• Under 35 – most often a sexually transmitted pathogen such as Chlamydia trachomatis and Neisseria gonorrhoeae.
• Over 35 – most often a non sexually transmitted gram negative enteric organisms causing urinary tract infections. Particular risks include recent instrumentation (such as prostate biopsy and vasectomy) or catheterisation.
• There is cross over between these two groups, and complete sexual history taking is imperative.
• Men who engage in insertive anal intercourse are at risk of epididymo-orchitis secondary to sexually transmitted enteric organisms.
• Abnormalities of the urinary tract are common in the group with gram negative enteric organisms.
• All patients with confirmed urinary tract pathogen should have further investigations of the urinary tract.
• Ureaplasma urealyticum is found in men with epididymo-orchitis but is often associated with C. trachomatis and N. gonorrhoeae. Evidence for its role in the development of epididymo-orchitis is lacking.
• Mycoplasma genitalium has been identified in some cases of epididymo-orchitis, but evidence for its role in the development of epididymo-orchitis is so far lacking.

Mumps – unilateral or bilateral orchitis can occur in up to 40% of post pubertal men who have mumps.

Tuberculosis - epididymo-orchitis is a rare presentation of TB (see BASHH).

• Rare infective causes include Brucella, fungi such as candida and schistosomiasis.
• Brucellosis – consider in the context of travel history to an endemic area, failure to respond to first line therapy and / or history of preceding fever, lethargy and night sweats.

• Behcet’s Disease.
• Adverse effect of amiodarone treatment.
• Rare manifestation of Henoch–Schonlein purpura.
• Other rare non-infective causes include Mediterranean fever and polyarthritis nodosa.