CONTROL MEASUR2ES
Vector control
Medical Parasitology
CBIO4500
April 24, 2012
Silvia N J Moreno
PARASITE CONTROL
Prevention of Environmental Contamination: Antiparasite drugs.
Treated hosts should be protected from re-infection. Sanitation.
Destruction of Free-Living Stages: Protozoan cysts, helminth eggs
and infective larvae are often extremely resistant to toxic chemicals.
Destruction of Intermediate Hosts and Vectors: chemicals to kill snails, insecticides.
Altering the environmental conditions so the the target species do not find the suitable habitat
for its survival. Does not rely on public cooperation.
Destruction of Reservoir Hosts: dogs in leishmaniasis, game animals in trypanosomiasis
Prevention of Infection: Many infective stages gain entry in drinking water,
Cryptosporidium, Giardia, Dracunculus etc. These can be controlled by safe water supplies.
Meat inspection to prevent Taenia. Bednets prevents mosquito bites. Shoes stops hookworm
larvae burrowing through the skin.
Prevention of Parasite Maturation: chemoprophylaxis and
vaccination
Integrated Control
400,000 long-lasting
insecticide-treated bed nets
start the last leg of their
journey from a Red Cross
warehouse in Mozambique
into the hands of families
who need them
VECTOR CONTROL
Major vector-borne diseases of humans, and associated aetiological agents and
arthropod vectors
VECTOR CONTROL
Vector control has proven successful for
disease control:
Malaria: eradication of malaria from most of the temperateclimate countries in the northern hemisphere
Onchocerca: the insecticide phase of the Onchocerciasis
Control Prgramme (OCP) in West Africa has almost
eliminated onchocerciasis from11 countries
(www.who.int/ocp/index.htm)
However there is paucity of effective vectorcontrol programmes: past neglect in this area of
research, potential environmental impact of existing
agents, reduction of their effectiveness because of
resistance and other biological complexities of vector
populations.
DTT spraying does reduce disease
transmission but environmentalists
have opposed to their use on large
scale. Bednets impregnated with
pyerethroids are simple and
effective at reducing vector-human
contact and transmission. Limited
adoption of this approach, improper
use and the emergence of vector
populations with resistance to
pyrethroids limit their effectiveness.
Abandoned village near the Volta river high blindness rates resulted in the
depopulation of entire villages.
The result has been increased activity
and productivity in many of these areas.
New villages in areas formerly
uninhabitable because of river blindness
VECTOR CONTROL
Advances in vector genomics: an important
advance is the sequencing of the genome of the
A. gambiae mosquito (Science, 2002 298:129). The
genome of Aedes aegypti (Science (2007) 316:1718-23)
(yellow fever) was also sequenced, and a project
is in place for the genome of Glossina morsitans
(Tsetse fly vector). This information will provide
opportunities for understanding better vectors
and devising new methods for their control.
Vector biology is poised to explore new
strategies for vector-based disease control:
Mosquitos refractory to pathogen infection
Novel insecticides targets
Understanding the molecular basis for vector
behavior, ecology and host-parasitevector interactions
Manipulation of insect vector by their parasite
• Parasites alter their host in some way.
Does this alteration affect transmission?
• Parasite undergo a period of growth,
development and sometimes reproduction
within their vector:
Lutzomyia longipalpis feeding
Parasite fitness is linked to vector fitness.
Two factors that would improve parasite
success:
Vector longevity
Fast Development
Anopheles albimanus mosquito feeding on a human arm. This
mosquito is a vector of malaria and mosquito control is a very
effective way of reducing the incidence of malaria.
Manipulation of insect vector by their parasite
Host manipulation by parasite:
Hematophagy: Infection results in
altered vector feeding behavior.
Leishmania and plasmodium.
Fecundity, blood feeding, and
infection: Blood is important for the
Lutzomyia longipalpis feeding
vector and the parasite. Parasiteinduced fecundity reduction. Allows
vector survival until transmission can
occur?
Infection and survivorship: Do
infected mosquitoes live longer? High
infectivity also results in increase in
mortality. Studies are not conclusive.
Anopheles albimanus mosquito feeding on a human arm. This
mosquito is a vector of malaria and mosquito control is a very
effective way of reducing the incidence of malaria.
Manipulation of insect vector by their parasite
Hematophagy:
Malaria-infected mosquitoes: Infected
mosquitoes makes many attempts to feed, each
time depositing parasites at the feeding site. ADP
stimulates platelet recruitment to the wound site.
Apyrase is an enzyme which degrades ADP and
is present in the mosquito saliva and would inhibit
host hemostasis promoting easier and longer
blood feeding.
Apyrase activity is reduced in the salivary glands
of P. gallinaceum- infected Aedes Aegypti
resulting in an increase in the times of the
median blood location time.
One study showed that P. falciparum infected An. gambiae
bite more hosts and also that infected mosquitoes were
most likely to take multiple meals and become fully
engorged. This would increase their chances for
transmission. Is infection increasing the threshold blood
volume at which blood-seeking behavior is inhibited?
More studies are needed in the natural infection scenario.
Leishmania manipulates sandfly feeding to
enhance its transmission
Hematophagy:
•Difficulty in ingesting a blood meal leads to
more probing. Increased probing favors
transmission of the parasite.
•The blockage by the parasites interferes with
feeding and limits the volume of blood a fly
can obtain; this could explain why infected flies
probe the skin more frequently and spend more
time feeding.
• Experiments showed that flies were more likely to
transmit L. donovani to hamsters when they probed
and took no blood compared to those infected flies
that took a blood meal.
Transmission from the Vertebrate host
• Host attractiveness is enhanced
possibly due to modified host odor and
vectors such as tsetse flies feed more
frequently on infected hosts.
• Host defensive behaviors may be
reduced making feeding on infected
hosts less risky.
Pathology associated with the infection
could assist the blood-feeding efforts of
vectors:
Thrombocytopenia induced by
malaria, African trypanosomiasis
and babesiosis decrease
hemostasis.
Manipulation of insect vector by their parasite
FECUNDITY, BLOOD FEEDING, AND INFECTION
• Blood feeding is essential for both: vector and parasite.
• Blood meal quantity and quality are important for egg production.
• There is a reduction in fecundity (ability to reproduce) in plasmodium-infected
mosquitoes, specially when oocysts are developed.
• This loss of fecundity may be attributed to reduced blood intake or intake of
impoverished blood when mosquitoes fed on infected hosts.
• How could reduced fecundity benefit the parasite?
Since increased reproductive effort decreases lifespan; if vector survival
results from fecundity reduction then the parasite increases its chances of
successful transmission.
Recommendations for vector control
• Genetic manipulation of vectors:
Transgenic mosquitoes with impaired ability to transmit the parasite. This
is attractive because is likely to be economically viable and relatively “low
technology”.
• Vector immunity and vector-parasite interactions:
Genome projects would help identify novel targets in the mosquito gut and
salivary glands involved in digestion of the blood meal and host-parasitevector interactions which could be used to develop vaccines that block
the transmission of parasites and mosquito immune regulators or ‘smart
sprays’ that disrupt the development of the parasite in the mosquito.
• Vector behavior and other approaches to vector control:
Elucidating the molecular basis of many mosquito behavior may be an
expensive research investment, but the simple traps and repellant devises
anticipated from this research could be easily adopted in malaria-endemic
countries
Genetic manipulation of vectors
Two broad categories:
• Genetic modification of mosquito
populations. Could be achieved by
releasing transgenic mosquitoes
carrying genes whose products impair
pathogen development.
• Population suppression: use of
sterile insect technique (SIT) in
conjunction with “release of insects
carrying a dominant lethal” (RIDL).
Insects are transformed with a
transgene whose product suppresses
offspring production, leading to a
decrease of the vector population.
A transgenic mosquito carrying a gene that confers
resistance to the malaria parasite. The mosquito can be
recognized as transgenic by the green fluorescence of the
eye facets. http://news.mongabay.com/2007/0319-mosquitoes.html
Genetic manipulation of vectors
Controversial but attractive and potentially selfpropagating. Many questions need to be
addressed first about the feasibility and
consequences of this approach. Serious issues
are: reduced fitness of modified vectors, the
ecological impact of transgenic arthropods and
the evolutionary consequences of their release.
In addition scientists need:
• Methodologies to introduce foreign genes into
vectors mosquitoes (e.g. transposable elements)
• Promotores that can drive the expression of
foreign genes in the correct tissues and at the
appropriate times need to be characterized
• Find “blocking gene products” capable of
interfering with parasite development in the
mosquito. Deleterious effects of these gene
products on the mosquito should be considered
and avoided.
Laser scanner image of transgenic mosquito larvae. The white
areas of high fluorescent intensity indicate a high level of
transgene expression - particularly in the gut of the larvae
The transformation technique involves the microinjection of
the recombinant DNA into the posterior end of mosquito
embryos (fresh laid eggs) prior to pole cell formation.
Genetic manipulation of vectors
• Transgenic: A genetically modified
organism (GMO): an organism whose
genetic material has been altered
using genetic engineering techniques.
The Transgene is a segment of DNA
containing a gene sequence that has
been isolated from one organism and
introduced into a different one. This
DNA will be transcribed and
translated. In transgenic mosquitoes
the idea is to transfer genes that will
alter the transmission of the malaria
parasite.
• Experiment using the gene for the
Green Fluorescent Protein (EGFP).
The gene was inserted into a plasmid
and pre-blastoderm embryos were
injected with it and an average of 29%
of the injected ones survived. From
these, 50% were fluorescent.
Confocal fluorescence and transmission microphotographs of putative
homozygous (left) transgenic larvae expressing EGFP compared with
a wild-type mosquito (right).
Effector strategies that could inactivate malaria
parasites within the mosquito
CHITIN
One example of a Parasite-secreted
protein: CHITINASE
• Ookinetes secrete chitinases that
facilitate their crossing of the peritrophic
matrix.
• Parasite strains carrying mutations or
gene knockouts of chitinase are
markedly impaired in their ability to form
oocysts.
• Feeding anti-chitinase antibodies to
mosquitoes also impaired the
development of oocysts.
Ookinete (O) penetration of the chitin-containing peritrophic matrix
(PM) of the mosquito midgut. The parasite is exiting the bloodmeal
on the left, producing chitinase focally to disrupt the peritrophic
matrix, en route to the microvilli of the midgut epithelial surface at
the right. Trends in parasitology, (2001) 17:269
Genetic manipulation of vectors
• The ookinete cross the midgut epithelium and
differentiate into oocyst. The sporozoites cross the
salivary gland epithelium.
• Scientists identified a peptide (SM1 for salivary
gland- and midgut-binding peptide 1) that binds
specifically to the two epithelia: the distal lobes of
the salivary glands and the lumenal surface of the
midgut. SM1 inhibits crossing of the two epithelia by
the parasites.
• The idea is that if you can produce SM1 into the gut
lumen with a blood meal the Plasmodium
development would be blocked.
• They created a synthetic gene AgCP[SM1]4.
They used a promoter which is activated by a blood
meal, and a signal sequence to drive secretion of
the protein into the midgut lumen. The gene was
inserted into a vector and transformed into the germ
line of Anopheles stephensi. Nature (2002)
417:452.17
• The expression of the peptide produced a reduction
in the number of oocysts formed (69-95% inhibition)
and also these transgenic mosquitos had fewer
sporozoites in their salivary glands
Detection of AgCP[SM1]4
transgenic mosquitoes by
transformation marker-mediated
fluorescence. Top, a wild-type
(non-transgenic) larva (middle)
flanked by transgenic larvae
viewed from the dorsal (top) or
ventral (bottom) sides. Bottom,
the head of a wild-type (left) and
a transgenic (right) mosquito.
The entire eye expresses GFP.
http://www.genomenewsnetwork
.org/articles/05_02/transgenic_m
osquitoes.shtml
Population reduction: SIT and RIDL
SIT: Sterile Insect technique: male insects are mass-reared, sterilized by irradiation
and then released in large numbers in the infested areas in order to contribute to sterile
mating with wild mosquitoes.
RIDL: release of insects with a dominant lethal .
RIDL uses males carrying female dominant lethal
transgenes that can produce purely male offspring. In
the laboratory, this strain is maintained by using a
repressible system to control transgene expression;
absence of the repressor from the insect diet in the field
activates the lethal trait.
Sterilized male flies are mass released
This approach does not
appear suitable for malaria
control because of its
susceptibility to immigration
from outside the target area.
across the target area by airplanes. The
sexually sterile males, which outnumber
the wild-type males, mate with wild-type
females resulting in infertile mating
events. This results in a decrease of the
pest levels and, if continued over several
generations, the potential eradication of
the pest from the target area. Nature
Biotechnology 23:433
SIT has been used successfully to eradicate
tsetse flies from Burkina Faso, Tanzania, Nigeria
and Zanzibar where it eradicated Glossina austeni
from the 1600 km2 Unguja Island.
Population replacement
An important issue to consider before
the release of genetically manipulated
organisms is the fitness of the
mosquitoes carrying the transgene.
The transgenic insect needs to
compete with the local populations to
efficiently introgress the effector genes
into the wild gene pool.
Fitness is the relative success with
which a genotype transmits its genes to
the next generation. Survival and
reproduction are the important
components.
A transgenic
mosquito
(Left) with
green
fluorescent
eyes, and a
nontransgenic
mosquito
(Right), with
no eye
fluorescence.
The
transgenic
mosquito
carries a gene
that confers
resistant to
the malaria
parasite.
Insect immunity
This approach studies vector immune responses,
and its effect on parasite transmission.
The idea is to develop vaccines that block
parasite transmission and antimalarial agents
that target vector immunity and parasite
development.
Killing mechanisms:
• Antimicrobial peptides: defensin and cecropin
genes in A. gambiae. Highly induced by malaria
infection
• Melanotic encapsulation: Two mechanisms:
Cellular encapsulation mediated by haemocytes
that surround and attach to the microorganism to
form a capsule that becomes melanized.
Humoral encapsulation is the formation of a
melanized proteinaceous capsule around the
microorganism without participation of
haemocytes.
• Phagocytosis: killing of microorganisms through
engulfment and subsequent degradation by
haemocytes.
Middlepanel: Organs and cell types involved in Plasmodium
interaction and immune response, and the parasites life cycle.
Gametocytes (GC) enter the posterior midgut (PMG) through the
anterior midgut (AMG), fertilize and develop into an ookinete (OK) that
traverses the peritrophic matrix and midgut epithelium to form an
oocyst (OC) under the basal lamina. After maturation, sporozoities (S)
translocate from the oocyst (OC) to the salivary glands (SG).
Lower panel: Melanized ookinetes in the midgut epithelium (MOK1,
MOK2). Melanization is stronger on the apical end that is facing the
haemolymph. Ookinetes (OK1, OK2) can be visualized in the midgut
epithelium when stained with an ookinete surface protein-specific
antibody. Intact mature oocyst on the basal side of the midgut
epithelium (OC1). Cellular Microbiology (2003) 5, 3-14
Other research initiatives
Understanding vector biology
Mosquitoes show a remarkable preference
for humans as hosts for blood-feeding
They are highly susceptible to infection
Olfaction plays a crucial role in shaping
behaviors such as host seeking and feeding
and determines their vectorial capacity.
Research on the behavior of vectors:
• Modification of vector behavior for disease
control
• Basic research to understand the genetic and
environmental components of vector behavior
and reproductive biology
• Mosquito genomics: comparative genomics to
provide information about lineage-specific
adaptations, population biology, ecology and
genetics, dynamics, regulation and variation of
vector populations, and vector survival
strategies.
Other research initiatives
Investigating mechanisms of insecticide resistance
• At present, the control of malaria vectors relies
extensively on the use of indoor house spraying with
residual insecticides and the use of insecticideimpregnated bednets.
• The problem is the lack of available licensed insecticides
and the growing resistance.
• Novel targets and the understanding of resistance are
also important areas of research.
• Scientists are trying to identify specific members of the
detoxification enzymes whose expression are elevated in
insecticide-resistant populations
SUMMARY
• Remarkable progress has been made
towards a the understanding of vector
biology and the potential manipulation of its
host for its own benefit (transmission and
metacyclogenesis).
• New tools are available and are being
produced for the production of transgenic
vectors.
• Fitness of the transgenic population in the
wild is a problem to be solved.
• Mosquito control measures are often
complicated by the presence of multiple
vectors in the same area. In Africa, many
different anopheline species can function
as malaria vectors, either simultaneously or
seasonally.
• The A. gambiae genome has been
completed so the number of neutral genetic
markers for estimating gene glow between
populations has increased.
Strategies for malaria control