Image Courtesy of L A County Agricultural Commissioner's Office.

All lice are ectoparasites; they live and feed on the outside of their host's body. Although lice can have detrimental effects on the host, an infestation is rarely lethal. All ectoparasites have a number of advantageous characteristics in common. Most are quite small, flattened, wingless and possess backward-projecting spines on their bodies. Their small size makes it more difficult for the host to detect their presence. Being flattened and possessing backward projecting spines that can lodge in hairs or feathers makes it more difficult to dislodge them from the host. Finally, because the host represents an unlimited source of food and typically is infested with many of these ectoparasites, there is no need for wings to find food or a mate.


There are 2 orders of lice, namely the chewing lice (Mallophaga) and the sucking lice (Anoplura).  Chewing lice mainly attack birds although a few species inhabit mammals. They feed on hair, skin debris, blood serum and other body secretions. Eggs (nits) typically are glued either to feathers or hair. The life cycle is generally completed quite rapidly with many species developing from egg to adult in as little as 30 days. Chewing lice rarely leave the host, and some have been collected from museum specimens long after the death of the animal. All are host specific with none living on humans. Heavy infestations of chewing lice on poultry will result in loss of weight, reduced eggs production, and a general rundown condition in an animal’s health.

Typical Chewing Louse with Wide Head and Chewing Type Mouthparts. Image Courtesy of Department of Parasitology, Sao Paulo, Brazil.

ORDER: ANOPLURA (SUCKING LICE). The sucking lice share the characteristics of all ectoparasites but differ from chewing lice by their mouthparts and width of their heads. The chewing lice have large, heavily sclerotized chewing type mandibles for gnawing on the host. Consequently, they possess large muscles to move the mandibles and a large head that is needed to house the muscles. Sucking lice have small needle-like mouthparts to suck the blood of their host, and correspondingly have smaller mouthpart muscles and a smaller head. With Mallophaga and Anoplura, the head is wider and narrower than the thorax, respectively.

Comparison of Head Width of a Sucking Louse and Chewing Louse. Image Dr. Kaae

 Crab Lice. Sucking lice are chiefly found feeding on mammals. Almost all are host specific with three types attacking humans. These are the head, body and crab lice. Crab lice (Pthirus pubis) or pubic lice attack humans and a few species of gorillas. Their broad flat bodies, overly large claws on the middle and hind legs combined with a characteristic sluggish behavior have resulted in the pubic louse also commonly referred to as crab lice. Infestations typically are restricted to the groin and perianal area of adult humans; however, with very heavy infestations, these lice can occur all over the body. If a child who hasn't reached puberty is attacked, the lice frequently are found on the eyelashes. Unfortunately in some cases, this is a sign of sexual abuse. Apparently finding these lice on the eyelid are a result of a critical factor in crab louse environment- namely the proper distance between and thickness of individual hairs. This distance and coarseness in eyelashes is ideal and consequently is occupied in the absence of pubic or other body hair in children.            

Crab Louse. Image Courtesy of Los Angeles County Agricultural Commissioner’s Office.

Females lay a relatively small (at least when compared to many other insects) numbers of eggs (100 to 150) on body hairs. Development is quite rapid with a single life cycle being (eggs, nymphs and adult) completed in 30 to 40 days. Crab lice are nearly immobile and typically attach to body hairs with their opposing claw-like tarsi and tibia while feeding. They frequently become buried in their own excrement and can only live for a short time when off the host.

Because of host dependency and relative immobility, these lice normally are acquired by sleeping with an infested person. It is possible (but not probable) that crabs could be contacted from situations such as using the same shower towel immediately after an infested individual, using a toilet seat immediately after use by a heavily infested individual or similar situations. As might be expected, there is a correlation between the degree of crab louse infestations in the general public in a given area or during a particular era and the degree of sexual promiscuity. For example, crab lice infestations in the United States during the 1960’s and 1970’s were more common than today. The 60’s and the 70’s were the decades of social unrest, sexual experimentation and "free love." Today, we are hopefully much more careful because our awareness of more serious repercussions (AIDS, for example) has risen.

Crab Lice on Eyelashes. Image Courtesy KostaMumcuoglu (Talk) at en.wikipedia

Crab lice do not vector any disease but can cause a rash and corresponding itching. As do most blood sucking insects, crabs inject a saliva when they feed that contains both an anticoagulant to keep the host's blood flowing and an anesthetic to minimize detection of their presence. Prolonged infestations of this species can result in subcutaneous blue spots (essentially bruising) ranging in size from 1/16 to 1/2 inch in diameter. Of course, the blue spots are a result of the anticoagulant in the saliva which results in bruising.

There are several products available for control of head lice. However, historically Kewell shampoo was probably the most effective material available. It is formulated as a lotion that contains the insecticide lindane. This insecticide has a relatively long residual activity and readily penetrates the skin, qualities that are not found in other products. Most doctors are not prescribing Kwell anymore because of possible side effects. There are new medications used recently that approach Kwell in effectiveness but do not have the same side effects. Permethrin based shampoos are currently one of the doctor prescribed products for control of crab lice.

Head Lice. Pediculus humanus capitis and Pediculus humanus humanus are different subspecies of the same species. When placed side by side, they look almost exactly alike; however, they have quite different types of biology and are easily to distinguish from crab lice (see above). It is thought that primitive humans were infested with head lice, but as we became more civilized and began wearing clothing, this opened up a different ecological niche that was ultimately occupied by body lice (these are found on clothing). Because they are both the same species, they are capable of inter-mating, but their offspring all exhibit characteristics of head lice, thus possibly reverting to the ancestral form.                                                                                                                                                                                                        

Head lice were called mechanized dandruff during World War II; typically they were found above the shoulders on scalp hair, mustaches, sideburns and beards. In addition their presence could have been confused with moving dandruff. Heavily infested individuals may have the lice on all hairy parts of their bodies. They normally are gray colored but tend to take on the color of the hair they are infesting.

File:Headlice.jpgFile:Male human head louse.jpg

Top. Relative Size of Head Llce Adults. Image Courtesy CDC. Bottom. Closeup of Head Louse. Courtesy Gilles San Martin-CC SA 2.0

Their eggs (also referred to as nits) typically are deposited on the base of hair immediately next to the scalp. Generally, they hatch by the time the hair has grown ¼ inch. Occasionally head lice will lay their eggs on long hair that is lying on the scalp so unhatched eggs can be found some distance from the base of the hair. Lice use a cement to attach their eggs to hair.  This cement is impervious to normal influences such as shampoo; consequently, the egg shells remain attached to the hair long after hatching. The empty egg shells are also referred to as nits. Head lice are significantly more prolific than crab lice because a single female can deposit up to 200 eggs and development can be completed in as little as 30 days. It should be noted an infestation of head lice has no correlation to unsanitary conditions as these lice have no socio-economic barriers. The rich are as likely to be infested with these pests as are the poor.


Nits on Human Hair. Image Courtesy of CDC.

There does seem to be a correlation between the probability of being infested with head lice and hair length. This is possibly due to the fact that head lice are more difficult to control in longer hair.  Also it is less common to find head louse infestations in African-Americans: however, in Africa these lice readily attack black individuals. Individuals infested with these pests typically harbor only 10 to 20 lice, but in heavy infestations the hair can become matted with nits, nymphs and adult lice. Typically, only the head or scalp of the host is infested; however, these pests can occur in other hairy parts of the body such as leg hairs. As with the other species of lice that attack people, head lice feed on human blood and itching from louse bites is a common symptom of an infestation. Treatment typically includes application of topical insecticides such as permethrin.  A variety of folk remedies are also common, some of which can be dangerous. The use of gasoline or other flammable liquids can be especially dangerous. A distant acquaintance of ours grew up in Turkey and as a kid on occasion had head lice treated by rinsing his hair with kerosene.  Unfortunately, on one occasion he got too close to a flame.  Forty years later he is still blind due to the accident.


Head Louse Bite on Neck . Image Courtesy of CDC.

Head-louse infestations are widely endemic and a worldwide problem in children.  Such infestations are of some concern in public health. This concern unquestionably is due to the fact that infestations are much more common worldwide than the other species of lice that infest humans; however, unlike human body lice, head lice are not carriers of infectious diseases.  Infestations from head lice are most commonly found on children ages 3 to 10 and their families. Females typically get head lice more often than males (again possibly due to difference of hair length).  Infestation in individuals of Afro-Caribbean or other black descent is rare because possibly due hair consistency.

Lice in general pass through 3 stages of development--egg, nymphs and adults. From each egg or "nit" hatches one nymph that will grow passing through several nymphal instars to grow reach the adult stage. Full-grown head lice are about the size of a small grain of rice. Lice feed on blood once or more a day by piercing the skin with their tiny needle-like mouthparts.

Life Cycle Head Lice. Body Lice Similar. Public Domain.

Diagnosis. Generally speaking it is not difficult to diagnosis the presence of head lice.  They will generally let you know of their presence. If needed, the entire scalp can be combed repeatedly with a louse comb to check for their presence. One effective way to determine the presence of living lice in children, especially those with long and/or curly/frizzy hair, is to part it and examine the scalp at short intervals.  With each of the above techniques, attention should be paid to the area near the nape of the neck and close to the ears.  Both processes should be accomplished with precision as the early instars and eggs of the lice are tiny.  A magnifying glass may help with either process. .

It is worth noting that nits may be empty shells of a previous louse infestation, and their presence alone is not necessarily an accurate symptom of an active infestation.  However, individuals with nits on their hair have a 35-40% chance of harboring living lice. If lice are detected on a child or any individual, the entire family should be checked (especially children up to the age of 13 years) as head louse infestations are readily transferable, especially within a family unit. Only those who are infested with living lice should be treated: however, louse control products that are registered for use in the U.S. should be considered safe.  Excessive use of any pesticide product, if not needed, is a poor practice. As long as no living lice are detected on a suspected individual, they should be considered not having a head louse infestation. Accordingly, a child should be treated with a pediculicide only when living lice are detected on his/her hair (not because he/she has louse eggs/nits on the hair and not because the scalp is itchy.


Louse Comb. Image Courtesy of

As previously indicated, the most characteristic symptom of infestation is itching on the head. Once infested by head lice, the degree significantly intensifies by the 3rd or 4th week. The victim’s reaction to the bite is normally very mild and can be rarely seen between the hairs. However, these symptoms can be seen on the nape of the neck in those individuals with long hair.  In rare cases, excessive scratching and itching from the bites can lead to secondary infection. Even more rare are swelling of the lymph nodes and fever. Unlike body lice, head lice are not known to transmit any disease causing organism to humans. Head lice only cause symptoms with mild itching. More severe results are due to the victim's reaction to those symptoms. In the absence of discomfort or skin lesions, an infestation is therefore only a cosmetic and possible psychological problem for the victim. 

Worldwide the number of cases of head louse infestations (or pediculosis) has increased tremendously since the middle 1960s annually reaching hundreds of millions. There is no product, treatment or method which will assures 100% destruction of various stages of this louse with a single treatment. However, there are several treatments that are available which will work with varying degrees of success. These methods include use of chemicals, natural products, shaving the head, nit combs, hot air treatment, silicone-based lotions and nit picking.

These insects are the most common insect pests of children. Approximately 40% of the children attending primary or secondary schools in the United States will be infested with these insects at one time or another. Teachers are trained to watch for children constantly itching their heads in class. If this behavior is observed, the child is sent to the school nurse who inspects for nits, especially around the upper nape area. If lice are found, the child is sent home and it then becomes the responsibility of the parent to control the lice before the child can be readmitted to school.

The widespread occurrence of these pests in schools partially is due to the way lice are spread from person to person. The most common vehicles of distribution are trading hats and combs, a common practice of school children. Also in many classrooms, hats and jacket are stored on adjacent or common hangers when not being worn. This gives any lice that may be on these articles a chance to crawl from one item to the next. Also, school children love to "sleep over" at friend’s houses. All participants at one slumber party can carry home head lice to their siblings or parents. The possibilities are staggering!<iframe width="854" height="480" src="" frameborder="0" allowfullscreen></iframe>

Video Courtesy Sage Rose-Creative Commons International.  

For Information on Head Lice treatment and Prevention See-Head Lice Prevention and Treatment-Courtesy CDC

Body Lice. The behavior of body lice or cooties is totally different than that of head lice. Females lay more eggs than head lice (up to 300 eggs) and the life cycle can be completed in a shorter time than head lice (in as little as 25 days). The eggs are deposited on clothing and the nymphs and adults remain on clothing even while feeding. Body lice infestations are not common in California, but typically develop when a large number of people live closely together under poor sanitary conditions (war time). One individual can harbor a large colony of this species. Over 30,000 body lice have been removed from a single individual. Even though such an infestation is rare, these insects have such a rapid reproductive capacity that huge numbers can develop rapidly unless control is quickly initiated. Close contact with someone who is heavily infested can result in a transfer of several hundred lice.

As in the other two species of lice that attack humans, body lice cause dermatitis and considerable irritation. Scratching can result in secondary infection and sometimes blood poisoning. In long term infestations, a condition called vagabond’s disease may develop. In this case, the victim’s skin becomes thickened and deeply pigmented. This condition is not due to a microorganism but is due to the victim’s skin reacting to the long term feeding of the lice. Finally, individuals who are heavily infested with lice may develop a systemic or overall body effect. The following is taken from a report made by a scientist who recorded his symptoms after allowing 700 to 800 lice to feed on him two times a day. "I started feeding twice a day and almost immediately a general tired feeling was noticed in the calf of my legs and along the shin bones, while on the soles of my feet and underneath my toes the tired feeling was so intense as to often prevent sleep until late in the night. An irritable and pessimistic state of mind developed. An illness resulted with symptoms very similar to grippe and a rash similar to German measles was present, particularly over the shoulders and abdomen. Once the experiment was discontinued the symptoms disappeared within a few days."

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Vagabonds Disease. Image Courtesy Univar Corp.

Body lice are capable of vectoring diseases; the most important is epidemic typhus. Based on actual human deaths, this is one of the top three insect-vectored diseases in the world. The etiological agent is Rickettsia prowazeki and the primary vector is the body louse. Body lice are the main louse transmitters; head lice are less mobile and they don’t reach the high populations on the body louse—therefore they are less frequently passed from person to person. Epidemics usually occur in the winter when people are huddled closely together and the lice migrate nightly from one pile of clothing to the next.

The rickettsias multiply in the epithelial cells of the louse intestine and are voided with the feces. After feeding, body lice defecate next to the bite; subsequently, the rickettsia can be scratched into the bite by the host. In addition, after a period of time the feces dries and the rickettsia become airborne resulting in transmission through inhalation.

Historically, this disease has been instrumental in the outcome of several wars. In some cases, it has killed more soldiers than fighting. Epidemic typhus was a major factor in Napoleon's withdrawal from Russia. During World War I this disease killed over 3 million Russians. Typhus was again threatening in the early years of World War II. During 1942 there were 83,000 cases in North Africa. When allied forces landed in Italy in l943, a typhus epidemic was well underway in Naples, Italy. There was congestion, unsanitary conditions, food scarcity and malnutrition. The death rate was around 81%, and it was quite likely that the entire city would have been annihilated, if not for a very effective louse control program initiated by the allies. The program consisted of applying 10% DDT dust to the clothing of the majority of the city’s inhabitants. The people were lined up for blocks. The DDT was applied from a backpack duster with an attached elongated tube. The tube was inserted into the shirt and/or pants of the individual to be treated and then the duster was turned on—as a result the dust was blown throughout the clothing.

Epidemic typhus is characterized by severe headache, high fever and a rash caused by small hemorrhages. Today the disease is kept in check by vaccines, modern antibiotics, and the use of pesticides to prevent the buildup of louse populations. According to the CDC, a body lice infestation is treated by improving the personal hygiene of the infested person and assuring a regular (at least weekly) change of clean clothes. Clothing, bedding, and towels used by the infested person should be laundered using hot water (at least 130°F) and machine dried using the hot cycle. Sometimes, the infested person also is treated with a pediculicide, a medicine that can kill lice; however, a pediculicide generally is not necessary if hygiene is maintained and items are laundered appropriately at least once a week. A pediculicide should be applied exactly as directed on the bottle or by your physician.


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