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Abstract
The systemic, equilibular, and cytopathic effects of gentamicin and streptomycin were tested on young male chicks injected daily with the antibiotics.
At all but the lowest (25 mg) dosage, gentamicin progressively affected weight gain, with the chicks appearing noticeably smaller and lighter after two and one-half weeks of treatment with dosages ranging from 25 to 600 mg/kg/day. These same chicks showed retarded development, behavioral impairments and physiological abnormalities. Mortalities were high. The presence of a head tremor and perching changes were early indicators of gentamicin intoxication. After 25 injections at the lower (25, 50 mg) dosages, most ampullary hair cells, dark cells, and nerve terminals appeared undamaged. However, at higher dosages (200, 400 mg), dark cells appeared vacuolated, nerve terminals were grossly swollen and hair cells were moderately to severely vacuolated.
Streptomycin was less systemically toxic on a mg/kg basis than gentamicin. Chicks gained weight normally at dosages up to 800 mg/kg/day. Mortalities were low. Perching was impaired at all dosages tested (400, 800, 1200 mg). A head tremor began by the third injection at the highest dosage but occurred later and less severely at the two lower dosages. Streptomycin caused sublethal cell damage in the chick's ampulla. Damage was confined to dark cell shrinkage, nerve terminal swelling and clearing, and formation of small vacuoles throughout the cytoplasms of the hair cells after 16 daily injections at the 1200 mg dosage and 30 injections at the 400 mg dosage.
The chick, like the guinea pig and rat, required larger doses of gentamicin and streptomycin than humans, squirrel monkeys and cats before demonstrable loss of vestibular function and sensory cell damage.
