Dr. Philip Johnson
Following exposure to an infected horse or a contaminated fomite, the earliest clinical manifestations of strangles include a sharp rise in body temperature, lethargy, disinterest in food and reluctance to work. These early clinical signs are nonspecific, however, and could occur with numerous other diseases. An area of diffuse swelling commonly develops in the region of the throat, usually under the jaw but sometimes behind the jaw in the throat-latch area. This is due to swelling of the lymph nodes, which is usually quite painful when examined. This area of swelling may remain localized or, in some cases, it may expand to affect other parts of the head and the jaw.
As the disease progresses, the following may occur:
Affected horses are often reluctant to lift up their head and may appear to have a stiff neck.
Salivating and gagging, coughing, and groaning are not unusual. At this time, the infected horse may show signs of difficulty with swallowing and the act of eating may provoke frustration and more coughing.
Snoring sounds may be heard during breathing in some horses if thickening of the palate occurs. After a few days, the more throaty signs may dissipate, and the area of swelling develops into an abscess that ruptures through the skin (there may also be some bleeding).
Thick yellow liquid (exudate) may leak from the abscess for several days. It should be noted that this exudate is a rich source of environmental contamination by Streptococcus equi. In most cases, after the abscess has ruptured, the clinical signs gradually resolve and most horses have made a full and complete recovery within two to three weeks.
Horses tend to lose weight while they are fighting a strangles infection but are able to recover lost weight within a few weeks of resolution of this disease.
Although a majority of horses eliminate Streptococcus equi from their system by the time the symptoms have resolved, a small number of recovered horses continue to harbor Streptococcus equi equi, but do not exhibit any clinical evidence of disease.
Infection of Sinuses and Guttural Pouches
Two areas commonly infected secondary to the initial throat and gland infection include the sinuses and guttural pouches. If you have a horse with a persistent nasal discharge after strangles in the barn, you might be looking at one of these two complications. Sinusitis produces a one-sided (unilateral) nasal discharge, and guttural pouch infections can cause nasal discharge from one or both sides. The key fact is that the nasal discharge is thick and viscous, often white or yellow-white in color. The infection can collect in these regions for long times, so sinusitis or guttural pouch infections can crop up later. The guttural pouch is an area where the strangles organism can survive for a long time, and may explain the presence of some carriers.
Atypical Clinical Presentations
Less commonly, the clinical results of infection may be atypical. The disease leads to early onset extensive swelling under the skin in some infected horses, especially at the head. In other horses, the development of an abscess leads to rupture of the abscess into the throat (instead of rupturing through the skin), which causes a thick yellow nasal discharge. In some horses, a large abscess develops deep in the throat where it interferes with both breathing and swallowing – in those cases, it may be necessary to provide a tracheotomy (hole in the wind pipe) to allow the horse to breathe. Internal rupture of an abscess into the throat sometimes leads to infection of the horse's guttural pouches and the horse may need surgical treatment.
The immune system appears to be relatively slow in eliminating the organism from some horses. In these cases, more than one abscess may develop in the throat area or under the jaw before the infection is finally eliminated.
Another frustrating and difficult manifestation of infection is the rare development of abscesses at other locations in the body. Although abscess development is localized to the throat in most infected horses, strangles abscess formation can arise in remote areas of the body such as the brain, the spinal cord, the lungs, the intestinal system, the udder or in the kidneys.
Additional clinical symptoms are determined by the specific location of these additional abscesses:
Abscess development in the lung leads to pneumonia and pleurisy.
Abscesses in the intestinal system lead to colic and weight loss.
Abscesses in the brain may lead to seizures, behavioral abnormalities and stumbling.
The development of abscesses at an area remote to the upper respiratory tract (throat) is commonly referred to as metastatic strangles or bastard strangles. The prognosis for most horses affected with metastatic strangles is unfavorable. Fortunately, metastatic strangles is a rare complication of this disease.
Purpura hemorrhagica is another uncommon complication of strangles. In these horses, swelling in the limbs or under the trunk occurs several days or weeks following resolution of a typical strangles infection. This swelling is a result of an immune reaction against the horse's blood vessels that has been provoked by the development of immunity against the streptococcus organism. In rare instances, purpura hemorrhagica may affect the intestinal tract and cause severe colic or diarrhea.
Other rare clinical effects of strangles infection in horses include immune-mediated damage to the heart, the skeletal muscles and the kidney. Heart damage leads to irregularity in the heartbeat, muscle damage leads to weakness and muscle destruction, and kidney damage may lead to weight loss, increased urine loss and enhanced thirst.