Chapter 44 - Bacterial Otitis Externa in the Guinea-pig Model

Abstract

This chapter describes a guinea-pig model of bacterial otitis externa. Otitis externa is also known as "swimmer's ear", and it is a common problem treated by primary-care practitioners and otolaryngologists. Acute bacterial otitis externa is usually caused by Pseudomonas, but sometimes Proteus, Klebsiella, or Escherichia coli are also involved. In this model, Hartley guinea pigs with an average weight of 300 g are used. Any strain of guinea pig can be used, but the size should be large enough to allow examination of the external ear canal, which is quite tortuous in the guinea-pig. It is important to ensure that the guinea pigs are examined and made free of middle and external ear infections prior to the experiment. A surgery is performed to transfer infection of Pseudomonas aeruginosa from a human with known otitis externa into a guinea pig external ear canal to develop an external otitis in the guinea pig. This allows experimental manipulations in accordance with different treatment regimens. The guinea pigs are anesthetized with an intramuscular injection of ketamine hydrochloride (35 mg/kg) and xylazine hydrochloride (7 mg/kg). Ketamine and xylazine hydrochloride in combination provide a short period of anesthesia, which is enough to allow cleaning of the ear canals and inoculation of the ears with bacteria. The ear canal of each guinea pig is mechanically cleaned with the use of suction and cotton wrapped about the Lathbury probe, and inoculated with a washed suspension of Pseudomonas aeruginosa. Otitis externa infection is evidenced by the presence of purulent debris in the ear canal, erythema (redness), and edema (swelling) of the ear canal skin. Otitis externa, when treated, shows gradual resolution of the various signs of infection, with edema being the last parameter to resolve.