Small organisms such as lice, larvae, pinworms, roundworms and tapeworms, which live in dependence on another living being, are called parasites. Parasites are microorganisms that can live only in dependence on a host and harm the host on which they live; for this reason they cause many diseases. Parasites are divided into two groups: internal and external parasites. Intestinal worms are the best example of internal parasites, and lice, ticks and scabies mites of external parasites. Parasitic diseases are even more common in low-income, poor communities.
Development
Because parasites are living beings, they need to feed in order to develop and grow. They obtain their food from the host organism. As their numbers increase, the amount of food they take increases, causing the host to weaken and to be unable to make full use of the food it eats, and in cases where the parasite load increases they can cause death.
Examples of Parasitic Skin Diseases
* Scabies
* Lice infestation
* Fungal diseases
* Skin disorders caused by bites from insects such as fleas, bedbugs and ticks can also be included.
Scabies
Scabies has an important place in the history of medicine: in 1687 it was described as the first human disease with a known cause, and today it is known to affect 300 million people. Its incubation period is reported to be 2–3 weeks on average. The most prominent feature of scabies is severe itching at night; the itching increases when the person gets into bed and warms up and can be severe enough to wake them, while during the day a milder, tolerable itch is felt. At first the itching is in the area of the burrow and, related to the activity of the mite, has a burning character; it then becomes widespread. The widespread itching is due to sensitivity to the mite that develops later and usually begins 4–6 weeks after the primary infestation. In fact, it is thought that most of the itching and rashes seen in scabies are due to a hypersensitivity reaction, as symptoms appear about 1 month after the first encounter with the mite but within 24 hours on reinfestation, reflecting a sensitisation response.
The pathognomonic lesion, the burrow (sillon), is a slightly raised, brownish, pinkish-white tunnel lesion that can be 0.2–0.5 mm wide and 10–15 mm long and lengthens by 0.5–2 mm a day; at the end of the tunnel there are pearl-like vesicles containing female Sarcoptes mites.
Burrows are seen most often on the wrists (85%) and between the fingers, in the armpits, on the buttocks and on the penis. In babies they are common on the palms and soles; in adults, burrows are rarely found on the trunk, but they may often be seen there in elderly people and infants. It is also accepted that itching is beneficial in reducing the mite population. When scrapings from a burrow or vesicle are examined under the microscope, female Sarcoptes mites, their eggs and faecal pellets can be seen.
Burrow (Sillon)
The scabies mite (Sarcoptes scabiei) lives and lays its eggs in tunnels it digs in the stratum corneum of the epidermis of human skin. Only female scabies mites cause scabies. The female scabies mite moves through the tunnels called "burrows", especially in the warmth of the night. These movements cause severe itching. Scabies spreads easily from person to person through close contact. Sharing a bed with an infected person, sexual intercourse, wearing an infected person's clothes and shaking hands are routes of transmission.
Pearly Vesicle (Vésicule Perlée)
The female parasite Sarcoptes scabiei enters the human body and begins to dig tunnels in the skin, while the male fertilises the eggs in these tunnels. Interestingly, after fertilising the eggs the male parasite dies and remains in the skin. Because of the parasite and its faeces, lines in the form of black curves can form in the skin. These S- or I-shaped lines improve with treatment. As mentioned, the female parasite digs a tunnel as it enters the skin, and at the entrance of this tunnel it creates a raised bump that looks as though it contains fluid. In medicine this bump is called a pearly vesicle (vésicule perlée). People with scabies have many of these bumps on the trunk, hands and arms. These bumps are extremely itchy because the parasite's saliva and eggs irritate the dermis. When the patient scratches and bursts these bumps, they actually spread the eggs (new larvae) located there to other places with their own hands. Bursting the bump is in fact a factor in the spread of the disease; thanks to this feature, the parasite spreads to different places on the human body. Wherever the person rubs this fluid on their hand, they carefully carry the parasite there too.
Norwegian Scabies
The causative agent is Sarcoptes scabiei. It is a very itchy disease. In immunosuppressed people an abnormal degree of crusting occurs; this condition is called Norwegian (crusted) scabies. The mite is an obligate parasite of humans and feeds on tissue fluid. It passes through the egg–larva–nymph–adult stages. The male dies after fertilising the female. Sarcoptes mites live in burrows, and the condition becomes more common in the autumn and winter months. It is transmitted through sexual intercourse, sharing a bed and physical contact. The incubation period is 2–3 weeks, and there is itching that wakes the person from sleep at night. The best diagnostic method is epidermal shave biopsy.
Pediculosis (Lice Infestation)
Three types of lice can infest humans.
1. Pediculus humanus capitis (head louse)
2. Pediculus humanus humanus (body louse)
3. Pthirus pubis (pubic louse)
Lice infestation, known as pediculosis, remains a problem worldwide. Lice feed by sucking human blood. They lay their eggs on the hair shaft or in the seams of clothing. Chewing lice affect farm animals, causing major economic losses, and rarely affect humans. Lice infestation in humans is almost always caused by sucking lice. Lice are arthropod insects of the Pediculidae or Pthiridae family.
Adult lice live for about 20–30 days. During this time the body louse lays about 200 eggs, the head louse 100 and the pubic louse 50. They survive 3–4 days away from the host. Lice infestation causes itching and resulting excoriations. When secondary bacterial infection develops, regional lymphadenopathy occurs. A hypersensitivity rash called pediculid can mimic a viral exanthem.
Head lice can be seen in all walks of life, whereas body lice are seen particularly in homeless people. Body lice are major vectors of diseases such as typhus, trench fever and relapsing fever. Among refugees these diseases can cause thousands of deaths. Trench fever transmitted by lice causes endocarditis among homeless people in cities. Pubic lice are often transmitted through sexual contact; for this reason, people with pubic lice should also be investigated for other sexually transmitted diseases.
Head lice usually affect children, primarily girls. Transmission is usually through head-to-head contact, but can also occur through items used on the head. The head louse is 2 mm long and can travel 23 cm per minute. In dry climates they lay their eggs on the hair within 1–2 mm of the scalp. The eggs (nits), popularly called "sirke" in Turkish, are oval and firmly attached to the hair. The larvae hatch from the eggs in 1 week and mature in about 1 week. Head lice feed on blood every 4–6 hours.
In head lice infestation, the nape of the neck and the area behind the ears are the most commonly affected. Lice, and especially their eggs, can easily be seen in the occipital (nape) and retroauricular (behind the ear) areas of the scalp. Lice are most easily detected by combing. Although patients may be asymptomatic, itching is common. Bite reactions, excoriations, cervical lymphadenopathy and conjunctivitis are the most common findings. Eczematisation and infection may develop secondary to itching. It is most often confused with eczema and pyoderma. Pediculid, a hypersensitivity rash, can mimic a viral exanthem.
A definitive diagnosis is made by seeing lice and/or their eggs on the scalp. Using special lice combs increases the chance of finding live lice. Eggs alone are not diagnostic of active infestation. Eggs in the hair should be distinguished from dandruff and piedra. On microscopic examination of the hair, the eggs, firmly attached to one side of the hair shaft, are easily recognised. The eggs are oval, yellow-grey and about 1 mm in size.
Psocids are louse-like insects that rarely infest the human scalp. Bark lice and book lice belong to this group. They are 2–3 mm long and have long antennae, chewing mouthparts, larger heads and larger hind legs.
Pubic lice (Pediculosis pubis)
Pubic lice (pediculosis pubis) are usually transmitted through sexual contact. Sexually transmitted diseases such as HIV, syphilis, gonorrhoea, chlamydial infections, herpes, genital warts and trichomoniasis are more common in these patients. The pubic louse is about 1 mm in diameter. It clings firmly to the base of the pubic hairs and can move 10 cm a day. During its life cycle of about 2 weeks, the female lays an average of 25–50 eggs.
Apart from the pubic area, pubic lice can also settle in short-haired areas such as the chest, abdomen, legs and buttocks. They may settle on the eyebrows and eyelashes, and rarely on the scalp. The lice cling firmly to the base of the hairs, so they look like black-grey or grey-yellow dots at the hair roots. Their eggs are firmly attached to the hairs. Itching is usually mild. One of the characteristic findings is maculae caeruleae: blue-grey macules that develop secondary to louse bites and are seen on the lower abdomen and thighs. These spots are associated with deep dermal haemosiderin deposition due to the bites.
In pubic lice infestation the lice can be seen with the naked eye. The diagnosis is made by seeing the lice and their eggs. Eggs on the pubic hairs should be distinguished from trichomycosis (piedra) pubis.
Leishmaniasis
Leishmaniasis is a protozoal disease caused by Leishmania protozoan parasites of the genus Leishmania that are pathogenic to humans, transmitted by the bite of the sandfly insect vector (Phlebotomus). Depending on factors related to the parasite and the host, leishmaniasis shows a clinical spectrum ranging from subclinical to localised (cutaneous leishmaniasis) or disseminated infection (cutaneous, mucosal or visceral).
The prevalence of the disease is increasing day by day. It is estimated that there are 2 million new cases each year (1.5 million CL, 0.5 million VL, very few MCL). Each year about 70,000 people die from kala-azar (VL, Dum-Dum fever). Leishmaniasis is a significant problem in 88 countries worldwide, in areas where about 350 million people live. It is seen in a variety of geographical areas, from the rainforests of Central and South America to the deserts of Western Asia. It is not seen in Australia or the South Pacific. Incidence is highest in tropical and subtropical regions where conditions suit the sandfly population. Formerly common in rural areas, the disease is increasingly seen in cities as well.
All age groups living in or travelling to areas where leishmaniasis is found are at risk. Adventurers, peace volunteers, missionaries, birdwatchers and especially soldiers are at increased risk. The disease is seen in Europe and the United States, particularly in returning travellers and immigrants. More than 90% of cutaneous leishmaniasis cases occur in Afghanistan, Pakistan, Iran, Türkiye, Syria, Saudi Arabia, Algeria, Brazil and Peru, and more than 90% of visceral leishmaniasis cases in India, Bangladesh, Nepal, Sudan and Brazil. Looking at these countries, it can be seen that leishmaniasis, as a neglected disease, is closely intertwined with poverty.
Cutaneous leishmaniasis can be seen worldwide except in Antarctica, and its incidence is quite high particularly in countries bordering the Mediterranean, the Middle East, Central and South Asia, South-West Africa and Latin America. Leishmaniasis, especially in its cutaneous forms, has for many years been endemic, hyperendemic and sometimes even epidemic in certain regions of our country. The disease, which in recent years has come to threaten not only rural areas but also large cities, continues to affect thousands of people and put millions at risk.
In Türkiye, before the 1950s the disease was quite common, especially in the South-Eastern Anatolia region; after that time its incidence began to fall steadily owing to the DDT used against mosquitoes for malaria eradication. However, from the 1980s onwards the incidence rose again, particularly in and around the provinces of Şanlıurfa, Diyarbakır, Adana and Osmaniye, and the disease has become endemic in these regions. Most of the cases reported in Türkiye over the last 15 years have been from Şanlıurfa, Osmaniye, Adana, Hatay, İçel, Kahramanmaraş and Diyarbakır.
Because of improved transport and permanent or seasonal migration from endemic to non-endemic regions, the disease is spreading and has increasingly come to threaten not only rural areas but also large city centres and non-endemic regions.
Changes in the ecological balance affecting vectors and reservoirs due to large hydroelectric dams and irrigation projects may be one of the important factors in the renewed increase observed in some regions in recent years.
L. tropica and L. major cause disease mainly in the Mediterranean basin, the Middle East, the Caucasus and Central Asia; L. infantum, which often causes visceral disease, in countries bordering the Mediterranean; and L. aethiopica in South-West Africa.
In our country the disease is transmitted by infected females of Phlebotomus sandflies, known locally as "tatarcık", "üvez", "yakarca" and "mucuk". In our country Phlebotomus sergenti and papatasi species act as
vectors. In L. tropica infections, dogs and infected people are the reservoir; in L. major infections, certain rodents (gerbils and other rodents). The sandfly becomes infected by taking up Leishmania amastigotes while feeding on the blood of reservoir animals and infected people; the parasite turns into a promastigote in its gut, and the sandfly then inoculates it into healthy people, again while feeding on blood.
The parasite, phagocytosed by macrophages in the dermis, loses its flagellum and turns into an amastigote. The amastigotes multiply by binary division, burst the macrophages and leave the cell, then infect other macrophages. Depending on various factors related to the host and the parasite, the parasite is either eliminated or infection develops. Disease develops in only a very small proportion of parasite inoculations. After a period that varies according to the species of parasite, the size of the inoculum and the host's immunity, a clinical lesion begins to develop. This lesion contains parasite-laden macrophages, lymphocytes and plasma cells.
In chronic cases, a histological picture is seen consisting of classic epithelioid histiocytes and tuberculoid granulomas, sometimes accompanied by Langhans-type giant cells. In these cases it is quite difficult to find the parasite. Histologically, it is difficult to distinguish these cases from other granulomatous diseases, chiefly lupus vulgaris, but also sarcoidosis, deep mycosis and blastomycosis. The specific cell-mediated immune response that develops plays a major role in healing of the disease, with or without treatment. In cases where a sufficient specific cell-mediated immune response does not develop, relapses may occur months or years after healing with treatment.