Insect-Borne Diseases

Plague

Insect: Oriental Rat Flea
Organism: bacteria, Yersinia pestis

The plague bacteria can be transferred to humans by several routes:

• flea bites;
• handling contaminated animals or their body parts and fluids, including contaminated soil and surfaces;
• infectious air-borne particles, from infected people coughing or infected products.

The rat was the main carrier of the disease in the well-known historical epidemics in the middle ages that killed millions, when it was transported along land and sea trade routes. In current times the plague is present in low rates in a wide variety of small mammals and their predators in parts of Asia, Africa, South America and the US.

Three types of the plague are recognised, according to how the infection is caught:

• bubonic plague: the most common sign is swollen and painful lymph nodes (buboes) where the bacteria multiply and can spread from if not treated. There is also sudden onset of fever conditions and extreme weakness.
• septicemic plague: fever conditions, extreme weakness, diarrhoea, delirium, abdominal pain, shock and bleeding in the skin and other organs. The skin and other tissues can turn black and die especially on fingers, toes and the nose.
• pneumonic plague: fever conditions, shock and pneumonia, causing breathing difficulty, chest pain, cough and bloody mucous.

The plague is treatable with antibiotics. In bubonic plague death can occur in less than two weeks, in septicemic plague death can occur before symptoms appear, and in pneumonic plague all untreated patients die

Dengue

Insects: Mosquitoes
Organism: Dengue virus

Dengue is the most important mosquito-borne viral disease in the world, according to WHO, which has targeted it as one of 17 Neglected Tropical Diseases for major campaigns for awareness and eradication. It is endemic in over 100 countries across the tropics, from central and South America, Africa, South and Southeast Asia and to the Pacific Islands. It has increased rapidly over the last few decades in urban sprawls that provide ideal conditions for breeding. The WHO estimates that there are 50-100 million infections a year and half the world’s population live in countries where it is endemic.

The threat of Dengue is greater than the current outbreaks as the mosquito vectors are present in a far wider geographic range, and the Tiger Mosquito Aedes albopictus, especially is widening its range. New cases were reported in Croatia, France, Madeira Islands, Florida (USA) and Yunnan (China) in the last few years.

The majority of cases have no symptoms or mild fever, while about 5% have severe illness that shows as sudden fever 3-14 days after infection, with headache, muscle pain, joint pain, and a body rash. There is no vaccine and no specific treatment.

West Nile Fever

Insect: Mosquito.
Organism: Flavivirus, West Nile virus.

The disease was virtually unknown outside Uganda, where it was first identified in 1937, until the 1990s, when there was an outbreak in Algeria. It is now present in all temperate and tropical continents, with even the US experiencing an outbreak in 2012 that killed 286 people. The European Centre for Disease Prevention and Control monitors infections in Europe and in 2015 reported eight human cases of WNF in EU Member States up to August: Austria (1), Italy (4), Bulgaria (1), Romania (1) and Austria (1). Eight cases were detected in neighbouring countries: Israel (7) and Serbia (1). It even produces an interactive map of cases.

Around 80% of people infected do not show any symptoms. In the remainder, symptoms appear in 2-15 days, including fever, headache, body aches, joint pains, vomiting, diarrhoea or rash. Most people recover completely, but weakness can last for weeks or months. A small number of patients develop encephalitis, meningitis or poliomyelitis. There is no specific treatment and no vaccine.

Lyme disease

Vectors: deer tick
Organism: bacteria, Borrelia spp, Borrelia burgdorferi sensu lato

Lyme disease is the most common tick-borne disease in the northern hemisphere, maintained by many species of mammal that the ticks feed on becoming reservoirs of the disease. Infections are reported to be 7.9 cases per 100,000 population in the USA (WHO) and there are over 6,000 cases reported in England and Wales annually. 

Symptoms of Lyme disease include: a pink or red rash which in most cases has a central red spot and outer red circle — like a ‘bull’s eye’; temperature of 38°C/ 100.4°F or above; flu like symptoms such as headache and joint pain; swollen lymph nodes. It can be treated with antibiotics, but if untreated, symptoms can last for years, including arthritis, numbness, paralysis and around 10% of people infected develop central nervous system disease.

Asthma

Insect: House dust mites & Cockroaches.
Organism: none known (allergic response).

Asthma is not caused by an infectious organism but by an allergic reaction to particles shed by organisms, mainly house dust mites and cockroaches. So it does not strictly come under vector-borne disease.

It is, however, of great significance to the health and economies of developed countries. It is one of the most common chronic diseases of children and in the US is the leading cause of hospital admissions for children. The WHO estimates that 300m people have asthma worldwide, ranging from 1% in rural Africa to 7-20% in Europe and 25-40% in some cities of the US and Australia.

The main cause is house dust mites, which are arachnids less than one millimetre long that feed mainly on human skin scales. Pollen, spores of microorganisms, fungal mycelia and bacteria have also been found in the gut of house dust mites. They live in the small spaces provided by mattresses, carpets, duvets, pillows and furniture materials, whether natural or synthetic, as both provide ideal shelter.

The mites depend on a suitable relative humidity and temperature for survival, so heating, ventilation and moisture control in places that they inhabit affects their numbers. Optimal conditions are 25°C and 75% relative humidity, which allows development from egg to adult in about 25 days. D. farina can survive low humidity conditions by forming a ‘protonymph’ stage that is drought resistant. It is the most common species in regions with prolonged dry weather such as the US. In areas with higher humidity D. pteronyssinus and E. maynei are the most common.

The German cockroach is the main cockroach pest species, inhabiting human buildings on all continents except Antarctica, but needing the building (or ship) environment to survive in cold climates. It occurs especially in restaurants, hotels, hospitals, nursing homes and food processing facilities. Many studies have shown that exposure to cockroach allergens is associated with asthma. In some studies in the US a high proportion of asthmatics had cockroach sensitivity, especially in urban areas, showing the prevalence of the pest even in the US.

Amoebic dysentery

Insects: cockroaches, flies
Organism: protozoan, Entamoeba histolytica

Insects are vectors of amoebic dysentery by mechanical transmission of the protozoa from direct contact with human faeces or contaminated products and surfaces. It is estimated that 50 million people are infected worldwide, but the majority of cases are asymptomatic, with only 10% showing disease symptoms.

The disease causes bloody diarrhoea, weight loss, fatigue and abdominal pain. The organism can invade the intestinal wall, producing ulceration. It can then pass into the blood stream and enter other organs, particularly the liver.

Babesiosis

Insect: ticks.
Organism: protozoan, Babesia spp.

Babesiosis is an uncommon but emerging disease caused by several species of Babesia protozoa. The main vectors are ticks of the genus Ixodes, but various species of voles and mice or deer are required as intermediate hosts to complete the lifecycle.

It has long been known to affect cattle and was only identified in humans in the 1950s. It occurs mainly in north-eastern US and temperate regions of Europe. In most cases there are no symptoms or mild flu-like symptoms, but in people with weakened immune systems it can become more severe or even fatal.

Chikunguya fever

Insect: mosquitoes
Organism: virus, Chikunguya virus.

Chikunguya fever occurs across a broad band of the tropics from the Atlantic coast of Africa to Papua New Guinea in the western Pacific. Cases have also been reported in South and Central America, US, southern Europe.

The main symptoms are fever, severe joint pain for several weeks, skin rash, muscle pain, headache, nausea, fatigue. Most patients recover but the joint pain may last for months or years. There is no vaccine or specific treatment. It is similar to Dengue fever for the vectors, symptoms, apart from joint pain, and geographical area.

Crimean-Congo haemorrhagic fever

Insect: Tick
Organism: virus, Nairovirus

The virus is transmitted to humans either by a tick bite or by coming into contact with blood or tissue of an infected animal, which can be cattle, sheep or goats. The tick Hyalomma marginatum is widespread across North Africa and Asia and is present in southern and Eastern Europe. It is commonly spread via migrating birds and livestock.

The virus is endemic in Africa, Middle East, the Balkans, west and south Asia and is considered an emerging pathogen in Europe, where there has been both new cases in several countries and the detection of virus antibodies in other countries, according to the European Centre for Disease Prevention and Control.

Symptoms include fever, muscle ache, dizziness, neck pain, backache, headache, sore eyes, sensitivity to light, vomiting, diarrhoea. The fatality rate is 5-40% and there is no validated therapy and no safe vaccine.

Epidemic typhus

Insect: Louse.
Organism: bacteria, Rickettsia prowazekii

As with other louse-borne fevers, typhus tends to occur in conditions of overcrowding and poor hygiene such as refugee camps and prisons. It occurs mainly in central and eastern Africa, central and South America, and Asia. Recent outbreaks have occurred in Burundi, Ethiopia and Rwanda. Infection occurs from crushing the lice or rubbing the skin where the lice are feeding and defecating and carrying the bacteria to wounds and mucous membranes. Some cases in the US have been associated with flying squirrels nesting in houses over winter, but the means of infection is still unknown as the patients did not have lice.

The symptoms can include high fever, headache, severe muscular pain and after 5-6 days a rash of dark spots.

Japanese encephalitis

Insect: mosquito
Organism: flavivirus, Japanese encephalitis virus.

Japanese encephalitis occurs mainly in rural areas of East, Southeast and South Asia. Infection rates are associated with the rainy season and flooding of paddy fields. The main hosts of the virus are pigs and wading birds, with humans as incidental hosts that rarely pass on the disease through further bites. It is closely related to West Nile virus and St. Louis encephalitis virus.

The majority of infections do not show any symptoms, but in a few cases, especially children, the virus invades the brain and high fever symptoms and headache start showing after 5-15 days. This develops into coma, tremors and convulsions. There is no specific treatment but vaccines are available.

Leishmaniasis

Insect: Sandflies
Organism: protozoa, Leishmania spp

Leishmaniasis is spread by bites from female sandflies. These are tiny flies 1.5-3.0 mm long with large black eyes and hairy bodies, wings and legs. They breed in forest areas, caves and adobe brick houses where most of the infection of humans takes place. Rodents, dogs and other mammals are reservoirs for the disease.

There are more than 20 species of the protozoan that produce several forms of the disease: cutaneous, mucosal and visceral, causing variously, skin sores, chronic ulcers, mucosal infections, and infections of the spleen, liver, bone marrow and lymph nodes. The disease occurs in many countries in tropical and subtropical regions, with the different forms more prevalent in certain countries, although Brazil is a major source of all three forms.

WHO has estimated that 310 million people are at risk from the disease. There are 300,000 cases of visceral leishmaniasis and over 20,000 deaths a year, while 1 million cases of the cutaneous form were reported over a five-year period (to 2012).

The bite leaves a non-swollen red ring and symptoms of infection are fever and anaemia.

Murine typhus/ endemic typhus

Insect: Fleas
Organism: bacteria, Rickettsia typhi

The disease occurs worldwide, transmitted by fleas that infest rats and less frequently by mice, cats and opossums. It is more common in rat infested buildings and living areas such as refugee camps.

The infection is passed on by contact with the faeces of the flea, through skin wounds or rubbing hands on infected areas then passing it onto mucous membranes and the mouth.

Symptoms are similar to the louse-borne epidemic typhus, but it lasts for a shorter time and is less severe. They include a rash six days after infection, headache, fever, muscle and joint pain, nausea, vomiting. The symptoms may resemble measles or rubella. Murine typhus is treatable with antibiotics.

Pappataci fever

Insect: Sandfly
Organism: virus, Phlebovirus serotypes

There are three serotypes of the virus: Toscana, Sicilian and Naples. It occurs in a band around the Mediterranean, across the Middle East to northern India and southwest China. Symptoms appear a few days after infection: fever, severe frontal headache and muscle and joint ache, rapid heart rate and facial flushing, subsiding after two days. It is also known as three-day fever, phlebotomus fever and sandfly fever. There is no specific treatment.

Rickettsial fevers

Insect: fleas, ticks
Organism: bacteria, Rickettsia species

Rickettsia is a group of primitive bacteria that live only within animal cells and rely on host cell biochemical processes to survive. They are carried by many insects and arachnids, such as ticks, fleas, lice and mites.

Rickettsia species are responsible for a number of diseases, including the groups of typhus and spotted fevers that occur in different geographical areas with their associated species, for example:

• Epidemic typhus (Rickettsia prowazekii) — worldwide;
• Murine typhus (Rickettsia typhi) — worldwide;
• Rocky Mountain spotted fever (Rickettsia rickettsii) — western hemisphere;
• African tick bite fever (Rickettsia africae) — South Africa;
• Siberian tick typhus (Rickettsia sibirica) — Siberia, Mongolia, northern China;
• Boutonneuse fever (Rickettsia conorii) — Mediterranean countries, Africa, Southwest Asia, India;
• Australian tick typhus (Rickettsia australis) — Australia;
• Flea-borne spotted fever (Rickettsia felis) — North and South America, southern Europe, Australia;
• Oriental spotted fever (Rickettsia japonica) — Japan.
• Rickettsialpox (Rickettsia akari) — USA, Russia, Ukraine, South Africa, Korea, the Balkan states.

The more important diseases are covered under their individual headings.

Rickettsialpox

Insect: house-mouse mites.
Organism: bacteria, Rickettsia akari.

Human infection is most likely during natural die-offs of mice or after pest control when the mites seek new hosts for a blood meal. Infection is transmitted by the bite of the mite. It has been reported in urban areas of Ukraine, Russia, South Africa, Korea, Croatia, France and the US.

Rickettsialpox is regarded as a mild disease that takes 2-3 weeks to recover from. The first symptom is a bump around the bite that appears about a week after the bite, which turns into black crusty scab. A few days following this, a fever may occur, similar to flu, with a rash that covers the body.

Salmonellosis

Insect: house flies, blow flies, flesh flies, cockroaches
Organism: bacteria, Salmonella enterica

Salmonella bacteria can be carried by flies and cockroaches that inhabit ‘filth’ by mechanical transmission after contact with infected faeces from humans or animals and contaminated products.

In the US alone it is estimated to cause one million infections, 19,000 hospitalisations and nearly 400 deaths a year, according to CDC.

The Salmonella species of bacteria has a complex classification, having six subspecies which are further classified into 2500 serovars, a few of which are important causes of disease in humans.

Salmonellosis is caught mainly from contaminated water, foods, especially raw poultry, minced beef, and raw eggs. Poor hygiene in vegetable and fruit harvesting, and pets, especially reptiles, baby chicks and ducklings are also sources of infection. Avoidance of contact, general hygiene and hand hygiene are the most important means of prevention.

Symptoms include diarrhoea, fever, vomiting, and abdominal cramps. Most people recover in a few days without treatment other than replacement of fluid lost by the body. Once a person is infected however, the disease is easily transmitted to other people through poor hand hygiene and poor sanitation.

Typhoid fever
One serovar of Salmonella (Salmonella enterica subsp. enterica serovar Typhi) causes more severe infection and spreads from the intestines to the blood and lymphatic system and then to other body sites. It is endemic in many developing countries, where it affects around 27 million people a year, especially in India and in children. It only occurs in humans and is spread through faecal contamination, therefore poor hygiene plays a major role in maintaining its presence. It is treatable with antibiotics and a vaccine is available.

Tick-borne encephalitis

Insect: Ticks
Organism: virus: Flavivirus

Three types of the disease are recognised: European/Western, Far Eastern and Siberian. It is present in a band stretching across the Eurasian continent, from Italy in southern Europe to Finland in the north, through southern Russia and bordering countries to northern China and Korea in the Far East. Around 5-7,000 cases are reported annually in Europe and Russia.

Symptoms include influenza-like symptoms, high fever, severe headache, nausea, vomiting and back pain. The disease infects the central nervous system in about 30% of cases, which can result in paralysis and in 1-2% of cases death.

Bibliography

Wikipedia: www.wikipedia.com

US Centers for Disease Control: http://www.cdc.gov/

WHO. Public Health Significance of Urban Pests. WHO, Copenhagen, 2008.

European Centre for Disease Prevention and Control: http://ecdc.europa.eu/

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

Cases of sleeping sickness drop to lowest level in 75 years http://www.who.int/trypanosomiasis_african/cases_drop_to_lowest_since_75_years/en/

WHO. Investing to overcome the global impact of neglected tropical diseases: third WHO report on neglected diseases 2015. WHO, Geneva.

WHO. Global strategy for dengue prevention and control 2012-2020. WHO. Geneva, 2012

The European Food Safety Authority http://www.efsa.europa.eu/

Graczyk TK, Knight R and Tamang L. Mechanical Transmission of Human Protozoan Parasites by Insects. Clin Microbiol Rev. 2005 Jan; 18(1): 128–132. doi: 10.1128/CMR.18.1.128-132.2005