(Note: This article was written in 1996. Therefore, newer drugs may be available. Ed.)
This is the second in a series covering antimicrobial drugs. We will examine antibacterial agents, which we most familiarly call antibiotics.
Antibacterial drugs can be broadly and somewhat generally divided into two classes: bacteriocidal and bacteriostatic. Bacteriocidal drugs are designed to kill bacteria when the drugs contact the organisms. Bacteriostatic drugs hold the organisms in stasis or prevent them from multiplying without directly killing them. Bacteriostatic drugs may seem like an odd approach, but they are quite effective against certain types of bacteria. Some drugs fall into an intermediate position between these two classes and have qualities of both, usually depending on the dose of drug administered.
The choice of a drug depends on the species and age of the bird, the type of bacterium attacking the bird, the overall medical status, and sometimes even the client due to routes-of-administration problems. So, one commonly asked question I get is why did I choose a particular medication. The answer is complicated because the circumstances are so variable. The most important component in rational drug choice is sensitivity. Are the bacteria we are treating sensitive to the drug we are using? Usually, we have to start treatment before the culture/sensitivity results are back from the lab, so we make our decision based on experience and veterinary intuition.
Many other factors govern rational drug choice: availability, cost, route of administration (injectable, oral, topical), and age of the patient. For example, if the patient is a neonate or juvenile, I prefer to use Claforan (cefotaxime) over Baytril. They have similar killing abilities, but Claforan is much milder to the system. However, Claforan is very expensive. A 1-gram vial is about $11.00 vet cost, but a 1-gram vial, if kept frozen, will treat many birds.
Bacteriocidal drugs are used in cases of extreme urgency in our avian patients. The 'cidal drugs most commonly used are: penicillins, cephalosporins, quinolones, and aminoglycosides. Representatives of each of these subclasses are: amoxicillin, cefotaxime (Claforan), enrofloxacin (Baytril), and amikacin (Amiglyde), respectively. All these drugs are designed to target mainly Gram-negative bacteria. Some, such as amikacin, should be used with great care because they can be quite toxic to the kidneys. Amikacin is often used with highly virulent strains of Pseudomonas bacterial infections and can be used safely with careful monitoring. This is why you should consult your vet before using any drug—you can severely injure your bird while trying to do good. The 'cidal drugs work by a variety of methods, such as preventing cell walls from forming and interfering with the way the DNA replicates.
Bacteriostatic drugs are compounds such as the tetracyclines. We use these drugs specifically for Chlamydia infections. Due to the unusual nature of the chlamydial organism, only the tetracyclines or tetracycline-like drugs tend to be effective. The 'static drugs essentially hold the chlamydial organisms in stasis until the immune system eliminates them or the bacteria die. These drugs function by interfering with protein synthesis of the bacteria.
The drugs most often identified as intermediate between 'cidal and 'static are the sulfas. Trimethoprim-Sulfamethoxazole (Bactrim) can be either 'cidal or 'static depending on dosage. The sulfas act by interfering with folic acid, an essential bacterial metabolite.
All antimicrobial agents must be used with care and good reason. The pell-mell usage of drugs by both vets and aviculturists has led to a legion of resistant strains of bacteria. And just a note on resistance—antimicrobials do not mutate the bacteria but select for resistance already present in the population. So, indiscriminate use of drugs can lead to the growth of resistant bacterial strains.
Most of the agents we have discussed have targeted the Gram-negative bacteria. We do occasionally see Gram-positive infections, but these are much less common in birds.
The routes of administration are mainly oral and injectable. In most situations and depending on the drug, the injectable route is much easier on the bird and the aviculturist. And, an injectable often delivers a more effective dose than oral administration. It is sometimes difficult to convince people of this because many of us are a bit squeamish about giving shots.
This has been a very cursory and generalized view of antibiotics. I hope that this article may have answered some questions or perhaps generated some new ones we can address in the future.