On Mosquitoes, Fleas, Sandflies and Ticks. Who Will Bite us and Give us What Disease?

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We know them all too well. While their bites can cause allergic reactions, the microorganisms they carry have caused pandemics. The bubonic plague or the “black death” is perhaps the most infamous pandemic known to humanity, resulting in widespread illness along Eurasian trade routes in the 14th century and leading to about 200 million deaths1. We still come across these carriers of pathogens – also called vectors – relatively frequently in everyday life. While they are seasonal in some regions, they are perennial in others. With improvements in healthcare, sanitation and public awareness, the infectious diseases that are transmitted through mosquitoes, fleas, sandflies, and ticks diminished to a large extent by the 20th century. Nevertheless, these vectors are resilient; and hence, the diseases that they can transmit still pose a threat to human life and well-being. In 2018, 405,000 people died from malaria alone, and most of the victims were children2. The number of cases of Lyme disease too is increasing each year3. It is hence important to learn about these vectors and how we can protect ourselves from vector-borne illnesses.

Mosquitoes

What they are:

Mosquitoes are arthropods of the Culicidae family of insects that can grow up to 20mm in length, although the ones we commonly encounter are under 10mm. They are best known for their ability to transmit diseases while biting and extracting blood from mammals, reptiles, birds, and amphibians in order to nourish their eggs. Female mosquitoes alone have the mouth parts necessary to bite us; in fact, both female and male mosquitoes consume plant sugars for food4. Their lifespan ranges from two weeks to six months.

How they make us sick:

Mosquitoes are the preferred mode of travel for several bacterial, viral and other parasitic microorganisms. Although the bites themselves can cause allergic skin reactions such as reddening and itchiness, mosquitoes are cause for concern primarily because they can carry potentially deadly pathogens.

Female anopheles albimanus mosquito

Photo: Female Anopheles albimanus mosquito, a vector for malaria.

Among the thousands of strains of mosquitoes, three are most clinically significant – anopheles, culex and aedes mosquitoes5. The anopheles mosquitoes can be distinguished by the 45° angle between their bodies and the surface on which they rest; they are the primary vectors for the Plasmodium parasites, which spread malaria. They can also spread encephalitis and elephantiasis. Culex mosquitoes keep their bodies parallel to their resting surface. They too spread Japanese encephalitis and elephantiasis, together with the West Nile fever. Aedes mosquitoes are the easiest to identify, since they usually have distinctive black and white patches on their bodies. The two major sub-types that carry human diseases are the Asian tiger mosquito (Aedes albopictus) and the yellow fever mosquito (Aedes aegypti). They are the primary vectors of the viruses that cause the Zika fever, yellow fever, dengue fever, and chikungunya6. Many mosquito-borne illnesses are characterized by fever, body pain and nausea, although the exact symptom varies between diseases. The Zika virus often manifests without early symptoms.

Where we find them: 

Mosquitoes have been located everywhere except Antarctica. They prefer warmer weather and breed in standing water. They do not thrive in harsh winters or desert summers. Thus, while mosquitoes are restricted to the warmer seasons in sub-tropical regions, they are present year-round in regions near the equator.

What are our best defences against them?

While there are vaccines against several mosquito-borne illnesses such as Japanese encephalitis and yellow fever, there are no vaccines for others such as chikungunya and the Zika virus. Preventive malarial drugs recommended for travelers to infected areas, pregnant women, and children. Another effective way to prevent disease transmission is vector control. This can be done using insecticide-treated mosquito nets, indoor spraying with insecticides and adding larvicides to standing water. Other preventive steps too can be effective – such covering open drains and preventing water from collecting in empty paint cans or buckets (since these can be breeding ground for mosquitoes), wearing protective clothing while outside, staying indoors during peak mosquito activity, using mosquito nets on windows and doors, and using insect repellent coils, vaporizers and sprays. Further, since the Zika virus can also be transmitted via sexual intercourse, those who are likely to be exposed to the Zika virus should practice safe sex.

Fleas

What they are: 

Fleas are ectoparasites (i.e., they live outside the bodies of their hosts) belonging to the order Siphonaptera. Fleas are wingless insects 1-8 mm long and are dark brown in colour. Adult fleas feed on blood from their host. Like mosquitoes, fleas too are vectors of pathogenic micro-organisms, primarily bacteria. They are most notorious for having carried the bacteria Yersinia Pestis which caused the bubonic plague pandemic1.

How they make us sick: 

Humans are not the fleas’ preferred host. When they do bite humans, they are most commonly appear as a series or clusters of small red spots in the legs, especially near the ankle. This is different from the bites of mosquito or bedbugs, since the latter cause larger bumps on the skin.

Oriental rat flea

Photo: Oriental rat flea, a vector for plague.

In cats and dogs, fleas can cause a severe allergic reaction on the skin called the flea allergy dermatitis. Fleas usually travel via their hosts – for instance, via rats in the case of the bubonic plague. In the case of bubonic plague, infected fleas transferred the pathogenic bacteria to humans, and these bacteria consequently attacked the lymphatic systems, causing buboes or swollen lymph nodes. Other diseases that can be spread via fleas include flea-borne spotted fever, cat scratch disease, and murine typhus7. In addition to pathogens, fleas also spread tapeworms between dogs.

Where we find them: 

Fleas have a global distribution, and use mammals and birds as hosts, but can survive without a host for up to 100 days8. Like mosquitoes, fleas too prefer warmer weather, although they can survive winters if they are attached to a suitable host. These days, household pets are the easiest ways for fleas to enter our homes. At least 15 species of fleas have been reported in domestic dogs alone. Flea-borne illnesses are relatively rare in most countries these days; however, plague is endemic to certain parts of Africa and South America, the largest cases having been reported in Madagascar, the Democratic Republic of Congo, and Peru.

What are our best defences against them? 

The most important step to keep flea-borne illnesses at bay is by not attracting the hosts (rats) and carriers (fleas). To accomplish this, it is recommended that we (1) rodent-proof our houses by removing piles of junk, firewood, etc. and keeping foodstuff closed in airtight containers, (2) keep pets free of fleas by regular cleaning and using veterinarian-approved flea control products such as flea collars and medication, (3) use gloves while handling potentially infected animals and (4) use insect repellents while hiking and camping. Vaccines are currently not available for flea-borne illnesses. When infected, flea-borne diseases can be managed using antibiotics, but in the case of cat scratch disease, usually does not require treatment. It is to be noted that even when they do not carry pathogens, flea bites can lead to secondary bacterial infection. It is hence important to not scratch flea bites and to monitor the wound.

Sandflies

What they are: 

Sandflies or Phlebotominae, belong to the the Psychodidae family. Sandflies are akin to mosquitoes in some ways, although they are much smaller (< 3.5mm in length). For instance, though both male and female sandflies depend on plant juices for food, the female sandflies feed on the blood of vertebrate hosts to develop their eggs9. Further, they act as vectors transmitting pathogenic parasites from host to host. Sandflies can be identified by the v-shaped wings on their backs held above their bodies10 .

How they make us sick: 

Sandfly bites are painful and can cause red blisters, leading to inflammation, secondary infections, or dermatitis. More critically, the bites can also transmit protozoan parasites called Leishmania, which can cause a serious tropical disease called leishmaniasis.

Phlebotomus papatasi sandfly

Photo: Phlebotomus papatasi sandfly, a vector for leishmaniasis.

In severe cases, leishmaniasis affects the visceral organs, causing fever, a low concentration of red blood cells and an enlarged liver and spleen. Milder cases are cutaneous or mucocutaneous, resulting in ulcers on the skin, mouth, and nose. Malnourishment, especially with regards to proteins, vitamin A, and zinc deficiencies can increase the severity of disease.

Where we find them: 

Although they can survive in sandy deserts, sandflies are typically found in animal burrows, tree holes, caves and rocks. They are nocturnal and can only fly short distances of less than 300m (although in the desert, they have been reported to travel up to 2300m). The larvae develop in humid, organic environment such as the soil or animal excreta, and not standing water, unlike mosquitoes. Over 500 species of sandflies exist, and about a dozen of these are known to carry pathogens that can infect humans. Sandfly population has also been reported to be affected by climate change, which in turn affects humidity and rainfall patterns. In line with this, leishmaniasis has been linked to environmental changes such as deforestation, building of dams, or urbanization. Most cases of leishmaniasis have been reported in South America, eastern Africa, and south and south-west Asia11.

What are our best defences against them? 

There are no vaccines for leishmaniasis, and hence, the best methods are to control sandflies using physical barriers (nets and clothing) and chemical barriers (e.g. insecticides). For instance, the chemical dichloro-diphenyl-trichloroethane, or DDT, was used on a large scale in countries such as India, China, the Soviet Union, and Brazil to eliminate sandflies. It is also important to be aware of sandfly prevalence and feeding habits in an area. Although cutaneous leishmaniasis can heal spontaneously, getting medical help is highly recommended owing to the risk of infections.

Ticks

What they are:

Unlike the other three, ticks are not insects, but rather arachnids belonging to the family of spiders; however, similar to the other three, they too feed on the blood of animals including humans12. Typically, adult ticks are the size of a sesame seed. Ticks can be distinguished from other arthropods by the presence of four pairs of legs and the absence of clear segmentation of their body characteristic of insects.

How they make us sick:

Like insect bites, tick bites too result in itchiness. However, the ticks do not bite us immediately on climbing atop our body. They may take hours to explore, and usually latch on to hidden spots such as behind the ears, in the armpits, at the hairline or under folds of skin. They automatically fall off once satiated; however, this process could take two weeks. Once detected, ticks need to be quickly removed, since they can spread the dreaded Lyme disease via the bacterium Borrelia burgdorferi (also, rarely the bacterium Borrelia mayonii)13. Typical symptoms of the Lyme disease include fever, headache, fatigue, and characteristic “bull’s eye” skin rashes called erythema migrans. In severe cases, the infection can spread to the joints, heart, and the nervous system. Ticks also carry other bacterial pathogens causing borreliosis, rickettsial diseases, and tularemia, as well as viral pathogens causing the Crimean-Congo haemorrhagic fever virus and tick-borne encephalitis14.

 

Photo: Ixodes hexagonus or the hedgehog tick, a potential vector for Lyme disease.

Where we find them:

Ticks are usually active from March to November. They spend winters underground but become active when the air gets warmer than 8°C (~46°F)12.  They prefer warm and moist environments, and thrive in forests, meadows, parks and gardens. They usually latch onto the skin of humans and animals when we brush against them amongst shrubs or tall grass. Most reports of the Lyme disease come from the northeastern United States, Europe, and Asia.

What are our best defences against them?

The best defenses against ticks are preventive – (1) the use of insect repellents and insecticides such as permithrin on clothes and gears and (2) awareness of the prevalence of ticks in an area. It is also important to examine our bodies (especially hidden spots such as under the arms, behind the ears, belly button etc.), clothing, gears and pets for ticks soon after returning from being outdoors. Ticks should be removed carefully using tweezers and disposed of appropriately. Although a vaccine for the Lyme disease was available in the 1990s, it is no longer manufactured due to concerns over side effects. Currently, new vaccines are under development15.

References:

  1. De-coding the Black Death. BBC News (2001).
  2. CDC. Malaria’s Impact Worldwide. (2020).
  3. CDC. Concern about Lyme disease | CDC. Centers for Disease Control and Prevention (2017).
  4. Mosquitoes | National Geographic. Animals (2010).
  5. Seda, J. & Horrall, S. Mosquito Bites. in StatPearls (StatPearls Publishing, 2020).
  6. Mosquito factsheets. European Centre for Disease Prevention and Control.
  7. Dobler, G. & Pfeffer, M. Fleas as parasites of the family Canidae. Parasites & Vectors 4, (2011).
  8. Fleabites: Symptoms, causes, risks, and treatment. (2017).
  9. Claborn, D. M. The Biology and Control of Leishmaniasis Vectors. J Glob Infect Dis 2, 127–134 (2010).
  10. Fly Bites: Symptoms and Treatments. Healthline (2017).
  11. Leishmaniasis. WHO.
  12. What are ticks and how can they be removed? – Tick bites – informedhealth.org.
  13. CDC. Lyme disease home | CDC. Centers for Disease Control and Prevention (2019).
  14. Vector-borne diseases. WHO.
  15. Are We Close to a Lyme Disease Vaccine? Healthline (2019).

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