Mosquitoes bite birds and are capable of transmitting diseases between avian populations and, in some cases, to humans. These insects are classified as generalist feeders, meaning their blood meals are not restricted to a single host class. This feeding behavior is biologically significant because it creates a pathway for pathogens to move through the environment, affecting both wildlife health and public health. The interaction between mosquitoes and birds establishes a natural cycle of infection.
Feeding Preferences and Host Selection
Female mosquitoes require a blood meal to produce eggs, and they locate a host by following a precise sequence of sensory cues. The primary attractant for mosquitoes over long distances is the carbon dioxide (CO2) plume exhaled by the host, which indicates the presence of a living animal. Once in closer proximity, the insect uses thermal cues, detecting the host’s body heat, and specific odorant chemicals to confirm the host type.
Some mosquito species, known as ornithophilic species, have a distinct preference for avian hosts, guided by specific volatile compounds. Research has identified the chemical compound nonanal as a dominant attractant in the odorant profiles of birds. This chemical is detected with high sensitivity by the olfactory receptor neurons on the antennae of bird-feeding mosquitoes, particularly species in the Culex genus. Nonanal works synergistically with CO2 to increase the mosquito’s attraction, leading to a higher likelihood of a bite.
Mosquitoes cannot easily penetrate a bird’s dense feathers, so they typically target unfeathered areas of the body. They focus feeding efforts on the feet, legs, and the skin around the eyes or the base of the beak. The specific host a mosquito chooses can shift depending on environmental conditions, host availability, and population density, which directly influences disease transmission dynamics.
Diseases Primarily Affecting Avian Populations
Mosquitoes transmit several severe diseases that significantly impact bird health and survival, acting as vectors for pathogens that rarely affect humans. One widespread example is Avian Malaria, caused by the protozoan parasite Plasmodium relictum. This parasite reproduces within the bird’s red blood cells, and a high parasite load causes severe anemia.
In susceptible populations, such as the native Hawaiian honeycreepers, Avian Malaria has caused drastic population declines and extinctions. The loss of red blood cells leads to profound weakness, lethargy, and in many cases, death, especially in birds without developed resistance. Transmission is confined to the bird-mosquito-bird cycle, with the Culex quinquefasciatus mosquito being a particularly efficient vector.
Another disease transmitted by mosquitoes is Avian Pox, caused by avipoxviruses. This disease manifests in two distinct forms: the cutaneous, or dry, form and the diphtheritic, or wet, form. The dry form presents as wart-like growths on unfeathered areas, such as the legs, feet, and eyelids. The wet form is more dangerous, causing lesions on the mucous membranes of the mouth and upper respiratory tract, which can obstruct breathing and feeding, often resulting in death. Mosquitoes primarily transmit Avian Pox mechanically; the virus adheres to the mosquito’s mouthparts after feeding on an infected bird and is transferred to the next bird during a subsequent bite.
Birds as Vectors for Human Disease
Birds are not only victims of mosquito-borne illness but also serve as crucial hosts in the transmission cycle of viruses that can spill over into humans and other mammals. This public health risk is mediated by “bridge vectors”—mosquito species that feed on both birds and mammals. The most prominent example of this zoonotic threat is West Nile Virus (WNV). In the natural cycle of WNV, birds act as the primary reservoir and amplification hosts. When a mosquito, often from the Culex genus, feeds on an infected bird, it picks up the virus.
Within the bird, the virus replicates to high concentrations, known as viremia, which is sufficient to infect the next mosquito that bites it. The American robin and the House sparrow are considered important amplification hosts because they sustain high viral loads without immediately succumbing to the disease. The bridge vector then transmits the virus to a human or a horse during a subsequent blood meal, completing the spillover event. Humans and horses are considered “dead-end hosts” because the WNV concentration in their bloodstream is too low to infect another mosquito.