Phylum Arthropoda. Class Arachnida. Class Insecta Lice, fleas, bugs

 

Three-fourth of all animal species belong to the Phylum Arthropoda. The name “arthropod” comes from two Greek words, arthros, jointed, and podes, feet. All members of the Phylum Arthropoda share the following characteristics:

1.                 Arthropods have jointed appendages. Appendages are extensions of the body and include legs, mouthparts of various kinds and antennae.

2.                  The arthropod body is segmented. A pair of appendages is attached to each segment. In some species the appendages have been lost or reduced in size during the course of evolution. The members of some classes of arthropods have many body segments. In others, the segments have become fused together into functional groups, or tagmata (singular, tagma), such as the head or thorax of an insect, by a process known as tagmatization, which is of central importance in the evolution of the arthropods. All arthropods have a distinct head, sometimes fused with the thorax to form a tagma called the cephalothorax.

3.                  Arthropods have an exoskeleton. An exoskeleton is a hard external covering that provides protection and support.

4.                  Arthropods have a ventral nervous system, an open circulatory system, a digestive system, and specialized sensory receptors.

The phylum Arthropoda is divided into three subphyla: Chelicerata, Crustacea, and Uniramia. The chelicerates are characterized by chelicerae, mouthparts that often take the form of pincers or fangs, which evolved from the anterior appendages. Members of the other two subphyla have mandibles, originally biting jaws that also evolved from appendages, but from the second or third pair back from the anterior end. All appendages in crustaceans are fundamentally biramous, or two branched, whereas those in Uniramia are uniramous, or single branched.

Medical Arachnoenthomology studies the representatives of Phylum Arthropoda, which have medical importance. The members of this phylum can be the provisional or constant parasites of human beings, the infection carriers, and the poisonous animals. Phylum Arthropoda includes 3 classes of medical importance:

1.Class Crustacea: cyclops, crabs.

2. Class Arachnida (Octapoda): scorpions, spiders, ticks and mites.

 3.Class Insecta (Hexapoda): mosquitoes, flies, bugs, lice, fleas.

Class Crustacea. Most crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of pairs of legs. All of the appendages of crustaceans, with the possible exception of the first pair of antennae, are basically biramous. Crustaceans differ from the insects in that they have legs on their abdomen as well as on their thorax. They are the only arthropods with two pairs of antennae. Most crustaceans are small. Copepods no larger than a comma inhabit the surface waters of oceans, lakes, and streams. Large, primarily marine crustaceans such as shrimps, lobsters, and crabs, along with their freshwater relatives the crayfish, are collectively called decapod crustaceans. The term decapod means “ten-footed”. Most of their body segments are fused into a cephalothorax, which is covered by a dorsal shield, or carapace, which arises from the head. The crushing pincers common in many decapod crustaceans are used in obtaining food, for example, by crushing mollusk shells. Crabs differ from lobsters and crayfish in proportion; their carapace is much larger and broader, and the abdomen is tucked under it. Shrimps and their relatives also have different proportions; their carapace is proportionately smaller than that of lobsters or crabs.

       Subclass Entomostraca(Cyclops, diaptomus, and eudiaptomus).

Morphology: size 1-3 mm, pear shaped; body is divided into cephalothorax and abdomen. Cephalothorax is 5-segmented and carries a single median eye, 2 pairs of antennae (17 and 4 segmented), 4 pairs of legs. Abdomen has 4 segments in female, 5 segments in male, ends by two branches (each branch has 2 bristles).

Medical importance: Cyclopes are intermediate hosts of the fish tapeworm (Diphyllobothrium latum) and Dracunculus medinensis.      

        Subclass Malacostraca (crabs and other decapod crustaceans). Crab is second intermediate host of the lung fluke (Paragonimus westermani).

Class Arachnida. The Class Arachnida is a group of more than 100,000 species, including spiders, scorpions, ticks, and mites. Most arachnids are adapted to kill prey with poison glands, stingers, or fangs. Like crustaceans, arachnids have a body that is divided into a cephalothorax and an abdomen. Attached to the cephalothorax are 4 pairs of legs, a pair of chelicerae, and a pair of appendages called pedipalps. The pedipalps aid in chewing; in some species pedipalps are specialized to perform other functions. Arachnids undergo incomplete metamorphosis.

 Class Arachnida includes 3 orders of medical importance:

1.Order Scorpiones.

2. Order Araneae (spiders).

3.Order Acari (ticks and mites).

The scorpions are familiar group of arachnids whose pedipalps are modified into pincers. Scorpions use these pincers to handle their food and tear it apart. The venomous stings of scorpions are used mainly to stun their prey and less commonly in self-defense. The sting is located in the terminal segment of the body, which is slender toward the end. The elongated, jointed abdomens of scorpions are distinctive; in most chelicerates, the abdominal segments are more or less fused together and appear as a single unit. The adults of this order of arachnids range in size from 1 to 18 centimeters. There are some 1200 species of scorpions, all terrestrial, which occur throughout the world, although they are more common in tropical, subtropical, and desert regions. The courtship of scorpions is elaborate, with the spermatophores being fixed to a substrate by the male and then picked up subsequently by the female. The young are born alive, with 1 to 95 in a given litter. Scorpions differ from spiders in two ways. Scorpions have greatly enlarged pedipalps, which they hold in a forward position. They also have a large stinger on the last segment of the abdomen. Most scorpions hide during the day and hunt at night. Scorpions seize their prey with their pincerlike pedipalps. Then the fang injects paralyzing venom, the chelicerae tear the prey, the animal is ingested, and digestion begins. Only a few species have a sting that may be fatal to humans. They do not sting a man unless attacked.

Pathogenicity. The local symptoms of bite include severe pain, inflammation and swelling. Sweating, nausea and vomiting are common systemic symptoms. Muscular spasm and convulsions can occur in severe cases. Fatal outcome is caused by respiratory failure, pulmonary edema and shock. Control: spraying of insecticides. 

Spiders. There are about 35,000 named species of spiders (order Araneae). These animals play a major role in all terrestrial ecosystems, where they are particularly important as predators of insects and other small animals. Spiders hunt their prey or catch it in webs. The silk of the webs is formed from a fluid protein that is forced out of spinnerets, modified appendages on the posterior portion of the spider’s abdomen. Spiders have poison glands leading through their chelicerae, which are pointed and used to bite and paralyze prey. Some members of this order, such as the black widow spider (Latrodectus mactans), Australian black widow spider (Latrodectus seville), caracurt (Latrodectus tredicimguttatus), brown recluse (Loxosceles reclusa), tarantula (Lycosa singoriensis), have bites that are poisonous to humans and other large mammals.

Order Acari. Most mites are small, less than 1 mm long, but the adult length of different species ranges from 100 nm to 2 cm. In most mites, the cephalothorax and abdomen are fused into an unsegmented ovoid body. Respiration occurs either by means of tracheae or directly through the exoskeleton. Many mites pass through several distinct stages during their life cycle. In most, an inactive eight-legged prelarva gives rise to an active six-legged larva, which hatches from the egg and in turn produces a succession of three eight-legged stages and finally the adult males and females. Mites are very diverse, not only in their structure but also in their habits. They are found in virtually every terrestrial, freshwater, and shallow marine habitat known and feed on fungi, plants, and animals; they act as predators and as internal and external parasites of both invertebrates and vertebrates. Many mites are well known to human beings because of their irritating bites and the diseases that they transmit. Follicle mites (Desmodex folliculorum) live in the hair follicles and wax glands of the human forehead, nose, and chin, but usually cause no symptoms. Other mites cause mange in dogs and cats, often with severe consequences. A number of species of mites can cause house-dust allergy, which is aggravating to millions of people throughout the world; they are inhaled along with dust.

Sarcoptes scabiei (itch mite). Morphology: male (0,2 mm) is smaller than female (0,4 mm). Body is oval, convex dorsally and flat ventrally. The cuticle is striated and carries several hairs. Mouthparts: a) rudimentary hypostome without teeth. b) chelicerae end in pincer-like structures. c) pedipalps are short and 3-segmented. Legs: female mite has 4 short pairs (2 anterior and 2 posterior). The anterior pairs are completed by sucker-like structures. The posterior pairs are completed with long bristles. Male mite legs are similar, but the 4^th pair is ended with suckers instead of bristles. Larva has 3 pairs of legs (2 anterior and 1 posterior). Posterior pair is completed with bristles. Nymphs are usually quite similar to the adults, differing mainly in their smaller size.

 

 

 

 

 

 

 

 

Sarcoptes scabiei: 1 – mouthparts; 2 – walking leggs.

Life cycle: 1) mating occurs on the skin, then male dies and female burrows a tunnel in the superficial layers of the skin and lays eggs. 2) Larvae hatch within one week. 3) Larvae molt into nymphs (one nymph stage for males, two for females). The whole cycle takes about 2 weeks. Pathogenicity: Sarcoptes scabiei is the causative agent of scabies. Interdigital spaces, flexor surfaces of wrists and forearms, armpits, groin and genitals are preferential sites of infection. Usually head and neck are not affected in adults. Burrowing is activated by warmth, so itching is exaggerated during night. The lesions are reddish tracts with minute vesicles. Scratching spread infestation and induces secondary bacterial infection. Scabies is transmitted by close personal contact (congested population, poverty, in slums and jails). Diagnosis is made by 1) itching of affected surfaces at night 2) specific lesions 3) finding of the parasite in the tunnels by a needle. Control is provided by 1) proper treatment of infected persons 2) sterilization of clothes and bedding 3) personal cleanliness.

Follicle mites (Demodex folliculorum) live in the hair follicles and wax glands of the human forehead, nose, and chin, but usually cause no symptoms.

 

 

 

 

 

 

 

Demodex folliculorum:1 – mouthparts; 2 – walking leggs.

 

 Ticks. Ticks are blood-feeding ectoparasites (parasites that occur on the surface of their host). They are larger than most other mites and cause discomfort directly by sucking the blood of human beings and other animals. Some of them also inject toxins into their hosts. Ticks are divided into hard-bodied ticks (family Ixodidae) and soft-bodied ticks (family Argasidae). Morphology: 1) 2-5 mm in size, males are smaller than females. 2) Capitulum (false head) consists of a base (which carries the mouthparts and pedipalps). 3) Mouthparts consist of hypostome (ventral elongated structure with backwardly directed teeth) and two dorsal chelicerae with hooked digits. 4) Two four segmented pedipalps lie lateral to the chelicerae. 5) Dorsal surface may be covered by chitinous plate (Scutum). 6) Ventral surface carries: a) 4 pair of legs with one tarsal segment and a pair of claws, b) the genital pore between the first coxae, c) the anus behind the last coxae, d) one respiratory spiracle on each side.

Life cycle: 1) female ticks lay eggs. 2) Larvae hatch, feed on blood and then drop to the ground. 3) Larvae molt into nymphs. 4) Nymphs feed on blood and molt into adults. 5) Adult male and female feed on blood. 6) Life cycle takes several months (1-2 years in some species). 

         Some differences between hard-bodied and soft-bodied ticks

	Hard-bodied ticks	Soft-bodied ticks
Sexual dimorphism	Easy distinguishing of sexes by the scutum	Not
Posterior margin	Festooned	Not
Type of parasitism	Permanent ectoparasites (drop from host for molting and oviposition	Temporary ectoparasites (visit host for blood meal)
Representatives	Ixodes persulcatus, Ixodes ricinus, Dermacentor pictus, Dermacentor marginatus.	Argas persicus, Ornithodorus moubata, Ornithodorus papillipes.

                    

Medical importance. 1) Mechanical injury by the bite. 2) Tick paralysis: progressive flaccidity due to a failure of acetylcholine liberation in the neuromuscular junction. Tick’s toxin produces a block in the motor nerve fibers. The toxin is elaborated by the tick’s ovaries and is secreted by salivary glands. 3) Vectors of diseases: a) hard-bodied ticks (Ixodidae). Hard-bodied ticks are carriers of rickettsial, spirochaetal, viral, bacterial, and protozoan diseases. Ricketsial diseases: 1) American spotted fever (Rocky mountain spotted fever) caused by Rickettsia rickettsi, 2) Q-fever caused by Coxiella (Rickettsia bumeti). Modes of transmission of rickettsial diseases: a) by bite of the tick, b) contamination of bite wound with feces or coxal fluid. Rickettsiae are transmitted to progeny of ticks by transovarian mode. Spirochaetal diseases: Lyme disease caused by Borrelia burgdorferi. Viral diseases: 1) Colorado tick fever, 2) Encephalitis. Bacterial diseases: Tularemia caused by Francisella tularentis. Protozoan diseases: 1) Babesiosis caused by Babesia divergens. Mode of transmission of tick-borne diseases (spirochaetal, viral, bacterial, protozoan) is a tick bite; b) soft-bodied ticks (Ornithodorus) are vectors of endemic relapsing fever (caused by Borrelia duttoni) and Q-fever.

Control of ticks: 1) Repair of cracks. 2) Insecticide spraying on floors and cracks. 3) Infested animals are dusted by insecticide agents or dipped in special solution.

Careful prevention when working or playing outdoors in endemic areas is the safest approach.

In endemic areas: wear light-colored, tightly woven slacks and a long sleeved shirt; spray clothing with an insect repellant containing N,N-diethyltoluamide; try to stay out of dense brush; check yourself often for ticks; watch for early signs of spirochaetal diseases (e.g. Lyme disease), which can include a small red bump surrounded by a circular red rash, and/or fatigue, chills, headache, low-grade fever, and muscle and joint aches. Caught at an early stage, antibiotics can usually stop the infection.

 Insects, class Insecta, are by far the largest group of organisms on earth, whether measured in terms of numbers of species or numbers of individuals. Insects live in every conceivable habitat on land and fresh water, and a few have even invaded the sea. More than 70% of all named animal species are insects, and the actual proportion is doubtless much higher, because millions of additional forms await detection, classification, and naming. Most insects are relatively small, ranging in size from 0,1 mm to about 30 cm in length or wingspan. Insects have three body sections, the head, thorax, and abdomen; three pairs of legs, all attached to the thorax; and one pair of antennae. In addition, they may have one or two pairs of wings. Most insects have compound eyes, and many have ocelli as well. The mouthparts of insects are elaborate. They usually consist of the jaws, or mandibles, which are tough and unsegmented; a secondary pair of mouthparts, the maxillae, which are segmented; and the lower lip, or labium, which probably evolved from the fusion of another pair of maxilla-like structures. The upper lip, called the labrium, is of less certain origin. The hypopharynx is short, tonguelike organ (in chewing insects) that lies between the maxillae and above the labium; the salivary glands usually open on or near hypopharynx. Within this basic structural framework, the mouthparts vary widely among groups of insects, mainly in relation to their feeding habits. Many orders of insects – such as Coleoptera, the beetles; Hymenoptera, the bees, wasps, and ants; Isoptera, the termites; and Orthoptera, the grasshoppers, crickets, and their relatives – have chewing, or mandibulate, mouthparts. In other orders, the mouthparts may be elongated or styletlike. For example, in some flies (order Diptera) such as mosquitoes, blackflies, and horseflies, there are six piercing, fused stylets: the labrium, the mandibles, the maxillae, and the hypopharynx; the labium sheaths the stylets. In more advanced flies, the labium may be the principal piercing organ or may be expanded into large soft lobes through which liquid food is absorbed. The insect thorax consists of three segments (tagmata), each of which has a pair of legs. Ocasionally one or more of these pairs of legs is absent. Legs are completely absent in the larvae of certain groups – for example, in most members of the order Hymenoptera, the bees, wasps, and ants – and among the flies, order Diptera. If two pairs of wings are present, they are attached to the middle and posterior segments of the thorax; if only one pair of wings is present, it is usually attached to the middle segment. The thorax is almost entirely filled with muscles that operate the legs and wings. The wings of insects arise as saclike outgrowth of the body wall; in adult insects, they are solid, except for the veins. The internal features of insects resemble those of the other arthropods. Insects possess sophisticated means of sensing their environment, including sensory hairs to detect touch, tympanal organs to detect sound, and chemoreceptors to detect chemical signals called pheromones. Most young insects hatch from fertilized eggs laid outside their mother’s body. The zygote develops within the egg into young insect, which escapes by chewing its way out or by bursting the shell. During the course of their development into adults, young insects undergo ecdysis a number of times before they become adults and stop molting permanently. Most insects molt 4 to 8 times during the course of their development. The stages between the molts are defined as instars. There are two principal kinds of metamorphosis in insects: simple and complete. In simple metamorphosis, the wings, if present, develop externally during the juvenile stages; ordinarily no “resting” stage exists before the last molt. In complete metamorphosis, the wings develop internally during the juvenile stages and appear externally only during the resting stage that immediately precedes the final molt. During this stage, the insect is called a pupa or chrysalis, depending on the group to which it belongs. A pupa does not normally move around much, although the pupae of mosquitoes do move around freely. Avery large amount of internal reorganization of the insect’s body takes place while it is a pupa or chrysalis. In the insects with simple metamorphosis, the immature stages are often called nymphs. They are usually quite similar to the adults, differing mainly in their smaller size, less well-developed wings, and sometimes in their color. More than 90% of the insects, including the members of all of the largest and most successful orders, display complete metamorphosis, in which the juvenile stages and adults often live in distinct habitats, have different habits, and are usually extremely different in form. In these insects, development is indirect. Larvae in insects are immature stages, often wormlike, which differ greatly in appearance from the adults of the same species. Larvae do not have compound eyes. They may be legless or have legs as well as sometimes having leglike appendages on the abdomen. Pupae do not feed and are usually relatively inactive. As pupae, insects are extremely vulnerable to predators and parasites, they are often covered by a cocoon or some other protective structure. Groups of insects with complete metamorphosis include the moths and butterflies; beetles; bees, wasp, and ants; flies; and fleas.

Class Insecta is divided into 4 orders of medical importance:

1.     Order Anoplura (lice).

2.      Order Siphonaptera (fleas).

3.      Order Hemiptera (bugs).

4.      Order Diptera (mosquitoes and flies).

Order Anoplura (lice). Body is flattened dorso-ventrally. Lice are wingless insects with short legs. Order Anoplura displays incomplete metamorphosis.                                                                                                                                               

Lice are divided into: a) head louse (Pediculus humanus capitis);

 

 

 

 

 

 

 

 

 

 

                                                            Pediculus humanus capitis

b) body (clothes) louse (Pediculus humanus humanus); c) crablouse (Phthirus pubis).

Morphology (Pediculus humanus): 1. Adult louse is 2-5 mm in size, male is smaller than female. Body is divided into head, thorax and abdomen. Conical head carries 2 simple eyes, 2 five-segmented antennae, and retractile proboscis. Thorax consists of 3 fused segments and carries 3 pairs of short legs (which end with single tarsal segments and single claws) and one pair of respiratory spiracles. 8-segmented abdomen carries 6 pairs of respiratory spiracles on the each side of the first 6 segments, spring-like aedeagus protruding from the posterior end (male). The last abdominal segment of female is notched. Two triangular processes (gonopods) are located on the vagina’s sides.

2. Eggs (nits) are whitish, oval, operculated (crown-like), 0,7 – 0,8 mm in size. 3. Nymph resembles the adult louse but is smaller and sexually immature.

Life cycle. 1. Female louse lays about 10 eggs daily. 2. Eggs are cemented to hairs or to fibers of clothes. 3. Eggs hatch in about one week. 4. Nymphs feed on blood and pass through 4 instars. 5. Adult louse emerges in about two weeks.

Habits. Lice are permanent ectoparasites, they die out of the host’s body. If the body temperature rises (e.g. fever) or falls (e.g. death) lice leave the host, trying to find another host. They feed on blood several times a day.

Disease transmission (body louse). 1.Epidemic typhus caused by Rickettsia provaceki. Rickettsia invade the gut wall of the louse and multiply. After rupture of epithelial cells they are liberated into louse’s lumen and come out with the feces. The life of louse is shortened by infection from 4-6 weeks (normal span of life) to approximately 10 days. Mode of transmission of epidemic typhus: a) contamination of bite wound or mucous membranes by feces; b) inhalation of dust containing rickettsia. 2. Trench fever caused by Rickettsia quintana. The mode of transmission is the same as in case of epidemic typhus, but the life of the louse is not endangered. 3.Epidemic relapsing fever caused by Borrelia reccurentis. The organisms disappear from the gut of the louse and appear in the body fluid. Infection occurs by crushing the louse and contamination of the bite wound with the body fluid containing borrelia.

Pediculosis (Vagabond’s disease) means heavy infestation of hair with lice (Pediculus humanus capitis). The main symptoms of pediculosis are cutaneous lesions and pruritis.

Crablouse (Phthirus pubis) infests pubic region and armpits, and is the causative agent of pediculosis pubic. Morphology: a) crablouse is smaller than head louse or body (clothes) louse; b) body is short, thorax and abdomen are practically fused together. Medical importance: crablouse is an ectoparasite, the causative agent of pediculosis pubic; does not take part in disease transmission.

  Phthirus pubis

 

Prevention of lousiness (pediculosis) includes regular body washing with simultaneous change of linen, maintenance of cleanliness. Public accommodations should be kept clean. Special insecticide soaps and sprays are used for extermination of lice and nits.

Order Siphonaptera. Fleas are wingless insects. They are bilaterally compressed and covered with stiff hairs directed backwardly. Strong legs help fleas to jump. Order  Siphonaptera displays complete metamorphosis.

Morphology: 1. Adult flea is 2-3 mm in size. Body is divided into head thorax and abdomen. Head (1) is round or angular and carries 2 simple eyes (may be absent). Ocular hairs are located in front or below the eyes. Post-cephalic hairs may be present. Two short 3-segmented antennae (4) lie in the grooves behind the eyes. Proboscis is used for piercing and sucking. Thorax (2) consists of 3 segments and carries 3 pairs of strong legs (5). 10-segmented abdomen carries a sensory plate (pygidium or sensilium) on the dorsal side of the 9th segment. A stiff bristle (antepygidial bristle) arises dorsally from the 7^th segment.

 

 

 

 

 

 

 

 

 

                              Flea Pulex irritans

 2. Eggs are pearly white, oval, 0,5 mm in size and have the blunt ends. 3. Worm-like larva consists of head, thorax (3 segments), and abdomen (10 segments). All segments carry hairs directed backward. The last abdominal segment has a pair of conical processes (anal struts). 4.Pupa is enclosed in a cocoon.

Life cycle. 1. Eggs are laid on the ground (chaps, floor cracks, burrows of rodents). 2. Eggs hatch after a few days. 3.Larvae come out and pass through 3 instars. 4. The mature larva (about 2 weeks) spins a cocoon of viscid substance. 5. The adult is developed in about one week and comes out. The whole cycle takes from 1 to 3 months.

Habits. Fleas are either temporary ectoparasites (human flea) or permanent ectoparasites (rat flea). They feed on blood more than once daily, but can withstand starvation for a long time.

Medical importance. 1. Vector of plague caused by Yersinia pestis. The bacilli multiply and block the gut. Infection occurs by the bite of the blocked flea. 2. Vector of murine typhus caused by Rickettsia typhi. Rickettsia invade the gut wall of the flea and multiply. After rupture of epithelial cells they are liberated into flea’s lumen and come out with the feces. Mode of transmission of murine typhus is contamination of bite wound or mucous membranes by feces. 3. Flea may be a vector of tularemia. 4. Occasionally flea serves as intermediate host for Hymenolepis nana. 5.Bites of fleas cause itching and scratching with possible development of secondary (bacterial) infection.

Control of fleas. 1. Human flea: a) cleanliness and sweeping of dust from floor and carpets, b) application of insecticides and repellents (naphthaline). 2. Dog and cat flea: animals and their resting-places should be dusted with insecticides. 3. Rat flea: a) rodent control (use of rodenticides), b) rodent burrows should be dusted with insecticides.

Order Hemiptera (bugs). Body is flattened dorso-ventrally. Wings are present or may be in rudimentary state. Bugs display incomplete metamorphosis. Order Hemiptera includes 2 families of medical importance (Cimicidae and Reduvidae).

Family Cimicidae. Cimex lectularius (bedbug). Morphology. Adult bug is about 5 mm in size, male is smaller than female. Body is dark brown and divided into head, thorax, and abdomen.

 

    Cimex lectularius

Habits. Bugs are temporary ectoparasites. They feed by night and hide during the daytime. They can withstand starvation for a long time. Bedbugs emit a characteristic odor. Medical importance. Naturally bedbugs are not known to transmit any disease. Very rare mechanical transmission can occur. Experimentally could transmit relapsing fever (Borrelia reccurentis) and Chagas’ disease (Trypanosoma cruzi). The bites of bedbug are irritating and may lead to insomnia. Control: a) cleanliness, b) repair of cracks, c) manual collection of bugs and their destruction, d) application of insecticides to hiding places.

Family Reduvidae (Triatoma, reduviid bug, cone-nose or kissing bug). Triatoma bug is the vector of Chagas’ disease (American trypanosomiasis). Chagas’ disease occurs primarily in rural Central and South America. The reduviid bug lives in the walls of rural huts and feed at night. It bites preferentially around the mouth or eyes, hence the name is “kissing bug”. Infection occurs by contamination of bite wound with feces. Prevention of Chagas’ disease involves protection from reduviid bite, improved housing, and insect control.

 

 General characteristics of order Diptera (two-winged).  Mesothoracic pair of wings. Metathoracic pair of wings is modified into halters or balancers. Mouthparts are sucking or piercing and sucking. Complete metamorphosis.

Order Diptera is divided into 3 suborders:

	NEMATOCERA	BRACHYCERA	CYCLORRAPHA
Families	1.Culicidae (mosquitoes): Anopheles Culex Aedes 2.Psychodidae: Phlebotomus 3.Simuliidae: Simulium 4.Ceratopogonidae: Culicoides	1.Tabanidae: Chrysops Tabanus	1.Muscidae: Musca Stomoxis Glossina 2.Calliphoridae: Calliphora Lucilia Chysomiya Sarcophaga Wohlfahrtia 3.Oestridae: Oestrus Dermatobia Hypoderma

There are many species of flies. Broadly they can be divided into bloodsucking and nonbloodsucking types, and the mouthparts for each are appropriately adapted. The role of the nonbloodsucking flies in disease is one of mechanical transmission, since their feeding habits bring them in intimate contact with all sorts of filth. Bacterial, viral, protozoan, and helminthic agents of disease are known to be mechanically transferred by nonbiting flies. Although the bloodsucking flies also act as mechanical vectors of disease, they are far more important as intermediate hosts in the transfer of such human diseases as leishmaniasis, trypanosomiasis, onchocerciasis, and loa loa.

General morphology. Flies have general morphology of insects. They have only one pair of wings, the second pair being rudimentary knobs called halters. For this reason the mesothorax (middle section) is enlarged, and the prothorax and metathorax are rudimentary rings which unite the head and abdomen to the thorax. The position, venation, and markings of the wings are important in species identification. Flies possess three pairs of jointed legs that end in hooked claws or in hairy pads which may secrete a sticky substance. The head possesses a pair of large, compound eyes that may or may not meet in the midline and a pair of sensory antennae which are jointed in different ways depending upon the species. The mouthparts of the bloodsucking flies are highly developed; skin penetration is effected through saw-like or file-like modifications of the mandible or maxilla. In the female, the terminal segment of the abdomen contains the ovipositors.

Maggots. Flies complete their life cycle by complex metamorphosis through four stages: ovum, larva, pupa, and adult. Most flies procreate through the production of ova; a few species are viviparous. Although nonbloodsucking flies generally spread disease only by mechanical means, man can become infected directly by fly larva, known as maggots. Myiasis is the term used for this condition. Various tissues or organs may be involved, including the skin and wounds, the intestine, the urinary tract, nasal atria, ears and eyes. Adult flies may deposit their ova directly into wounds or necrotic tissue, or man may become infected by handling soil, filth, or excreta contaminated with fly ova. The worm-like larvae which hatch are equipped with chewing mouthparts with which they can feed on organic material or human tissue.

The adult flies most commonly concerned in myiasis belong either to the Muscidae (housefly) or the Oestridae (bot fly) families.Myiasis resulting from infection of ova from muscid flies, which includes the common domestic housefly, is considered accidental in man, since the larvae usually develop in decaying organic material. In contrast, the larvae of the bot fly are obligate parasites of the intestinal tract or other tissues of mammals, including man. In this sense, they must be considered true parasites.  

Family Muscidae. This family includes three flies of medical importance (Musca, Stomoxys and Glossina).

Musca domestica. Housefly is a medium-sized fly, measuring between 6 and 10 mm in length; the female is generally slightly larger than the male. Distribution: cosmopolitan. Morphology. Adult fly has a gray color. Body is divided into head, thorax, and abdomen. Life cycle. 1. Eggs are laid into masses of decaying organic substances, garbage, refuse or manure. 2. Larvae hatch in 6-24 hours and feed on organic matter. 3. They molt 2 times giving 3 larval stages. 4. The third larva pupates inside the larval skin. 5. The adult emerges after a few days through a circular cut of the puparium. 6. The whole cycle takes about one week.

Medical importance. 1. Indirect mechanical transmission of microorganisms (as typhoid, poliomyelitis and bacillary dysentery), eggs of helminthes and cysts of protozoa. 2. Accidental myasis.

Control. 1. Sanitary disposal of refuses, garbage and manure (breeding media) by dumping, burning or application of insecticides. 2. Control of adult flies by screening or space spraying of insecticides.

         Myiasis is invasion of tissues of animals or man by larvae of dipterous fly.

Classification of myiasis. 1. According to the habits of the flies:

 a) Specific myiasis. In this case flies are obligatory tissue parasites; larvae develop only in living tissues (obligatory sarcobiots). The place of flies’ oviposition or larvaeposition is located in or near living tissues. Examples. 1. Members of family Oestridae: Oestrus, Hypoderma, Dermatobia. 2. Gasterophitus. 3. Cordilobia (lay eggs on ground or clothing, larvae penetrate unbroken skin). 4. Some species of Chrysomiya or Wolfahrtia (larvae do not penetrate unbroken skin, only wounded or diseased tissues).

 b) Semi-specific myiasis. In this case flies are obligatory necrobiots; they lay eggs or larvae on decaying matter but may attack tissues (facultative sarcobiots) attracted by specific emanating odor from discharges of diseased tissues or wounds, e.g. members of family Calliphoridae.

 c) Accidental myiasis (larvae may accidentally get in the tissues, e.g. Musca, Stomoxis and Fannia.

2. According to habitat (type of invaded tissue):

 a) intestinal e.g. Musca, Calliphora, Lucilia,and Sarcophaga.

      b) gastric e.g. Eristalis.

      c) urogenital e.g. Fannia (lays eggs on urethral opening).

      d) cutaneous: 1. Traumatic (wound) myiasis invade wounds or ulcers e.g. members of family      

Calliphoridae. 2. Creeping eruption e.g. Hypoderma. 3. Nodular e.g. Dermatobia.

      e) ocular e.g.  Oestrus, Wohlfahrtia and Sarcophaga.

      f) aural e.g.  Wohlfahrtia and Sarcophaga.

      g) nasopharyngeal e.g.  Wohlfahrtia and Sarcophaga.

Diagnosis is based on finding of larvae in the lesion. Larvae are identified by the characteristic posterior spiracles. Living larvae may be reared to adult stage for identification.

Treatment: removal of larvae.

Bloodsucking flies.

Fly genus	Morphology	Habits, distribution	Symptoms of bite	Parasite and disease transmitted
Phlebotomus  Sandfly  Moth fly  Owl midge	1.Thorax is humpbacked 2.Small size (3 mm) 3.Body and wings quite hairy 4.Wings devoid of scales 5.Wings at rest held in V at 60° angle 6.Wings veined in parallel lines 7.Mouthparts structured for piercing and biting. 8.Yellow-buff color	Active nocturnal feeders; female alone are bloodsuckers Cosmopolitan in tropical and subtropical climates Breeding places hard to find: under stones, in masonry cracks, poultry houses, hollow trees; breeding places nonaquatic	Rose-colored papule at site of bite, with 10-20 mm zone of erythema Stinging pain and itching Nausea, fever, and malaise in sensitive individuals	Leishmania donovani (kala-azar) Leishmania tropica (Oriental sore) L.braziliensis (American leishmaniasis) Phlebotomus fever (pappataci fever) Bartonella bacilliformis (Oroya fever, Carrion’s disease)
Simulium  Black fly  Buffalo gnat	1.Small size (2-3 mm) 2.Boy black 3.Body sturdy, legs short 4.Conspicuous compound eyes 5.Short proboscis with blade-like cutting organs 6.Wings broad and usually clear 7.Body appears longitudinally striped due to fine silver	Cosmopolitan in distribution Breeds along shaded watercourses or woodland streams Females bite in daytime	Bite initially painless; wound bleeds profusely Pain, itching, and swelling develop later	Onchocerca volvulus (onchocerciasis)
Chrysops  Deer fly  Tabanid  Mangrove fly	1.Highly colored: yellow banded abdomen with dark stripes 2.Mouthparts fitted for stabbing and cutting 3.Wings clear with dark band along anterior margin; wings held spread away and horizontal to body	Cosmopolitan, but more common in Americas Female bite in early morning or late afternoon Breeds in woodlands, marshy ponds, or roadside ditches	Several thrusts of cutting mouthparts leave an unsightly puncture wound Pain and swelling develop within a few hours	Loa loa Pasteurella tularensis (tularemia)
Glossina   Tsetse fly	1.Slightly larger than housefly 2.Brownish color 3.Resting wings overcrossed like scissor blades 4.Slender proboscis held horizontal to ground	Found in equatorial Africa G.palpalis: hot, dump areas along borders of rivers, lakes, and streams in West Africa Males and females bite by day	Bite of minor significance	Trypanosoma rhodesiense T. gambiense (African sleeping sickness)

 

Mosquitoes

Mosquitoes are annoying, swarming, biting pests, and some are carriers of malaria, yellow fever, dengue fever, filariasis, and encephalomyelitis. Mosquitoes can be distinguished from other Diptera by their fragile, small appearance. Females have elongated mouthparts and a distinct proboscis well suited for piercing and sucking blood. Scales are present along the wing veins and along the lower wing margins. The sexes can be distinguished by the antennae: in females they are long and slender with a few short hairs; in the male they are feathery and plumose. Close examination shows the antennae to be composed of 14 or 15 segments. The venation of the wings is also characteristic for mosquitoes and is helpful in species identification. Mosquitoes are particularly attracted to man and animals, especially to bright light and dark-colored clothing. The females are the bloodsuckers and cannot produce fertile eggs without ingesting blood. It is postulated that the blood supplies the serotonin and epinephrine necessary for production of gonadotropic hormone by the mosquito, required for ovulation. The eggs are laid on the surface of water and hatch into aquatic “wiggletails”. These subsequently metamorphose into adults. Three genera -  Anopheles, Aedes, Culex- transmit disease to man.

 

Differences between Anopheles and Culex.

        

     Mosquito control. 1. Mechanical methods. Wire screens for windows and doors. Mosquito nets. Repellants; certain lotions or creams applied to the skin repel mosquitoes from biting e.g. citrgnella oil, Indalone. Animal barrier: placing farm animals between the breeding places and human habitations would deviate mosquitoes (particularly zoophilic) from biting man. 2.Natural or physical methods (ecological interference). Changing the environment so as to become unsuitable for the mosquito e.g. filling or drainage of the breeding place. Developing of shade if larvae require sunshine and vice versa. Changing of water level, water current, pH. 3.Biological methods. Introduction of a natural enemy (predator): a) fish (Gambusia affinis) feed on larvae and pupae b) release of sterilized males (X-ray or chemical agents) which compete with males in nature, mating with females, and resulting in reduction of the forthcoming generation. 4. Chemical methods (insecticides): a) against adults (adulticides). Insecticides act on contact being absorbed through the cuticle (contact poison). These are applied as: 1. Space spray with an insecticide, which causes immediate knockdown of mosquitoes e.g. pyrethrum (active ingredient in Flit), but it has no residual action. 2. Residual spray of resting places of adults (walls) using an insecticide of long action (residual insecticide) e.g. hydrocarbons as DDT and Gammaxane, organophosphorous compounds as Malathion and Diphterex, carbamates as Sevin. The development of resistance is drawback of insecticides. b) against aquatic stages. Special agents are applied to water surface. Stomach poison: Paris green applied as 1% in light dust to water surface. Anopheline larvae are affected more than culicines, being surface feeders. Pupae are not affected, as they do not feed. Respiratory poison: applying non-volatile oils suffocates larvae and pupae by blocking the respiratory spiracles. Residual insecticides are sprayed on water surface mixed with dust or oil. They exert their effect by contact and ingestion.

 Practically no single method alone is efficient in control. The combined use of two or more methods (integrated control) would provide a higher and more efficient level of abatement.  Insects escaped one method, are killed by the other.