Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance/Sucking lice

Lice (Phthiraptera)

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Characters of lice

Lice are insects that live as obligate parasites; that is, they are all so specialized that they can only feed and develop as parasites. Each feeding stage of the life-cycle takes repeated small meals. Lice are flattened dorso-ventrally, and never develop wings. On their hosts lice show a dull surface and are colored from pale yellow through to dark brown.

Photograph shows a live female Head-louse, genus Pediculus, that infests the scalp of humans. Note its size against the strands of hair. (Photo by Gilles San Martin).

Lice live entirely on their hosts; they crawl from one individual host to another when hosts are in close contact. Some species are able to survive for a short time off their host. Females attach their eggs in rows onto hairs or feathers of their hosts. These glued eggs are called nits and are easily recognized as a sign of louse infestation. Lice steadily lay small batches of large eggs and their survival rate is high as they develop into nymphs. This good survival enables louse infestations to increase to high densities per host under favourable conditions, typically when hosts are housed close together for long periods. However, sparse infestations by lice typically cause little or no signs of disease.

Each species of louse is specialized for feeding on one species of host, or on a few closely related species. This permits specific adaptations for digestion of blood and evasions of immune reactions by their hosts. These associations also simplify identification of these parasites. The Order Phthiraptera is thought to have multiple origins, making their classification difficult and varied. (Older groupings of the lice included the blood-sucking Siphunculata and the skin-chewing Mallophaga). However, lice converged during evolution, and for clinical purposes are easily grouped into those specialized to suck blood (sub-Order Anoplura), and those specialized to chew on skin scales, hair or feathers. There are two sub-Orders of chewing lice, the Amblycera and the Ischnocera .

Glossary

• Abdomen = Posterior part of body of insects, containing gut, gonads and other organs.
• Bristle = A large thick type of seta (6 on Pediculus).
• Claw = Legs of most insects and acarines end in hard sharp gripping organs (5 on Haematopinus).
• Eye = Most insects have prominent eyes on their head, either compound eyes of many sensory units, or simple eyes of one sensory unit (2 on Pediculus).
• Granuloma = Scar tissue, as often forms in skin where an insect or acarine has fed and made an inflamed wound.
• Nit = Vernacular and clinical term for egg of a louse glued to hair or feathers of host (10 on Haematopinus).
• Obligate = A form of parasitism where the parasite can only feed parasitically, in contrast to facultative parasitism where the animal can optionally feed in a free-living or non-parasitic way.
• Paratergal plate = Hardened (sclerotized) plates on the lateral margins of abdominal segments of some lice (5 on Pediculus).
• Pediculosis = Clinical term for the disease state of heavy infestation with lice.
• Pruritus = Itching.
• Seta = A pivoted moveable extension of the body wall of insects and acarines (5 on Solenopotes).
• Sclerotize = When part of the body wall of an insect or acarine becomes tanned and harder than surrounding areas.
• Spiracle = Opening in body wall to allow respiration (4 on Pthirus).
• Sternal plate = A sclerotized part of the body wall on the ventral surface of a louse (8 on Linognathus).
• Sucking = This term is used to distinguish the anopluran lice, which suck up blood from their hosts with needle-like mouthparts, from the amblyceran and ischnoceran chewing lice, which have no blood-sucking mouthparts (see Chewing lice ).
• Tubercle = A hump shaped protrusion of the body wall (4 on Pthirus).

Characters of Sucking lice (Anoplura)

The mouthparts are thin and long for piercing, and are retracted when not in use. There are no sensory palps. Antennae usually consist of five segments. The thorax has its three segments fused together, without obvious divisions. Spiracles are usually visible dorsally; a pair on the thorax and one at each side of most abdominal segments. Eyes occur on some species, as a pair on their head. These lice parasitize many species of mammals.

Photograph shows a female Linognathus louse from an infestation on cattle, showing gripping of host hair, and outlines of two eggs (arrowed). Note that the mouthparts of sucking lice are retracted within the head between sessions of blood feeding.

Diagram of feeding at skin shows a typical sucking louse penetrating dermal capillaries of host with long fine mouthparts (proportions of louse to skin are not drawn accurately).

Pediculus (Pediculidae)

Characters: female, dorsal. 1- Width of head is equal to its length. 2- Antenna has 5 segments. 3- Eyes are prominent. 4- Abdomen is elongate, bulging centrally. 5- Abdominal paratergal plates are prominent. 6- Tibial spurs have small bristles.

Hosts: The two species (or sub-species) P.humanus humanus, the Body-louse, and P.humanus capitis, the Head-louse parasitize only humans .

Signs and symptoms: Body-lice reside and lay their eggs on clothing of humans. They crawl onto their host's skin to feed on blood, resulting in small localized inflamed and pruritic sites. Chronic heavy infestations lead to a thickening and added pigmentation of the skin. Head-lice reside in the hair of the head and lay eggs (known as nits) on those hairs. These eggs or their empty cases are the most easily seen sign of infestation. Inflammation and pruritus result from these lice feeding. Infestations of humans by Body-lice are commonest either on individuals living in severe poverty, or on larger groups of people forced to crowd together under conditions of social collapse, warfare or refugee camps .

Disease: The Head-louse is not known to transmit any pathogens. Heavy infestations of the Body-louse cause the condition is known as pediculosis, experienced literally as feeling lousy. The Body-louse transmits to humans three species of bacteria: Rickettsia prowazekii causing epidemic or louse-borne typhus. ; Bartonella (formerly Rochalimaea) quintana causing trench fever ; and Borrellia recurrentis causing louse-borne relapsing fever .

Pthirus (Pthiridae)

Characters: female, dorsal. 1- Head is blunt anteriorly; eyes are present. 2- Fore-legs are thin; mid- and hind-legs are stout. 3- Thorax is wide whilst abdomen is narrow and short. 4- Abdomen bears lateral tubercles and dorsal spiracles. 5- Mid- and hind-legs have large claws which close onto a tibial spur.

Hosts: The human Pubic-louse or Crab-louse, P. pubis, is specific for humans. The other single species of this genus is specific for gorillas.

Symptoms and disease: Infestations are usually confined to pubic hair but hair in axillae, eyebrows and beard may become infested. Pruritus is the commonest symptom of infestation but spots of grey pigmentation of skin may occur where there is chronic infestation. Pthirus pubis is not a vector of pathogens .

Haematopinus (Haematopinidae)

Characters: female, dorsal, claw and egg. 1- Point at which mouthparts protrude when in use. 2- Head is elongate. 3- Eyes are absent, but there is an ocular point posterior to the antenna. 4- All legs are of similar size. 5- All legs bear a large claw that closes onto a tibial spur. 6- Next to the tibial spur is a tibial pad. 7- Abdominal segments bulge laterally. 8- Abdominal segments bear hardened (sclerotized) paratergal plates. 9- Body is large with distinct brown areas. 10- Egg glued by a female onto a hair of its host. Also: sternal plate on ventral side of thorax is large and dark.

Hosts: The only species of louse found on pigs is Haematopinus suis (Hog-louse); infesting the neck, flanks and insides of legs. Horses and other equids are infested by H. asini on their head, neck, back, brisket and between legs. Cattle are infested by H. eurysternus (Short-nosed louse) which occurs all over the body, and H. quadripertusus (Tail-louse) which infests the tail.

Signs: These lice cause irritation, pruritus, and dermal granulomas.

Disease. Piglets may suffer severe anemia if heavily infested. Biting-stress and lost production in pigs and cattle is caused. Also economically significant losses to processors of leather hides may be caused by dermal granulomas at the feeding sites of these large lice .

Linognathus (Linognathidae)

Characters: female, ventral. 1- Head is usually elongate. 2- There is no ocular point posterior to the antenna. 3- Fore-legs are smaller than the mid- and hind-legs; claw on the fore-leg forms a smaller gripping mechanism than on the mid- and hind-legs. 4- Body is dark grey and these lice are medium sized. 5- Each segment of the abdomen bears two rows of setae. 6- Abdomen is without paratergal plates. 7- Sternal plate on ventral surface of thorax is narrow or absent.

Hosts: Cattle are infested by Linognathus vituli (Long-nosed cattle louse) on their head, thorax and abdomen. Sheep are similarly infested by L. ovillus (Blue louse), whilst L. pedalis infests feet of sheep. Dogs are infested by L. setosus and goats by L. stenopsis.

Signs: Irritation, pruritus, dermal granulomas, and dermal induration are caused. Heavy infestations lead to the hair-coat having a lousy appearance: matted, staring and dull. Sheep infested with L. ovillus may lose areas of wool due to combination of inflammation and persistent self-grooming. (Note that the feeding of these and similar lice does not result in the disruption and heavy scabbing of the skin surface that is the main sign of infestation with psoroptic scab mites. Inspection by eye reveals intact skin even when louse infestations are fairly heavy.)

Disease: Heavy infestation causes biting-stress, and anemia leads to loss of production. Hides for leather manufacture are damaged by granuloma formation at feeding sites of these lice .

Solenopotes (Linognathidae)

Characters: female, ventral. 1- Head is without eyes and ocular points, and has a blunt profile. 2- Fore-legs are smaller than the mid-legs and hind-legs; they lack a tibial spur. 3- Abdomen is without paratergal plates. 4- Abdominal spiracles are borne on tubercles. 5- Abdominal segments bear one row of setae. 6- Sternal plate on the ventral surface of thorax is large and dark. Also: body is small and grey.

Hosts: Cattle are infested by Solenopotes capillatus (Little blue louse) on their head, neck, shoulders, back and tail.

Signs and disease: Irritation and pruritus, and the signs listed above for Linognathus are caused. Effects on host are similar to those for Linognathus but this smaller louse is less likely to cause lousiness.

Polyplax (Polyplacidae)

Characters: female, ventral. 1- Head is short and blunt; eyes and ocular points are absent. 2- Antennae are large relative to size of head. 3- Fore-legs are small relative to mid- and hind-legs; they have no tibial spurs. 4- Paratergal plates are present on the abdomen. 5- Abdominal segments bear two rows of setae. 6- Sternal plate on ventral surface of thorax is large. Also: body is small.

Hosts and Disease: Lice of this genus infest rodents. Such infestations may become a problem to pet rodents and in laboratory colonies.

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