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Azithromycin for Veterinary Use
by Barbara Forney, VMD
Azithromycin is an azalide or advanced generation macrolide antibiotic. Macrolide antibiotics work by inhibiting protein synthesis by susceptible bacteria and usually are considered bacteriostatic. Advanced generation macrolides characteristically produce high tissue concentrations and comparatively lower serum concentrations of antibiotic. Azithromycin concentrates within polymorphonuclear leucocytes (PMN), which gravitate by chemotaxis towards the site of infection. Upon phagocytosis of the PMN, the intracellular pathogens are exposed to very high, potentially cidal antibiotic-concentrations. Azithromycin in particular has an extended tissue-elimination half-life. The prolonged, high concentration of azithromycin at the site of infection permits once a day dosing and may allow for a shorter duration of treatment.
Dogs and Cats
Azithromycin has been used successfully in dogs to treat Babesia Gibsoni (Asian genotype). Babesia is a haemotropic protozoal parasite that is transmitted by ticks. When treating Babesia Gibsoni, azithromycin usually is combined with atovaquone. Azithromycin in combination with other drugs has been suggested as a treatment for systemic non-tubercular mycobacterial disease of dogs and cats. M. avium and M. fortuitum are the most common cause of systemic mycobacterial infection in companion animals.
These infections frequently are resistant to traditional anti-tubercular medications such as isoniazid and ethambutol. Systemic infection with M. avium is particularly difficult to treat and carries a poor prognosis. Basset Hounds and Siamese cats may have increased susceptibility to these infections.
Azithromycin is used to treat Bartonella in cats. Bartonella is a gram negative hemotropic bacterial organism found primarily in erythrocytes and endothelial cells. It is a zoonotic disease. Human infection is generally called Cat Scratch Disease (CSD).
Bartonella is transmitted between cats primarily through fleas although most naturally infected cats display no clinical disease. Ocular Bartonella is well documented in humans and some veterinary ophthalmologists are beginning to recognize and treat ocular Bartonella in cats. The diagnosis of feline bartonellosis is difficult and optimal anti-microbial protocols for clinical feline bartonellosis are a source of controversy.
In 2006 a panel of the American Association of Feline Practitioners published a comprehensive report on the diagnosis, treatment and prevention of Bartonella spp. infections in cats (available online). Doxycycline appears to be their first choice antibiotic and azithromycin or a fluoroquinolone are used in cases that do not respond to doxycycline. Because Bartonella is a zoonotic disease, particular care should be used to avoid being bitten or scratched while administering antibiotic therapy.
Azithromycin is used in foals to treat Rhodococcus equi infection. It may be used alone or in combination with the antibiotic, rifampin. R.equi pneumonia is the most severe bacterial pneumonia in foals. Other sites of infection due to R.equi include the gastrointestinal tract and joints or physis. All forms of R.equi infection can be very difficult to diagnose and abscess formation due to R.equi can make these infections difficult to treat. Fatalities can occur even with prompt diagnosis and treatment.
For many years, erythromycin or erythromycin combined with rifampin was the standard of care to treat R.equi infections. Recently the newer advanced generation macrolides, azithromycin and clarithromycin have gained popularity. Azithromycin has better absorption characteristics than erythromycin. The terminal half-life of azithromycin in serum, bronchial alveolar lavage (BAL) cells and pulmonary epithelial lining cells is significantly longer than for either erythromycin or clarithromycin. Peak drug activity in BAL cells is similar for both azithromycin and clarithromycin and is significantly higher than erythromycin. Recent research at Texas A&M University has looked at the prophylactic use of azithromycin in foals on R.equi endemic farms (AAEP 2007).
Azithromycin Maleate Side Effects
No information was found in the literature on the side effects of the atovaquone and azithromycin combination in animals. Malarone (atovaquone combined with proguanil hydrochloride) should not be used in dogs due to a high incidence of gastrointestinal side effects.
In humans, the most commonly reported side effects to the drug combination of atovaquone and azithromycin were diarrhea and rash and the most commonly reported side effects to Malarone were related to the digestive tract. They included abdominal pain, nausea, vomiting, anorexia and diarrhea.
The most-common side effects in all species are gastrointestinal. Vomiting may occur in dogs. Mild to moderate diarrhea may occur in foals. Hyperthermia is a serious and potentially fatal side effect that may be seen in foals. There is ample clinical evidence that foals on erythromycin are very sensitive to heat and possibly to bright sunlight and there have been anecdotal reports of similar hyperthermia with both azithromycin and clarithromycin. Because of this problem, many veterinarians do not turnout foals on macrolide antibiotics in the daytime and may severely limit their turnout time in general. Should hyperthermia occur, aggressive cooling using water and fans or air conditioning is helpful.
Azithromycin should not be used in animals on cisapride. Azithromycin may be combined with rifampin. Oral antacids may reduce the rate of absorption of azithromycin. Overdose Overdose of any macrolide antibiotic can cause severe GI side effects. Some veterinarians use oral probiotics in foals being treated with macrolide antibiotics in an attempt to decrease the likelihood or severity of antibiotic induced diarrhea
No information was found in the literature regarding overdose.
About the Author
Dr. Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania. She has a master's degree in animal science from the University of Delaware and graduated from the University of Pennsylvania School of Veterinary Medicine in 1982.
She began to develop her interest in client education and medical writing in 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
You can purchase books by Dr. Forney at
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