Humans can be infected by a variety of organisms such as bacteria, viruses, fungi, protozoa, and worms. In addition, we can also be infected by mites and other insects (arthropods).
Lice (Pediculus humanus) are wingless, parasitic insects than can infect the human body. These small insects are about 2 to 3mm in size and can be transmitted from one person to another. Lice live on the human body and feed on human blood. Lice infestation, also known as pediculosis, can happen in three areas: the head, the pubic area, and the body.
- Head lice - Head lice are the most common form of lice infestation. Head lice live on the hair and on the scalp. The adult lice feed on the blood from the scalp, and the females lay their eggs (called nits) on the shaft of the hair. These eggs appear as tiny whitish to yellowish oval structures that are attached to the shaft of the hair. Head lice can cause itchiness of the scalp because the saliva of the lice can irritate the scalp as the insect feeds. Sharing of personal belongings like comb, brush, hat, and other hair accessories can transmit this parasite.
- Pubic lice - Pubic lice are found on the pubic area, particularly on the pubic hair. Like the head lice, pubic lice can also cause intense itchiness especially when the insect is feeding. Pubic lice are usually sexually transmitted but can also be transferred by direct contact to infested objects like clothes, towel, and beddings.
- Body lice - Body lice are usually hiding in the folds and seams of clothing and beddings. They can be transmitted by direct contact or by sharing of personal belongings. They are common in cramped areas and in populations of low socioeconomic status. Body lice, just like head lice and pubic lice, can cause itchiness on the infested area. Moreover, they can also cause some rashes on the area where they bite.
The diagnosis of pediculosis is made by documentation or visualization of lice or nits.
The treatment of pediculosis consists of medicated shampoo or lotion designed to kill the insect. These medications include pyrethrin, permethrin, lindane, and malathion. For body lice, the treatment also consists of decontaminating the clothing and beddings by thorough washing and drying in warm temperature.
Scabies is another parasitic infection that is caused by the mite Sarcoptes scabiei. The female mite burrows and lays eggs under the skin, causing intense itchiness of the affected area. The burrow tracks can be seen as red lines and bumps on the skin that are often associated with scratch marks because of intense itchiness. Common infested areas include the webs of the digits and intertriginous sites like the armpits and groin.
Scabies is transmitted by direct contact. The mite can also survive on objects like clothes, towels, and beddings, thus making its transmission even more efficient. In households where one person is infected, it is very common for the entire household to eventually contract the infection.
The diagnosis of scabies is made by history and physical examination. A history of intense itchy rashes found on the intertriginous areas or webs of digits, along with history of contact with a person having the same rashes, often clinches the diagnosis. This is further supported by examination of the rashes under magnification to detect the burrow tracks. Skin scrapings can also be studied under the microscope to document the presence of mites or eggs.
The treatment for scabies consists of medicated creams or lotions that are designed to kill the parasite. These medications include permethrin, lindane, and ivermectin. Antihistamines can also be taken to relieve the itchiness. Family members who were in close contact with the infected person may also benefit from getting treatment preemptively. Clothing and beddings should be decontaminated to prevent the further spread of infection. Decontamination is done by washing the items in hot water and drying them in a hot drier.