Itch

Events of "contagious itch" are common occurrences and may be more than a localized phenomenon in the place one scratches.

In polycythemia vera itching is particularly caused by skin contact with warm water, such as in bath or shower.

Studies done in the last decade have shown that itch can be inhibited by many other forms of painful stimuli, such as noxious heat,^[9] physical rubbing or scratching, noxious chemicals, and electric shock.^[10]

• Body louse, found in substandard living conditions
• Bed bug, lives in mattresses and bedding. May crawl into cracks, baseboards.
• Cutaneous larva migrans, a skin disease caused by hookworm infection
• Head lice, if limited to the neck and scalp
• Herpes, a viral disease
• Arthropod bites, such as those from mosquitos or chiggers
• Pubic lice, if limited to the genital area
• Scabies, especially when several other persons in close contact also itch
• Shaving, which may irritate the skin
• Swimmer's itch, a short-term immune reaction
• Varicella – i.e. chickenpox, prevalent among young children and highly contagious
• Tungiasis, ectoparasite of skin

• Allergic reaction to contact with specific chemicals, such as urushiol, derived from poison ivy or poison oak, or Balsam of Peru, found in many foods and fragrances.^[11][12] Certain allergens may be diagnosed in a patch test.^[13][14]
• Foreign objects on the skin are the most common cause of non-pathological itching.
• Photodermatitis – sunlight reacts with chemicals in the skin, leading to the formation of irritant metabolites.
• Urticaria (also called hives) usually causes itching.

• Dandruff, an unusually large amount of flaking is associated with this sensation.
• Punctate palmoplantar keratoderma, a group of disorders characterized by abnormal thickening of the palms and soles.
• Skin conditions (such as psoriasis, eczema, seborrhoeic dermatitis, sunburn, athlete's foot, and hidradenitis suppurativa). Most are of an inflammatory nature.
• Scab healing, scar growth, and the development or emergence of moles, pimples, and ingrown hairs from below the epidermis.
• Xerosis, dry skin, frequently seen in the winter and also associated with older age, frequent bathing in hot showers or baths, and high-temperature and low-humidity environments.

• Diabetes mellitus, a group of metabolic diseases in which a person has high blood sugar
• Hyperparathyroidism, overactivity of the parathyroid glands resulting in excess production of parathyroid hormone (PTH)^[15]
• Iron deficiency anemia, a common anemia (low red blood cell or hemoglobin levels)
• Cholestasis, where bile acids leaking into the serum activate peripheral opioid receptors, resulting in the characteristic generalized, severe itching
• Malignancy or internal cancer, such as lymphoma or Hodgkin's disease^[16]
• Polycythemia, which can cause generalized itching due to increased histamines. In polycythemia vera itching is particularly caused by skin contact with warm water, such as in bath or shower.
• Psychiatric disease ("psychogenic itch", as may be seen in delusional parasitosis)
• Thyroid illness
• Uraemia – the itching sensation this causes is known as uremic pruritus
• mast cell activation syndrome (MCAS).^[17]
• fibromyalgia^{[18][19][20][21]}

• Drugs (such as opioids) that activate histamine (H1) receptors or trigger histamine release
• Chloroquine, a drug used in the treatment and prevention of malaria
• Bile acid congeners such as obeticholic acid

• Gestational pemphigoid, a dermatosis of pregnancy
• Intrahepatic cholestasis of pregnancy, a medical condition in which cholestasis occurs
• Pruritic urticarial papules and plaques of pregnancy (PUPPP), a chronic hives-like rash

• Menopause, or changes in hormonal balances associated with aging
• Terminal illness^[22]

A ground squirrel scratching itself

A wolf scratching itself

A lioness scratching herself

Itch originating in the skin is known as pruritoceptive, and can be induced by a variety of stimuli, including mechanical, chemical, thermal, and electrical stimulation, or infection. The primary afferent neurons responsible for histamine-induced itch are unmyelinatedC-fibres.^[1]

Nociceptors. Two major classes of human C-fibrenociceptors exist: mechano-responsive nociceptors and mechano-insensitive nociceptors. Mechano-responsive nociceptors have been shown in studies to respond to mostly pain, and mechano-insensitive receptors respond mostly to itch induced by histamine. However, it does not explain mechanically induced itch or itch produced without a flare reaction that involves no histamine.^[1] Therefore, it is possible that pruritoceptive nerve fibres have different classes of fibres, which is unclear in current research.^[2]

Histology and skin layers. Sensitivity to pruritic stimuli is evenly distributed across the skin and has a clear spot distribution with similar density to that of pain. The different substances that elicit itch upon intracutaneous injection (injection within the skin) elicit only pain when injected subcutaneously (beneath the skin).

Itch is often classified as that which is histamine mediated (histaminergic) and nonhistaminergic.

Itch is readily abolished in skin areas treated with nociceptor excitotoxincapsaicin but remains unchanged in skin areas rendered touch insensitive by pretreatment with anti-inflammatorysaponins. Although experimentally induced itch can still be perceived under a complete A-fiber conduction block, it is significantly diminished. Overall, itch sensation is mediated by A-delta and C nociceptors located in the uppermost layer of the skin.^[26]

Gene expression. Using single-cell mRNA sequencing, clusters of genes expressed in itch-related tissues were identified, e.g. NP1-3, transmitting itch information; where NP3 expresses neuropeptides Nppb and Sst as well as genes involved in inflammatory itch (Il31ra, Osmr andCrystrl2). The histamine receptor gene Hrh1 was found in NP2 and NP3, suggesting that histaminergic itch is transmitted by both these pruriceptive sub clusters.^[27]

Infection. Staphylococcus aureus, a bacterial pathogen associated with itchy skin diseases, directly activates pruriceptor sensory neurons to drive itch. Skin exposure to S. aureus causes robust itch and scratch-induced damage.^[28]

After the pruriceptive primary afferent has been activated, the signal is transmitted from the skin into the spinal dorsal horn. In this area, a number of interneurons will either be inhibited or activated to promote activation of projection neurons, mediating the pruriceptive signal to the brain. The GRP-GRPR interneuron system has been found to be important for mediating both histaminergic and non-histaminergic itch, where the GRP neurons activate GRPR neurons to promote itch.^[29][30]

Neuropathic itch can originate at any point along the afferent pathway as a result of damage of the nervous system. They could include diseases or disorders in the central nervous system or peripheral nervous system.^[24] Examples of neuropathic itch in origin are notalgia paresthetica, brachioradial pruritus, brain tumors, multiple sclerosis, peripheral neuropathy, and nerve irritation.^[31]

Neurogenic itch, which is itch induced centrally but with no neural damage, is mostly associated with increased accumulation of exogenous opioids and possibly synthetic opioids.^[24]

Itch is also associated with some symptoms of psychiatric disorders such as tactile hallucinations, delusions of parasitosis, or obsessive-compulsive disorders (as in OCD-related neurotic scratching).^[24]

Inflammatory mediators—such as bradykinin, serotonin (5-HT) and prostaglandins—released during a painful or pruritic inflammatory condition not only activate pruriceptors but also cause acute sensitization of the nociceptors. In addition, expression of neuro growth factors (NGF) can cause structural changes in nociceptors, such as sprouting. NGF is high in injured or inflamed tissue. Increased NGF is also found in atopic dermatitis, a hereditary and non-contagious skin disease with chronicinflammation.^[32] NGF is known to up-regulate neuropeptides, especially substance P. Substance P has been found to have an important role in inducing pain; however, there is no confirmation that substance P directly causes acute sensitization. Instead, substance P may contribute to itch by increasing neuronal sensitization and may affect release of mast cells, which contain many granules rich in histamine, during long-term interaction.^[2]

Noxious input to the spinal cord is known to produce central sensitization, which consists of allodynia, exaggeration of pain, and punctuate hyperalgesia, extreme sensitivity to pain. Two types of mechanical hyperalgesia can occur: 1) touch that is normally painless in the uninjured surroundings of a cut or tear can trigger painful sensations (touch-evoked hyperalgesia), and 2) a slightly painful pin prick stimulation is perceived as more painful around a focused area of inflammation (punctuate hyperalgesia). Touch-evoked hyperalgesia requires continuous firing of primary afferent nociceptors, and punctuate hyperalgesia does not require continuous firing which means it can persist for hours after a trauma and can be stronger than normally experienced. In addition, it was found that patients with neuropathic pain, histamine ionophoresis resulted in a sensation of burning pain rather than itch, which would be induced in normal healthy patients. This shows that there is spinal hypersensitivity to C-fiber input in chronic pain.^[2]