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Mosquito-borne Disease

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Mosquitoes are generally considered an annoyance, but some species have the potential to transmit diseases in humans and animals. Mosquito borne diseases such as malaria, yellow fever, dengue fever, chikungunya, filariasis and others occur worldwide, but presently remain relatively rare in the United States. For example, about 500 million cases of malaria with about 2 million deaths occur annually, but only about a thousand cases were reported in this country (in 1988). Most U.S. malaria cases (about 99%) were acquired in other countries. Yellow fever has been responsible for many epidemics with high mortality rates in many parts of the world including the United States. This disease has been greatly reduced worldwide and has not been present in this country for many years. Filariasis is a common human illness in many warm parts of the world, but, like malaria, it has not been significant in the United States for some time.

Dengue fever (breakbone fever) and chikungunya remain concerns in many parts of the world. Dengue is was uncommon in the United States but, there have been some localized outbreaks of dengue in Florida, Texas and Hawaii in the last several years. Chikungunya, a new mosquito transmitted virus, spread rapidly throughout the Caribbean and into the United States in 2014. There were numerous cases of dengue and chikungunya that were imported from travelers coming from endemic locations. Symptoms of dengue and chikungunya are very similar and some cases individuals are asymptomatic, which makes these viruses difficult to track.

Encephalitis diseases have many causative agents, some of which are transmitted by mosquitoes. In Florida, eastern equine encephalitis (EEE), St. Louis encephalitis (SLE) and west Nile virus (WNV) are the leading forms of mosquito-transmitted encephalitis. Eastern equine encephalitis is the most virulent of these forms with a mortality rate in humans of 30 to 60 percent. The disease has been reported statewide but seems to be more common in the northern part of the state. Fortunately, this disease remains rather rare in humans with only a few human cases reported each year. There have been some larger outbreaks of St. Louis encephalitis in Florida (The last in 1990). The mortality rate for this disease is much lower than that of eastern equine encephalitis, only being 3 to 20 percent. West Nile virus was first reported in the New York City area in 1999 but has rapidly spread throughout the eastern United States. A few cases have been reported in Florida over the last several years, but it is difficult to determine how many people were infected because a large majority of the cases have very mild symptoms or no symptoms at all.

In order for a disease to be transmitted by mosquitoes, certain conditions must be met; (1) the disease agent (the actual organism that causes illness, for example a virus) must be ingested by the vector (the organism which carries the disease), (2) the disease agent must be able to survive and reproduce inside the vector and finally (3), the disease agent must be injected into the new host (the individual who is sick from the disease). Most diseases such as influenza (flu) viruses, colds, and bacterial diseases cannot be transmitted by mosquitoes because the disease agent cannot survive and reproduce inside the mosquito. For example, if a mosquito bites a person with the flu, the flu virus is easily ingested by the mosquito but it cannot survive and reproduce inside the mosquito. In contrast, diseases such as dengue fever or west Nile virus are easily ingested by a mosquito from the infected host. The virus is able to survive and reproduce within the mosquito and finally, the agent moves to the mosquitoes salivary glands where it can be injected into a new host, thus transmitting the disease.

Not all species of mosquitoes are equally efficient at transmitting diseases. For example, yellow fever, dengue fever and chikungunya are transmitted primarily by the Yellow Fever Mosquito (Aedes aegypti) and malaria is vectored only by some mosquitoes of the genus Anopheles. The Asian Tiger Mosquito (Aedes albopictus) has been implicated as a vector of dengue in some parts of the world. Mosquitoes of the genus Culex are the major suspects for both St. Louis encephalitis and west Nile virus. The transmission mechanism for eastern equine encephalitis is poorly understood. It must be remembered that without the disease agent, a mosquito cannot transmit a disease.

Yellow Fever mosquitoes, tiger mosquitoes and Anopheles mosquitoes are very common in this area, but, since the disease agents that they can transmit are not present, there are no illnesses from their bites. The mosquito borne encephalitis diseases that are found in Florida (EEE, SLE and WNV) generally show up first in birds or veterinary cases (mainly horses). Because of this, most mosquito control districts monitor sentinel chicken flocks and horses to detect if the disease becomes a significant threat in a given area.

Sentinel Chicken Program

The District deploys four flocks within the District each year. The chickens are purchased from a local vendor at a young age when they haven’t had the chance to be exposed to encephalitis viruses. Each flock consists of six chickens and are strategically located throughout the county from July through to November each year. 

Chickens are utilized to provide early warning that disease-carrying mosquitoes are in the vicinity. The chickens are unaffected by west Nile virus or any other of the encephalitis viruses, however, they develop antibodies, which show up in their blood approximately 10 days after infection. Inspectors draw blood from each of the six chickens within the flock each week and send the blood to the State laboratory to detect the presence of these antibodies. 

If an initial “Reactive” result is observed, it means that the chicken may have been infected.  Upon notification of the test results, Mosquito Control staff inspect and treat the area to eliminate any infected mosquitoes. If secondary testing results in a “Positive” result, the area is evaluated again and treated as necessary. Most “Reactives” however, turn out to be false alarms.