Mosquitoes as vectors of diseases
Mosquitoes can transmit a number of human diseases including West Nile, Zika, dengue and encephalitis. Mosquitoes that transmit diseases are referred to as “vectors” and the diseases they transmit as “vector-borne diseases”. However, not all mosquitoes can transmit diseases, and not all mosquitoes can transmit all mosquito-borne diseases.
The association of a specific disease with a species of mosquito is a result of co-evolution, and the different disease agents are often highly adapted to their vectors. For example, the pathogens that cause human malaria can be transmitted only by Anopheles mosquitoes, which in turn are unable to transmit yellow fever or dengue. Viruses such as HIV or measles cannot be transmitted by mosquitoes at all, but are transmitted directly from human to human.
In order to transmit a virus or pathogen, a mosquito must first feed on blood from an infected person or animal. Then, a number of barriers must still be overcome before she becomes infectious herself. First, the pathogen must invade and replicate in cells of the midgut (the digestive system of the mosquito). Next it must escape the midgut and disseminate throughout the body until it finally reaches the salivary glands. Here again the pathogen must pass yet another barrier to penetrate the salivary glands. Only if success is achieved in crossing all these barriers, the mosquito becomes infectious and is able to transmit the disease agent to the next host in its saliva during the bloodfeeding process. This process takes time, and this time required for a pathogen to develop inside the mosquito (between ingestion and transmission) is called the “extrinsic” incubation period. The process is also temperature dependent and generally takes between 10 to 14 days during the summer months.
When an infected mosquito bites a human, the period between the infectious bite and onset of disease symptoms is called the “intrinsic” incubation period and may range from 2 to 8 days depending upon the virus. But not every person bitten by an infected mosquito will become sick. For example, only about 1 in 5 individuals bitten by a mosquito infected with West Nile virus will develop symptoms.
Mosquito biology and host feeding preferences play a major role in determining how mosquito-borne viruses are transmitted. Species in the genus Culex have a tendency to prefer feeding on birds, while Aedes mosquitoes feed more often on mammals. West Nile (WN) and St. Louis encephalitis (SLE) viruses normally cycle between birds and Culex mosquitoes. LaCrosse encephalitis has a disease cycle that involves small woodland mammals, such as chipmunks and squirrels, and Aedes mosquitoes. Some viruses such as dengue, Zika and Chikungunya cycle primarily between humans and a small group of Aedes mosquitoes such as Ae. aegypti and Ae. albopictus. These species are closely associated with human populations; resting, feeding and reproducing in close proximity to human habitations.
Some species feed on a wide range of hosts, such as both birds and mammals. These mosquitoes are referred to as bridge vectors as they can move a virus from its natural cycle and infect other hosts, such as humans, that would normally not be involved. In most zoonotic disease cycles, humans become infected as incidental hosts when levels of circulating virus are high.
Mosquito-borne viruses are a recurring public health problem each summer in the United States. In 2016, the following numbers of human cases were locally acquired in the U.S.: West Nile (2039), Zika (224), Lacrosse encephalitis (34), St. Louis encephalitis (9) and Eastern equine encephalitis (5). Cases are also diagnosed in travelers returning to the U.S. from countries where mosquito-borne diseases are prevalent. These imported cases account for most of the dengue, chikungunya, Zika and malaria cases reported in the U.S.