Azithromycin is an azalide, a subclass of macrolide antibiotics. It is rapidly absorbed and known to be well distributed throughout the body of mice, rats, dogs, and humans following oral administration. High concentration within cells results in significantly higher concentration in tissues than in plasma or serum. Its extended half-life results in therapeutic levels for several days after treatment is discontinued. It has activity against both gram positive and gram negative aerobes commonly associated with respiratory and integumentary infections. It has in vitro activity against some anaerobes including Clostridium perfringes and Bacteroides bivius as well as other organisms such as Borrelia and mycoplasma. The spectrum of activity, once a day dosing, and extended half-life make this an attractive antibiotic for use in marine mammals.

Serum levels of azithromycin were obtained using both bioassay and high - performance liquid chromatography tHPLC) in three species of cetaceans (Delphinapterus leucas, Orcinus orca, and Tursiops truncates).Currently, the dosage used at Sea World is an initial loading dose of approximately 6.7 mg/kg followed by 3.7 mg/kg every 24 hours for a total of 10 days. We have continued treatment for as long as 40 days. Side effects consisting of inappetence, abdominal discomfort as evidenced by a "tucked up" appearance, and elevated triglycerides were observed in two beluga calves. These signs disappeared when the medication was discontinued.