevents immediate
reinfection
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Antibodies consist of five classes of
proteins called immunoglobulins that have specific functions in host immunity
...

The IgGs are relatively small proteins, typically reaching high concentration several weeks after
infection, and may persist at detectable and protective levels for years
...
In
contrast, immunoglobulin M (IgM) is a large macroglobulin that appears shortly after infection but
decays rapidly relative to IgG
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A 4-fold increase in parasite-specific
IgG antibody concentration in these paired sera provides diagnostic evidence of infection
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T cells and
macrophages are cells that are responsible for the recognition and elimination of parasites
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Clinically, the host response to infection ranges from asymptomatic (i
...
,
inapparent) to severe
...

For most parasites, interactions with the vertebrate host are a delicate balance between the need for
adequate virulence to amplify parasite concentrations in the host’s blood to increase infectiousness to
vectors and the need to avoid excessive virulence that could cause adverse effects on the host and result
in illness or death that would reduce the host’s infectious period
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The Vertebrate Host
One or more primary vertebrate hosts are essential for the maintenance of parasite transmission,
whereas secondary or incidental hosts are not essential to maintain transmission and may or may not
contribute to parasite amplification
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An amplifying host is permissive to infection, which results in an increase in
the number of parasites sufficient to infect susceptible vectors
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A reservoir host supports parasite development and remains infected and potentially
infectious to vectors for long periods but usually does not develop acute disease
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- The vertebrate host must be abundant and fed on frequently by vectors
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e
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For persistence of the parasite within the host population,
abundance must be above the critical community size, which is the minimal number of hosts necessary
to ensure an adequate number of susceptible hosts to maintain chains of transmission
...
For
example, the birds that are the primary hosts of eastern equine encephalomyelitis virus (EEEV)
generally begin nesting in swamps coincidentally with the emergence of the first spring generation of
the mosquito vector, Culiseta melanura, thereby bringing EEEV, susceptible avian hosts, and
mosquitoes together in time and space
...

Diel activity patterns of primary hosts must coincide with that of vectors, and some parasites themselves
exhibit diel periodicity that maximizes potential transmission
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bancrofti move to the peripheral circulatory system of the human host during specific hours of the night
that coincide with the peak biting period of the mosquito vector, Culex quinquefasciatus
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- A primary host must be susceptible to infection with a parasite and permissive to its
development and reproduction such that the host eventually becomes infectious to vectors
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Ideal reservoir hosts permit parasites to persist in the peripheral
circulatory system (or other suitable tissues) in sufficient numbers for sufficiently long periods to be an
effective source for vector infection
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Strategies vary, and asexual parasites such as viruses
and bacteria typically produce intensive infections that produce large numbers of infectious parasites
for relatively short periods, during which the host either succumbs to infection or develops protective
immunity
...
In contrast, highly evolved parasites produce
comparatively few individuals during a longer period
...
However, because both the
worms and the human host are long-lived, transmission is enhanced by repeated exposure of hosts to
vector biting rather than by a high parasite load over a period of a few days
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The Arthropod Vector
Literally, a vector is a “carrier” of a parasite from one host to another
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Abundance
...
Vectors with small population sizes may play
minor roles in transmission but would not provide enough total bites to ensure that the parasite would
reliably be transferred between hosts
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Host fidelity
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Host-seeking or biting activity during times or places where primary hosts are inaccessible or feeding
on incompetent hosts will reduce contact with infective hosts and reduce the efficiency of transmission
...

Anthropophagic vectors feed selectively on humans and are important in the transmission of parasites
that use humans as their primary host
...
Vectors that
rarely enter houses are termed exophilic (“outside loving”)
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Mammalophagic vectors blood feed
primarily on mammals and are important in the maintenance of mammalian parasites, whereas
ornithophagic vectors feed primarily on avian hosts and are important in the maintenance of avian
parasites
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With zooprophylaxis the dead-end host typically exhibits natural immunity in which host
tissues are not conducive to parasites (i
...
, do not permit growth or reproduction)
...
West Nile virus (WNV), for example, can
cause serious illness in humans, which are considered to be a dead-end host because they rarely produce
a viremia sufficient to infect mosquitoes
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- The vector must be competent (i
...
, susceptible to infection and capable of
becoming infectious to vertebrate hosts) and must survive long enough for the parasite to complete
reproduction and/or development within the vector
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For example, the mosquito Cx
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Modes of Transmission
The transmission of parasites by vectors may be horizontal or vertical
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Vertical transmission is the passage of parasites directly to subsequent
life stages or generations within vector populations
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Transstadial transmission is the sequential passage of parasites acquired during one life stage, or
stadium, through the molt to the next stage(s) or stadium
...
Lyme disease spirochetes, for example, that are acquired by larval ticks must be
passed transstadially to the nymphal stage before they are transmitted to vertebrates
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Some parasites may be maintained transgenerationally for multiple generations,
whereas others require horizontal transmission for amplification
...

Another type of transgenerational transmission is transovum transmission, in which the parasite remains
on the surface of the egg after being laid by an infected vector and eventually infects the offspring on
eclosion
...

Anterior-station transmission occurs when parasites are liberated from the mouth parts or salivary
glands during blood feeding (e
...
, malaria parasites, encephalitis viruses, filarial worms)
...
The trypanosome that causes Chagas disease, for example, develops to the
infectious stage within the hindgut and is discharged onto the host skin when the triatomid vector
defecates during feeding
...

Mechanical transmission occurs when the parasite is transmitted among vertebrate hosts without
amplification or development within the vector, usually by contaminated mouthparts
...
For blood-feeding vectors, mechanical transmission
may be accomplished by contaminated mouthparts, particularly if the vector is interrupted while blood
feeding and then immediately refeeds on a second host in an attempt to complete the blood meal
Multiplicative (or propagative) transmission occurs when the parasite multiplies asexually within
the vector and is transmitted only after a suitable incubation period is completed
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This is exemplified by arboviruses such as WNV, which are not transmitted until the virus infects and
escapes the midgut of the mosquito vector, disseminates throughout the hemocoel, and enters and
replicates within the salivary glands
...

Venereal transmission is the passage of parasites between male and female vectors during mating and
is relatively rare
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Cyclodevelopmental transmission occurs when the parasite develops, but does not multiply, within
the vector
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These filarial worms do not
reproduce asexually within the mosquito vector; that is, the number of worms available for
transmission is never greater than the number ingested
...
In the life cycle of the malaria parasite, for example, gametocytes that are
ingested with the blood meal unite within the mosquito gut and then change to an invasive form
(ookinete) that penetrates the gut and forms an asexually reproducing stage (oocyst) on the outside of
the gut wall
...
Malaria
parasites also reproduce asexually within the liver and blood cells of the intermediate human host
...
Similarly, the intrinsic incubation period is the time
from infection of a vertebrate host to potential parasite transmission (infectiousness) from the host to
vectors
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A vector-borne anthroponosis is a disease resulting from a parasite that normally infects only humans
and one or more anthropophagic vectors
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Humans serve as reservoir hosts for these parasites, which
may persist for years as chronic infections
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Widespread transmission of an anthroponosis with an increase in the number of diagnosed human cases
during a specified period of time is called an epidemic
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Vector-borne zoonoses are diseases caused by parasites of animals that occasionally infect humans
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Such infections may cause illness, but humans rarely circulate sufficient
numbers of parasites to infect vectors and thus are termed dead-end hosts
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When levels of
enzootic transmission escalate, transmission may become epizootic, characterized by a sharp increase
in transmission among animals
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e
...
Often, different vector species are responsible for enzootic,
epizootic, and tangential transmission
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Integrated Vector Management (IVM) of Vector Borne Diseases
Integrated Vector Management (IVM) is a rational decision-making process to optimise the use of
resources for vector control
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Use of IVM strategies help vector control programmes find and use more local evidence, integrate
interventions where appropriate and collaborate within the health sector, and with other sectors, as
well as with households and communities
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Although not a new process, global uptake of the IVM concept remains very low, due in part to the
requirements for capacity and capability building at central, district and village levels, and also
because of the lack of new and innovative tools (or access to current tools and strategies)
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Vector & its role in disease transmission
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Geographical and seasonal abundance
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Its life cycle in terms of development
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Breeding sites
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Location where adults emerge, mate, feed, rest, hide and lay eggs
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Host preference, feeding behavior, dispersal and migration behavior
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Its life cycle in relation to human habitats
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Effectiveness
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Safety
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Simplicity of use- indications, contraindications, skills, difficulties in using
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Cost
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Type of benefit
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Social acceptability- local context
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Support requirements
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The present trend is to adapt a IVC
approach
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Integrated vector management (IVM) is the strategic approach to vector control promoted by WHO
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

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Monitoring and evaluation - indicators as per the local requirements and targets set
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Case Study (Malaria control)

The following are key elements in the Integrated Vector Management process in malaria
control:
1
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2
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3
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4
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5
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