27th June 2023

Parasitic STIs

Trichomoniasis

Trichomoniasis is the most common non-viral STI worldwide, caused by the flagellated protozoan Trichomonas vaginalis. Symptoms in females can include abnormal or offensive vaginal discharge, itching of the vulva, and dysuria; however, up to half of infected women are asymptomatic. Men can become infected, usually as a result of sexual contact with an infected female; transmission between male partners is uncommon.

Trichomoniasis is associated with co-infection with other STIs, increased risk of HIV transmission, as well as fertility and perinatal complications such as preterm birth.

Current guidelines advise treatment with oral metronidazole or oral tinidazole. Any sexual partner within four weeks prior to presentation should also be treated and offered testing for other STIs.

Pubic lice

Pubic lice infestation (pediculosis pubis), commonly coined ‘crabs’, is caused by an ectoparasite called Pthirus pubis. As the name suggests, infestations typically occur within the pubic hair, however, they can also occur in other coarse hair such as axillary hair, chest/back hair, beards, eyebrows, and eyelashes. They do not typically infest head hair, unlike pediculosis capitis (head lice).

Symptoms of a pubic lice infestation include pruritis, often worse at night, and complications such as skin infection can occur due to excoriation. Lice and eggs may be seen on examination.

Transmission is commonly through sexual contact or close body contact, however, fomite transmission can also occur, e.g. via infested bedding or towels.

Treatment includes topical permethrin or malathion and decontamination of infested materials with hot water.

Scabies

Scabies is a contagious infestation transmitted by any prolonged skin-to-skin contact, but in young adults is commonly sexually transmitted. It is an infestation caused by the parasitic mite, Sarcoptes scabei. The female mites burrow into the skin (which may be seen as silvery lines in the skin) and lay 2-4 eggs per day in the stratum corneum layer. The eggs produce larvae which hatch and develop into adult mites over a period of two weeks. The symptoms of pruritus, erythema, papules, and nodules can take up to four weeks to present following initial infestation; this is due to the symptoms being a result of the host immune response to the mites. Commonly affected sites include between the fingers, wrists, elbows, and genitals. Norwegian scabies (also known as crusted scabies) is a highly contagious variant that typically affects people who are immunocompromised. It results in hyperkeratotic, crusting plaques affecting the torso, extremities, palms, and soles.

First-line treatment for non-crusted scabies is with permethrin 5% dermal cream (washed off after 12 hours), with repeat treatment after seven days. All close household contacts and sexual partners from the past two months must be treated at the same time, and recent bedding and clothing should be washed at 60 degrees Celsius or more.

Crusted scabies is usually treated with a combination of permethrin 5% and ivermectin.