Equine Strangles, Why all the pus?

By Lauren Lamb

Strangles, which is caused by Streptococcus equi ssp equi (S. equi) bacteria, is a disease commonly seen in horses. This bacterial infection frequently affects the upper respiratory tract, but can affect other locations of the body. Strangles was first documented by Jordanus Ruffus in 1251. S. equi is highly contagious, which can result in large Strangles outbreaks in locations with large horse populations (stables, shows race tracks). The following paper will discuss the clinical signs, treatments options, preventatives and disease outbreak recommendations. S. equi bacteria is an intracellular bacteria, which means the bacteria live and reproduce within cells. The bacteria especially like cells within lymph nodes, i.e. lymphocytes. The lymph nodes in the throat latch region and the area under the jaw are particularly susceptible to S. equi infection and abscess formation.

The diagnostic test of choice is a culture of nasal swabs, nasal washes or pus aspirated from an abscessed lymph node. A PCR can also be performed to identify DNA for S. equi, but this test will not differentiate between DNA from live bacteria or dead bacteria. A PCR is best used to diagnosis asymptomatic carriers as well as determining if S. equi is eliminated from the guttural pouches.

Clinical signs are usually characterized by abrupt onset of fever of more than 102 degrees  Fahrenheit, lethargy, depression, decrease appetite and nasal discharge. The fever will be present for 24-48 hours prior to onset of swelling in the lymph nodes in the throat latch region and nasal drainage. The lymph nodes will continue to swell until they abscess and drain. In some cases the lymph nodes may not rupture. This could result in suffocation (hence the name Strangles) and death if the horse does not receive proper veterinary attention. Clinical signs tend to be much worse in younger horses compared to older horses. The severity of clinical signs also depends of the amount of bacteria infecting the horse. The higher the bacterial load the worse the clinical signs.

Bacterial shedding will begin two to three days after the onset of a fever. The S. equi bacteria will continue to be shed for two to three weeks or more. The bacterial can be transmitted directly or indirectly between horses. Direct transmission is when a horse comes in direct contact with another horse shedding S. equi i.e. muzzle to muzzle. Indirect transmission occurs through sharing of contaminated housing, water sources, feed or feeding utensils, tack and caretakers.

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