Mosquito Borne Diseases

Mosquito-borne diseases of humans offer special challenges to those endeavoring to prevent or control disease outbreaks. A health-related mosquito problem is the ability of a mosquito to transmit infectious disease. In Bay County, health related problems include Eastern Equine Encephalitis, St. Louis Encephalitis, and West Nile Virus. However, nearby in the Caribbean and Central American, other mosquito-transmitted diseases are common (e.g., dengue, malaria, and yellow fever.) Dog Heartworm is a chronic disease of dogs (and sometimes cats) that is due to a mosquito-transmitted filarial worm parasite.

Information on diseases:

Eastern Equine Encephalitis (EEE)

Eastern Equine Encephalitis (EEE) is caused by a virus transmitted to humans and horses by the bite of an infected mosquito. Birds are the source of infection for mosquitoes. The principle vector in avian populations is the mosquito Culiseta melanura. This mosquito does not feed on humans or horses, but in rare cases the virus can escape from its marsh habitat in other mosquitoes that feed on both birds and mammals (including horses and humans) and then transmit the virus to mammals, including people. Horses and humans are “dead end” hosts, meaning that they do not develop enough virus in their blood to transmit the virus (therefore sick horses or humans can't transmit the disease to mosquitoes, only birds can).

The virus is normally found only in wild songbirds and mosquitoes that live in and around wooded swamps; not just any swamp, but a swamp where there is a certain species of mosquito. The EE mosquito or “black-swamp-mosquito” has the scientific name Culiseta melanura (cue-la-see-ta mel-ah-nur-ah). Culiseta melanura, which translates to “curly black hairs,” is indeed a dark mosquito that has a very long proboscis or probe that it uses to draw blood from its hosts. Culiseta melanura has very specific breeding requirements. It occurs in most states east and a few states west of the Mississippi River. The larvae are found only in the underwater root systems of deciduous trees that grow in swamps. Fortunately for us, they get their blood from songbirds; rarely does it bite humans or other mammals. And, since Culiseta melanura flies no further than 5 miles from its breeding sites, most cases of EE occur within 5 miles of these swamps.

Since Culiseta melanura does not bite people, the key to human and horse infection is tied to the short period when birds have high concentrations of virus in their blood. When other mosquitoes feed on infected birds they can become infected as well. It is these “secondary” mosquito species that carry the virus to other vertebrate hosts, including horses and humans. For those secondary mosquitoes to transmit the virus from birds to humans, an individual mosquito must successfully blood feed on both groups of animals. Not all mosquito species do that, Culiseta melanura for example.

There is another species of mosquito that is most often associated with outbreaks of EE in horses and humans. This mosquito, the “Salt-and-pepper mosquito”, has the scientific name Coquillettida pertubans (Coke-qua-la-tid-e-ah purr-tur-bans) or “cokes” for short. This is a large black and white mosquito that looks for blood around dusk. Cokes have a geographic distribution similar to Culiseta melanura, but rather than breed in wooded swamps, they breed in cattail or grassy marshes that have a mucky bottom. These types of marshes are often next to the swamps that produce Culiseta Melanura.

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St. Louis Encephalitis (SLE)

St. Louis Encephalitis (SLE) virus is a flavivirus that is transmitted to humans and other vertebrates primarily by mosquitoes of the genus Culex. Infection with SLE results in inapparent infection in a variety of birds and mammals with a resultant period of viremia and lasts a matter of days. Humans represent an incidental, deadend host. The first human disease outbreak recognized to be caused by the SLE virus occurred in St. Louis, Missouri, in 1933.

The annual cycle of the SLE virus in Florida is still not completely understood. We know, however, that wild bird populations are central to the transmission cycle. When a bird becomes infected by the bite of a carrier mosquito, it produces more virus in its blood that can infect other susceptible mosquitoes that bite the bird. These mosquitoes can then further spread the virus in one or two days, but the virus disappears two to three days later, so a bird remains “infective” for only a few days. Birds do not show any symptoms of disease and become immune after exposure.

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West Nile Virus Encephalitis (WNV)

West Nile virus is carried by mosquitoes and if transmitted to humans, can cause serious encephalitis. The virus was first reported in 1937 from a woman in the West Nile province of Uganda in Central Africa. West Nile virus was first documented in the United States in New York City during an epidemic in August 1999 and reached Northwest Florida in summer of 2001.

The disease caused by the virus is known as encephalitis and can be severe in the elderly, but it is usually mild in healthy adults and children. Mosquitoes obtain the virus from feeding on infected birds. The virus is then passed along to humans when an infected female mosquito takes a second blood meal from a human instead of a bird. As they are feeding on human blood, they release saliva that contains the virus. The virus enters the human bloodstream along with the mosquitoes' saliva. The mosquito species responsible for the 1999 New York City outbreak have not yet been identified with certainty. The peak time for mosquito blood-feeding is during the time between sunset and sunrise.

Personal protection against biting arthropods, particularly when they are infected with dangerous pathogens, remains one of the most important ways to avoid disease. Avoid mosquitoes. Make sure screens are in good repair to prevent mosquitoes entering areas where there is a threat of encountering infected mosquitoes, wear protective clothing. Fortunately, the USA has some of the best mosquito and arthropod control programs in the world. Vector control and personal protection against vectors and the diseases they carry are the best way to avoid infection with vector-borne pathogens.

The 5 D's of PREVENTION:

  • Dusk & Dawn: Avoid being outdoors when mosquitos are most active.
  • Dress: Cover your skin with clothing.
  • DEET: Use mosquito repellant on bare skin and clothing.
  • Drain: Remove standing water in which mosquitoes can lay eggs.
Credit: J. Rutherford, Monroe CHD

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Dengue Fever

Dengue is a disease with symptoms ranging from simple flu-like illness to severe hemorrhagic symptoms, shock, encephalitis, or death, and is caused by any of four distinct dengue virus species (DEN-1, -2, -3, or -4). Nearly unique among arboviruses, the dengue viruses utilize humans as their only natural vertebrate host. The mosquitoes Aedes aegypti and Ae. albopictus are the principal vectors in most of the world; both are common in Florida. Dengue has become an increasingly serious threat throughout the Caribbean and Central and South America in the past 15 years, and all four dengue viruses occur in the region. Although dengue transmission has not been documented within Florida in recent years, epidemics of this disease had a major impact during the early development of the state. Dengue was first noticed in Florida in 1850, and by 1934 an epidemic in Miami was estimated to include more than 15,000 cases.

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Malaria in humans is a disease transmitted by mosquitoes in the genus distributed throughout the world. Human malaria was endemic in the United States in the 19th and 20th centuries. It was “eradicated” from the U.S. by mid 1950. From 1957 through 1996, 78 human cases of introduced malaria, have been reported from 22 states. Three of the cases were in Florida, one was a woman camping in the panhandle's Gulf County in 1990, and two in men living in Palm Beach County in 1996. These patients had never lived or visited a malarious area. These cases are the first reported in Florida since malaria was eradicated in the State in 1948. Presently, the major hazard of malaria transmission in Florida stems from people who have relapses, or cases recently acquired in foreign countries where malaria is common.

The parasite is transmitted from person to person by the bite of Anopheles mosquitoes, and ONLY Anopheles mosquitoes. The malaria parasite inhabits the human red blood cells, where it multiplies asexually. After reaching maturity in 48-72 hours, the red blood cells burst and release large numbers of new parasites, most of which enter new red bloods cells, thus, reinitiating the cycle. The typical malaria symptoms, chills and fever, are associated with this rupturing of infected red, cells.

Infection of the mosquito takes place when an Anopheles female feeds on an infected person who is carrying gametocytes. The human infection is initiated when sporozoites are injected during the bite of the infected mosquito. To avoid the risk of malaria, avoid mosquito bites - it is that simple. Humans cannot get malaria from wild animals, domestic animals or pets. Transmission of malaria from human to human is accomplished by Anopheles mosquitoes or by reuses of needles contaminated with the blood of an infected person.

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Yellow Fever

Yellow fever is an acute viral disease of varying severity. Currently occurs in portions of South America and Africa. Two forms of yellow fever exist: the urban, and the sylvatic/jungle. Although two forms are clinically and etiologically identical, their epidemiology is different. Urban yellow fever is an epidemic viral disease of humans transmitted from infected to susceptible persons by the bite of Aedes aegypti mosquitoes. Ae. aegypti mosquitoes breed in containers such as jars, barrels, and tires. Sylvatic yellow fever is an enzootic viral disease transmitted among nonhuman primate hosts by various mosquito vectors. Sylvatic yellow fever usually occurs in forested areas.

By the early 1900s, Ae. aegypti was found in the Untied States and every other country in the Western Hemisphere except Canada. Ae. aegypti is still found in Florida. Since viremic humans are the source of infection for yellow fever transmitted by Ae. aeypti, it is possible that infected travelers returning from abroad could cause an outbreak of yellow fever in the United States. Aedes albopictus, the Asian “tiger mosquito,” is also found in the United States and it is a competent laboratory vector of the yellow fever virus. However, there is no evidence that Ae. albopictus has served as a vector of any cases of any human diseases in the United States. The last documented Ae. aegypti-borne yellow fever epidemic in the Western hemisphere occurred in Trinidad in 1954. The last reported case of yellow fever in Florida occurred in 1918.

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Dog Heartworm Disease

Heartworm disease in dogs and related canines is caused by a filarial nematode (a large thread -like roundworm), Dirofilaria immitis. The adult worms live in the right side of the heart (right ventricle) and adjacent blood vessels (pulmonary arteries), and because of their location, are commonly called “dog heartworms”. Heartworm disease occurs worldwide in most tropical and subtropical regions, with increasing frequency in temperate climates. For most of North America, the danger of infection is greatest during the summer when temperatures are favorable for mosquito breeding. In the southern U.S., especially the Gulf Coast and Florida, where mosquitoes are present year-round, the threat of heartworm disease is constant.

Visible signs of heartworm disease may not appear until a full year after a dog is bitten by infected mosquitoes. In fact, the disease may be well advanced before the dog shows any symptoms. Typically, dogs with heartworm disease fatigue easily, cough, and appear rough and not thriving. Heartworm disease in dogs and cats is a serious and potentially fatal disease. Always consult your veterinarian for treatment and prevention of the heartworm disease in dogs and cats.

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