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Azithromycin for Dogs, Cats, and Horses (Foals)

By Evan Ware, DVM

Last reviewed: 7/12/2022

Commonly prescribed for: Bacterial Infections

Species: Dogs, Cats, and Horses (Foals)

Therapeutic Class: Semi-synthetic Macrolide Antibiotic

Basic Information

Azithromycin is a semi-synthetic macrolide antibiotic derived from erythromycin. It is used in veterinary medicine to treat certain bacterial infections. Azithromycin is a more popular choice than erythromycin in the treatment of dogs and cats because it has a longer half-life and is better absorbed by both species.

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 lethal antibiotic-concentrations. Azithromycin has an extended tissue-elimination half-life. The prolonged, high concentration of azithromycin at the site of infection allows once a day dosing and can allow for a shorter duration of treatment.

Veterinary Medicine Uses for Azithromycin

Veterinarians use azithromycin to treat a wide range of bacterial infections in dogs and cats including streptococci, staphylococci, bartonella henselae, some species of Chlamydia, haemophilus spp, mycoplasma spp, borrelia burgdorferi, and others.

The medicine works by binding to the P site of the 50S ribosomal subunit of those microorganisms that are susceptible to it, thereby interrupting the microorganism's RNA-dependent protein synthesis.

Azithromycin is a popular treatment choice in veterinary medicine for many types of infections including dermatological infections, urogenital infections, respiratory tract infections, and otitis media.

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 often 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 can 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; optimal antimicrobial 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. Doxycycline appears to be the 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.

In horses, azithromycin is used in foals to treat Rhodococcus equi infection. It can 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).

Potential Side-Effects of Azithromycin

The most-common potential side-effects associated with azithromycin include gastrointestinal problems like abdominal discomfort, vomiting, and diarrhea. Angioedema and jaundice can also result from taking azithromycin, although the chances are rare. More -serious potential side-effects include cardiac arrhythmia, ventricular tachycardia, and issues with renal function.

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.

Drug Interactions with Azithromycin

The drug can interact with other medications. Common contraindications include ergotamine, dihydroergotamine, triazolam, pimozide, cisapride or other macrolide antibiotics, oral antacids, and any drugs metabolized by cytochrome P450 (e.g. cyclosporine, hexobarbital, phenytoin, terfenadine, and carbamazepine).

Azithromycin can be combined with rifampin. Oral antacids can reduce the rate of absorption of azithromycin. Overdose of any macrolide antibiotic can cause severe GI side-effects. Some veterinarians use oral probiotics in foals being treated with macrolide antibiotics to decrease the likelihood or severity of antibiotic induced diarrhea.

Precautions for Using Azithromycin

The most-common side effects in all species are gastrointestinal. Vomiting can occur in dogs. Mild to moderate diarrhea can occur in foals. Hyperthermia is a serious and potentially fatal side-effect that can 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 can severely limit their turnout time in general. Should hyperthermia occur, aggressive cooling using water and fans, or air conditioning is helpful.
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