Page 1

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,


birds, sheep,

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

control others

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 e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 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

in Europe.

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


Blood meal

Of the same order as flies, Phlebotominae sand flies are silent

and active

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.

Nearly 1,400

different species

have been counted, with about

90 reported in France.

Scientists are discussing the possibility of recent colonisation of the Mediterranean basin by Culicoïdes imicola.

Disease present

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






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

in fast-flowing

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



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


Presence Absence

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

and appropriate



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


lifetime. Emergence

Blood meal

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

The five

larval stages and adults of both sexes are haematophagous. Triatominae become active at nightfall

The parasite breeds in the digestive tube of the bug

Mammal hosts

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 a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 a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 ^


FLEAS plague

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


of mammals.


Pupa Larva

Geographic distribution Present distribution of plague

Fleas are present all

over the world.

Pulex irritans has 'specialised'

in human

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.

Present Absent

Vector transmission Fleas transmit the bacterium, Yersinia

killed 25


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

rat poison

Treat cases of plague with antibiotics

A traumatising historical disease that still exists today 14





Vector diseases,

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, 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

(6-12 days)

Ticks are not



insects but arachnids that feed on blood

Moult 2

at each stage of their development (larva, nymph, adult).


5-25 weeks

Ticks hunt, attaching themselves to vertebrates and each 'meal'

Blood meal 2 (4-7 days)

can last for several days.

20-50 days


accidental host



Moult 1

2-8 weeks

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

Exhibition Vectors and diseases  

What do plague, malaria and chikungunya have in common? These diseases and many others—or more precisely the causal agents responsible—are...

Exhibition Vectors and diseases  

What do plague, malaria and chikungunya have in common? These diseases and many others—or more precisely the causal agents responsible—are...