Insect blood feeding impacts directly on human and animal health. When a blood meal is taken, pathogens of some of mankind's most devastating diseases are transmitted either directly through the food channel or indirectly via excreta deposited in the skin. Among these are malaria with 300-500 million infections a year and 1.5-2.7 million deaths a year; lymphatic filariasis with 120 million cases; dengue with 50 million cases; Chagas disease with 16-18 million infected people and 45,000 deaths per year; onchoceriasis with 18 million infections and 270,000 cases of blindness, and leishmaniasis with 12 million cases (Nuttall 1997). Other diseases include African trypanosomiasis, yellow fever, typhus, and plague. For these reasons, a better understanding of hematophage physiology is important for the development of biorational control programs (Adams, 1999).
The most important factor to be considered in this further analysis is understanding the ways in which insects overcome the difficulties and dangers of blood feeding. In order to overcome the threat they pose to us, we must have extensive knowledge of the ways in which they obtain their blood meal, the mouth-parts involved, the ways they neutralize any infections from the prey, and how they deal with other problems presented to them. This knowledge will help us in creating ways to immunize ourselves against the dangers transmitted by them. For example, speculations are being made about the possibilities of injecting insecticides into the bloodstream of cattle that will disrupt the metabolitic processes of the insects, which will result in their deaths. Such solutions can not be created without extensive knowledge of the insect's blood letting and digestive mechanisms.
It is believed that hematophagous insects first developed from dietrous feeding insects attracted to animal nests to feed on sloughed skin and feathers that tend to gather in burrows. Eventually mouth parts developed for the feeding of these directly from the host. While these were not designed to pierce the skin, as in the Mallophaga, some did feed on blood. Menacanthus stramineus, a current day mallophagan, feeds at the base of the feathers or on the skin of the chicken. It will often break through the dermis, exposing blood for the insect to feed upon (Emmerson et al. 1973). Blood has a higher nutritional value than skin and is far easier to digest. This is reflected in the increased fecundity of blood feeding Anoplure compared to the skin feeding Mallophaga (Marshall 1981). Once exposed to this higher nutrient source, a blood feeding line of insects was evolutionarily favored. This developed progressively through physiological, behavioral, and morphological adaptations. First to favor hematophagy, then to require it. Members of the mallophagan suborder the Rhynchophthirina, such as the elephant louse, Haemotomyzus elephantis posess typical mallophagan biting-type mouthparts (Ferris 1931, Mukerji and Sen-Sarma 1955). These are not particularily well designed for obtaining blood, but by holding the mouthparts at the end of an extended rostrum, the insect manages to use them to penetrate the thick epidermal skin layers of the host to get to the blood in the dermis.
Blood sucking insects feed from a wide range of hosts including mammals, birds, fish, amphibians, reptiles, insects, arachnids, and annelids (Hocking 1971). Host selection is the primary crucial element in insect blood feeding. An improper host may not have the nutrients needed in the blood stream, carry parasites that are harmful to the insect, or even be able to retaliate against the invasion with a deadly swat. Host choice is defined as the species of host animal or animals from which a blood sucking insect obtains its blood meals. This can even become so specific as to specify certain individuals in a population that are to be chosen. The most common hosts tend to be large social herbivors, as they provide a more than ample easily accessible food source for many of the temporary ectoparasites. They also move quite slowly from one pasture to another. Carnivores tend to be less abundant, more solitary, less visible, less predictable, and occupy a large home range. For these reasons, carnivores tend to be less likely choices as hosts for the bloodsucking insects. Also, larger animals tend to be more favorable than smaller ones. Small rodents and deer tend to be able to hide and burrow easier than a larger bovine. Anopheles albimanus studied on the costal plains of Chiapas, in southern Mexico were found to have a large range of hosts, including human, bovine, equine, porcine, canine, and chicken. They were found to have fed more frequently on bovines than on any other host, but the forage ratio indicated that there was also a high preference for equines. The feeding patterns varied according to the availability of hosts and to ecological factors including proximity of animal hosts to houses and access to human hosts. Traditional animal holding practices affected signifigantly the feeding patterns of An. Sergentii. There was a low human blood index when animals were kept in sheds close to human dwellings, indicating the insects' higher preference for the animals over humans (Gonzales 1993). Aedes albopictus exhibit this same patterning and are described as opportunistic feeders which selectively feed on an array of hosts depending on the season and the microhabitat sampled (Niebylski 1994).
Host location is a complex series of behavioral patterns that gains momentum as the host is tracked down. The patterns are not a strict sequence of events, but rather a mixture of all possible avenues of action. This gives the insect versatility in its possible actions to choose from, depending on the situation it finds' itself in. For convenience, these behavior patterns have been classified into three phases (Sutcliffe 1987). Different species will exhibit variations along these lines.
It is believed that hormones play a signifigant role in host selection and blood feeding ability. The most studied is the indolalkylamine serotonin. Serotonin is an important neuroactive compound found in both vertebrates and invertebrates. Relatively small numbers of serotonergic neurons occur in insects, but they extend to large areas of the central and peripheral nervous systems (Nassel 1988). Although most serotonergic neurons are interneurons, some extend to peripheral sites and innervate muscles and organs (Davis 1987, Peters et al. 1987). Alternatively, networks of serotonergic fibers have been observed on the surface of peripheral nerves and organs in Calliphora (Nassel and Elekes 1985), Periplaneta (Davis 1987) and Rhondnius (Lange et al. 1988, Orchard et al. 1988). These networks of varicose fibers suggest neurohormonal functions for serotonin (Nassel and Eleckes 1985).
Serotonin and other major biogenic amines in insects, dopamine and octopamin, have been reported to modulate a variety of physiological and behavioral functions. One or more of these biogenic amines have been implicated in the control of circadium activity, locomotory behavior, and diapause (Evans 1980, Klemm 1985, Nassel 1988). In addition to influencing these functions, biogenic amines have been implicated in the control of feeding behaviors in insects. Trimmer (1985) demonstrated that serotonin induced salivation in the blow fly, Calliphora vicina (Robineau-Desvoidy). General amine depletion, induced by reserpine or d-amphetamine, increased the response thresh hold to sucrose in Phormia regina (Meigen) (Brookhart et al. 1987). Feeding was found to be a natural stimulus for the release of serotonin into the hemolymph of the blood sucking bug Rhodnius prolixus Stal (Lange et al. 1989) and serotonin depletion led to the reduced blood meal size in this insect (Cook and Orchard 19990).
In these insects, AMTP was found to reduce serotonin levels, while AMT reduced dopamine levels. It was found that neither affect the host seeking activity even though both reduce the spontaneous flight activities (unpublished observations). When presented with a host stimuli, both amine-depleted mosquitos responded positively and without signifigant differences. The diffenrences arise in the ability to ingest a blood meal. It was found that AMPT treated insects were less successful at obtaining a blood meal than the control groups. This indicated that serotonin is important in some way for the blood feeding behavior, although it is unknown exactly how. The only change exhibited by mosquitos treated with AMT is the previously stated alteration in flight activities. This leads one to believe that dopamine is not an integral part of the blood feeding behavior (Novak 1994).
Feeding is carried out in two primary ways. The most common is insect's feeding from pools. They have mouthparts which are used to rip, tear, or otherwise cut the skin, and then to lap or suck blood from the haemorrhagic pool which forms. These mouthparts are seen in the tabanids, blackflies, and biting flies. Some insects are oportunists and will feed on blood when it leaks to the surface through a wound. For example, the nonbiting flies in the family Muscidae such as Fannia benjamini and Hydrotaea armipes, feed from wounds inflicted by tabanids (Garcia and Radovsky 1962). They may even become impatient and crowd the biting fly, sometimes even feeding at the same time as it (Tashiro and Schwardt 1953). This sometimes interrupts the feeding of the biting fly, forcing it away. Some species are not so opportunistic and do not depend on chance. Philaematomyia lineata have prestomal teeth,which may be sufficiently well developed to break through the partially dried and clotted surface of a wound (Patton and Cragg 1913).
The second method of feeding is insects that feed within a blood vessel of the animal with a probiscus inserted into the epidermal layer of the host's skin. Piercing and sucking mouthparts are seen in the bugs, lice, fleas, and mosquitos. In all of these the mandibles and/or maxillae have been modified to form long, thin, piercing stylets which are also interconnected to form a long tube through which blood can be sucked. Although it is common to have these two distinct methods of feeding, some insects like the mosquito have piercing mouthparts, which they use for feeding directly from a blood vessel or from a pool of blood on the skin. The line is not concretely drawn between the two methods.
The problems that insects encounter in blood feeding are quite numerous and can often be deadly. Mammalian blood is 55% plasma and 45% cells by volume with an osmolarity of .15M with K+-rich cytoplasm and plasma rich NaCl (Dow 1986). Whole human blood contains 80g water, .6g lipid, .08g carbohydrate, and 20.5g protein per 100 ml, respectively (Lehane 1992). Thus, approximately 80% of the blood meal is water. Adult temporary ectoparasites commonly take meals, which are twice their unfed body weight, while nymphal stages of the blood sucking Hemiptera may take meals of ten times their unfed body weight. These large blood meals certainly impair the mobility of the insect, increasing the short-term chances of it being swatted by the host or eaten by a predator. For example, the flight speed of Glossina swynnertoni is reduced from 15 to 3-4 mph after feeding (Glasgow 1961). The majority of this cumbersome weight is water that is completely unnecessary for the insect. As a result, mechanisms have been derived to quickly expunge this excess water. To achieve this the meal is held in a distinct region of the midgut where the epithelium is adapted for rapid water transfer. In tsetse flies and truatomine bugs, water movement across this epithelium is linked to a ouabain sensitive, Na+ K+ ATPase located in the basal membranes of the epithelium with chloride as the counter ion (Gooding 1975, Peacock 1981, 1982, Farmer et al. 1981). This very efficient pump works by generating an osmotic gradient across the epithelium, which drags water passively after it. These systems are so efficient that tsetse flies can shed about 40% of the weight of the meal in the first 30 minutes following feeding (Moloo and Kutuza 1970, Gee 1975), and most of the fluid in the very large blood meals of the triatomine bugs is disgarded within four hours of ingestion (Wiggelsworth 1931).
Another problem associated with blood feeding is the very high chance of detection by the host and possible termination. Insect blood feeding is often highly invasive and this quickly alerts the host to the presence of the hematophage. After detection, most hosts will take evasive action to remove the troublesome pest, such as a flick with the tail or a roll in the dirt. For permenant ectoparasits, the animals' gooming activities are most often the primary threat. Preening in birds serves very often to remove lice and ticks, as well as maintaining the condition of the feathers and their water proof properties.
Blood feeders also face possible infection by parasites the vector may ingest from the host. Given the brief association of parasite and vector during mechanical transmission, it is not surprising that little if any, pathological effect is seen. However, in the cases of extensive association, considerable damage can be caused. In the most extreme cases the ingested parasite can lead to the death of the vector insect. For example, Rickettsia prowazekii and R. typhi both cause death of the louse, Pediculus humanus (Weyer 1960, Jenkins 1964). Increased mortality will ususally only occurs when the insect has ingested either an unusually large number of parasites, or an uncommonly large number survived entry and are developing successfully inside the insect. Aedes trivittatus is a fine example of this when infected with the filarial worm Dirofilaria immitis. A. trivittatus can tolerate low numbers of microfilarire, but when they feed on dog blood with concentrations as high as 347 per 20 mm3 a significantly increased rate of mortality occurs in the vector population (Christensen 1978). Reduced longevity of the vector can also be seen in malaria infections of mosquitoes (Gad et al. 1979, Klein et al. 1982).
Parasites can also have sublethal effects on vector insects. Infected mosquitos may produce a reduced number of eggs (Maier and Omer 1973, Hacker and Kilama 1974, Freier and Friedman 1976). Also, the invasion of the salivary glands of Aedes aegypti by sporozoites of Plasmodium gallinaceum reduces salivary apyrase by as much as two- thirds (Rossingnol et al. 1984). This is dangerous for the host because it increases the probing time in mosquitos, thereby increasing the chance of the insect being disturbed or killed while feeding (Gillett 1967). The flight capabilities of the tsetse flies can also be signifigantly impaired because trypanosomes have used a considerable proportion of the reserves normally reserved for flight (Bursell 1981).
In some cases, the behavior of the insect can actually increase its chances of infection by a parasite. Anopheles gambiae is commonly infected with Plasmodium falciparum, and it has been found that prior blood meals can signifigantly decrease infection if the pathogen is ingested in a later meal. Prior blood feeding accelerated digestion of the infective blood meals and the faster the digestion, the less time afforded for the parasite for fertilization/transformation/penetration. The prior feeding in An. Gambiae had detrimental effects on the ability of P. falciparum ookinetes to penetrate the midgut (Vaughan, Noden, Beier 1994).
Continued research such as this can only help us to find new ways in
which to control these insect pests. Stopping parasitic vectors before
they can contract their dangerous companions is the most effective way
for us to contain the often devastating spread of infectious diseases.
Unfortunately, the majority of these outbreaks occur in areas that can
not afford such research and controlling methods.
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