History and Signalment
A 4 month-old Thoroughbred foal is presented for investigation of a swelling under its jaw. This and other foals have also been noted to cough occasionally and the stud suspect a viral infection circulating. On examination you identify that the sub-mandibular lymph nodes are increased in size and one is particularly large. Other clinical parameters are all within normal limits.
You perform an ultrasonographic examination of the lymph nodes:
Q From the ultrasound examination you conclude that:
1. The horse has lymphoma
2. There is a foreign body present
3. The lymph node is reactive consistent with viral infection
4. The lymph node is abscessated
A. 4. The hyperechoic node has an anechoic centre consistent with the presence of purulent material.
You confirm the presence of purulent material by performing an aspirate and then lance the abscess releasing approximately 10ml of pus. You submit the pus for culture and this yields a profuse, pure growth of Rhodococcus equi.
Q What further investigations would you want to perform?
1. A tracheal wash
2. PCR on a faeces sample
3. Ultrasonographic examination of the thoracic cavity
4. Haematology and biochemistry
A. All of the above may provide useful information but pulmonary abscessation is the hallmark of R.equi infection. Ultrasonographic examination, possibly combined with radiography, would be the best means of identifying pulmonary pathology.
At the right 10th intercostal space you identify the following:
Q You advise:
1. That the foal has pulmonary abscessation and the abscess ought to be drained percutaneously as it is easily accessible.
2. The foal has pulmonary abscessation and ought to be treated with clarithromycin and rifampin
3. The foal has pulmonary abscessation and ought to be treated with erythromycin and rifampin
4.That the foal has focal necrosis of the right lung and should be euthanase
A. Either 2 or 3 would be appropriate but compared to erythromycin, clarithromycin has better oral bioavailability, requires less frequent dosing, and reaches more reliable concentrations in phagocytes; hence it tends to be preferred.