There are 2 orders of lice: the chewing lice and sucking lice. Both are ectoparasites, meaning they live and feed on the outside of the body of their hosts.  Chewing lice are mainly external parasites of birds and with a few feeding on mammals.   As their name implies, chewing lice chew on their hosts feeding on skin, feathers and blood serum.  Sucking lice suck the blood of their hosts that are primarily mammals.

It is quite easy to distinguish a chewing louse from a sucking louse. Chewing lice have well developed mouthparts that are equipped with large jawlike mandibles while the mouthparts of sucking lice are mainly small needle-like structures for piercing and sucking the blood of their host.  As a consequence the head of a sucking louse is small and is much narrower than the thorax.   The head of chewing lice is relatively large and as wide as or wider than the width of its thorax. The difference in the size of the head of these 2 types of lice can be attributed to the size of the muscles in the head, which are necessary to move the different types of mouthparts.


A chicken louse illustrating the wide head of chewing lice.   Image courtesy of Marcelo de Campos Pereira, Department of Parasitology, University of Sao Paulo, Brazil.

                                                                                                                                                               CHEWING LICE THAT ATTACK HUMANS.

The head louse, Pediculus humanus capitis, body louse, Pediculus humanus humanus, and crab louse, Pthirus pubis, are the 3 lice species that specifically infest humans.  The head and body lice are the same species but different subspecies.  They look almost exactly alike but have distinctive biologies and are found on totally different areas of the body. 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 occupied by body lice (body lice are found on clothing). Because they are both the same species, they are capable of mating but their offspring all exhibit the characteristics of head lice.

Head Lice. Pediculus humanus capitis, and body lice, Pediculus humanus humanus are different subspecies of the same species. Placed side by side, they look almost exactly alike; however, they have quite different biologies and are easily distinguish from crab lice. 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 occupied by body lice (body lice 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.




                                                                                            A head or body louse (right) compared to a crab louse (left).

Head lice were called mechanized dandruff during World War II as typically they were found above the shoulders on scalp hair, mustaches, sideburns and beards. 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.

                                                                                                         Relative Size of Head Lice Adults.

The eggs (nits) typically are deposited on the base of the 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 that unhatched eggs can be found some distance from the base of the hair. Lice use cement that is impervious to normal influences such as shampoo; consequently, the eggshells remain attached to the hair long after hatching. Head lice are more prolific than crab lice because the females 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 and 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.

 There does seem to be a correlation between the probability of being infested with head lice and hair length. Probably this is due to the fact that these 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.

 lndividuals infested with these pests 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, although the disease can occur in other hairy parts of the body, like leg hairs. Head lice feed on human blood (hematophagy), and itching from louse bites is a common symptom of this condition. Treatment typically includes application of topical insecticides such as a pyrethrin or permethrin, although a variety of folk remedies are also common.

Head Louse Bite on Neck

From each egg or "nit" may hatch one nymph that will grow and develop to the adult louse. Full-grown lice are about the size of a sesame seed. Lice feed on blood once or more often each day by piercing the skin with their tiny needle-like mouthparts. Lice cannot burrow into the skin.

Diagnosis. In order to diagnose infestation, the entire scalp should be combed thoroughly with a louse comb and the teeth of the comb should be examined for the presence of living lice after each time the comb passes through the hair. The use of a louse comb is the most effective way to detect living lice. In cases of children with dirty, long and/or curly/frizzy hair, an alternative method of diagnosis is examination by parting the hair at one inch intervals to look for moving lice near the scalp. With both methods, special attention should be paid to the area near the ears and the nape of the neck. The examiner should examine the scalp for at least 5 minutes. The use of a magnifying glass to examine the material collected between the teeth of the comb could prevent misdiagnosis.

The presence of nits alone however, is not an accurate indicator of an active head louse infestation. Children with nits on their hair have a 35-40% chance of also being infested with living lice and eggs. If lice are detected, the entire family needs to be checked (especially children up to the age of 13 years) with a louse comb and only those who are infested with living lice should be treated. As long as no living lice are detected, the child should be considered negative for 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

The most characteristic symptom of infestation is itching on the head which normally intensifies 3 to 4 weeks after the initial infestation. The bite reaction is very mild and it can be rarely seen between the hairs. Bites can be seen, especially in the neck of long-haired individuals when the hair is pushed aside. In rare cases, the itch scratch cycle can lead to secondary infection with impetigo and pyoderma. Swelling of the local lymph nodes and fever are rare. Head lice are not known to transmit any pathogenic microorganisms.

The most common symptom of lice infestation is itching. Excessive scratching of the infested areas can cause sores, which may become infected.

Head lice may cause local infections, but are not a vector for epidemic microbial disease (see below). Head lice per se are a disease only if they cause symptoms. In the absence of discomfort or skin lesions, infestation is therefore only a condition and cosmetic problem for the carrier (not strictly a disease), though they may present a disease-transmission-risk for those others who will develop symptoms, if infected.

The number of cases of human louse infestations (or pediculosis) has increased worldwide since the mid-1960s, reaching hundreds of millions annually. There is no product or method which assures 100% destruction of the eggs and hatched lice after a single treatment. However, there are a number of treatments that can be employed with varying degrees of success. These methods include chemical treatments, natural products, combs, shaving, hot air, and silicone-based lotions.

Treatment.  Lice on the hair and body are usually treated with medicated shampoos or cream rinses. Nit combs can be used to remove lice and nits from the hair. Laundering clothes using high heat can eliminate body lice. Efforts to treat should focus on the hair or body (or clothes), and not on the home environment. Some lice have become resistant to certain (but not all) insecticides used in commercially available anti-louse products. A physician or pharmacist can prescribe or suggest treatments. Because empty eggs of head lice may remain glued on the hair long after the lice have been eliminated, treatment should be considered only when live (crawling) lice are discovered. There are many home remedies for lice control most of that are ineffective and some can be dangerous. The authors have a Turkish friend who had head lice as a child and the method used on him consisted of soaking the head with kerosene. Unfortunately, the chemical caught on fire and the individual has been blind for virtually all his life. Other flammable materials such as alcohol are also not advisable.

Prevention. Examination of the child’s head at regular intervals using a louse comb allows the diagnosis of louse infestation at an early stage. Early diagnosis makes treatment easier and reduces the possibility of infesting others. In times and areas when louse infestations are common, weekly examinations of children, especially those 4–13 yrs old, carried out by their parents will aid control. Additional examinations are necessary if the child came in contact with infested individuals, if the child frequently scratches his/her head, or if nits suddenly appear on the child’s hair. Keeping long hair tidy could be helpful in the prevention of infestations with head lice. In order to prevent new infestations, the hair of the child could be treated with 2–4 drops of concentrated rosemary oil every day, before he/she leaves for school or kindergarten. The oils can be combed through the hair using a regular comb or brush. Clothes, towels, bedding, combs and brushes, which came in contact with the infested individual, can be disinfected either by leaving them outside for at least 3 days or by washing them at 60°C (140 degrees F) for 30 minutes. An insecticidal treatment of the house and furniture is not necessary.

 Prevalence. Despite improvements in medical treatment and prevention of human diseases during the 20th century, head louse infestation remains stubbornly   prevalent. In 1997, 80% of American elementary schools reported at least one outbreak of lice. Lice infestation during that same period was more prevalent than chicken pox.

About 6-12 million people, mainly children, are treated annually for head lice in the United States alone. High levels of louse infestations have also been reported from all over the world including Israel, Denmark, Sweden, U.K., France and Australia.  Normally head lice infest a new host only by close contact between individuals, making social contacts among children and parent child interactions more likely routes of infestation than shared combs, brushes, towels, clothing, beds or closets. Head-to-head contact is by far the most common route of lice transmission. The number of children per family, the sharing of beds and closets, hair washing habits, local customs and social contacts, healthcare in a particular area (e.g. school) and socio economic status were found to be significant factors in head louse infestation . Girls are 2-4 times more frequently infested than boys. Children between 4 and 13 years of age are the most frequently infested group.

The United Kingdom's National Health Service, and many American health agencies, report that lice "prefer" clean hair, because it's easier to attach eggs and to cling to the strands, however, this is often contested.

Because head louse infestation occurs primarily in children, much of the effort to prevent head lice transmission has focused on school and day care settings—places where large numbers of children come into close contact. Schools in the United States, Canada, and Australia commonly exclude infested students, and prevent return of those students until all lice, eggs, and nits are removed. This is the basis of the "no-nit policy". Data from a primarily American study during 1998-1999 found that no-nit policies were present at 82% of the schools attended by children suspected of louse infestation. A separate 1998 survey revealed that 60% of American school nurses felt that "forced absenteeism of any child who has any nits in their hair is a good idea.

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!


It should be noted an infestation of head lice has no correlation to unsanitary conditions and these lice have no socio-economic barriers. The rich are as easily infested with these pests as the poor. There does seem to be a correlation between the probability of head louse infestation and hair length. Probably this is due to the fact that these 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.

               School Head Lice Policy. This involves a number of issues:

               Establishing criteria to infer infestation

               Screening asymptomatic children for evidence of infestation

               Establishing the circumstances under which children will be inspected

               Immediate exclusion of infested children

Establishing criteria to permit previously infested children to return to school (e.g., the no-nit policy).All of these policies are controversial. In particular, a number of health researchers and organizations object to the required removal of nits (i.e., the no-nit policy). Opponents to the no-nit policy point out that nits, being empty egg casings, have no clinical importance. Transmission can only occur via live lice or eggs. Time-consuming nit removal, therefore, has no direct effect on transmission. This has led to the perception that the no-nit policy serves only to ease the workload of school nurses and punish the parents of infested children.

Proponents of the no-nit policy counter that only a consistently nit-free child can be reliably shown to be infestation-free. That is, the presence of nits serves as an indirect proxy for infestation status. Proponents argue that such a proxy is necessary because lice screening is prone false negative conclusions (i.e., failure to find lice present on actively infested children).For example, a 1998 Israeli study found that 76% of live lice infestations were missed by visual inspection (as verified by subsequent combing methods). Although lice cannot fly or jump, they are fast and agile in their native environment (i.e., clinging to hairs near the warmth of the scalp), and will try to avoid the light used during inspection. Lice colonies are also sparse (often fewer than 10 lice), which can contribute to difficulty in finding live specimens. Further, lice populations consist predominantly of immature nymphs, which are even smaller and harder to detect than adult lice.

Body Lice. The behavior of body lice, which are also called cooties, is totally different than that of head lice. Females lay up to 300 eggs and the life cycle can be completed 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, as those common during wartime. One person 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 high 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 to the non-infested individual.




 Extremely heavy infestation of body liceImage Courtesy of Vopak Inc.


As in the other 2 species 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. The victim’s skin becomes thickened and deeply pigmented. This condition is not due to a microorganism but to the skin’s reaction to the feeding of the lice.





Deeply pigmented and thickened skin (Vagabond’s disease) due to

long term feeding of body lice.  Image courtesy of VoPak Inc.


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 2 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 (influenza) 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.


Body lice are capable of vectoring diseases; the most important of which is epidemic typhus that is also known as tarbardillo (Mexico), Brill’s disease, jail fever, war fever and European typhus. This disease reached its worldwide peak prior to 1870 but since that time has decline in occurrence except during wartime or during times when large numbers of people are living under crowded and poor sanitary conditions. Based on actual human deaths, this is one of the top 3 insect-vectored diseases in the world. It is estimated that during WWII over 2 million Russian died from this disease. Body lice are the main transmitters, perhaps because head lice are less mobile and they don’t reach the high populations on the body—therefore they are less frequently passed from person to person. Epidemics usually occur in the winter when people are huddled closely together and the body lice migrate nightly from one pile of clothing to the next.


The etiological agent is Rickettsia prowazeki and the primary vector is the body louse.  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, thus establishing the disease. Also 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, a city of about one million people. 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--blowing the dust 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.


Louse Born Relaspsing Fever. There are two forms of this disease being caused by bacteria in the genus Borrelia and transmitted through louse or soft-bodied tick bites.  Borrelia recurrentis is the agent of louse-borne disease vectored by Pediculus humanus, namely the body louse. It is more severe than the tick-borne variety. Epidemics most commonly occur with poor living conditions, famine and war in the developing world; it is currently prevalent in Ethiopia and Sudan. Mortality rate is 1% with treatment; 30-70% without treatment. Prognostic signs include severe jaundice, severe change in mental status and severe bleeding.

Lice that feed on infected humans acquire the Borrelia organisms that then multiply in the gut of the louse. Subsequent the bacterium can be passed onto another human by crushing the louse or by the feces which are scratches into feeding wounds. B. recurrentis can also infect a victim via mucous membranes and then invades the bloodstream. No other animal reservoir exists.

 Tick-borne relapsing fever is found primarily in Africa, Spain, Saudi Arabia, Asia, and certain areas in the Western U.S. and Canada. It is vectored by soft ticks and caused by Borrelia duttoni. 

Most people who are infected get sick around 5-15 days after they are bitten by the tick. The symptoms may include a sudden fever, chills, headaches, and muscle or joint aches, and nausea; a rash may also occur. These symptoms continue for 2-9 days, and then disappear. This cycle may continue for several weeks if the person is not treated. Relapsing Fever is easily treated with 1-2 weeks of antibiotics. Most people improve within 24 hours of starting antibiotics. Complications and death due to relapsing fever are rare.

Relapsing fever is a candidate etiology for a mysterious series of plagues in late medieval and early renaissance-era England referred to at the time as sweating sickness but which have not recurred in epidemic form since the 16th Century.

Antibiotics of the tetracycline class are most effective, but may induce a Jarisch-Herxheimer reaction, which occurs in over 50% of patients. This reaction produces apprehension, diaphoresis, fever, tachycardia, and tachypnea with an initial pressor response followed rapidly by hypotension. Recent studies have shown that tumor necrosis factor-alpha (TNF-alpha) may be partly responsible for the reaction.


Crab Lice. Crab lice, Pthirus pubis, also known as pubic lice, attack humans and a few species of gorillas.  The broad squat bodies, large claws on the middle and hind legs, and characteristic sluggish movements have resulted in the pubic louse aptly being called the crab louse. Infestations typically are restricted to the groin and perianal area in adult humans; however, in very heavy infestations they can be found all over the body. If a child who hasn't reached puberty is attacked, the lice frequently are found on the eyelashes. Apparently this is because one critical factor in crab louse environment is the distance between, and thickness of, individual body hairs. This distance and coarseness in eyelashes is ideal and consequently is occupied in the absence of pubic or other body hair.




Figure 41.  A crab louse, Pthirus pubis. Image courtesy of L.A. County Department of Agriculture.


Females lay a small number of eggs (100 to 150) on body hairs. Development is quite rapid with the life cycle being completed in 30 to 40 days. This is partially due to the high body temperature of the host and consequent high environmental temperature for the louse. Crab lice are nearly immobile and typically attach to body hairs with their opposing clawlike tarsi and tibia while feeding. They frequently become buried in their own excrement and can only live for a short time 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 infestation 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 much more careful because our awareness of more serious repercussions (AIDS, for example) has risen.


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 ranging in size from 1/16 to ½ inch in diameter. The blue spots are a result of the anticoagulant in the saliva, which causes a sort of bruising.


There are several products available for control of head lice. However, Kwell shampoo is 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. Some doctors are not prescribing Kwell anymore because of possible serious side effects. There are new medications being used recently that approach Kwell in effectiveness but that do not have the same negative side effects.


22.                    Body lice are the same species as head lice but typically lay their eggs and are found as nymphs and adults on clothing.

23Symptoms of the presences of a large infestation of crab lice over a long period of time include thickening and dark coloring of the skin.

23.                   Crab lice are typically found feeding around the groin area but in children can be found along the eyelashes.

24.                   Head lice are commonly found in children and are commonly brought home from school to infest the whole family.

25.                   The eggs of lice are referred to as nits and in the case of head and crab lice are laid on hairs.

26.                   Crab lice may cause blue spots on the skin as a result of their feeding.