If you notice lymph swelling on your horse's belly, it could result from a number of causes. Call your veterinarian to take a look at it. If there's lymph swelling and your horse or other horses on your farm recently have had a bout of strangles, have your vet come out as soon as possible. It could become a matter of life or death.
The Equine Lymphatic System
Your horse's lymphatic system is a circulatory system, not unlike that delivering blood to the body. It consists of lymphatic vessels throughout the body that send lymphatic fluid -- containing antibody-producing white blood cells -- for waste collection, and small glands called lymph nodes that filter the lymphatic fluid. Lymph circulates slowly throughout your horse's body primarily via involuntary muscle contraction. Inactivity and disease can cause fluid to collect in lymph nodes, producing swelling.
If you keep your horse in a stall, you've probably experienced him "stocking up" if he's been in the stall longer than usual. The swelling in the rear legs generally goes away once he's able to move freely. On the abdomen, edema might appear as a reaction to bug bites. More serious causes of abdominal -- or ventral -- edema include diseases such as equine viral arteritis, Potomac horse fever, salmonellosis and equine infectious anemia -- the dreaded swamp fever, which generally requires euthanasia. If a horse is not dewormed regularly, an extensive parasite load can result in ventral edema.
While horse owners dread a diagnosis of the highly contagious disease known as strangles, most horses will survive a bout of this malady. Strangles results in fever, nasal discharge and the telltale abscessing in between the animal's jawbones. Once the abscesses break or a vet lances them, a horse is generally on the road to recovery. "Bastard" -- or metastatic -- strangles is another story. Bastard strangles occurs when the infection travels elsewhere in the body, often causing abscesses in the abdominal lymph nodes. Horses are more likely to succumb to bastard strangles than the common variety.
Found primarily in warmer regions of the U.S., pigeon fever received its nickname because an affected horse's chest swells up like a pigeon's breast; abdominal swelling is also common. Caused by exposure to Corynebacterium pseudotuberculosis, a bacterial infection, the horse develops abscesses intramuscularly and often internally. Transmission occurs via houseflies biting horses and possibly between equines. Your vet must drain the intramuscular abscesses, after determining the location via ultrasound. Expect copious amounts of pus to discharge from your horse. Take care to collect and properly dispose of this infectious material so it can't spread to other horses.