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An Introduction to Medical Entomology

For educational purposes. 

 

PHTHIRAPTERA

ANOPLURA

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[Also see:  Phthiraptera Key]

 

       Anoplura are the sucking or true lice that are ectoparasitic on mammals (primates, ungulates, canines and rodents). They are a small group of insects with about 230 species. Most important on humans are the head louse, (Pediculus capitis) body louse (Pediculus humanus) and crab louse (Pthirus pubis).  The evolution of these lice with their hosts is closely paralleled.  They are small wingless insects that live entirely as ectoparasites on mammals and birds and in the clothing of humans. Their mouthparts are adapted for piercing the skin and sucking the blood of their hosts. The eyes are poorly developed or absent, and there are no ocelli. Antennae are very short with 3-5 joints.  The legs are very short and the single-jointed tarsus carries a large curved claw that is well adapted for clinging to the host.  The thoracic segments are fused, and a flattened abdomen of nine segments has large pleural areas allowing the body to swell on feeding.  There is no metamorphosis.

 

       .  The mouthparts are adapted for piercing and sucking blood.  The biting lice, Mallophaga, that are not of great medical importance to humans, have chewing mouthparts that feed on scales, feathers, and skin waste (Matheson 1950).

 

        Sucking lice are all permanent ectoparasites of mammals.  They have highly modified mouthparts, which when at rest are pulled back within a diverticulum that opens into the lower part of the pharynx at its anterior end.  The thoracic segments are fused save for the genus Haematomysus.  The tarsi have only one segment and end in a single claw that is adapted for grasping and clinging to hair.  Eggs are attached to the host's hair (Fig. 4).

 

LIFE CYCLE

 

       Both male and female lice suck blood during days and nights from their hosts.  They spend their entire lifetime on their hosts and in the case of humans on clothing as well.  The eggs, which have been called "nits", are white and 1 mm. or less long.  The eggs hatch in 5-11 days if located on the body.  If warm hatching conditions are not available, the eggs may survive more than a month.  There is a hemimetabolous life cycle in which nymphs resembling small adults hatch.  The nymphs can suck blood and pass three stages, which take about two weeks.  Nymphs that have left the body for clothing require a longer time to complete their development.  Service (2008) noted that lice deprived of a blood meal couldn't live more than four days, whereas those that are able to take 3-5 blood meals daily can live for about a month.  Those hosts with a fever of about 40 deg. Centigrade are not suitable for louse survival.

 

       Usually there are less than 100 lice on a single person, but some people may harbor an infestation of up to 500 lice on their bodies and clothing.  Service (2008) even refers to an exceptionally high infestation of 20,000 lice being recorded!

 

 

MEDICAL IMPORTANCE

 

       All Phthiraptera are bloodsucking ectoparasites of mammals, and among the four families only the Pediculidae have species that are of medical importance to humans.  Humans develop a rash from the salivary secretions.  Pediculus humanus, the body louse is associated with the spread of many diseases, such as Ricketts, Typhus and Relapsing Fever. This insect also transmitted the disease known as Trench Fever, which reduced Napoleon's Army and was prevalent in all war areas during World War I (see ent79):

 

 

          The group as a whole includes the most important vectors of Typhus Fever.  During World War II, DDT treatment of the Italian population was required to rid it of a louse epidemic.  Although the crab louse is not a disease vector, it can be acquired either through bodily contact or indirectly from bedding, etc.  It is known to attack only humans and wild gorillas in Africa.

 

 

DISTINGUISHING CHARACTERISTICS

(Derived from Service 2008)

 

Pediculus humanus- Body Louse

 

       The adults are small brown or grey and wingless, with a soft but tough integument.  The males average 2-3 mm and females larger at 3-4 mm.  Two black eyes are present and short antennae with 5 segments.  The thorax consists of three fused segments and the legs are proportionately large and well developed.  There is a short spur on the tibia that also bears a tiny spine.  The legs are all very close in size.

 

       The mouthparts are distinctive because there is no extended proboscis but rather a sucking snout that projects into the haustellum.  This bears tiny teeth that are able to grip the skin.  The stylets pierce the skin and saliva is injected to the wound to deter clotting while the blood is sucked out to where it enters the stomach.

 

       The darkened sides of the abdomen are sclerotized.  Males have dark bands on the abdominal dorsum and the posterior is round, which contrasts with females where it is forked, which aids in holding fast during oviposition.

 

       Both sexes draw blood at any time during the day or night, and both adults and immatures pass their lives entirely on humans that includes their clothing.  The eggs, known as nits, are ovoid, white and about 1 mm. in length.  There are openings on the egg that allow air to enter for breathing and for egg expansion at hatching.  Females may live for 2-4 weeks during which 150-300 eggs can be laid.  Some humans may actually sustain as many as 500 lice on their body and clothing.

 

Pediculus capitis - Head Louse

 

       There are few morphological differences between P. capitis and P. humanus, but rather their location on the body distinguish them.  The life cycles are also similar but the eggs of P. capitis are usually glued to single hairs located on the head with hatching occurring within seven days.  Most infestations do not exceed 20 lice on a head, but there are exceptions.  Female lice generally lay about eight eggs per day with not more than 150 per lifetime, which is not more than two weeks.  Eggs hatch in 5-10 days.

 

       Head lice can be of serious public health concern all over the world.  Overcrowding contributes to the louse population size in any given area.  However, unlike body lice that are vectors of typhus, head lice may only be minor vectors of a relapsing fever.

 

Pthirus pubis - Pubic Louse

 

       Pubic lice are smaller than those in the Pediculus genus, and their bodies are almost completely round.  The legs also differ as the middle legs are thicker than the front legs and they possess large claws, which gives them the common name of "crab lice".  Females lay only about three eggs per day, totaling not more than 200 during their lifetime.  The eggs are a bit smaller than the other two species, and their location is primarily in the pubic area of humans, although other parts of the body are occasionally infested.  Their activity is much less than the other species.

 

CONTROL

 

       Sucking lice rank number one in livestock pests with three different species attacking cattle, two species goats and one species hogs. Cleanliness is of the utmost importance in keeping down infestations of sucking lice.  Because the eggs will not survive for more than a month infested clothing not worn for over a month should be louse-free.  For livestock it is important to maintain the animals in a healthy state.  Rotenone applied twice a year in autumn and spring has been effective for the control of both adults and eggs.

 

 

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  Key References:     <medvet.ref.htm>    <Hexapoda>

 

Burgess, I. F.  1998.  Head lice: developing a practical approach.  The Practitioner 242:  126-69.

Burgess, I. F.  2004.  Lice and their control.  Ann. Rev. Ent. 49:  457-81.

Burgess, I. F., C. M. Brown & P. N. Lee.  2005.  Treatment of head louse infestation with 4% dimeticone lotion:  randomised controlled equivalence

      trial. BMJ 330:  1423-25.

Buxton, P. A.  1948.  The Louse:  An Account of the Lice which Infest Man, Their Medical Importance & Control, 2nd ed., Edward Arnold, London.

Chetwyn, K. N.  1996.  An overview of mass disinfestation procedures as a means to prevent epidemic typhus.  IN:  Proc. 2nd Intern. Conf. on Insect

      Pests in the Urban Environment.  ICIPUE:  pp 421-416.

Matheson, R. 1950.  Medical Entomology.  Comstock Publ. Co, Inc.  610 p.

Service, M.  2008.  Medical Entomology For Students.  Cambridge Univ. Press.  289 p

Meinking, T., C. N. Burkhart & C. G. Burkhart.  1999.  Ectoparasitic diseases in dermatology:  reassessment of scabies and pediculosis.  Adv.

      Dermatology 15:  77-108.

Nuttall, G. H. G.  1917.  The biology of Pediculus humanus.  Parasitology 10:  80-185.

Orkin, M. & H. I. Maibach (eds.).  1985.  Cutaneous Infestations & Insect Bites.  Marcel Dekker, NY., Chapt. 19-26.

Service, M. W. (ed.).  2001.  The Encyclopedia of Arthropod-transmitted Infections of Man & Domesticated Animals.  CABI: pp. 70-3, 170-4, 295-9.

Zinsser, H.  1935.  Rats, lice and history.  Boston Globe.

 Zinsser, H.  1937.  The rickettsia diseases:  varieties, epidemiology and geographical distribution.  Amer. J. Hyg. 25:  430-63.

 

 

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