Cerebral Abscess in a foal- Streptococcus equi
A 4 month old intact female Quarterhorse with nasal discharge and mandibular lymphadenopathy. Despite treatment the foal became neurologic: ataxia, wobbly, and head pressing. Neurologic exam revealed an intention tremor when head raised and no left menace but direct PLRs were normal. A CBC revealed 600mg/dl fibrinogen and 11,100/ul neutrophils. A CT scan performed 7/14/09 revealed large mass in the right cranium. The owner elected euthanasia.
There is a current strangles outbreak on the farm. The brain mass indicates strong suspicion of abscess as complication to strangles.
The carcass was in good body condition, fresh with no autolytic changes. The guttural pouches bilaterally were filled with thick, firm inspissated pus.
The mandibular and retropharyngeal lymph nodes were enlarged bilaterally and several were completely effaced with purulent material, and had fibrous capsules enclosing the pus.
The right caudal lung lobe had a focal 2cm firm nodule elevating the pleural surface of the dorsal and medial aspect of the lung lobe. The nodule was composed of a thick fibrous capsule surrounding a 2 mm diameter central zone containing yellow purulent material.
The right cerebral hemisphere was swollen, with flattened sulci and gyri and contained a 5 cm x 4 cm elliptical abscess filled with yellow to light green pasty pus.
The small intestine contained moderate numbers of large roundworms (Ascarids) (incidental finding)
The non-glandular gastric mucos had multifocal to coalescing ulcers.
1. Brain: Focal cerebral abscess, unilateral
2. Mandibular and retropharyngeal lymph nodes: Suppurative lymphadenitis, multifocal, severe, chronic
3. Guttural pouch: Bilateral empyema, severe, chronic
4. Lung: Focal chronic abscess, unilateral
5. Small intestine: intra-luminal ascarids, moderate
The gross findings are consistent with bastard strangles, a disseminated form of Streptococcus equi infection. Typical strangles results in empyema and lymphadenitis of the retropharyngeal lymph nodes. If the infection progresses, septicemia can seed other organs. In this case the brain abscess resulted in severe neurologic disease and euthanasia was elected. The lung abscess was originally presumed to be a Streptococcus equi infections, however cultures identified only Rhodococcus equi. This is a common location for Rhodococcus equi infections, and these may be clinically silent for a long time.
Other complications of Streptococcus equi infections include Purpura hemorrhagica, which is a Type III hypersensitivity to certain anitgens related to this bacteria. This conditions requires a first infection that sensitizes the horse to the bacteria, and then when re0infection happens antigens released systemically form complexes with circulating antibodies that lodge in small vessels, activate complement, recruit neutrophils, and cause a leukocytoclastic vasculitis with petechial hemorrhage and edema. Hemorrhages may only be visible in the gingival mucosa. Edema can be seen in the head, neck, and chest.
Lung: Rhodococcus equi
Brain, swab: Streptococcus equi