Insect-borne illness and disease

• ALSO KNOWN AS: Arthropod-borne illness and disease, vector-borne illness and disease

Definition

An insect-borne illness or disease is a disease transmitted by an insect that carries bacteria or viruses from one host to another. Many types of insects can extract bacteria and viruses from infected birds, animals, and humans while feeding on blood. Pathogens are transmitted when these insects bite the animal or human, causing illness, disease, and sometimes death. Several of these diseases are found worldwide and others are regional.

Mosquito-Borne Diseases

The majority of insect-borne diseases and illnesses are transmitted by mosquitoes. As a group, these diseases lead to over one million deaths each year, with infants, children, and pregnant women showing particular vulnerability to illness and mortality. Controlling mosquito populations and avoiding mosquito bites are the two most important ways to avoid these diseases. The main preventive mechanisms include eliminating standing water, using larvicidal chemicals or bacteria in standing water that cannot be drained (such as rice paddies), wearing protective clothing, avoiding being outdoors during times when mosquitoes are prevalent, and having secure screens on all windows and doors. Additional protective measures include spraying pesticides in the home (if necessary), soaking bed nets in pyrethroid pesticide, and using a repellant containing N,N-diethyl-meta-toluamide (DEET). Insects developing a resistance to pesticides and pathogens becoming resistant to medications pose significant challenges in treating and preventing insect-borne diseases.

For many mosquito-borne diseases, there is a cycle between birds and mammals that involves the mosquito picking up the virus from a bird or mammal and transmitting it to a human. For some of these illnesses, vaccines or prophylactic drugs are available, but for many others, the search for preventive medication remains.

One particularly severe and widespread mosquito-borne disease is malaria, which is caused by four parasite species in the genus Plasmodium: falciparum, vivax, malariae, and ovale. P. falciparum is responsible for the most deaths, particularly in Africa. P. vivax is responsible for most cases of relapsing malaria, which stays dormant in the liver during treatment and reinfects the individual at a later time. P. ovale and P. malariae also cause malaria, but the symptoms are generally less severe and do not reoccur. In the early and mid-2020s, about half the world’s population was at risk for malaria. Plasmodia are transmitted by the Anopheles mosquito, and the incubation period lasts from seven to thirty days, depending on the Plasmodium species transmitted. Symptoms of malaria include shivering, fever, headache, vomiting, and sweating. Severe malaria can involve such symptoms as impaired consciousness, seizures, coma, anemia, pulmonary edema, and cardiovascular collapse. Drug treatments for malaria include Artemether-lumefantrine (Coartem), Atovaquone-proguanil (Malarone), Quinine sulfate (with doxycycline, tetracycline, or clindamycin), Primaquine, Clindamycin, and Hydroxychloroquine. Certain pathogens have developed resistance to some malaria treatments, such as Chloroquine phosphate (resistance nearly globally), Mefloquine (resistance primarily in Cambodia, Thailand, and Vietnam), and Sulfadoxine/pyrimethamine (widespread resistance; no longer used).

Preventing malaria infection is a top priority for many health and research organizations, which are trying to establish vaccines and better treatments. In areas such as sub-Saharan Africa, where malaria is hyperendemic, anti-malaria therapy is recommended for pregnant women and children because of their increased susceptibility; however, this therapy decreases the chance of becoming infected only and does not ensure immunity. Between 2000 and 2021, the World Health Organization expanded its recommendations and tools addressing malaria infection and deemed indoor residual spraying of homes to be a major mechanism for controlling malaria. Bed nets soaked in pesticides are also recommended to reduce exposure to mosquito bites, as is the use of a DEET-containing repellant.

Another mosquito-borne illness is dengue fever, a viral infection that occurs in approximately one hundred million people each year, leading to thousands of deaths. Dengue fever is endemic to Southeast Asia and Latin America (including Puerto Rico) and is rarely found in the United States. It is most commonly transmitted by the mosquito Aedes aegypti. There are four viruses from the Flaviviridae family (DENV 1, 2, 3, and 4) that cause dengue, which has a three-to-eight-day incubation period. Symptoms of dengue include high fever, rash, nosebleeds, and pain in the muscles, joints, and bones. Most people recover from these symptoms after seven to ten days. The more severe form, dengue hemorrhagic fever (DHF), has additional symptoms of vomiting, abdominal pain, and the failure of the circulatory system when the capillaries become too permeable. No vaccine exists for dengue fever. Reducing exposure to mosquito bites is the recommended prevention.

West Nile virus (of the Flaviviridae family) infects individuals in Europe, North America, Africa, Australia, and Asia. Most people (80 percent) who are infected with West Nile virus do not experience any symptoms. About 20 percent of those infected have symptoms such as fever, aches, swollen lymph nodes, a rash, and vomiting, and the incubation period is two to fourteen days after being bitten. Less than 1 out of 150 people infected suffer from the most severe symptoms, which include vision loss, disorientation, convulsions, and coma; these symptoms are caused by encephalitis (brain swelling) and can lead to death.

Yellow fever virus (of the Flaviviridae family) is chiefly transmitted by the mosquitoes A. aegypti and A. africanus. Yellow fever is present in many countries of South America and sub-Saharan Africa, regions with an estimated six hundred million people. The incubation period is three to six days. Mild cases involve fever, aches, and muscle weakness, which typically dissipate within forty-eight hours. Severe cases result in hemorrhagic symptoms such as black vomit, nosebleeds, and bleeding gums. The disease gets its name from the skin-yellowing jaundice caused by liver failure in severe cases, as cellular necrosis occurs in the liver. There is a vaccine available for yellow fever; however, it continues to carry a significant disease burden, especially in impoverished areas. During epidemics, the fatality rate for those infected is 15 to 50 percent.

Japanese encephalitis virus is also a member of the Flaviviridae family and is typically carried by the mosquito Culex tritaeniorhynchus. This illness is found primarily in Asia and the western Pacific and affects 100,000 people each year, causing around 25,000 deaths. Outbreaks generally occur in rural areas. The incubation period is generally five to fifteen days. The disease involves acute-onset encephalitis and other manifestations, including seizures, paralysis, coma, and death. The fatality rate ranges widely, between 0.3 and 60 percent. Vaccines are available but are quite costly and come with adverse side effects.

St. Louis encephalitis is a member of the Flaviviridae family and is transmitted mainly by the Culex species of mosquitoes. The incubation period for the virus is between five and fifteen days. Symptoms include fever, headache, stiff neck, disorientation, coma, and convulsions, depending on the severity of the infection. The fatality rate is between 3 and 30 percent. There is no vaccine or therapy, other than supportive care, for St. Louis encephalitis.

Chikungunya is a member of the genus Alphavirus in the family Togaviridae and is present mainly in Asia and Africa. The mosquito A. aegypti is the primary vector, but A. albopictus is also known to transmit the virus. The incubation period ranges from two to twelve days but usually averages two to four days. Symptoms include fever, headache, nausea, vomiting, rash, and joint pain. Recovery usually takes about seven to ten days, and the disease is not usually fatal. Supportive care such as rest, fluids, and nonsteroidal anti-inflammatory drugs is recommended. Arthritic symptoms, particularly in the wrists, hands, and ankles but sometimes in the larger joints, continue in about one-third of cases. Chikungunya is one of the few mosquito-borne diseases that has a milder course in children than in adults. No vaccine or antiviral treatment is available for Chikungunya virus.

Tickborne Diseases

Tickborne diseases are caused by tick bites and are most commonly contracted in heavily wooded areas. Measures to prevent tickborne diseases involve the evasion of ticks, primarily, and the avoidance of tick bites. Measures include wearing light-colored protective clothing, checking for ticks after being outdoors, and using an insect repellant containing 20 to 30 percent DEET. Another measure is to maintain yards and gardens in a way that keeps ticks away. This includes applying acaricide pesticides in May or June, using wood chips between wooded areas and grassy areas to minimize tick migration into the yard, and ensuring that patio and playground equipment is away from trees and bushes. Controlling the deer population by erecting fences and by not feeding deer help to reduce the incidence of deer ticks. Tick removal should be done with fine-tipped tweezers, grasped as close to the tick head as possible. This method reduces the chance that an infected tick will release more bacteria-containing saliva into the bloodstream when removed from the skin.

Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted by Ixodes scapularis (black-legged tick/deer tick) and I. pacificus (Western black-legged tick) and is carried by deer, squirrels, and mice. Most cases in the United States occur on the East Coast and in the Midwest. Symptoms of Lyme disease include fever, headache, fatigue, and a skin rash that often looks like a bull's eye (erythema migrans); a rash occurs in about 80 percent of cases. The incubation period lasts between three and thirty-two days. Lyme-infected tick season is typically from May through July. Early detection is critical to avoid infection of the joints and the cardiac and nervous systems. A course of antibiotics administered for a few weeks is typically effective against Lyme disease. If left untreated, severe arthritis, meningitis, heart palpitations, and Bell’s palsy (loss of muscle tone on one or both sides of the face) can occur. Most people recover after a course of antibiotics; however, some people experience long-term effects of fatigue, arthritis, and cognitive deficits. A Lyme disease vaccine was available until 2002, when the company making it discontinued production, citing a lack of demand.

Rocky Mountain spotted fever is caused by the bacterium Rickettsia rickettsii and is typically transmitted by Dermacentor variabilis (American dog tick) or D. andersoni (Rocky Mountain wood tick). The incubation period is typically five to fifteen days after the tick bite. Early symptoms of the disease include fever, muscle pain, loss of appetite, and severe headache. Later symptoms of Rocky Mountain spotted fever are rash, joint pain, and diarrhea. The majority of cases in the United States have been reported in the Southeast. Other regions affected include North, Central, and South America. Treatment with tetracycline antibiotics for five to ten days is usually successful.

Flea-borne Disease

One flea-borne disease is plague, caused by the bacterium Yersinia pestis. The bacterium is usually transmitted by infected rodent fleas, with an incubation period of two to six days. The Plague, also called the Black Death, killed millions across Europe in the fourteenth century. Fleas also carry infection between squirrels, chipmunks, rabbits, and prairie dogs, resulting in transmission to humans. Each year, there are one thousand to two thousand cases. In Asia, South America, and Africa, the most common carrier is the flea species Xenopsylla cheopis. Plague occurs in the western United States and parts of South America, Asia, and Africa. One of the telltale symptoms is a painful, hot, and swollen lymph node. Additional symptoms are fever, headache, and exhaustion. Antibiotics can treat the plague if treatment starts early enough. Without antibiotics, the disease continues to progress to infection of the bloodstream and lungs (plague pneumonia) and has a high fatality rate. The plague vaccine is not commercially available in the United States.

Impact

Insect-borne diseases affect hundreds of millions of people each year and kill around one million, especially children in tropical countries. Determining mechanisms for fighting the diseases and the insects that carry them is a top research priority worldwide.

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