Disease vectors

A disease vector is any living organism that transmits an infectious disease to humans (or in agriculture to animals and plants). A vector picks up the disease from an infected host or the environment then transfers it to a new host through a bite when feeding or by mechanical transmission such as defecating on the skin or from particles on the outside of the body.

The most important vector borne diseases at a global level and their vectors are shown below:

• Malaria (protozoan): Anopheles species of mosquito.
• Lymphatic filariasis (nematode worm): Culex, Anopheles, Aedes species of mosquito.
• Dengue (virus): Aedes species of mosquito.
• Leishmaniasis (protozoan): mainly Phlebotomus species of sandfly.
• Schistosomiasis (trematode worm): tropical and subtropical freshwater snails.
• Chagas disease (Trypanosoma species of protozoan): Assassin bugs, triatomine bugs.
• Trachoma (bacterium): flies (various species).
• Onchocerciasis (nematode worm): Simulium species of blackfly.
• Japanese encephalitis (virus): Culex species of mosquito (also some domestic animals, wading birds).
• Other arboviruses: various vectors.
• Enteric diseases (various microorganisms): various vectors.
• Human African trypanosomiasis (Trypanosoma species of protozoan): Tsetse fly, Glossina species (also animals).

The number of countries with active transmission of vector-borne diseases is given in the chart below. This shows that virtually every human being is at risk of being infected with a vector-borne disease.

The number of people infected with malaria worldwide is 265 million, lymphatic filariasis is 120 million and Chagas disease 10 million. The population at risk, however, is far greater:

• Malaria: 3.2 billion
• Dengue: 2.5 billion
• Lymphatic filariasis: 1.4 billion
• Leishmaniasis: 200 million
• Chagas disease: 30 million

Mosquitoes are the greatest threat globally and consequently they are also the main target of vector control by governments and international organisations such as WHO.

Mosquitoes

Mosquitoes cause more deaths than any other disease vector. Over 3 billion people are at risk from mosquito-borne diseases such as malaria, dengue fever and lymphatic filariasis (elephantiasis). The most dangerous mosquitoes are:

• Aedes aegypti, Ae. albopictus, and other Aedes spp: Chikungunya virus, dengue virus, West Nile virus, yellow fever virus, Zika virus, lymphatic filariasis/ elephantiasis (parasitic roundworm);
• Anopheles species (60+ species transmit diseases): malaria (parasitic protozoan), lymphatic filariasis (parasitic roundworm), West Nile virus;
• Culex species: West Nile virus, lymphatic filariasis/ elephantiasis (parasitic roundworm);
• Haemagogus species; Sabethes species: yellow fever virus.

Many other species of mosquito can transmit diseases but are not so prevalent in the human environment or as effective at transmission as the genera and species listed.

There is no vaccine for the majority of mosquito-borne diseases, and no specific treatment for viruses such as dengue, Chikungunya, Zika or West Nile virus — there is a vaccine only for yellow fever. Therefore disease reduction and elimination is totally dependent on vector control for all these other diseases.

For more information see:

• Insect-borne diseases;
• Zika.

Rodents

Rats (both Rattus norvegicus the brown or Norway rat; and R. rattus the black or roof rat) and the house mouse are the most significant rodent pests or urban environments. Ground squirrels, chipmunks, woodrats and marmots are also significant vectors of disease in some areas.

Diseases: rodents carry a large number of diseases including bacteria, viruses, protozoa and helminths (worms). Specific diseases of importance include:

• Leptospirosis via urine, caused by two bacteria Leptospira interrogans and L. biflexa (it is also common in the urine of other wild and domestic animals and causes illness in them). Rodents cannot control their urine so contaminate every surface they run across;
• various food-borne diseases such as Salmonella, from rodents contaminating food with faeces and filth;
• murine typhus and the plague carried via fleas;
• many other diseases carried by rodent ectoparasites: flea, tick, lice and mite-borne diseases.

Rodents have been associated with human activities for millennia, infesting homes, domestic buildings, food stores, businesses handling food, and ships and vehicles carrying food. They are a major health and economic threat worldwide for homes and businesses.

For more information see:

• Rodent-borne diseases;
• Pest control in food processing;
• Pest control in the hospitality sector;
• Connected rodent control.

Cockroaches

The main pest species are: German Cockroach, Blattella germanica; American Cockroach, Periplaneta americana; Oriental Cockroach, Blatta orientalis.

Diseases: a wide range of pathogenic bacteria, including Salmonella, Staphylococcus, Listeria, E. coli, Campylobacter and viruses, fungi, protozoans and parasitic worms. They also produce particles that can cause asthma.

Cockroaches are one of the most serious pests of homes and businesses wherever there is food, such as in food storage facilities, food processing factories, restaurants, healthcare facilities and pharmaceutical manufacturing plants. They feed on virtually any source of food including mould, decaying and faecal matter, which they can then carry on their bodies to hygiene-critical areas. In addition: they defecate as they travel, secrete saliva to taste their surroundings, and cast skins and egg cases, leaving a foul odour wherever they travel — on surfaces and in equipment, furnishings, packaging and food.

Flies

Filth flies, including house flies, drain flies, black flies and flesh flies.

Diseases: Flies are known to carry over 100 disease-causing bacteria, viruses and parasites, such as Salmonella spp., E. coli, Campylobacter sp., Trypanosoma brucei (a protozoan parasite that causes African trypanosomiasis), Onchocerca volvulus (nematode worm parasite that causes onchocerciasis, carried by Simulium species of blackfly).

Flies are a hazard from contaminating surfaces and food. They are attracted to food sources around homes and businesses, including food waste. They also feed and breed on dead animals, drains and faeces, where they come into contact with many pathogenic microorganisms.

Fleas

Rodent fleas, Xenopsylla cheopis and Nosopsyllus fasciatus; cat flea, Ctenocephalides felis; human flea, Pulex irritans; Oropsylla montana on ground squirrels in the US.

Diseases: Bubonic plague, bacteria Yersinia pestis; flea-borne typhus (also called murine typhus, endemic typhus, urban typhus), bacteria Rickettsia typhi.

There are over 2,200 flea species worldwide, but most are only found on specific animal hosts. Fleas are usually brought into contact with humans by domestic or wild animals, including rats and mice, cats, dogs, foxes, birds, and rabbits. They generally prefer to feed on their host animal, but will attempt to feed on other hosts when they cannot find their preferred host.

The most important pest fleas are rodent fleas, which carry disease, and cat fleas which are a biting pest. They are a serious urban pest for humans and other urban animals. Both species are nest fleas, which means they are therefore dependent on the host animals returning to their nests each day for feeding. When the animals permanently leave their nest or are exterminated, the fleas will then move out of the nests to seek new hosts. This is often the cause of infestation of buildings.

Rat fleas become infected by feeding on an infected rat. The bacteria multiplies in the rat intestine, but does not appear in saliva, so the flea bites do not transmit the plague or typhus. People are infected when a rat flea (or rarely other species of flea) defecates onto its host’s skin and the person scratches a flea bite or squashes the flea. The bacteria enter the bite wound or an existing break in the skin.

Louse

Body louse, Pediculus humanus humanus; Head louse, Pediculus humanus capitis.

Diseases: epidemic typhus, Rickettsia prowazekii; murine typhus, Rickettsia typhi; trench fever/ bartonellosis, Bartonella quintana; relapsing fever, Borrelia recurrentis.

The head louse and body louse are the same species and virtually indistinguishable even using genetic analysis, but are thought not to interbreed. Lice can be passed from person to person by close contact in situations of poor sanitation such as in homeless person shelters and refugee camps. They can also be spread by contact with clothing and personal items such as hats and scarves.

In developed countries head lice most commonly affect children and are not known to transmit disease. They are more of an irritant, causing itching and distress.

People catch the infections from the lice faeces when scratching and rubbing bites, which carries the bacteria into broken skin and from hands can be passed into mucous membranes.

Sandflies

Phlebotomine sandflies are the most important disease vectors. Phlebotomus species occur from Europe across Africa and Asia to Australia and the Pacific. Lutzomyia and other genera occur in North and South America.

Diseases: The most important diseases are cutaneous and visceral leishmaniasis caused by Leishmania species of protozoa. Sandflies also transmit several bacterial and viral diseases including bartonellosis and pappataci fever. The majority of cases worldwide occur in India, Sudan, Nepal, Bangladesh and Brazil.

Sandflies live for about a month, of which 20 days are as larvae. The females lay eggs in soil and cracks and crevices with relatively cool and humid a environment and dead organic matter for the larvae to feed on. This can include dense vegetation, animal burrows, household rubbish. In areas such as rural India the construction of homes with mud walls plastered with cow dung creates an ideal environment.

Only the females feed on blood to produce eggs. Human to human transmission via sandfly bites is the main means of transmission. However, dogs, foxes and other canids are animal reservoirs of the parasite in parts of South America, Mediterranean and the Middle East. Rodents have been found with the parasite in Sudan.

Control methods include residual insecticide spraying inside houses, insecticide treated nets, environmental protections and personal protection.

For more information see the WHO Leishmaniasis page.

Ticks

There are around 900 species of tick, classified into two main families: hard ticks, Ixodidae, which includes most ticks of medical importance, and soft ticks, Argasidae.

Diseases: Some of the main diseases are: Lyme disease (bacteria), Crimean-Congo haemorrhagic fever (virus), Kyansur Forest disease (virus), tick typhus (bacteria), tularaemia (bacteria), Crimean Congo haemorrhagic fever (virus), relapsing fever (bacteria), tick-borne encephalitis (virus). The US Centers for Disease Control and Prevention (CDC) lists 15 diseases carried by ticks in the US. Even in developed countries new diseases are being discovered. In the US two new tick-borne viruses have recently been discovered: Heartland virus in 2012 and the Bourbon virus in 2014.

The reservoirs of these diseases are rodents, dogs, cattle, rabbits, other mammals and birds.

Ticks cause more cases of disease in Europe and North America than any other arthropod (insects and arachnids), according to WHO — the most common disease in all areas being Lyme disease.

Ticks occur in forests shrubland, grassland and moorland, where they feed on many kinds of wild and domestic animals, and passing humans. Carried by farm animals, rodents, birds and pets, ticks can infest buildings in rural and urban environments.

Homes in rural and semi-urban areas are also at risk of having ticks in the surrounding vegetation. Increasing suburbanisation is bringing more homes into contact with tick infested areas in both Europe and North America. Ticks can be controlled near properties surrounded by vegetation by spraying the vegetation with a residual insecticide.

Birds

Pigeons, gulls and sparrows are the main pests in the human environment.

Diseases: Birds carry many human diseases including viruses, bacteria, fungi and protozoa. The more common pathogenic microorganisms include Salmonella, E. coli and Campylobacter. Strains of bird flu such as H1N1 can be carried long distances by migrating wild birds, but are mainly a threat to people working close to farmed birds.

Bird nesting and roosting sites can be sources of infestations of arthropods such as bird mites and fleas. Many wild bird species can spread ticks that are vectors of a number of diseases, including Lyme disease.

Birds are mainly a health threat in urban and industrial environments where they are attracted by food and shelter. Their droppings foul buildings, vehicles, paved areas and building entrances. The droppings, nesting material and feathers can contaminate sensitive surfaces, machinery and food products.

Triatomine bugs

The main species causing human infection are Triatoma infestans in S. Peru, Rhodnius prolixus in Colombia, Venezuela, and Central America, Triatoma dimidiata, Central America. However, over 150 species of Triatomine bug carry the disease.

Disease: Chagas disease/ American trypanosomiasis. Protozoan parasite, Trypanosoma cruzi.

The disease occurs only in the Americas, from about 42°N in California and Maryland to 42°S in Argentina. Cases of Chagas disease occur mainly in rural areas of Mexico and Central and South America.

Chagas disease has been recorded in over 100 species of domestic and wild mammal. In domestic situations, cats and dogs are the most important hosts. In some towns in Argentina and Venezuela, infection rates in dogs were found to be around 40%, and in the high Andes where guinea pigs are a common domestic animal, 10-60% were infected with the parasite. (WHO, 2003 c).

Triatomine bugs occur in the wild in forested and dry areas in burrows and nests of wild animals, including birds, bats, squirrels, opossums. Near human habitation they will feed on both domestic animals and humans.

They are night time feeders, hiding during the day in small spaces such as cracks and crevices of walls made from coarse brick and plaster work, furniture, beds, thatched roofs, and piles of firewood. The parasite is carried in the faeces of the bug and can infect a person when scratching the skin around a bite or from touching a contaminated surface and then the eyes, mouth, skin break or food.

Vector control for triatomine bugs includes residual insecticide spraying of walls in homes, home improvement to remove cracks in walls and floors, replacing thatch with corrugated iron roofing, bednets and general hygiene practices.

Tsetse fly

Glossina species.

Disease: African trypanosomiasis/ sleeping sickness, caused by two subspecies of the protozoan Trypanosoma brucei: T. b. gambiense and T. b. rhodesiense.

As the name of the disease suggests, it mainly occurs in Africa, between 15°N and 20° S. The subspecies T. b. gambiense is mainly caught in areas with dense vegetation near rivers and lakes. The parasite is carried from human to human by the tsetse fly.

For T. b. Rhodesiense, however, the reservoir of the parasite is wild animals and cattle in savannah and forested areas. It causes a more acute infection but occurs more sporadically due to the habitats where it occurs being more remote. This subspecies is also a cause of serious disease in cattle.

The disease was virtually eliminated by the 1960s but vector control and surveillance programmes lapsed and it became an epidemic again in several places in the 1970s.

Vector control techniques for sandflies include the sequential aerosol spraying technique (SAT); ground spraying; insecticide-treated targets or live animals; baited traps or screens, and the sterile insect technique (SIT).

In 2015 WHO announced the lowest number of new cases in 75 years (below 4,000) and that it was on track to eliminate the disease by 2020.

Snails

Freshwater snails in the tropics and subtropics: Biomphalaria, Bulinus, Oncomelania and Neotricula species.

Disease: Schistosomiasis, trematode worm, primarily Schistosoma mansoni, S. japonicum, and S. haematobium. Other species infect humans in localised areas.

The snails live in shallow areas of freshwater bodies, ranging in size from ponds and streams to lakes and rivers. They are more abundant where there are water plants and organic matter present, such as from human sewage.

Schistosomiasis has been documented in 78 countries and is prevalent in 52 (WHO, 2014), infecting about 250 million people in 2012. The larvae of the trematode worm live in freshwater and penetrate the skin of people spending a long time in water, such as for bathing, fishing, farming rice and swimming. The adults live in blood vessels of the host and lay eggs that are expelled in faeces and urine. If they reach fresh water, the eggs hatch to release the larvae. These have to penetrate snails and undergo phases of development before being released into the water again, then find a human or animal to complete the cycle.

Infections are spread through poor sanitation, building of dams and irrigation projects, and migration of populations. It is thought that the parasite was introduced into the Americas by the slave trade.

The main strategy for controlling Schistosomiasis is by providing at-risk populations with anthelmintic drugs, improved sanitation, hygiene education and snail control.

Dogs and other mammals

Dogs, bats, other carnivorous mammals.

Disease: Rabies virus.

Rabies is present in over 150 countries and causes an estimated 59,000 deaths a year. However, it is believed to be greatly under-reported (Hampson, 2015) as most cases occur in poor communities in developing countries. Around 99% of cases are caused by dog bites or saliva coming into contact with broken skin or mucosa (eye, nose, mouth). In the Americas bats are also significant vectors. Rabies is almost always fatal and has no diagnostic tests to determine infection before symptoms show.

The disease usually takes 1-3 months to incubate but it can vary from one week to one year before symptoms start to show. Symptoms start as fever and pain, then after the virus progresses through the central nervous system it causes fatal inflammation of the brain and spinal cord. People can also show hydrophobia (fear of water) or aerophobia (fear of flying).

There is a safe vaccine for both humans and dogs and if immediate treatment is given following a bite the disease can be avoided. Post-exposure prophylaxis (PEP) involves washing the wound with soap and water for at least 15 minutes, application of povidone iodine (or equivalent) to kill the virus and vaccination.

Bibliography

Centers for Disease Control and Prevention. A–Z Index. (link)

CDC, Division of Vector Borne Diseases. (link accessed 7 Oct 2016)

Hampson K, et al. Estimating the Global Burden of Endemic Canine Rabies. PLOS Neglected Tropical Diseases, April 16, 2015. (link)

PAHO, WHO. 2003 a. Zoonoses and communicable diseases common to man and animals. VOl I. Bacterioses and Mycoses. Scientific and Technical Publication No 580. PAHO/ WHO, Washington.

PAHO, WHO. 2003 b. Zoonoses and communicable diseases common to man and animals. VOl II. Chlamydioses, Rickettsioses and Viroses. Scientific and Technical Publication No 580. PAHO/ WHO, Washington.

PAHO, WHO. 2003 c. Zoonoses and communicable diseases common to man and animals. Vol III. Parasitoses. Scientific and Technical Publication No 580. PAHO/ WHO, Washington.]

WHO. 2008. Public health significance of urban pests. WHO, Copenhagen.

WHO. 2014. A global brief on vector-borne diseases. WHO, Geneva.

WHO. 2016. Vector surveillance and control at ports, airports and ground crossings. WHO, Geneva.

WHO. Rabies fact sheet. (link accessed 14 Oct 2016)