How to Make the Most of Antibiotics for your Dog
Antibiotics are used to fight bacteria and fungus in and on the body. Bacteria are classified according to their ability to cause disease. Pathogenic bacteria are capable of producing a particular illness or infection. Nonpathogenic bacteria live on or within the host but do not cause illness under normal conditions. These bacteria are called normal flora. Some actually produce substances necessary to the well-being of the host. For example, bacteria in the bowel synthesize vitamin K, which is necessary for blood clotting. Rarely, nonpathogenic bacteria will overgrow and cause symptoms due to their sheer numbers.
Antibiotics fall into two categories: Bacteriostatic and fungistatic drugs inhibit the growth of microorganisms but don't kill them; bactericidal and fungicidal drugs destroy the microorganisms outright.
Why Antibiotics Fail
Antibiotics may not always be effective, for a number of reasons.
Inadequate Wound Care
Antibiotics enter the bloodstream and are carried to the source of the infection. Abscesses, wounds that contain devitalized tissue, and wounds with foreign bodies (dirt or splinters, for example) are resistant areas. Under such circumstances, antibiotics can't penetrate the wound completely. Accordingly, it is essential to drain abscesses, clean dirty wounds, and remove foreign bodies.
An antibiotic chosen to treat an infection must be effective against the specific type of bacteria that is infecting the body. The best way to determine susceptibility is to sample the organism, grow it on a culture plate, and identify it by the way its colony appears and by microscopic characteristics. Antibiotic discs are then applied to the culture plate to see which discs inhibit the growth of bacteria colonies. The results are graded according to whether the bacteria are sensitive, indifferent, or insensitive to the effects of the antibiotic. Laboratory findings, however, do not always coincide with results in the patient. Nonetheless, sensitivity testing is the best way to select the most effective antibiotic.
Route of Administration
An important medical decision is choosing the best route for administration. In a dog with severe infections, antibiotics may be given intravenously or by intramuscular or subcutaneous injection. Some oral antibiotics should be given on an empty stomach; others with a meal. Incomplete absorption is one cause of inadequate levels of antibiotics in the blood. If the dog is vomiting, oral medications may not be absorbed.
Dose and Frequency of Administration
The dose is computed by weighing the dog, then dividing the total daily dose into equal parts and giving them at prescribed intervals. Other factors that must be accounted for when computing the daily dose are the severity of the infection, the age of the dog, her overall health and stamina, and whether the dog is taking another antibiotic. When the total dose is too low or the antibiotic is not given often enough, the drug may not be effective.
Antibiotics can destroy the normal flora in the body, which crowds out pathogens. This allows harmful bacteria to multiply and cause disease. Furthermore, strains of bacteria may develop that are resistant to antibiotics and thus cannot be effectively controlled. This is particularly likely to occur when antibiotics are used:
- For too short a time
- In too low a dosage
- When the antibiotic is not bactericidal
Microorganisms that are resistant to one antibiotic are usually resistant to other antibiotics of the same class. The development of antibiotic-resistant bacteria is one of the main reasons why antibiotics should be used exactly as prescribed and only in situations in which they will clearly benefit the dog. If antibiotics are given at a lower than effective dose or for a shorter time period than prescribed, this may select for bacteria that have resistance to that drug. For example, if five days of treatment would kill all the bacteria and you administer antibiotics for only four days, the only bacteria left are those that managed to survive through four days of treatment. The most resistant bacteria are then left to reproduce.
Antibiotics and Steroids
Steroids are often combined with antibiotics, particularly in topical preparations for the eyes and ears, and on the skin. Corticosteroids have anti-inflammatory effects. By reducing swelling, redness, and tenderness, they often give the impression that the dog is getting better when actually, she is not.
Steroids have one other side effect that is undesirable: They depress the normal immune response. This can impair the dog's ability to fight the infection. Antibiotic medications that contain steroids should be used only under the guidance of a veterinarian.
This article is excerpted from “Dog Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.
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