WHAT IS A VECTOR? A vector is a transmission agent. Certain invertebrates - mainly insects carry microscopic organisms that cause diseases and transmit them to humans and animals when they feed. Fortunately, not all biting invertebrates are vectors!
A vast world of vectors s r o t c e v n w o n k t s The be o s l a g n a g e h t t u b s e o t i u q are mos . c t e , s e i ﬂ , s k c i t , s a e ﬂ , s g u b includes
What do plague, malaria and chikungunya have in common? These diseases and many others - or more precisely the causal agents responsible - are spread by certain
arthropodes, a zoological category that includes ﬂies, scorpions, shrimps, etc. Arthropod vectors feed on blood and this very special diet means that they can act as intermediaries.
in forests and between them all cover almost the world
These haematophagous vectors are themselves infected
from the tropics to temperate latitudes. The list of their 'victims'
by pathogens and inject the latter into their hosts when they feed on blood. They are found in the country,
in towns and
also shows their diversity: humans,
The infernal triad Host
Three players are involved in the transmission of what are known as vector diseases: a vector, a host (the future infected body) and a pathogen. The latter member of the trio can be a virus (like those of chikungunya and dengue), a bacterium (plague) or a parasite (malaria and sleeping sickness). A given pathogen is transmitted by only one or a small number of vectors. Likewise, only one or several vertebrate hosts are at risk from the pathogen.
The precision of these adaptations results from centuries of interactions between the three players.
What are scientists doing? Some vector diseases have aﬀected humans
for thousands of years (malaria, yellow fever). Others are emerging diseases (chikungunya, dengue) and result from recent global changes and globalisation. Finally, it has been possible to
to a considerable degree (plague, onchocercosis). Today, scientists are continuing to perfect new
anti-vector and prevention strategies to reduce vector transmission of pathogens and to
lessen the risk of the establishment
of a disease in a region.
Aedes M O S Q U I T O E S dengue and chikungunya
Name Aedes albop ictus Length 5 mm Longevity as a biting in sect 3 weeks Special feature black and white stripes, t h e reason why it is known as the 'tiger mosquito '.
Biology of the vector In towns, these mosquitoes ﬁnd 'homes' (recipients containing the water needed for the larval stages) and 'food' (human blood for the female insects). Larvae develop anywhere that water accumulates. Females bite during the daytime.
i t p y g e a s e d e A e m Na
m m 7 h t g n Le s k e e w 3 t c e s n i g n i t i b a s Longevity a n r e t t a p d e p a h s e r y l a e r u t a e f l a i c Spe x a r o h t e on th
Geographic distribution Distribution of Aedes albopictus in 2014
Aedes albopictus originated in Asia
and has recently spread
to Africa, Latin America and temperate regions in Europe and America. Aedes aegypti is found everywhere in the tropics. Eggs, can remain dormant for several months
Isotherm 10°C average in July 2013
and this has allowed the mosquito
to spread to diﬀerent continents. Original zone Extension zone Favourable area Isotherm 10°C – average in July 2013
Vector transmission Aedes aegypti and Aedes albopictus cause intense transmission of dengue and chikungunya viruses in the
tropics, with increasingly frequent and serious epidemics of both diseases. The number of new cases of dengue has
30-fold in the last 50 years. The chikungunya virus causes serious fever. It aﬀects articulations in particular and recovery is slow.
Dengue can be fatal. Today, the dreaded tiger mosquito is well established in Europe and North America. The vector now causes the local transmission of viruses imported by people infected during stays in areas in which the diseases are endemic.
Prevention and control There is no medication for the two diseases. Control of the vector is still the only solution: Elimination of stagnant
water that is propitious for larvae
Spraying of chemical or organic pesticides to reduce the density of the adult mosquito population Use of repellents to reduce bites
Dengue and chikungunya are spreading in the wake of the tiger mosquito 4
Anopheles M O S Q U I T O E S malaria
l e h p o n A Name
e a i b m a g s e
s m k e m e 6 w h 3 t g t c e Len s n i s g e n i o t t i i b u a q s s a o y t i m v r e g e d l Lon o y l n o e r u t a e f l a i c e Sp s r o t c e v e ar
Biology of the vector Only the female
bites, and especially during the night. This feeding on blood is essential for the development of eggs but is not necessary for the survival of the female. Adults of both sexes
feed on sweet plant juices.
Geographic distribution Distribution of malaria
Anopheles are found all
over the world
in zones including the cold temperate regions. However, a few isolated zones are
free of them: New Caledonia, the Seychelles and Polynesia.
Scientists are still discussing the reasons for these exceptions.
Strong transmission Weak transmission
Vector transmission Word Health Organization (WHO) estimates that malaria killed 600,000
lives annually. Deaths are mainly in tropical zones and particularly concern young children in Sub-Saharan Africa. This potentially deadly disease is caused by single-cell parasites belonging to the genus Plasmodium.
Mosquitoes become contaminated by intake of the blood of an infected person. They reinject the parasite in their saliva into other humans during subsequent feeds. Plasmodium develops in the vector for about ten
days and so only old females can spread it.
Prevention and control Drugs are available but in spite of research eďŹ€orts there is still no vaccine. Vector control is still the approach to be used: Apply repellents to the skin, wear clothes that cover the body well and use mosquito nets impregnated with insecticide TKill adult mosquitoes using insecticides
Dry out aquatic larval habitats to prevent the development of new generations
1 feed on blood = 200 eggs laid 5
Culex M O S Q U I T O E S West Nile disease
s n e i p i p x e l u C e m a N
m m 0 1 h t g Len s k e e w 2 t c e s n i g n i t i b a Longevity as s d r i b s e t i b o s l a e r u t a e f Special
Biology of the vector Female mosquitoes lay eggs
on the surface of the water,
where they form small ďŹ‚oating clumps. Culex mosquitoes are the main vectors of the West Nile Disease and are found in marshes, irrigated and in towns, even in polluted water.
Geographic distribution Distribution of the West Nile virus
Culex colonise all
ecosystems in all the continents. Since 1999, 1,550 deaths have been reported in the USA. Epidemics-sometimes urban-have also occurred
The number of cases in Africa, where the virus is endemic, is not known with any accuracy. Presence Absence (or information lacking)
Vector transmission The West
Nile virus, transmitted by Culex
other birds or humans
mosquitoes, mainly infects birds.
and horses that share their environment.
The West Nile virus targets the
Of African origin, the virus is carried to other continents
by migratory species.
European, Asian and North American mosquitoes are infected by birds carrying the virus. In turn they infect at a local level
nervous systems of hosts. It causes muscle pains, fever and headaches in humans. The disease can cause sometimes fatal encephalitis.
Prevention and control There is neither a vaccine nor a drug to combat infection by the West Nile virus. In view of this, measures must be taken for protection against mosquitoes:
Reduce the aquatic habitats suitable for larvae Use chemical
or biological insecticides
Protect against bites by using repellents, clothes
that cover the body well and sleep under a mosquito
The virus spreads from birds -the main target-to humans via mosquitoes 6
Phlebotominae SAND FLIES leishmaniasis
Name Phlebot omus papatasi Length 2-3 mm Longevity as a biti ng insect 1 mon t h Special feature m ultiple bites on a small are a of skin
Biology of the vector
Of the same order as ﬂies, Phlebotominae sand ﬂies are silent
at night. Eggs, larvae and pupae have a terrestrial habit with a preference for dark places. The males do not bite. Each blood meal taken by a female is suﬃcient for the development of about a hundred eggs.
3 larval stages
Geographic distribution Distribution of leishmaniasis (visceral and cutaneous) These sand ﬂies are found in the
tropical zones of all continents. Seen in both the Amazon forest and in the Andes, they are also present in the South of France and the
Mediterranean region. Transmission conﬁrmed Transmission not conﬁrmed or lack of information
Vector transmission 70 of the 800 Phlebotomus species are potentially vectors
The visceral form is fatal without
treatment, aﬀecting liver, spleen and bone
of leishmaniasis. The pathogens that cause this disease
are members of the genus Leishmania.
The World Health Organization (WHO) estimates that there
The most benign form of the disease causes cutaneous
persons at risk.
The parasite infects humans, dogs and other mammals.
ulceration. The mucocutaneous form is more serious.
are 2 million new cases per year and 350
Prevention and control The drugs available have serious undesirable secondary eﬀects and so preventive measures should be taken: Use cutaneous
repellents and insecticide-treated nets
Destroy any resting habitats of adult sand ﬂies around dwellings
Vaccinate dogs in zones where the disease is endemic using the vaccine developed by IRD and its partners
A discreet but fearsome insect! 7
MIDGES Culicoides bluetongue disease
Name Culicoides imicola Length 1-4 mm Longevity as a biting insect 20-30 days Special feature its bite is particularly painful
Biology of the vector The female culicoid midge bites
humans and animals to obtain the blood meal that is essential for maturation of her eggs. These are laid in moist and even wet soil. Several stages follow hatching, from that of semi-aquatic larva to adult.
Eggs 0.25 mm long, 25 to 150 per oviposition
Adult stage 1-3 mm long
4 larval stages
Pupal stage 2-3 days 1-3 mm long
Geographic distribution Distribution of bluetongue disease in 2012
Culicoïdes are widely distributed
in the world, but absent in Antarctica,
New Zealand and Hawaii.
have been counted, with about
90 reported in France.
Scientists are discussing the possibility of recent colonisation of the Mediterranean basin by Culicoïdes imicola.
Vector transmission Some Culicoides species are vectors of human
viruses such as the Oropouche
virus in South America.
But others are the cause of very serious damage to domestic animals. For example, bluetongue
disease in sheep, whose characteristic clinical feature is a violet tongue,
whence the common name of the disease.
European ﬂocks and herds have been seriously aﬀected since the 2000s. They are exposed to the recent introduction of exotic virus that can also be transmitted by local species of Culicoides.
Prevention and control Various preventive measures are proposed: Treat livestock with insecticide
Keep livestock in stables as adult midges do not enter buildings very much
Remove manure as it is propitious for Culicoides larvae Vaccinate livestock using the appropriate viral strain
1 400 species and considerable impact on animal health
S E D I C I T C E S N I W E N
S E I D U T S FIELD
H C R A E S E R C I T E GEN
INNOVATION IN SCIENTIFIC IMAGERY
PROGRESS IN RESEARCH Existing control methods have shown their limits in terms of eﬀectiveness in vector control and acceptability by the Les méthodes actuelles de lutte ont montré leurs limites tant en termes d’eﬃcacité contre les vecteurs que d’acceptabilité populations exposed to a risk of transmission. Innovative strategies and methods must be identiﬁed for an optimised par les populations exposées au risque de transmission. Pour une approche optimisée et respectueuse de approach that respects the environment. l’environnement, il faut identiﬁer des stratégies et des outils innovants. New substances are being developed to get around resistance to insecticides. Research is in progress on the behaviour De nouvelles molécules sont développées pour pallier la montée des résistances aux insecticides. Des études sont en cours of vectors, their chemical communication, their physiology and genetics in order to identify new control methods. sur le comportement des vecteurs, leur communication chimique, leur physiologie et leur génétique pour identiﬁer de The genome of the tsetse ﬂy was sequenced recently and will contribute to the development of innovative methods to nouvelles cibles. Le génome de la mouche tsé-tsé, décrypté récemment, contribuera au développement de moyens de control this vector. Tested by IRD and its partners, the release of sterile male Aedes albopictus mosquitoes will reduce lutte innovants contre ce vecteur de la maladie du sommeil. Le lâcher de moustiques mâles stériles d’Aedes albopictus, the number of adults in subsequent generations and thus reduce the transmission of dengue and chikungunya viruses. testé par l’IRD et ses partenaires, réduira le nombre de vecteurs adultes aux générations suivantes et donc la transmission des virus de la dengue et du chikungunya.
B L A C K F L I E S simulium onchocerciasis
Biology of the vector
Name Simulium da mnosum Length 3 mm Longevity as a biting ins ect 2-3 weeks Special feature capab le of active ﬂight for tens of kilomet res The onchocerciasis parasite cycle
Simulium are black ﬂies. Their larvae and pupae live
rivers. Adults emerging from their spun cocoons rise to the surface in an air bubble. Only the females feed on blood. They tend to bite in daytime.
Migration to the head and mandibles
Subcutaneous human tissue Larva L2 Adults in lumps under the skin Larva L1
IN THE VECTOR
Geographic distribution The distribution of onchocerciasis today
The biological constraints of larvae mean that Simulium are found in particularly
large numbers near rivers.
99% of cases of onchocerciasis or 'river blindness' have been recorded in
Vector transmission The main infectious agent transmitted
to humans by Simulium black ﬂies is the parasitic worm Onchocerca volvulus that causes onchocerciasis.
The microﬁlaria laid by adult worms burrow into the skin and ocular tissues, causing lesions that may result in irreversible
blindness. The disease is particularly prevalent in rural areas of African savannah where outbreaks are seen along watercourses.
In operations coordinated by the World Health Organization (WHO), with a primordial contribution by IRD, larvicides
were sprayed over rivers from 1974 to 2002 by the Onchocerciasis Control Programme in West Africa. This allowed the repopulation of fertile valleys that now carry no risks for the population. Today, 37
million Africans are still infected
and 90 million live in risk zones.
Prevention and control Two types of measure are recommended: Protect against bites using chemical
If prevention fails, treat using Ivermectine, an eﬀective medication against microﬁlaria
Monitoring onchocerciasis outbreak areas will make it possible to maintain the progress made 10
Tsetse F L Y sleeping sickness
s i l a p l a p a n i s s o l G e Nam
m m 0 1 h t g n Le s h t n o m 3 2 t c e s n i g n i t i b a s a y t i v e g n Lo e t a r n o i t c u d o r p e r w o l e r u t a e f l a i c Spe
Biology of the vector
Among vectors, the tsetse ﬂy (genus Glossina) has original features:
adults of both sexes feed
on blood only and the females do not lay eggs but 'give birth' to 3 to 5 large larvae during their
Gestation for 15 to 20 days
Pre-pupa Birth of a larva
Geographic distribution Distribution of sleeping sickness
Tsetse ﬂies are found only in Sub-Saharan Their thermal
optimum is around 25°C.
Furthermore, these ﬂies do not survive if the air is too dry.
Persistent vector transmission
Vector transmission Trypanosoma parasites are injected in human or animal blood with tsetse ﬂy saliva and multiply, causing swellings, and then reach the brain of the host. The sleepiness experienced is the source of the common name of the disease: 'sleeping sickness'. Infected
persons die without
treatment. More than 98 % of cases of human trypanosomiasis
are caused by Trypanosoma brucei gambiense. It is estimated that
70 million persons are at risk in 37 African countries and 50 million head of cattle are exposed. This is a problem of public health and a major obstacle to livestock farming.
Prevention and control There is no vaccine or preventive treatment for these parasitic diseases, whence the importance of vector control. Actions that can be taken: Reducing tsetse ﬂy population by means of insecticides and traps
Early screening for the disease Treatment of patients according to the stage of the disease
'Tsetse' is an African term referring to the sound of a fly in flight 11
B U G S Triatominae Chagas disease
s n a t s e f n i a m o t a i r T Name
Biology of the vector
m c 5 , 1 h t g n Le s r a e y 2 à 1 t c e s n i g n i t i b a s a y t i v e g Lon e t e r c x e y e h t e r u t a e f l Specia e t i b y e h t e r e h w e c a l p at the Vector bugs: Triatominae
larval stages and adults of both sexes are haematophagous. Triatominae become active at nightfall
The parasite breeds in the digestive tube of the bug
The parasite in blood is ingested by the bug
after remaining hidden in dry places during the day.
The parasite bursts cells and moves into the blood
They hide under
beds, in cracks in walls near people in bed who are bitten while they sleep.
The parasite infects several types of cell and multiplies there
Mammal hosts The host scratches and contaminates himself The parasite is in the excrements of the bug on the skin of the host
Geographic distribution Distribution of Chagas disease Of 141
triatome species, 121 are strictly American. These insects live in dry or humid forests from the north of the United States to Patagonia. After human colonisation of these areas, they adapted to human
- especially where these are poor and old.
Transmission levels Transmission interrupted No transmission Persistent and emerging transmission
Vector transmission Triatomes prefer to feed on mammals,
humans. They deposit the Trypanosoma cruzi in their excrement during these blood meals. Scratching results in penetration by the parasite at the site
of Chagas disease in South
America. It causes a death rate of about 5% among young children. A chronic form with damage to the heart or the digestive system develops in 30% of cases.
of the bite. Triatoma infestans is the main vector of the agent
Prevention and control Health professionals have scant resources. Drugs are ineﬀective in the chronic phase and have undesirable eﬀects. There is no vaccine and resistance to insecticides is observed. Vector control is therefore of crucial importance:
Eliminate bugs in dwellings Use appropriate insecticides
Improve makeshift housing Eliminate favourable habitats
A neglected and under-diagnosed disease that is strictly American 12
s i r o p r o c s u n a m u h s u l u c i d e P e Nam
m m 2 h t g n e L s k e e w 5 e t i s a r a p g n i t i b Longevity as a s n o i t i d n o c y r a t i n a s o t d e t a l e r e r Special featu
Biology of the vector Larva 1 Larva 2
Among the 550
louse species found as parasites in the fur of mammals, two live only on human beings: the famous 'crab' or pubic louse and two forms of the same species, one of which targets hair and the other is found in body hair.
Egg fixed on a hair Larva 3
Geographic distribution World distribution of lice Head and body lice are distributed
worldwide. The second is often seen in mountainous regions in the tropics. In temperate countries, they aďŹ€ect people living in precarious sanitary conditions. Public lice are dwindling.
Vector transmission In contrast with other species, body
lice are vectors for infectious agents. They carry bacteria that cause epidemic typhus or trench fever. Lice ingest bacteria with the blood of sick persons and spread them in their excrement.
Trench fever probably aďŹ€ected more than a million persons during World War 1 and typhus has caused more deaths than the sum of deaths in all wars. Today there are serious epidemics in refugee camps
They enter the body via bite wounds, scratching or contact
during wars or following natural catastrophes.
with ocular mucous membrane.
Prevention and control Preventive or curative measures must be planned: Apply measures for bodily
Treat infested persons and clothes with insecticides Use antibiotics in case of infection
Body lice cannot survive for more than 48 hours when separated from their host ^
s i p o e h c a l l y s p o n e X Name
m m 2 h t g n Le s h t n o m 0 1 t c e s n i g n i t i b a s a y t i v e g n o L f o s p m u j r o f d e t u p e r e r u t a e f l a i c e Sp e z i s s t i s e m i t 0 0 3 n a h more t
Biology of the vector Every ﬂea species lives in close
contact with an associated host species (land mammals and sometimes birds). Adult males and females live in the fur of their hosts and feed on blood alone. But the larvae live in the litter
Geographic distribution Present distribution of plague
Fleas are present all
over the world.
Pulex irritans has 'specialised'
hosts but is not a vector. The rat ﬂea, Xenopsylla cheopis, is the main vector of bubonic plague in humans. It is still present in more than 25 countries.
Vector transmission Fleas transmit the bacterium, Yersinia
from one rodent to another. However, the bacterium
can infect humans, causing catastrophic plague epidemics
before the discovery of a vaccine and antibiotics.
It is estimated that in the Middle Ages the Black
million people in Europe, that is to say 40% of the population in ﬁve years. No cases have been reported for decades in Europe. In contrast, cases are reported in the United States.
Prevention and control The vaccine for plague was discovered by Institut Pasteur. In countries where vaccine cover is absent or imperfect, measures must be taken in a precise order in case of epidemic: First kill ﬂeas using fast-acting
Then eliminate rats by slow-acting
Treat cases of plague with antibiotics
A traumatising historical disease that still exists today 14
N O I T A T S E R O F E D
S D O O L F
GLOBALISATION OF TRANSPORT
N O I T A S I N A B R U G N I S A E R C IN
CLIMATE CHANGE AND GLOBALISATION The geographic distribution of vectors will be less changed in the future by the increase in mean temperatures than, above all, by changes in thermal amplitudes, precipitation patterns and the occurrence of extreme events (droughts, ďŹ‚ooding, etc.). However, the impact of these more or less foreseeable climate changes on the development of vector diseases is diďŹƒcult to gauge. Climatic upset is only a tiny part of the environmental changes caused by humans: deforestation, increasing urbanisation, access to water and the globalisation of trade and transport also contribute to the rapid change in the environments where vectors and pathogens develop and adapt.
Lyme disease s u n i c i r s e d o x I e m a N
Biology of the vector
m m 4 à 5 , Length 0 s r a e y 7 à 2 r o t c e v g n i t i b a s a y t i v e g n Lo h t g n e l n i e l b u o d s e l a m e f e r u t a e f Special l a e m d o o l after a b Blood meal 3
Ticks are not
insects but arachnids that feed on blood
at each stage of their development (larva, nymph, adult).
Ticks hunt, attaching themselves to vertebrates and each 'meal'
Blood meal 2 (4-7 days)
can last for several days.
Blood meal 1 (3-5 days)
Geographic distribution Distribution of Lyme disease
More than 900
tick species are known, covering the whole world
from the tropics to temperate climates and as far as the poles. Habitats are varied in prairie
and forests (including
medium altitude mountain areas). However, the species observed are diﬀerent Persistent vector transmission
according to the region.
Emerging vector transmission
Vector transmission The eﬀects in humans range from skin
Ticks transmit a very great variety of infectious agents to humans and animals.
In the northern hemisphere, Lyme disease (also known as Lyme borreliosis) is the main human vector disease (12,000
to 15,000 new cases in France
every year). The bacterium involved is
burgdorferi, transmitted by ticks of the genus Ixodes.
lesions and joint pains to neurological or heart conditions that may occur 10 years after the bite. In the South, other tick species transmit viruses and bacteria that cause particularly serious damage to livestock.
Prevention and control Treatment of Lyme disease using antibiotics is long and diﬃcult. The best way of avoiding infection is to protect oneself from tick bites. For this, when in forest areas: Wear appropriate
clothes and footwear
Use repellents On returning check
whether there are any ticks
on your body and if so remove them with a tick removal device
Lyme disease is increasing strongly in Europe and the United States 16
What do plague, malaria and chikungunya have in common? These diseases and many others—or more precisely the causal agents responsible—are...
Published on Aug 25, 2015
What do plague, malaria and chikungunya have in common? These diseases and many others—or more precisely the causal agents responsible—are...