Aristotle, Pliny, and Plutarch recognized the numbing effect of electricity when referring to the application of the torpedo fish to the body. Scribonius Largus advocated the use of piscine electrotherapy for the relief of pain associated with gout, headache, arthritis, and hemorrhoids. Various means of providing electrical current were developed, including the use of the electric ray or great Nile catfish.“
more here about the great Nile catfish here.
The Egyptians reputedly used the electric shock from them when treating arthritis pain.[9] They would use only smaller fish, as a large fish may generate an electric shock from 300 to 400 volts. The Egyptians have depicted the fish in their mural paintings and elsewhere;[8] the first known depiction of an electric catfish is on a slate palette of the predynastic Egyptian ruler, Narmer, about 3100 BC.[7] It was suitably called "angry catfish" in ancient Egyptian. An account of its electric properties was given by an Arab physician of the 12th century; then as now, the fish was known by the suggestive name of raad, abo el raash, el raad or raash, which means "thunder"[8] (literally "trembler" or "shaker").
but beware the electric eels, whose shocks are more dangerous.
giving electric stimulation for pain is used in rehabilitation: And arthritis responds to this (probably in the same way that linaments and other irritants help wih the pain).
WAPO article on athlete who uses electric patches for his arthritis.
The Arthritis foundation discusses here.
medical analysis of electrical stimulation for Rheumatoid Arthritis.
The results showed that ES had significant benefit when compared to a control no treatment group in terms of muscle strength and fatigue resistance of the first dorsal interosseous. Most favourable results were obtained by using a patterned stimulation derived from a fatigued motor unit of the first dorsal interosseous in a normal hand rather than a fixed 10 Hz stimulation frequency. Side effects of the ES application were not reported.