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Pthiriasis

Last updated Dec. 20, 2018

Approved by: Maulik P. Purohit MD, MPH

Pthiriasis is an infestation of the human pubic region by the parasitic louse known as Pthirus pubis. This infection is commonly known as “crabs”.


What are the other Names for this Condition? (Also known as/Synonyms)

  • Crab Lice Infection
  • Pthirus Pubis Infection
  • Pubic Lice Infection

What is Pthiriasis? (Definition/Background Information)

  • Pthiriasis is an infestation of the human pubic region by the parasitic louse known as Pthirus pubis. This infection is commonly known as “crabs”
  • It primarily spreads via sexual contact with individuals who are infected with the parasite. Hence, it is classified as a sexually-transmitted disease (STD)
  • Pthirus pubis is an ectoparasite (meaning it lives on the outside of its host organism) that requires the blood of its host for survival
  • Infected individuals may present symptoms, such as mild to severe itching in the genital area, where the parasites reside. A healthcare provider may diagnose Pthiriasis upon inspection of the pubic area for lice or eggs of P. pubis
  • The treatment of Pthiriasis may involve over-the-counter or prescription medication, often administered topically (applied directly to the skin)
  • The prognosis of Pthiriasis is generally good with proper medical treatment. Symptoms often resolve within 10 days of the start of treatment with no long-lasting side effects being noted

Who gets Pthiriasis? (Age and Sex Distribution)

  • Individuals of all ages, races, ethnic groups, and both genders are susceptible to infection by P. pubis. However, more number of cases are reported in males than in females
  • Pthiriasis occurs more often in adolescents and adults
  • The incidence is much higher in sexually-active individuals, though sexually-inactive individuals can also contract the infection

What are the Risk Factors for Pthiriasis? (Predisposing Factors)

  • The major risk factor for developing Pthiriasis is sexual contact with the infected individuals
  • Contact with, sharing, or handling infected individual’s clothing or bedding

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases one’s chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Pthiriasis? (Etiology)

Pthiriasis is caused by infection of the human skin (generally in the pubic area) by the organism Pthirus pubis, called pubic lice. These organisms feed on the blood of their host to survive and propagate.

  • Sexual contact with infected individuals/partners is the most common method of transmission
  • The infection can also be contacted through infected clothes or bedding, although it is a less common route of disease transmission
  • Fully-developed adults feed on the blood of their hosts by biting the skin and sucking blood

Note: Humans are the only known host of the Pthirus pubis parasite.

What are the Signs and Symptoms of Pthiriasis?

The signs and symptoms of Pthiriasis vary in occurrence and severity depending on the duration and magnitude of infestation by Pthirus pubis. The signs and symptoms may include:

  • A tickling sensation caused by movement of hair in the infected area
  • Itching caused by the saliva of the P. pubis in the pubic hair area
  • This can be accompanied by pruritus (severe, intense itching) caused by bites of the crab lice. It has been observed that scratching will often not alleviate the itchiness
  • Skin discoloration after long-term infection

How is Pthiriasis Diagnosed?

The diagnosis of Pthiriasis is made by the following diagnostic tools:

  • A complete physical examination and an assessment of symptoms
  • An evaluation of the affected individual’s medical history
  • The finding of a nit (egg), nymph (undeveloped form), or adult louse surviving in the hair of the affected person. While an infestation is most commonly found in the pubic or perianal (surrounding the anus) hair of the individual, it can also be sometimes present in the underarm hair, moustache, chest hair, or even on the eyelashes
  • The identification of the morphology of the crab louse, as the pubic louse is morphologically different from that of head and body lice and is often easily identifiable by a trained medical professional

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Pthiriasis?

The potential complications of Pthiriasis may include the following:

  • Secondary bacterial infections due to intense scratching of skin, which can cause lacerations. These may be prone to superimposed infection
  • Presence of other sexually transmitted conditions: It has been observed that other STDs may accompany Pthiriasis (in the infected individual). It is therefore important to clinically address the possibility of other STDs by getting oneself adequately tested by a healthcare provider

Pthirus pubis are not capable of spreading disease from one individual to another. Thus, a pubic louse is unlikely to directly transmit any other disease.

How is Pthiriasis Treated?

Treatment for Pthiriasis can include either over-the-counter drugs or prescription medications. The common treatment measures are listed below:

  • The following medications are considered safe and effective, if used appropriately
    • Over-the-counter lotions for topical skin application, typically containing 1% permethrin
    • Over-the-counter mousses for topical hair application, typically containing pyrethrins and piperonyl butoxide
  • Prescription shampoos can kill both the eggs and lice. But, due to its potential toxicity to the brain/nervous system, it should be used as a therapy for healthy adults (and not in children), only when other treatments have proven ineffective
  • Certain prescription lotions have been shown to kill the lice and eggs. It is not currently approved by the US FDA for use in treatment

Following treatment with the above medications, nits and dead lice should be removed with a fine-toothed comb or using other mechanism, and all infested items of clothing appropriately treated.

  • If the infestation is not severe, in some cases, a fine-toothed comb is sufficient for removal of the organisms
  • All bedding, clothing, and other items that were in contact with the infected individuals (spaces used by them) should be machine washed and dried on high heat settings
  • All items that cannot be machine-washed must be cleaned and sealed for at least 2 weeks

How can Pthiriasis be Prevented?

The following factors may be considered towards the prevention or early diagnosis of Pthiriasis:

  • Avoiding sexual contact with infected individuals, until they have been clinically cleared of infection
  • Proper protection should be used during sexual activity, if any
  • Immediately seeking testing (by a healthcare provider) after having sexual contact with an infected individual
  • Avoiding shared bed linens or clothing with recently-infected individuals

Regularly cleaning clothes and bed linens can decrease the risk for infection through such items.

What is the Prognosis of Pthiriasis? (Outcomes/Resolutions)

  • The prognosis of Pthiriasis is generally good following treatment. Most cases of Pthirus pubis infection are completely cleared by treatment using over-the-counter or prescription topical medications and the removal of lice remnants using a comb
  • The Pthirus pubis parasite is unable to survive after 48 hours without blood. Due to their low viability, treatment is often highly-effective and should resolve the disease within about 10 days, in most cases

Additional and Relevant Useful Information for Pthiriasis:

  • If pubic lice are found on the head or eyelashes of children, it may be an indicator of some form of sexual abuse or exposure. This should alert the healthcare personnel to assess for a potential sex abuse in them
  • The adult forms of the Pthirus pubis have crab-like morphology, with a wide, flat body, and 6 arms and legs (including 2 sets of claws used for attaching themselves to the hair shaft). Eggs of the P. pubis have dark-brown coloration

What are some Useful Resources for Additional Information?


References and Information Sources used for the Article:


Helpful Peer-Reviewed Medical Articles:


Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: Aug. 4, 2017
Last updated: Dec. 20, 2018