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Azithromycin is an azalide or advanced generation macrolide
antibiotic. Macrolide antibiotics work by inhibiting protein synthesis
by susceptible bacteria and are usually 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.
Azithromycin has been used successfully in dogs for
the treatment of Babesia gibsoni (Asian genotype). Babesia is a haemotropic
protozoal parasite which is transmitted by ticks. When treating Babesia
gibsoni, azithromycin is usually combined with atovaquone.
Azithromycin in combination with other drugs has been suggested as a treatment
of 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 are frequently 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 in the treatment of 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 for the treatment 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 in the treatment of 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).
• 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
turn foals on macrolide antibiotics out 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 be used with caution in animals
with decreased liver function.
• Macrolide antibiotics are not used in horses one year of age or
older due to severe, potentially fatal diarrhea. Severe diarrhea has also
been reported in mares whose suckling foals are on erythromycin. Extra
care should be used to minimize or prevent any exposure to the mare to
macrolide antibiotics. The water buckets and the foal's skin are potential
sources of exposure.
• 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 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.
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 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 www.exclusivelyequine.com
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risks and benefits of using a particular product. You should consult your doctor
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