Vent Disease (rabbit Syphilis) (spirochetosis)
Barbi Brown's Bunnies offers this information based on personal experience and is not a substitute for proper veterinary care. Your vet can do scrapings or blood tests to confirm vent disease.
Vent disease is a sexually transmitted disease caused by the organism Treponema cuniculi. This is gram negative bacteria called a spirochete.
It shows itself as lesions on the genital area, and is passed to the nose and eyes when the rabbits grooms itself. It must be treated with long acting penicillin to completely eradicate it. "Rabbit Production" and the Merck Veterinary Manual recommend treating once a week for 3 weeks although I have found that 6 doses is more effective. I give one dose a day for 3 days and one a week for 3 weeks. Some vets recommend treating daily for 7 days but that really increases the risk of overgrowth of harmful bacteria in the gut. Giving beneficial bacteria supplements when using penicillin is always a good idea. Lesions will disappear in a few days but you must continue the full regimen in order to eradicate the disease.
The biggest issue with this disease is that it can be covered up with topical treatments that do not kill the pathogen but masks the symptoms. These animals can then become "silent carriers" and without symptoms will be used in breeding programs and the carrier will spread the disease to the rabbit it is bred to. Does can transmit it to her young who will generally not show symptoms until about 3 to 4 months of age.