Swollen Lymph Nodes in a Colt

History. A 2-year-old colt who just returned from training has shown marked bilateral nasal discharge for at least 2 days. The colt has not been eating for the last day and appears depressed.

Clinical Examination. On physical examination the colt has a temperature of 39.2^o C (102.7^o F) but a normal heart rate of 36 beats/min and normal respiratory rate of 12 breaths/min. The horse has bilateral mucopurulent nasal discharge. The lymph nodes underneath the jaw and in the throat area are both greatly swollen. No other abnormalities are present.

Comment. This colt was vaccinated for Eastern and Western encephalitis as well as rhinopneumonitis, influenza, rabies, tetanus, and West Nile virus. However, the colt was not vac­cinated against Streptococcus equi, the causative agent of strangles. Young horses not vaccinated against S. equi that have been traveling are at increased risk for disease. Based on the clinical signs of fever, bilateral mucopurulent discharge, and normal lung sounds and respiratory rate, this disease is likely an upper respiratory infection. The vaccination history and clinical signs indicate S. equi is the most likely pathogen.

Horses are exposed to S. equi through inhalation, inges­tion, or exposure to conjunctival surfaces; association with other infected horses; or a contaminated environment. The bacteria enter the cells of the tonsillar crypts and ventral sur­face of the soft palate and migrate throughout the upper respiratory tract, infecting the lymph nodes as well and repli­cating there extracellularly. Most cases resolve over a pro­longed period. In other horses, however, the guttural pouches can become infected, or the animal can become systemically infected and develop abscesses in other parts of the body (e.g., lymph nodes).

The immune response associated with infection begins with the mucosal innate response. Horses will have increased levels of neutrophils in their blood (neutrophilia). The number of lymphocytes in the blood may vary, but there is a marked reaction in the local lymph nodes to the infection. These horses have severe Iymphadenopathy of the regional nodes from increased numbers of neutrophils and lympho­cytes. A combination of both the innate and the acquired immune response is capable of controlling the infection, and eventually the horse eliminates the pathogen. Again, however, in a small percentage of the infected horses, S. equi migrates to the systemic lymph nodes to cause bastard strangles at some later time point in life.

A diagnosis can be made based on history, clinical signs, and combined diagnostic testing. Traditionally, these horses have neutrophilia in their peripheral blood. Definitive diagnosis is based on culturing the agent, often from the regional lymph nodes using an aspirate. In many cases, there is concern about an outbreak within a particular barn because the agent is highly infectious, and infected horses and barns must be quarantined to prevent further spread of the disease. Another test for a definitive diagnosis is polymerase chain reaction (PCR).

Treatment. The treatment of S. equi infection varies based on clinical signs and the population at risk. In the early stages of infection, with clinical signs of fever, depression, anorexia, and bilateral mucopurulent nasal discharge, but no or limited Iymphadenopathy, these horses can often be treated systemi­cally with antibiotics, and the disease will usually resolve. However, once the lymph nodes begin to become enlarged or abscessed, antibiotic treatment will often prevent further spread of infection but will not aid in clearance. When antibiotics are discontinued, the disease continues to progress. Some horses become greatly depressed, with marked Iymphadenopathy that can affect breathing. In these animals the lymph nodes are often drained. Sometimes a tracheostomy is needed, in which case horses are treated with systemic antibiotics to prevent secondary infections. Disinfecting the environment is critical to the control of strangles within populations.