Strangles Infection in Horses
One of the most contagious and potentially serious that occurs in horses is strangles. Strangles is an infectious disease caused by the bacteria Strep equi that mostly affects young horses although all horses are susceptible. The name strangles is due to the swelling of lymph nodes that occurs under the horse's throat and if this swelling is severe, it can actually cause the horse to strangle and not be able to breathe. Infection is transmitted from horse to horse or from contaminated tack or equipment. Once in the horse's nasal cavity, the organism migrates to the tonsils and then the lymph nodes in the throat.
The first symptom noted is fever and the presence of fever is an important sign because if your farm is involved in an outbreak, fever tells you that horse should be isolated. After a few days, an infected horse will develop a nasal discharge and may stop eating. At this time, lymph nodes under the throat may begin to swell.
Diagnosis of strangles is made by culture and PCR testing of nasal swabs or material collected from abscessed lymph nodes. It is important to make an accurate diagnosis because the disease is so contagious. Horses that are possibly infected must be quarantined until the diagnosis is confirmed.
It is common for horse owners to begin penicillin injections in horses with nasal discharge and this is not recommended. Many times these horses with a nasal discharge do not have an infection at all and do not need antibiotics. Using antibiotics in these horses causes resistant infections to develop. If the horse does have strangles and the lymph nodes are swollen, penicillin will delay rupture of the nodes and prolong the infection. For this reason, it is important to have your veterinarian examine and diagnose your horse's condition before beginning antibiotics.
Lymph nodes that are swollen must be allowed to rupture or can be lanced. After all lymph nodes have been drained, penicillin can be used but it is difficult to know that no other abscesses are developing. For this reason, I routinely do not use antibiotics unless I am concerned the horse will not survive without them. Horses with draining lymph nodes are very contagious and some horses can carry the organism for long periods of time and are capable of transmitting the infection.
It is difficult to determine when an infected horse is no longer contagious as some horses can have the organism in their upper respiratory tract and not show any symptoms. Because of this, it is recommended to run 3 separate cultures and PCR tests on samples obtained by flushing the horse's guttural pouches before allowing previously infected horses out of quarantine.
Prevention of the disease is by vaccination and prevention of exposure to infected horses. Vaccination is somewhat effective and there are two types of vaccines available. An injectable killed vaccine is available and is the vaccine we use at our clinic. There is also an intranasal live vaccine that can be used. The intranasal vaccine has been reported to actually cause the disease in some cases. Also, it should never be used when other injectable vaccines are to be given as abscesses have been reported at sites of the other vaccines. Another possible reaction following the disease or following vaccination in horses that have recently recovered from the disease is purpura hemorrhagica. This is an immune mediated disease that causes severe swelling in the lower legs and head. It is a serious disease that is usually treated with corticosteroids. If your horse develops a fever or a nasal discharge, call our office rather than starting them on penicillin.