One gene error explains a blood cancer where a warm bath brings itching
In polycythemia vera the bone marrow overproduces red blood cells, thickening the blood. Nearly every patient, about 98 percent, carries a mutation in a gene called JAK2, which is found in only a sliver of everyone else. A telltale sign is intense itching after contact with warm water, reported by roughly half of patients.
The condition is an uncommon myeloproliferative neoplasm, a type of blood cancer arising in the marrow. JAK2 codes for an enzyme in the signalling chain triggered by erythropoietin, the hormone that normally tells the marrow to make red cells. When mutated, most often in a variant called V617F, the cells multiply regardless of that signal. That is why patients typically have unusually low erythropoietin levels, the reverse of what happens in secondary forms of polycythemia, and doctors use that clue when the mutation is absent.
Many people have no symptoms at all. Others notice fatigue, night sweats, the warm-water itch, which may involve histamine release, or erythromelalgia, a burning pain in the hands or feet with reddish or bluish skin. That burning comes from tiny clots formed by overactive platelets and responds quickly to aspirin. The spleen can enlarge. Thick blood is the core danger, raising the risk of thrombosis, especially in people over 60 or with a previous clot; untreated disease carries a substantial risk of a clot in the liver's veins.
The World Health Organization revised the diagnostic criteria in 2016. The three major markers are a very high red cell count, a marrow biopsy showing crowded, abnormal cells, and the JAK2 mutation. Recent reviews cite haemoglobin above 16.5 grams per decilitre in men or 16 in women.
There is no cure, and treatment aims chiefly to prevent clots. Its mainstay is regular blood removal, much like donating blood, to hold the haematocrit below 45 percent, often weekly at first. Aspirin, the JAK2 inhibitor ruxolitinib and interferon drugs may be added, and in August 2026 the United States approved rusfertide, the first therapy that mimics hepcidin, the body's iron-regulating hormone. Median survival with controlled disease ranges from 10 to 20 years, mostly among people diagnosed in their 60s, though the disease can progress to myelofibrosis or leukaemia.
Source: Polycythemia vera
