Itch inhibition due to pain
Studies done in the last decade have shown that itch can be inhibited by many other forms of painful stimuli, such as noxious heat, physical rubbing/scratching, noxious chemicals, and electric shock. Causes
Infections
Body louse, found in substandard living conditions
Cutaneous larva migrans, a skin disease
Head lice, if limited to the neck and scalp
Herpes, a viral disease
Insect 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
Swimmers itch, a short-term immune reaction
Varicella – i.e. chickenpox, prevalent among young children and highly contagious
Tungiasis, ectoparasite of skin

Environmental and allergic
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. Certain allergens may be diagnosed in a patch test. 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. Skin disorders
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, athletes 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.
The primary afferent neurons responsible for histamine-induced itch are unmyelinated C-fibres.Two major classes of human C-fibre nociceptors 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. Therefore, it is possible that pruritoceptive nerve fibres have different classes of fibres, which is unclear in current research.Studies have been done to show that itch receptors are found only on the top two skin layers, the epidermis and the epidermal/dermal transition layers. Shelley and Arthur verified the depth by injecting individual itch powder (Mucuna pruriens) spicules and noting that maximal sensitivity occurred at the basal cell layer or the innermost layer of the epidermis. Surgical removal of those skin layers removed the ability for a patient to perceive itch. Itch is never felt in muscle or joints, which strongly suggests that deep tissue probably does not contain itch signaling apparatuses.Itch is often classified as that which is histamine mediated (histaminergic) and nonhistaminergic. Sensitivity to pruritic stimuli is evenly distributed across the skin and has a clear spot distribution with similar density to that of pain.