Lymphangitis is inflammation of the lymphatic vessels. These are low-pressure vessels similar to veins that collect the fluid that surrounds cells and return it to the bloodstream. Lymphangitis is usually caused by a bacterial infection that spreads to the lymphatic vessels.
Infection causing lymphangitis in horses can occur following infection with Corynebacterium pseudotuberculosis bacteria. The bacteria probably enter by way of skin wounds including injections, insect bites, or by contact with contaminated soil, tack, or grooming equipment. The condition is commonly known as pigeon fever, but is also called false strangles, pigeon breast, dry land distemper, and ulcerative lymphangitis. The infection presents in 3 ways: as small lumps and sores on the lower limbs (ulcerative lymphangitis), as inflamed and pus-filled sores (abscesses) on the chest and abdomen, and as abscesses on internal organs. It is a common infectious diseases of horses in California and Texas and is increasingly seen in other western and midwestern states of the US. In these areas, infections are seasonal, with a peak in summer and fall.
Signs of the ulcerative lymphangitis caused by pigeon fever develop slowly and may include painful inflammation, small lumps or swellings (nodules), and slow-healing sores, especially on the fetlock. Occasionally, the swelling extends up the entire limb causing lameness. The discharge is odorless, thick, tan, and blood-tinged. Usually, only one leg is involved. If the condition is not treated quickly, the disease can worsen and become longterm with relapses.
Infection can also result in pus-filled sores (abscesses) on the chest and lower abdomen. The infection can also spread to internal organs. Signs include widespread or localized swellings, fluid accumulation and inflammation of the skin on the lower abdomen, lameness, sores that drain pus, fever, weight loss, and depression. An increase in white blood cells may be present. A high or prolonged fever, unwillingness to eat, or weight loss may indicate complications such as deep and recurring abscesses, internal abscesses, or whole-body infection and abortion. Colic may also be a sign of internal infections. Abscesses can be large, up to 8 inches (20 centimeters) in diameter before rupturing, and take weeks to months to heal. Weight loss, colic, or lack of coordination may be signs of internal abscesses. Inflamed skin sores are painful and mildly itchy with a loss of hair, discharge, and crusting. They are also slow to heal.
To confirm the diagnosis of this disease, your veterinarian will take samples of material from the sores. Tests will then be done in a laboratory to identify the organism involved. Because the signs of this disease can easily be confused with infection by other bacteria, fungi, or worms, it is important to identify the bacteria so that proper treatment can be started. Abdominal ultrasonography can detect internal infections of the liver, spleen, or kidney. Blood tests may also be necessary.
Ulcerative lymphangitis and internal infections require treatment with antibiotics for a month or longer. Despite appropriate treatment, 30–40% of horses with internal abscesses die because of the infection. Early, external abscesses are often treated with hot packs, poultices, or flushing with water (hydrotherapy). In many cases, small sores are pierced to let the pus out and then flushed with iodine solution. Your veterinarian can recommend the most effective treatment for your horse. Abscesses deep within muscles often require ultrasonography and placement of a drain. For skin lesions and grossly contaminated limbs, veterinarians usually prescribe a daily scrubbing with a shampoo that contains iodine. Medications to relieve pain and promote healing may be needed. General supportive and nursing care is also important. Affected horses should also be separated from healthy ones. If treatment is successful, the swelling gradually goes down over days or weeks. Severe or untreated lymphangitis often become longterm, and scarring and hardening of the tissue of the leg occurs.
Unhygienic and wet conditions increase the risk of infection, particularly of the lower legs and abdomen. Maintaining good hygiene, good sanitation, and fly control, and avoiding prolonged exposure of horses to damp or wet conditions may help limit infections. However, the disease occurs even under excellent management conditions.