The question of host choice is an extremely important one because it defines patterns of disease transmission and economic damage caused by blood-sucking insects. Bloodsucking insects in general feed on a range of different hosts, including birds, reptiles, mammals, and amphibians. Even invertebrates such as annelids, arachnids, and other insects are sometimes included in the diet. But any particular bloodsucking insect generally feeds only from a small segment of the available hosts. This segment of choice is preferred but it is not immutable. This can be clearly seen around zoos where the exotic animals are quickly incorporated into the diet of the local blood-sucking insects.
The determinants of host choice are complex, but probably one of the most important factors is simply host availability. Changes in host availability because of more intensive animal husbandry, coupled with decreasing rural populations of humans and improved, mosquito-free housing, were a major factor in the disappearance of autochthonous malaria from Northern Europe in the past century. Despite our poor understanding of the factors determining host choice, there is a direct relationship between the number of hosts that blood-sucking insects utilize and the insects' locomotory abilities (which is often reflected in the amount of time they spend with the host). Thus, ectoparasites (which have poor locomotory abilities and usually remain permanently on hosts) are often restricted to a single host species. A good example is the louse Haematomyzus elephantis, which is restricted to elephants. At the other extreme, those flying blood-sucking insects such as mosquitoes that make contact with the host only long enough to take a blood meal often display a very catholic host choice. For example, a sample population of the mosquito Culex salinarius was shown to take 45% of its blood meals from birds, 17% from equines, and 15% from canines; moreover, 13% of the meals was a mixture of blood from more than one host!
In general terms, the most common hosts chosen are large herbivores. Large, social herbivores present an abundant, easily visible food source that is reliable and predictable from season to season. Carnivores in comparison are fewer in number, often solitary, and range unpredictably over wide areas. Another reason large herbivores are chosen is that they are poor at defending themselves from attack compared to small, agile animals that will often kill and/or eat attacking blood-sucking insects.
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