Diagnosis:
- Clinical symptoms (fever, nettle rashes, ocular and nasal discharges, swelling of legs or scrotum, stiffness of gait, with abortion in mares) - but these are variable.
- Carrier stallions, the main reservoir of infection, may show no symptoms.
- Virus isolation from field cases, eg from semen, requires repeated passage so that PCR on semen and ELISA for serum antibody are now often used.
- Test mating of a stallion with on 2 seronegative mares and then screening them for antibody is commonly used to say if a stallion, which has tested virus-negative by isolation, has actually recovered.