Diagnosing the problem by clinical (on site) and laboratory procedures in each field case would be ideal, but is often impractical. Additional information concerning past history of the herd and environmental factors provide a basis for medical decision making. Similarly, laboratory work is necessary at some point in the course of many problems if you are to expect effectiveness of treatment procedures.
When the presence of a specific pathogen has been established in the laboratory, antibiotic susceptibility (sensitivity) tests can be conducted to aid in proper drug selection. Confusion seems to exist about antibiotic susceptibility tests. There is an apparent lack of understanding about both their interpretation and the role such testing plays in a treatment program. In the laboratory, isolated bacteria are exposed to a variety of antibiotics. If the organism is killed or its growth restricted by one of these antibiotics, that bacteria is assumed to be susceptible to that drug. If the organism continues to multiply, it is assumed to be resistant to that antibiotic.
A single species of bacteria, such as Staphylococcus aureus, has many strains or types that may vary in their susceptibility to a given antibiotic. For this reason, it is important to determine in the laboratory if a certain strain in a particular animal or herd is susceptible or resistant to various antibiotics. If an organism is resistant to a drug such as penicillin in a laboratory test, selecting penicillin for treating the animal would not be advisable. On the other hand, susceptibility to a specific antibiotic in the laboratory tests does not necessarily mean that that antibiotic will be effective in controlling the infection in the animal. This apparent contradiction is due to the many uncontrolled factors present in the live animal as opposed to the controlled conditions of the laboratory. Many producers have seen this happen in their treatments--the laboratory report indicates susceptibility, yet that particular drug provides no, or poor, results. In spite of this, standardized susceptibility testing is a valuable means for assisting with the selection of an antibiotic. Professional interpretation and evaluation of herd health history and good individual animal records are also important factors to consider before final selection of the antibiotic. The point to be made is that a laboratory report indicating susceptibility of a bacterium to a particular antibiotic does not mean treatment with this antibiotic will guarantee elimination of the infection. For this reason, selecting antibiotics based on correct laboratory data and subsequent modifications of treatment regimen are best managed by a veterinarian.
Causes of Antibiotic Failure
Incorrect diagnosis--the disease may be noninfectious.
The disease may be due to organisms that don't respond to antibiotics (i.e., a virus).
Incorrect route of administration or dosage.
The drug may suppress normal body defense mechanisms.
The drug may encourage the development of resistant pathogens.
There is a possibility of many other complex incompatibilities.
Guidelines To Minimize Treatment Failures
Avoid using a combination of antibiotics in treatment.
Avoid using multiple treatment regimens for the same animal.
Treat early and long enough.
Consult a veterinarian about the animal.
Summary
The indiscriminate use of antibiotics is costly and can be a deterrent to supplying quality food animal products that are free of residues. The dangers of unsupervised antibiotic use are numerous and, under some circumstances, can result in liability and health risks. Veterinary counsel should be sought concerning the use and compatibility of drugs before treating animals. Although the actual administration of medicine may not appear to be difficult, the activity of the drug within the living animal is very complex and can adversely affect the animal, food animal products, and the consumer.
