“Crabs”

The order Anaplura includes over 400 species of sucking lice, which are ectoparasites of mammals. Sucking lice are dorsoventrally compressed, wingless, and small, with retractable piercing-suckling mouthparts. One species that is responsible for causing an STD/STI is known as Pthirus pubis aka pubic lice, or “Crabs” but is it really lice?

 

History

 
 
Lice have been constant companions of human beings since antiquity. DNA analysis of lice suggests that lice specific to human and lice specific to chimpanzees appeared 5.6 million years ago. Israeli scientists confirmed that the warriors in Bar Kochba’s Jewish revolt against Rome 19 centuries ago was were afflicted with lice. These lice were morphologically indentical to P. capitis (Anaplura: Pediculidae) that continue to afflict human populations today. The lice were discovered in the hair and clothes of archeological remains from caves in the Sudan desert.

 

Background

 
 
Lice have five stages in their life cycle: egg (or nit), three nymphal stages, and the adult stage. All stages occur on the host. Sucking lice undergo a simple metamorphosis in which immature lice are morphologic miniatures of adults but have no reproductive ability.

The egg or the crab louse is smaller than the eggs of either the head or body louse, which measure approximately 0.8 mm long and 0.3 mm wide. The nit is oval in shape and opalescent in color; it contains a cap (operculumn) that comes off intact when the egg hatches. An egg will hatch within 5-10 days after being incubated in the heat of the host’s body. Interestingly, the young nymph emerges through the cap by sucking air into its body and expelling it from its anus until a cushion of compressed air is formed, which then pops the cap open and allows the nymph to escape. Over a period of 8-9 days, the nymph produces three molts. The louse remains on the body and requires frequent blood meals after having hatched. When lice reach adulthood, mating occurs after approximately 10 hours and continues until the lice die. The female louse lays approximately four eggs per day.

In the case of the body louse, the egg cases is attached to hair or clothing by a cement like material secreted by the female louse. After hatching, the empty shell may stick to hair or clothing for some time. It is often difficult to remove by washing or shampooing or by vinegar or organic solvents. If all else fails, the empty egg cases are eventually moved away either by growth of the hair, by the use of a fine-tooth comb, or by cutting the hair.

Little is known about the actual life spam of the adult louse. Under artificial conditions, lice have survived for about 1 month. Ambient temperature, humidity, and availability of human blood are thought to influence  the life span of all types of human lice. Off the host, all stages of the louse can be expected to die within 30 days, regardless of temperature. Unfed adult head and body lice can survive for up to 10 days, whereas adult pubic lice rarely survive more than 24 hours off the host. Lice leave the host voluntarily only when the host has died or becomes febrile or when there is close personal contact with another host.

The life of the louse is dependent on human blood. When ready to feed, the louse anchors its mouth to the skin, stabs an opening through the skin, pours saliva into the would to prevent clotting, and pumps blood from the wound into its digestive system. During feeding, dark-red feces may be deposited on the skin.

The pubic louse has greatly enlarged middle and hind legs and claws; the abdomen is wider than it is long, giving it the appearance of a crab. Public lice are about 1 mm in length. Adult head and body lice have a longer body (approximately 3 mm in females and 2 mm in males) and relatively shorter middle and hind legs. The pubic louse has three parts: a head (with a pair of eyes), a thorax (with three pairs of legs), and a segregated abdomen At the end of each leg, there is a hook-like claw and opposing thumb, that enable the louse to maintain its hold on hair.

A difference between body and head lice and the pubic louse involves their grasping ability. The grasp of the pubic louse’s claw matches the diameter of pubic and auxiliary hairs; hence, public lice may be found not only in the pubic area but also have been recovered from the axillae, beard areas of the face, eyelashes, and eyebrows. It is rare in these other areas and is probably mechanically moved to these areas via fingers. The diameter of the head louse’s grasp seems to be uniquely adapted to the diameter of the scalp hair. Therefore, it is very difficult to transplant head lice to other areas of the body.

Another difference between the species is that of egg laying. Adult female pubic lice and head lice glue their eggs to hairs, while the female body louse usally oviposits on the fiber on in the seams of clothing.

The third difference is the rate of movement. Pubic lice seem to be most sedentary. Nuttal and Payor recorded pubic lice moving at a maximum of 10 cm in a day. Body lice can wander as much as 35 mm in a 2-hour period. Temperature, ambient humidity, and available of a blood meal also influence the rate of movement. Lice do not like light and will move frantically to escape light.

(Back to what is an STD from crabs)