The exact mechanism of the condition is unknown, though some studies have suggested the itching occurs in response to increased fibrinolytic activity in the skin. Later studies indicated inappropriate activation of the sympathetic nervous system may play a part.

The name is derived from Latin: aquagenic, meaning water-induced, and pruritus, meaning itch.

Drug-induced pruritus is itchiness of the skin caused by medication, a pruritic reaction that is generalized.

Biliary pruritus is caused by chronic liver disease with obstructive jaundice, characterized by a severe generalized itchy sensation.

One way to treat xerotic eczema is to avoid ing the affected area and to apply anti-itch or moisturizing lotion frequently.

A study published in 2005 found positive results from soaking the affected area in water for twenty minutes and then applying mid- to high-strength corticosteroid ointment.

Adverse reactions to biologic agents, such as imatinib, occur in more than 80% of patients, and can be characterized by edema and pruritus without dermatitis.

HIV-associated pruritus is a cutaneous condition, an itchiness of the skin, that occurs in up to 30% of HIV infected people, occurs when the T-cell count drops below 400 per cubic mm.

Generally, lichen nitidus is asymptomatic and self-limited; therefore, no treatment is required. However, if persistent pruritus is present, or the appearance “...interferes with daily activities or outlook...” topical glucocorticoids may be tried. If the disease process is symptomatic, generalized and extensive, oral glucocorticoids may be indicated. Other reported treatments include PUVA, UVA/UVB phototherapy, astemizole, acitretin, and etretinate.

When appears with sun/humidity; air conditioning (cool dry air) reduces swelling and discomfort.

Xerotic eczema (also known as "Eczema craquelé", "Pruritus hiemalis", "Asteatotic eczema", "Winter itch", "Desiccation dermatitis," and "Winter eczema") is a form of eczema that is characterized by changes that occur when skin becomes abnormally dry, itchy, and cracked. Lower legs tend to be especially affected, although it can appear in the underarm area as well.

Xerotic eczema is common in elderly people, though it is not uncommon for people in their 20s. It can appear in red, bumpy, pimple-like irritations. Shaving can cause it to become inflamed.

Prion pruritus is the intense itching during the prodromal period of the Creutzfeldt–Jakob disease.

Riehl melanosis is a form of contact dermatitis, beginning with pruritus, erythema, and pigmentation that gradually spreads which, after reaching a certain extent, becomes stationary. The pathogenesis of Riehl melanosis is believed to be sun exposure following the use of some perfumes or creams (a photocontact dermatitis).

If a specific cause for pruritus ani is found it is classified as "secondary pruritus ani". If a specific cause is NOT found it is classified as "idiopathic pruritus ani". The irritation can be caused by intestinal parasites, anal perspiration, frequent liquid stools, diarrhea, residual stool deposits, or the escape of small amounts of stool as a result of incontinence or flatulence. Another cause is yeast infection or candidiasis. Some diseases increase the possibility of yeast infections, such as diabetes mellitus or HIV infection. Treatment with antibiotics can bring about a disturbance of the natural balance of intestinal flora, and lead to perianal thrush, a yeast infection affecting the anus. Psoriasis also can be present in the anal area and cause irritation. Abnormal passageways (fistulas) from the small intestine or colon to the skin surrounding the anus can form as a result of disease (such as Crohn's disease), acting as channels which may allow leakage of irritating fluids to the anal area. Other problems that can contribute to anal itching include pinworms, hemorrhoids, tears of the anal skin near the mucocutaneous junction (fissures), and skin tags (abnormal local growth of anal skin). Aside from diseases relative to the condition, a common view suggests that the initial cause of the itch may have passed, and that the illness is in fact prolonged by what is known as an itch-scratch-itch cycle. It states that scratching the itch encourages the release of inflammatory chemicals, which worsen redness, intensifies itchiness and increases the area covered by dry skin, thereby causing a snowball effect.

Some authorities describe “psychogenic pruritus” or "functional itch disorder", where psychological factors may contribute to awareness of itching.

Ingestion of helminth (worm) "Enterobius vermicularis" (pinworm, or threadworm) eggs leads to enterobiasis, indicative of severe itching around the anus from migration of gravid females from the bowel. Severe cases of enterobiasis result in hemorrhage and eczema.

The term morbilliform refers to a rash that looks like measles. The rash consists of macular lesions that are red and usually 2–10 mm in diameter but may be confluent in places.

Patients with measles will have the rash but there are other syndromes and infections that will display the same symptom such as patients with Kawasaki disease, meningococcal petechiae or Waterhouse-Friderichsen syndrome, Dengue, congenital syphilis, rubella, Echovirus 9, drug hypersensitivity reactions (in particular with certain classes of antiretroviral drugs, such as abacavir and nevirapine, and also the antiepileptic drug phenytoin), or other conditions may also have a morbilliform rash.

One cause of morbilliform rash is an allergic reaction to transfused blood/blood components. In such a case, the skin lesions would develop within a few hours (Approx. 4hours) of transfusion along with pruritus. The condition may even present with other symptoms, such as conjunctival oedema, oedema in the lips and tongue, and even localised angioedema. On rare occasions, the condition may even escalate to anaphylactic shock where pulmonary restrictions are seen. The associated cause for this is a reaction against an allergen that is seldom identified during testing. Transfusing products with anti-IgA antibodies to IgA-deficient patients has also been a suspected cause for such reactions. Management usually relates to the stoppage of transfusion for around 30minutes, until given antihistamines take effect. Transfusion may even be continued after, if no further progression is seen.

Lichen nitidus is a chronic inflammatory disease of unknown cause characterized by 1–2 mm, discrete and uniform, shiny, flat-topped, pale flesh-colored or reddish-brown papules that may appear as hypopigmented against dark skin. Occasionally, minimal scaling is present or can be induced by rubbing the surface of the papules. The disease usually affects children and young adults and is painless and usually nonpruritic, although protracted itching may occur in some cases. It is sometimes referred to by dermatologists as "mini lichen planus".

Psychogenic pruritus is a common manifestation of chronic anxiety, usually a localized itch, especially in the anogenital area.

The condition is often managed with drugs including H1-antihistamines, tricyclic antidepressants, tetracyclic antidepressants, selective serotonin reuptake inhibitors, antipsychotics, or benzodiazepines.

Chronic and repetitive scratching, picking, or rubbing of the nodules may result in permanent changes to the skin, including nodular lichenification, hyperkeratosis, hyperpigmentation, and skin thickening. Unhealed, excoriated lesions are often scaly, crusted or scabbed. Many patients report a lack of wound healing even when medications relieve the itching and subsequent scratching.

Patients often:

- seek treatment during middle-age, although PN can occur at any age.

- have a history of chronic severe pruritus.

- have a significant medical history for unrelated conditions.

- suffer from liver or kidney dysfunctions.

- suffer secondary skin infections.

- have a personal or family history of atopic dermatitis.

- have other autoimmune disorders.

- have low vitamin D levels.

The cause of prurigo nodularis is unknown, although other conditions may induce PN. PN has been linked to Becker's nevus, linear IgA disease, an autoimmune condition, liver disease and T cells. Systemic pruritus has been linked to cholestasis, thyroid disease, polycythaemia rubra vera, uraemia, Hodgkins disease, HIV and other immunodeficiency diseases. Internal malignancies, liver failure, renal failure, and psychiatric illnesses have been considered to induce PN, although more recent research has refuted a psychiatric cause for PN. Patients report an ongoing battle to distinguish themselves from those with psychiatric disorders such as delusions of parasitosis and other psychiatric conditions.

Autoimmune estrogen dermatitis presents as a cyclic skin disorder, that may appear eczematous, papular, bullous, or urticarial. with pruritus typically present, skin eruptions that may be chronic but which are exacerbated premenstrually or occur immediately following menses.

The Mazzotti reaction, first described in 1948, is a symptom complex seen in patients after undergoing treatment of onchocerciasis with the medication diethylcarbamazine (DEC). Mazzotti reactions can be life-threatening, and are characterized by fever, urticaria, swollen and tender lymph nodes, tachycardia, hypotension, arthralgias, oedema, and abdominal pain that occur within seven days of treatment of microfilariasis. The Mazzotti reaction correlates with intensity of infection; however, there are probably multiple infection intensity-dependent mechanisms responsible for mediating this complex reaction.

The phenomenon is so common when DEC is used for the treatment of onchocerciasis that this drug is the basis of a skin patch test used to confirm that diagnosis. The drug patch is placed on the skin, and if the patient is infected with the microfilaria of "O. volvulus", localized pruritus and urticaria are seen at the application site.

A case of the Mazzotti reaction has been reported after presumptive treatment of schistosomiasis and strongyloidiasis with ivermectin, praziquantel and albendazole. The patient had complete resolution of symptoms after intravenous therapy with methylprednisolone.

Uremic pruritus (also known as uraemic pruritus or renal pruritus) is caused by chronic kidney failure and is the most common internal systemic cause of itching.

Nalfurafine, an orally-administered, centrally-acting κ-opioid receptor agonist, is approved to treat the condition in Japan.

Cholestatic pruritus is the sensation of itch due to nearly any liver disease, but the most commonly associated entities are primary biliary cirrhosis, primary sclerosing cholangitis, obstructive choledocholithiasis, carcinoma of the bile duct, cholestasis (also see drug-induced pruritus), and chronic hepatitis C viral infection and other forms of viral hepatitis.

Brachioradial pruritus (BRP) is a localized pruritus of the dorsolateral aspect of the arm. BRP is an enigmatic condition with a controversial cause; some authors consider BRP to be a photodermatosis, whereas other authors attribute BRP to compression of cervical nerve roots.

BRP may be attributed to a neuropathy, such as chronic cervical radiculopathy. The possibility of an underlying neuropathy should be considered in the evaluation and treatment of all patients with BRP.

The main cause of BRP is not known, but there is evidence to suggest that BRP may arise in the nervous system. Cervical spine disease may be an important contributing factor.

Patients with BRP may have underlying cervical spine pathology. Whether this association is causal or coincidental remains to be determined.

There is controversy regarding the cause of brachioradial pruritus: is it caused by a nerve compression in the cervical spine or is it caused by a prolonged exposure to sunlight?

In many patients, itching of the arms or shoulders is seasonal. Some patients reported neck pain.

BRP can be linked to the thyroid.

Shiitake mushroom dermatitis (also known as "flagellate mushroom dermatitis", "mushroom worker's disease", and "shiitake-induced toxicoderma") is an intensely pruritic dermatitis characterized by disseminated 1mm erythematous micropapules seen in a linear grouped arrangement secondary to Koebnerization due to patient scratching. It is caused by the ingestion of shiitake mushrooms and was first described in 1977 by Nakamura. Although it is rarely seen outside of China and Japan due to a lower incidence of shiitake consumption outside these regions, there is a well-established association between flagellate dermatitis and shiitake mushroom (Lentinula edodes) ingestion. Bleomycin ingestion may also cause similar findings. On physical exam, one key difference between the two is that post-inflammatory hyperpigmentation changes are usually seen with bleomycin-induced flagellate dermatitis and are not typically present with shiitake mushroom induced flagellate dermatitis. The median time of onset from ingestion of shiitake mushrooms is typically 24 hours, ranging from 12 hours to 5 days. Most patients completely recover by 3 weeks, with or without treatment. Although the pathogenesis of shiitake induced flagellate dermatitis is not clear, the theory most argued for is a toxic reaction to lentinan, a polysaccharide isolated from shiitake mushrooms. However, Type I and Type IV allergic hypersensitivities have also been supported by the 24-hour median time of onset, clearance in 3–21 days, severe pruritus, benefit of steroids and antihistamines, and lack of grouped outbreaks in people exposed to shared meals containing shiitake mushrooms. Most cases reported shortly after its discovery were due to consumption of raw shiitake mushrooms, but several cases have since been reported after consumption of fully cooked mushrooms.

Basophilia as an isolated finding is uncommon. However it is a common feature of myeloproliferative disorders and particularly prominent in chronic myelogenous leukemia.

Conditions Associated with Increased Numbers of Blood Basophils

- Allergy or inflammation

1. Ulcerative colitis

2. Drug, food, inhalant hypersensitivity

3. Erythroderma, urticaria

4. Juvenile rheumatoid arthritis

- Endocrinopathy

1. Diabetes mellitus

2. Estrogen administration

3. Hypothyroidism (myxedema)

- Infection

1. Chicken pox

2. Influenza

3. Smallpox

4. Tuberculosis

- Iron deficiency

- Exposure to ionizing radiation

- Neoplasia

1. "Basophilic leukemia" (see text)

- Myeloproliferative neoplasms (especially chronic myelogenous leukemia; also polycythemia vera, primary myelofibrosis, essential thrombocythemia)

- Carcinoma

Pruritus ani is the irritation of the skin at the exit of the rectum, known as the anus, causing the desire to scratch. The intensity of anal itching increases from moisture, pressure, and rubbing caused by clothing and sitting. At worst, anal itching causes intolerable discomfort that often is accompanied by burning and soreness. It is estimated that up to 5% of the population of the United States experiences this type of discomfort daily.