CHAPTER 2 Arthropods are invertebrates with jointed legs. Their bodies are bilaterally symmetrical and segmented. They are the most diverse of all animal life-forms. Two groups are of major veterinary importance: the insects with six legs and the arachnids with eight (see Figure 2.1). A third group, the Crustacea, contains just a couple of parasitic genera worthy of attention. Figure 2.1 The major groups of parasitic arthropods. Most, but not all, arthropods of veterinary interest are associated in various ways with body surface tissues (see Figure 2.2). They commonly cause skin trauma and inflammatory or allergic reactions, often accompanied by pain or itching (pruritus). These infestations therefore play a prominent role in clinical dermatology. Figure 2.2 Some associations between arthropod parasites and skin: a – blood-sucking (tick); b – surface feeding on secretions and exudates (muscid fly); c – flesh-eating (cutaneous myiasis); d – surface feeding on skin debris (chewing louse); e – burrowing mite; f – warble fly developing under skin; g – mite in hair follicle. Based on Jacobs, 1986 with permission of Elsevier. Warble fly redrawn after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins. Demodex redrawn after James and Harwood, 1969 from Cheng, 1986 with permission of Elsevier. This chapter focuses on the broad spectrum of the insect world, while the next covers parasitic acarina, crustacea and the drugs that are used to combat ectoparasites. The insects of greatest concern in clinical practice are fleas, lice and flies, although there are many others that can adversely affect animal and human health by means of bites, stings, blister-inducing venoms or irritating hairs, all of which can induce allergies and sometimes anaphylaxis. Insects also destroy and foul feedstuffs. Cockroaches are particularly adept at contaminating food and other surfaces by the mechanical transfer of pathogens. Along with many beetles, they can also act as transport or intermediate hosts for some helminth infections, e.g. the spiruroid stomach worms of dogs and pigs (see Table 2.2). Help box 2.1 A general appreciation of the body structure and function of insects and other parasites is needed at a practical level for two main reasons. Firstly, as an aid to diagnosis, since familiarity with the terminology used in books, on-line resources and identification keys facilitates the accurate recognition of pathogens. Secondly, to enhance understanding of key biological features that influence the epidemiology, pathogenesis or control of related disease processes. The insect body is divided into three distinct regions: head, thorax and abdomen. These divisions are clearly seen in the sheep ked illustrated in Figure 2.3. The entire surface is covered by a hard protective noncellular exoskeleton, which is secreted by an underlying epidermis. The exoskeleton has three cuticular layers containing chitin (a long-chained polysaccharide) which provides mechanical support. The chitin molecules are sometimes cross-linked for extra strength. The outer layer often has a waxy surface. Figure 2.3 The sheep ked, Melophagus ovinus. Redrawn after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins. The exoskeleton is composed of plates linked by flexible intersegmental membranes. These provide weak spots where contact insecticides can penetrate through to sensitive tissues beneath. The insect head is composed of a number of fused plates and acts as a support for eyes, mouthparts and a single pair of antennae covered with sensory hairs and bristles (see Figure 2.4). Figure 2.4 Head of the blowfly Lucilia. Adapted after Mönnig from Soulsby, 1982 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins. The eyes vary greatly in complexity from simple ‘ocelli’, which monitor light intensity, to compound eyes made up of a honeycomb-like congregation of thousands of tubes, each covered by a cornea and containing sensory cells. These are sensitive to movement. Insect mouthparts consist of several basic structures: upper and lower lips (the ‘labrum’ and the ‘labium’), two pairs of jaws (‘mandibles’ and ‘maxillae’) and a protuberance like a drinking straw that channels saliva to where it is needed (‘hypopharynx’). There may also be additional finger-like projections carrying sensory organs. The mouthparts differ in shape and size to suit the particular feeding habit of each type of insect. Two contrasting examples are shown in Figure 2.5. The mouthparts of the female mosquito on the left are long and slender, coming together to form a needle-like tube ideal for penetrating skin, probing for blood vessels and sucking blood, whereas the lower lip of Musca, on the right, is expanded into two retractable sponge-like structures (‘labellae’) which are used to mop up surface-lying pools of liquefied food. Figure 2.5 Heads of a mosquito (left) and Musca (right). Adapted from Urquhart et al., 1996 with permission of John Wiley & Sons, Ltd. Insects often have different feeding behaviours in their juvenile and adult stages with correspondingly dissimilar mouthparts. Compare, for example, the chitinous lacerating structures of blowfly larvae (see Figure 2.6) with the scraping and sponging mouthparts of the adult fly (similar to those of Musca illustrated in Figure 2.5). In many cases, the sexes utilise different food sources with males taking nectar from flowers while females also seek protein meals. This is often blood in the case of parasitic insects. Figure 2.6 Head-end of a blowfly larva (maggot) showing the chitinous mouthparts (arrowed). Redrawn from Zumpt, 1965. The form of the antenna varies greatly and is sometimes useful for identification (see Figure 2.7). Some antennae are long and segmented (e.g. mosquitoes) while others are short and squat (e.g. horse flies). Some are hairy, whilst others (e.g. houseflies and blowflies) carry special bristles (‘aristae’). The antennae of males and females of some species are also different. Figure 2.7 Antennae of a mosquito (top); a horse fly (centre) and an adult blowfly (below). Adapted from Urquhart et al., 1996 with permission of John Wiley & Sons, Ltd. The thorax is covered with fused plates supporting three pairs of jointed legs, each made up of several parts (see Figure 2.8). Insects that fly also have one or two pairs of wings mounted on the thorax. Figure 2.8 An insect leg: a – coxa; b – trochanter; c – femur; d – tibia; e – tarsus; f – claw. Redrawn after Cheng, 1986 with permission of Elsevier. The wing is a membranous outgrowth of the exoskeleton, supported and strengthened by a network of ‘veins’ which comprise breathing tubes (‘tracheae’) and blood vessels. The arrangement of the wing veins (‘venation’) is important in the identification of adult flies as illustrated in Figure 2.9 which compares the wing of a horse fly (which has a central ‘discal cell’) and a tsetse fly (which has a characteristic ‘butcher’s cleaver’-shaped cell). Figure 2.9 Wing of a horse fly (a) showing the ‘discal cell’ (arrowed) and a tsetse fly (b) showing the ‘butcher’s cleaver’-shaped cell (arrowed). Redrawn after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins. One important group of flies, the Diptera, is easily recognised as they have only one pair of wings (see Figure 2.10). The posterior pair has evolved into small gyroscopic balancing organs (‘halteres’). Figure 2.10 Wing of a dipteran fly showing the halteres. Redrawn and modified after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins. The abdomen is usually a clearly segmented, soft structure. Various appendages may be present such as copulatory claspers, an ovipositor or external genitalia. The majority of body systems are located in the abdomen. The respiratory system consists of a network of branching tubes (‘tracheae’), which are strengthened by spiral thickenings. Airflow is regulated by muscular contractions of the body wall. The tracheae communicate with the atmosphere by means of chitinous surface openings (‘spiracles’) which can be opened and closed (see Figure 2.11). The shape of these and the plates upon which they are mounted (‘stigmatic plates’) are used for the identification of blowfly larvae in forensic medicine. (This, together with knowledge of the rates of development of individual species, can be used in investigations of human murder or animal abuse and neglect cases to deduce, for example, time of death and whether there had been prior trauma.) Figure 2.11 Spiracles of a blowfly larva. Redrawn after Zumpt, 1965. The alimentary canal is divided into three main regions: a foregut which processes ingested food mechanically, a midgut for storage and enzyme secretion, and a hindgut for water resorption (see Figure 2.12). The midgut is also an outlet for the ‘malpighian tubules’, which are the insect equivalent of kidneys. The circulatory system comprises a dorsally-situated heart (essentially a thick-walled blood vessel with valves which permit the blood to flow only forwards) and branching blood vessels. There are no capillaries. Instead, the blood flows into the general body cavity (‘haemocoele’) which bathes the organs and tissues. It returns to the heart via openings in blood vessel walls. Figure 2.12 An insect in longitudinal section. Redrawn after Imms, 1957 and Urquhart et al., 1996 with permission of John Wiley & Sons, Ltd. The nervous system, which is the target for many insecticides, consists of a small brain, just above the pharynx, and a chain of fused ganglia, which lie on the floor of the thorax and abdomen and give off nerves to the various body parts. The ‘fat body’ is a large structure made up of cells containing numerous fat vacuoles. Its form is variable but in many insects it lines the body cavity and all the internal organs (like the peritoneum in mammals). The fat body has many metabolic functions and acts as a food reserve for use during periods of hibernation or starvation. It also absorbs fat soluble insecticides, reducing their potency. Most insect species have separate sexes. The reproductive tracts are directly analogous to those of vertebrates, i.e. testes, vas deferens and seminal vesicles (male) and ovaries, oviducts, common oviduct (or uterus) and vagina (female). Females also have an accessory organ, the spermatheca, which acts as a receptacle for spermatozoa after mating. This enables the female to fertilise subsequent batches of eggs in the absence of a male. Most adult female insects are ‘oviparous’, i.e. they lay eggs which hatch after deposition. Some species are ‘viviparous’, i.e. the egg ruptures at some stage within the reproductive tract so the female gives birth to a juvenile. There are two main types of insect life-cycle: simple and complex metamorphosis. In simple metamorphosis (see Figure 2.13), the insect which emerges from the egg resembles the adult and is called a ‘nymph’. This grows and undergoes several moults (‘ecdyses’) before attaining maturity. Figure 2.13 An example of simple metamorphosis (life-cycle of a louse): a – adult louse; b – egg; c – nymphal stages. Adult redrawn after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins, diagram partly based on Urquhart et al., 1996 with permission of John Wiley & Sons, Ltd. In complex metamorphosis (see Figure 2.14), the young insect which emerges from the egg shows marked differences in morphology and structure from the adult and is called a ‘larva’. After feeding, growing and moulting several times, the larva enters a quiescent phase and the outer cuticle hardens to form the ‘pupa’. Some species spin a silk-like protective covering (the ‘cocoon’) before they pupate. The adult form develops inside the pupal case before the ‘imago’ finally emerges. Figure 2.14 An example of complex metamorphosis – the life-cycle of a dipteran fly, Stomoxys: a – adult fly; b – egg; c – larval stages; d – pupa. Adult redrawn after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins. Diagram partly based on Urquhart et al., 1996 with permission of John Wiley & Sons, Ltd Help box 2.2 Adult fleas have an unmistakable appearance with a narrow mahogany-brown body that is superbly adapted for moving swiftly through fur or feathers (see Figure 2.15). They are wingless but their powerful hind legs enable them to jump many times their own height. Figure 2.15 Fleas are well adapted to living in the hair-coat of their host (SEM). Reproduced with permission of Bayer plc. As blood-sucking ectoparasites, fleas are of considerable importance in small animal dermatology. Large numbers cause pruritus, considerable annoyance and sometimes anaemia. Moderate numbers are usually well tolerated and may not be noticed by the owner. Some animals, however, become allergic to substances in flea saliva. In highly vulnerable individuals, just a few bites will trigger hypersensitivity reactions. In addition, fleas act as intermediate host for the tapeworm Dipylidium (see Section 5.3.5) and as vectors for microorganisms, such as those associated with cat scratch fever and bubonic plague in humans and myxomatosis in rabbits. Most flea species have evolved as parasites of nest-building mammals and birds. Many have a preferred host but will feed on other animals. Primates and herbivores, therefore, tend to be incidental victims. The most important fleas found on cats and dogs are listed in Table 2.1, although other species that feed mainly on wild-life may also be found occasionally on pet animals. In warmer regions, jiggers and stick-tight fleas can cause great distress to humans and pets as they burrow deeply into the skin. Table 2.1 Some important fleas found on dogs and cats Table 2.2 Some important lice of domesticated animals and poultry * Bovicola is also known as Damalinia. Adult fleas are small (1.5–4 mm) but easily seen with the naked eye. They are flattened from side to side (‘laterally compressed’). Each body segment has a row of backward pointing bristles (‘setae’) which ensure that they move through dense hair in one direction only – forwards (see Figure 2.16). This helps to protect them from host grooming activities. Identification requires some expertise but is aided by the rows of heavy chitinous spines (‘combs’ or ‘ctenidia’) that occur along the ventral (‘genal’) or posterior (‘pronotal’) aspects of the head (see Figure 2.17). Figure 2.16 Head-end of flea (SEM). Reproduced with permission of Bayer plc. Figure 2.17 A chart for identifying the fleas most commonly found on dogs and cats. Fleas redrawn after Smart, 1943 with permission of The Natural History Museum, London, UK and Smit, 1957 with permission of the Royal Entomological Society, St Albans, UK. Although all flea life-cycles follow the same general pattern, there is variation between species. Further discussion in this section is confined to the cat-flea, Ctenocephalides felis (see Figure 2.18). A full understanding of this species is required so that appropriate advice on flea control can be given to pet owners. Host-seeking adult C. felis use changes in light intensity, warmth and CO2 to locate a host (cat, dog or other animal). Within minutes of jumping onto the host, they start to take frequent small blood meals. Females are larger than males and have a greater appetite. Seventy fleas can withdraw 1 ml of blood per day. They produce copious amounts of dark red faeces, known colloquially as ‘flea dirt’. The life-span of a cat-flea is determined by the grooming efficiency of the host, but is typically around 7–10 days. The cat flea is an ‘almost permanent’ parasite. There may be a small degree of transfer of fleas between animals in close contact, but the great majority are reluctant to leave once they have found a suitable host. After they start to feed, any cat-fleas that fall off an animal survive only a few hours. The eggs start to hatch in 1–6 days (all off-host life-cycle phases are temperature dependent). The first larval instar is 2 mm long and after a period of growth moults to the second stage. The process is repeated until the third larval instar reaches about 5 mm in 1–7 weeks. The larvae (see Figure 2.20) are maggot-like and covered with bristles, which together with a pair of posterior protrusions (‘anal struts’) are used to assist rapid movement. They avoid light (i.e. they are ‘negatively phototropic’) and squirm deeply into the pile of carpets, under cushions and furniture and into dark crevasses. Their main source of nutrition is the flea dirt that falls off flea-infested animals, but they also eat shed skin flakes and other organic debris. They need high humidity to survive and are killed by near freezing temperatures. Figure 2.18 Life-cycle of the cat-flea Ctenocephalides felis: a – newly emerged adult flea jumps onto host; b – egg drops to ground; c – larva hatches, hides and develops; d – larva spins cocoon and pupates; e – following metamorphosis, adult flea emerges (details in text which uses same lettering as shown above). Adult (e) redrawn after Smit, 1957, with permission of the Royal Entomological Society, St Albans, UK; larva (c) drawn from photograph by © P. J. Bryant; larva (d) drawn from photograph in Krämer and Mencke, 2001. Figure 2.19 Flea eggs. Reproduced with permission of D.-H. Choe. Figure 2.20 Flea larvae among hatched eggs and flea faeces. Reproduced with permission of D.-H. Choe. Figure 2.21 Flea cocoons covered in sand particles. Reproduced with permission of D.-H. Choe. The time for an egg to hatch and the larva to become a fully formed adult in the cocoon is typically 3–4 weeks in summer but this can be faster or slower depending on temperature and humidity. The complex development of the immature stages is coordinated by a juvenile hormone which activates genetic switches that determine the sequential development of organs and tissues. Chemicals that disrupt this hormonal activity, called Insect Growth Regulators (IGRs), can be used to break the flea life-cycle (see Section 3.5.3). The fleas on an animal are greatly outnumbered by the developing eggs, larvae, pupae and newly-emerged host-seeking adults in its environment. The largest accumulations of eggs and larvae tend to be where infested animals spend most time. These are known as ‘hotspots’. They can be anywhere in the house, car or outbuildings and, in warmer climates, in humid, shady spots in the garden. Particularly high concentrations of eggs are found where pets sleep or where cats land after jumping. All animals develop pruritic papules when bitten by fleas. In heavy infestations, excessive grooming can lead to self-trauma. Some individuals become hypersensitive to flea bites, especially if exposure is long-term and intermittent. Several protein allergens occur in flea saliva, but not all are recognised by the immune systems of all cats or dogs. Responses therefore vary between individuals but often include IgE-mediated immediate hypersensitivity and delayed cell-mediated hypersensitivity reactions. In a small proportion of cases just a few flea bites can initiate extensive papular lesions with severe pruritus provoking self-harm through licking, scratching and biting. This disease process is known as ‘flea allergic dermatitis’, often abbreviated to FAD. Lice are dorsoventrally flattened, wingless insects with legs ending in claws. They are relatively small (just a few millimetres long), host-specific and spend the whole of their life-cycle on the same animal, causing a condition known as ‘pediculosis’. There are two types of louse: chewing lice (Mallophaga) and sucking lice (Anoplura). An ability to distinguish these is of clinical value as this influences chemotherapeutic choices, chewing lice being less vulnerable than sucking lice to systemic insecticides. Chewing lice are found on both mammals and birds. They have retained primitive insect mouthparts which are ideal for biting and rasping the surface of hair shafts, skin flakes and scabs, while those on birds also eat feathers and down. To accommodate this style of feeding, they have a broad head which is often wider than the thorax (see Figure 2.22). Their claws are relatively small (compared to those on sucking lice) and are single on mammalian parasites and double on avian species. Figure 2.22 A chewing louse. Redrawn after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins Sucking lice occur only on mammals. They have evolved highly specialised mouthparts for piercing skin and drawing up blood and tissue fluids. The head is adapted for exerting downward pressure and so is narrower than the thorax (Figure 2.23). In order to maintain a correct body position while feeding, they have powerful legs with big claws for holding onto hairs. When closed, each claw meets a thumb-like projection and pad on the leg to form a ring perfectly matching the circumference of their particular host’s hair shaft. Figure 2.23 A sucking louse. Redrawn after Mönnig from Lapage, 1962 with permission of Wolters Kluwer Health – Lippincott, Williams & Wilkins. Louse eggs, known as ‘nits’, are cemented individually onto hairs (see Figure 2.24). A nymph hatches from the egg (there is no larval stage) and moults three times before becoming an adult (see Figure 2.13). Parthenogenesis can take place in some species. The whole life-cycle occurs on the animal and takes 2–3 weeks. Figure 2.24 A louse egg attached to a hair (below-left) and a sucking louse (above-right). Reproduced with permission of AHVLA, Carmarthen, © Crown copyright 2013. Heavy infestations with lice irritate the skin. Host scratching and licking exacerbate the condition as allergenic substances in louse saliva and faeces are rubbed into the bite wounds. Pediculosis is therefore characterised by skin damage and loss of hair or feathers. It can result in reduced productivity in farm livestock. Sucking lice can also cause anaemia if present in large numbers. Transmission from host to host occurs only during close contact. Lice cannot survive more than a few days if accidentally separated from their host. Pediculosis in farm animals is usually seen in the winter when their coat is thickest. This creates a particularly favourable microclimate for parasitic development. Sick or debilitated animals often carry large numbers of lice as they do not groom efficiently. Cattle have one species of chewing louse and several sucking lice (see Table 2.2). Each has its own predilection site on the body and some tend to cluster. They are seen mostly in housed cattle in winter. Heavy infestations cause ill-thrift, anaemia and lead to downgraded leather. Sheep have one chewing louse and two sucking lice (the ‘foot louse’ and the ‘face louse’) which can result in damage to the fleece. They became rare in countries with compulsory dipping programmes (e.g. for sheep scab control – see Section 3.3.3), but their population numbers are rising again. This may be an unintended consequence of regulatory restrictions placed on the use of insecticides. Pigs have just one large (5 mm) sucking louse which is easily seen on the sparsely haired skin. It is very common, particularly on adult pigs. It can act as a vector for African swine fever and some other viruses and rickettsiae. Horses have one chewing louse and one sucking louse. The former favours skin with finer hair while the latter is seen mainly in the mane and tail, but they can spread over the whole body. Dogs have both a chewing louse and a sucking species, while cats have only a chewing louse. Heavy infestations are often, but not always, associated with neglect. Birds have many species of chewing lice, but no sucking lice. The most pathogenic belong to the genera Lipeurus (including the ‘wing louse’ and ‘head louse’) and Menacanthus (the ‘body louse’). Help box 2.3
Arthropods part 1: introduction and insects
2.1 Introduction
2.2 Insects
2.2.1 Key concepts
Body structure
Head
Thorax
Abdomen
Reproduction
2.2.2 Fleas (Siphonaptera)
Important fleas
Common name
Species name
Distribution
Notes (More information)
Cat flea
Ctenocephalides felis
Cosmopolitan (different subspecies occur in some locations)
The most common flea on dogs as well as cats; also bites humans and other animals (see Section 9.2.3)
Dog flea
Ctenocephalides canis
Cosmopolitan
Less common than previously; fairly host specific
Poultry flea
Ceratophyllus gallinae
Cosmopolitan
Mainly poultry; also bites humans and other animals
Rabbit flea
Spilopsyllus cuniculi
Cosmopolitan
Mainly rabbits; also bites other animals
Human flea
Pulex irritans
Cosmopolitan; now rare in many affluent countries
Mainly pigs and humans; also bites other animals
Jigger flea
Tunga penetrans
Parts of Africa, Asia and the Americas
Mainly pigs and humans; also bites other animals. Females burrow into skin
Stick-tight flea
Echidnophaga gallinacea
Tropics and warmer regions
Mainly poultry; also bites humans and other animals. Females burrow into skin
Host
Chewing lice
Sucking lice
More information in Section:
Cattle
Bovicola* bovis
Linognathus vituli, Haematopinus spp. etc.
Sheep
Bovicola* ovis
Linognathus spp.
Pig
None
Haematopinus suis
Horse
Bovicola* equi
Haematopinus asini
9.1.3
Dog
Trichodectes canis
Linognathus setosus
9.2.3
Cat
Felicola subrostratus
None
Poultry
Many
None
8.4.2
General characteristics
Life-cycle
Flea habitats
Pathogenesis
2.2.3 Lice (Phthiraptera)
Chewing lice
Sucking lice
Life-cycle
Pediculosis
Cattle
Sheep
Pigs
Horses
Dogs and cats
Poultry
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