Crab louse

Description
The pubic or crab louse (Phthirus pubis) is a parasitic insect which spends its entire life on human hair and feeds exclusively on blood. Humans are the only known host of this parasite. Humans can also be infested with body lice (Pediculus humanus humanus) and with head lice (Pediculus humanus capitis)

Morphology
The dorso-ventrally flattened body of the louse is divided into head, thorax and abdomen. A pair of eyes and a pair of antenna are clearly visible on the head. The mouthparts are adapted to piercing the skin and sucking blood. The second and third pair of legs, with their terminal claws are adapted to holding the hair-shaft, while the first pair of leg holds the skin during feeding. Abdominal protuberances on the side of the body are characteristic of this species. Males are slightly smaller than females. The eggs are oval-shaped and ca. 0.8 mm in length. Immediately after oviposition they are shiny, round, and transparent. Pubic lice are 1-2- mm in size, varying according to the stage of their development. They are usually whitish-grayish in color they become reddish-brown for some time after blood-meal.

Biology
Pubic lice are insect parasites, spending their entire life on the host's hair and skin and feeding exclusively on blood, 4 5 times daily. The life-cycle from egg to adult is 22-27 days. The egg hatches producing the first nymphal stage, which after three moltings develops to nymph 2, nymph 3 and subsequently to either a male or female louse. The incubation period of the egg is 7-8 days, while the rest of the cycle is taken up with the development of nymphal stages. The average adult female lives for 17 and the male for 22 days. .

Epidemiology
Pubic lice usually infest a new host only by close contact between individuals, making sexual contacts among adults and parent child interactions the more likely routes of infestation than shared towels, clothing, beds or closets. Adults are more frequently infested than children.

Diagnosis
A pubic louse infestation is usually diagnosed by carefully examining pubic hair for nits, nymphs and adults. Lice and nits could be removed either with forceps or by cutting the infested hair with scissors (with the exception of the eye area). A magnifying glass or a stereo-microscope can be used for the exact identification. If lice are detected in one family member, the entire family needs to be checked and only those who are infested with living lice should be treated.

Clinical Information
Although any part of the body may be colonized, crab lice favor the hairs of the genital (Fig. 1) and peri-anal region. Especially in male patients, pubic lice and eggs can also be found in hair on the abdomen (Fig. 2) and under the armpits as well as on the beard and mustache, while in children they are usually found in eye-lashes (Fig. 3). Infestation with pubic lice is called Phthiriasis or Pediculosis pubis, while infestation of eye-lashes with pubic lice is called Phthriasis palpebrarum. The main symptom is itching, usually in the pubic hair area. It results from hypersensitivity to louse saliva, and it becomes strong enough 2 or more weeks following initial infestation. In the majority of infestations a characteristic grey-blue or slate coloration appears (maculae caeruleae) at the feeding site, which may last for days and is also characteristic for the infestation. Pubic lice are primarily spread through sweat and body odour contact or sexual contact. Therefore, all partners with whom the patient has had sexual contact within the previous 30 days should be evaluated and treated, and sexual contact should be avoided until all partners have successfully completed treatment and are thought to be cured. Because of the strong association between the presence of pubic lice and classic sexually transmitted diseases (STD), patients diagnosed with pubic lice should undergo evaluation for other STDs. Infection in a young child or teenager may indicate sexual abuse.

Treatment
Permethrin 1% cream rinse and pyrethrins can be used for this purpose and are the drugs of choice for pregnant or lactating women. These agents should be applied to the affected areas and washed off after 10 minutes. Resistance of pubic lice to pyrethroids must be if at all very rare. A second treatment after 10 days is recommended. Pubic lice on the eyelashes can be treated with a permethrin formulation by applying the solution to the infested hair with an applicator. It is dangerous to remove lice or eggs in the eyelashes with forceps or scissors.

Lindane shampoo (1%), a pediculocide, is FDA approved as safe and effective when used as directed for the second-line treatment of pubic lice (crabs).While serious side effects have been reported, they are considered to be rare and have almost always resulted from misuse of medication, such as excessive application and oral ingestion. To minimize this risk, Lindane medications are now dispensed in small single-use bottles. The Centers for Disease Control and Prevention (CDC) notes that lindane should not be used immediately after a bath or shower, and it should not be used by persons who have extensive dermatitis, women who are pregnant or lactating or children aged <2 years. The FDA similarly warns against use in patients with a history of uncontrolled seizures disorders and premature infants, and recommends cautious use in infants, children, the elderly, and individuals with other skin conditions (e.g., atopic dermatitis, psoriasis) and in those who weigh less than 110 lbs (50 kg).

Evolutionary theory of origin
The most recent genetic research indicates that crab lice are related to the louse endemic to gorillas, Pthirus gorillae, having spread to early hominids from the ancestors of gorillas several millions of years ago, rather than having diverged from human head lice as was previously generally thought.