Image: iStock. By  Cameron Webb , University of Sydney .

What’s the worst thing you could be burdened with on Valentine’s Day? Unrequited love? Unwanted affection? Unpaid invoice from the local florist? How about an itchy dose of pubic lice?

Pubic lice, Phthirus pubis, or “crabs” as more commonly known, were once thought to be one of the most contagious sexually transmitted infections, but they rarely rate a mention in most sexual health research these days.

Perhaps their absence from the sexual health spotlight is due to the lack of serious health implications associated with infestation, the shifting grooming habitats of those most sexually active or, perhaps in the age of internet diagnosis, few people present their itchy nether regions to the local doctor.

Pubic crabs haven't gone anywhere. (iStock)

Our parasitic companions

Pubic lice are thought to have been our parasitic companions for more than 10,000 years. There is paleoparasitological evidence of lice infestation throughout human history.

They weren’t just a problem of the poor either. The remains of royalty have been found infested. Even the rise of sanitation and bathing in the Roman Empire wasn’t enough to stop the spread of pubic lice.

Pubic lice aren’t the only creatures to adapt to life on humans. Head lice are no doubt enjoying the end of summer holidays as they infest little heads around the country.

While head lice have adapted perfectly to life among the hairs on the human head, pubic lice have adapted to warmer and more humid habitats of “you know where” filled with an abundance of coarse hair that provides an ideal refuge.

Watch: Another awkward sex-related topic? Queefing. Here's why it happens. (Post continues after video.)

What are pubic lice?

Pubic lice are small six-legged insects about 1.5mm long. They have a crab-like appearance with large claws perfectly designed for a life clinging to and scurrying up and down pubic hair.

They don’t live long, perhaps less than a month, but a female louse can lay around 30 eggs during that time, ensuring a steady population of lice is maintained. Once off the hair, they’re clumsy and lucky to survive a couple of days.

They’re typically found on hair in the pubic region but can live anywhere coarse hair is found. This may include the beard, eyebrows and eyelashes, as well hair on the chest and in the armpit. Pubic lice are rarely, if ever, found in head hair.

There appears a strong link between the lice found on the ancestors of gorillas and our own pubic lice. That’s right: more than three million years ago our ancestors and those of gorillas were both in the right place at the right time (at least from the pubic louse’s point of view). The lice didn’t evolve as we did, they skipped between hosts much more recently than the time when gorillas and humans made the evolutionary step away from a common ancestor.

They don’t fly or jump. They move from host to host through direct contact and, for the most part, this is through sexual contact. Not always, but most commonly. (Post continues after gallery.)

Popular forms of birth control

Contraceptive pill

The Pillis the most popular form of birth control. There are two types: the combined pill, containing two hormones, and theprogestogen-only, or mini, pill, which only contains one female hormone. The combined pill works by preventing the ovary from releasing an egg each month. The mini pill changes the mucus at the entrance of the uterus, which stops sperm from entering. Both are 99.7% effective; however, they don't offer protection from STDs.

Intrauterine device

IUDs are small, T-shaped devices that are inserted into the uterus by a doctor in order to prevent pregnancy. There are two types of IUD: copper, which affects the movement and survival of sperm in the womb while changing the lining of the womb;and hormonal, which releases progestogen (a synthetic version of the naturally-occurring hormone progesterone) into the uterus. Although IUDs have a success rate of more than 99%, only around 2% of Australian women currently use one.

Condoms

The male condom is a barrier contraception that prevents both pregnancy and the transmission of STDs. They work by collecting semen so it doesn't enter theman's partner's body. Condoms are 95 to 99% effective at preventing pregnancy.

The female condom

The female condom is a pre-lubricated sheath, which loosely fits inside of the vagina. The condom has a removable ring, which helps to insert the sheath and keep the condom in place. A larger ring remains on the outside to cover the opening of the vagina. They are designed to fit all women and can only to be used once. Due to misuse of the product it is only 79 per cent effective.

Diaphragm

Like the male condom, the diaphragm is a barrier method of contraception. It's a shallow silicone dome that's inserted into the vagina andcovers the cervix, which is the entrance to the uterus. It's held in place by the pelvic muscles, and works by preventing sperm from entering the uterus. After sex, diaphragms need to remain in placefor another six hours, in which time the sperm inside the vagina die. This method is 88 to 94% effective at preventing pregnancy, although this rate increases with experience.

Birth control implant

This is a 4cm-longimplant that is inserted into a woman's upper arm, just under the skin, by a doctor. It releases a hormone called etonogestrel, which prevents ovulation and thickens the mucus in the cervix, which can stop sperm from entering. Contraceptive implants - usually sold as Implanon - are removed under anaesthetic every three years, and are close to 100% effective.

Vaginal ring

Vaginal rings are soft plastic rings that contain the same two hormones as the combined pill - oestrogen and progestogen. The ring is inserted into the vagina, where the hormones are absorbed into the body and then prevent the ovaries from releasing eggs. Once inside the ring can't be felt, and it needs replacing every three weeks (with one week's break in between). When used correctly, this method is more than 99% effective.

Depo Provera

Depo is a long lasting hormonal injection. The synthetic form of progesterone is administered every 12 to 14 weeks to woman, preventing pregnancy within this ongoing time period. The injection, which is given in the buttock or muscle of the upper arm, prevents ovulation and thickens the mucus in the cervix, helping to block sperm. The injections are 99 per cent effective but do not protect against STDs. Male contraceptive injections are currently being trialled

The morning after pill

Emergency contraception (popularly referred to as the morning after pill) is used to prevent a pregnancy following unprotected sex. Other reasons include missing a contraceptive pill or a condom breaking. It is common in Australia to use the progestogen-only method. This entails a dosage of EC pills containing progestogen or the synthetic version. A doctor can prescribe the pill or they can be bought over the counter at a pharmacy without a prescription. Emergency contraceptionis 85 per cent effective in reducing the risk of pregnancy.

Sterilisation

Sterilisation is a permanent form of contraception. For women there are two types of sterilisation: tubal ligation, in which clips are placed on the fallopian tubes, or tubal occlusion in which a device is inserted into the tubes. These methods work by preventing the sperm from reaching the egg. Tubal litigation is 99.5 per cent effective. The Vasectomy (for men) works by cutting and blocking the tubes called the vas deferens, preventing sperm from leaving the testicles. The chances of impregnation with a vasectomy are 1 in 1000.

A tangle of pubic hair provides a perfect opportunity for lice to “jump ship” to a new host. This migration most commonly happens when the lice move from hair strand to hair strand. They do so perhaps a little less athletically than Tarzan swinging on jungle vines from tree to tree but no less effectively.

Estimates put the prevalence rate of pubic lice infestation in adults at around one to two per cent. Rates can be a little higher in older individuals, especially men and men who have sex with men.

That means about 750,000 people who use Tinder have crabs. Ponder that.

What are the health risks?

As these parasites feed on human blood, they’re prone to causing skin irritation including raised, red lumps and an intense itchiness. The irritation is not caused by them scurrying around as most people would think. It’s caused by an irritation of the skin when the lice inject their saliva to get the blood flowing in order to feed. They feed a few times each day.

Luckily, they’re not associated with the transmission of any disease-causing pathogens. However, there’s evidence that pubic lice may be an indicator of other sexually transmitted infections, such as chlamydia. This relationship has led to the proposal that these lice may be the “canaries in the coal mine” for sexually promiscuous segments of the community and the health risks they face.

Unlike most other sexually transmitted infections, condoms won’t stop pubic lice swinging across to a new person during sex.

"Pubic lice don’t fly or jump. They move from host to host through direct contact." (istock)

Goodbye pubic lice.

Treatment is relatively straightforward. Insecticide creams are available and so long as insecticide resistance is kept at bay, should stop infestations. Remove your hair down there and pubic lice will disappear too (but keep in mind the public health considerations of pubic hair removal).

Trends in hair removal are often linked to loss of pubic lice habitat but there simply is not data to support the hypothesis that pubic lice are “going extinct”. Such claims make for great headlines but there is little evidence, let alone logic, behind the claim.

It may be true that complete hair removal will prevent pubic lice setting up home, but are all those currently sexually active bare down there? Not likely. Pubic lice have been with us for thousands of years and they’ll be with us for thousands more.

Do you think pubic lice has fallen off the public's radar?

Cameron Webb , Clinical Lecturer and Principal Hospital Scientist, University of Sydney

This article was originally published on The Conversation . Read the original article .