As the skin is constantly exposed to the environment, it represents a common portal of entry for different parasites. Infestations with parasites can cause systemic diseases in humans, but often result in cutaneous lesions which are on the rise in dermatology clinical practice due to the increase of travel and immigration.
Despite the considerable burden parasitic skin infestations can cause, their significance has often been neglected by the health-care providers and scientific community alike. Furthermore, the debilitating impact a persistent itch can have must be taken into account when assessing these conditions.
Protozoan and helminth infestations
Protozoan infestations characterized by a transient skin phase following infestation via the cutaneous route include African trypanosomiasis (sleeping sickness) and South American trypanosomiasis (Chagas disease). In the former vesicular lesion develops approximately five days after a bite of a tsetse fly, whereas the latter is characterized by a marked, usually unilateral periorbital edema known as Romaña’s sign.
Cutaneous amebiasis is a rare skin manifestation of a disease caused by Entamoeba histolytica – a protozoan parasite found worldwide. The disease initially presents as a deep-seated swelling that ulcerates and forms a necrotic base with blood and pus.
A myriad of helminthic diseases can present with various dermatologic signs and symptoms such as skin nodules, migratory skin lesions, cysts and itching. Various cestode or tapeworm species (such as Taenia solium, Echinococcus granulosus and Spirometra spp.) may form cysts under the skin.
Infestation with different nematode or roundworm species (most often zoonotic species that accidentally infect humans) can result in swelling and subcutaneous lesions at the point of entry. Some of the nematodes infecting the skin include filarial species such as Loa loa, Mansonella streptocerca, Onchocerca volvulus, as well as Gnathostoma spp. and Dirofilaria spp.
Trematode (fluke) infestations that involve the skin are scarce; for example, invading cercariae in schistosomiasis may provoke a transient allergic rash that lasts up to five days (cercarial dermatitis) while traveling to the blood stream. The non-human schistosome species can also cause similar type of dermatitis known as pelican itch or swimmer’s itch.
Scabies is a cosmopolitan cutaneous infestation caused by the human scabies mite (Sarcoptes scabiei var. hominis) and acquired by personal direct skin contact. The scabies mite lays eggs in a previously burrowed tunnel of up to 4 millimeters into the superficial layer of the epidermis.
Papules (with or without excoriation) and S-shaped burrows are the classical lesions of scabies, which together with intense pruritus support the diagnosis of this condition. Moreover, individuals of all age groups that suffer from chronic crusted scabies may present with eczema, impetiginized plaques and hyperkeratosis masking the typical clinical sings of scabies.
Three species of lice are known to infest humans – the head louse (Pediculus humanus capitis), the body louse (Pediculus humanus corporis) and the pubic louse (Pthirius pubis). All of them feed on the blood of the host, which causes inflammation, itching and skin discoloration.
Similarly, fleas bite humans in order to obtain a blood meal, producing in turn a localized inflammatory reaction. A clinical presentation of itching with papules, vesicles or small nodules (often in clusters) on both feet and lower legs is characteristic, while scratching can result in bacterial superinfections.