Antibiotics are medicines that fight infections caused by bacteria in humans and animals by either killing the bacteria or making it difficult for the bacteria to grow and multiply. Bacteria are microorganisms that live in the environment and all over the inside and outside of our bodies.
Antibiotic illustration. (Source: Beef Magazine)
No antibiotic, alone or in combination with another, has ever cured an animal or man. Antibiotics only buy time for self-healing. In other words, all antibiotics do is kill bacteria or stop their multiplication. Efficient repair of damaged tissue afterwards depends on the animal itself. This is greatly influenced by the management system, of which nutritional management is probably the most important.
Bacterial infection is rarely the primary cause of infection. There are usually precipitating causes that create favourable conditions for the overgrowth of bacteria. For example, Mannheimia bacteria rarely cause pneumonia, but their presence at the site of tissue damage in the airways or lungs is the result of damage to the respiratory system caused by dust, ammonia or viruses in the respiratory system. The inefficient immune system of the animal in a less-than-ideal management system (which includes nutrition), plays a major role in the occurrence of the disease.
When pathogenic bacteria are no longer effectively affected (killed or their growth suppressed) by the presence of antibiotics at the site of infection, the bacteria develop a resistance to the antibiotic in question.
The effectiveness of an antibiotic is determined in a laboratory, among other things, by plating the isolate on a growth medium and then determining to what extent growth suppression will occur around a paper slide impregnated with a particular antibiotic (the Kirby-Bauer method). This method is the in vitro method which does not take into account the contribution of the components of the animal’s cellular immune system. The larger the zone of inhibition, the “more effective” the antibiotic. Some standards determine whether the bacterium is sensitive, intermediate or resistant based on the distance between the centre of the paper slide and the outer edge of the zone of inhibition.
Kirby-Bauer disk diffusion
Resistance to antibiotics is aided by their excessive, untargeted use. The addition of antibiotics to livestock rations as a preventive measure is an example of that.
In some cases, cattle recover despite a pathogenic bacterium showing resistance to the antibiotic with which it was treated. The success of the outcome is then totally attributed to the antibiotic, which is not correct. In such a case, the cattle overcame the disease itself through, among other things, their cellular immune system.
This may give the impression that antibiogram results (in vitro) could be less significant for decision-making in the choice of an antibiotic. One can also easily conclude that the involvement of a laboratory in the evaluation of resistance is no longer worthwhile. Such an interpretation is flawed because in vitro efficacy ignores the role of the animal in self-healing.
Successful treatment with antibiotics will depend on the following factors:
- Make the correct diagnosis and determine, if practically possible, the exact reason for the infection.
- Identify the problem early and select an antibiotic that quickly reaches effective levels at the site of infection.
Bacteria multiply every 20 minutes in the lung tissue of an animal with pneumonia. A delay in treatment will increase the risk of a failed treatment.
- Treat with the most effective antibiotic.
The danger is that farmers often choose the cheapest antibiotic. When it doesn’t work, they try something else or ask the vet to help in a case where the prognosis has worsened due to poor decision-making, even if the first antibiotic was replaced by a “more effective” one. The approach of using two antibiotics at the same time “just in case” is not the ideal approach.
The veterinarian remains the key decision-maker in the choice of treatment but must never lose sight of the circumstances that led to infection.
Tetracyclines
Tetracyclines are broad-spectrum antibiotics used to treat a wide variety of infections. These antibiotics work by stopping the growth of bacteria. The different tetracyclines have similar antimicrobial features, but they differ somewhat from one another in terms of their spectra and pharmacokinetic nature.
The following figure illustrates the different tetracycline classes:
Tetracycline classes
Elimination times permit a further classification into:
- Short-acting tetracyclines (tetracycline, oxytetracycline, chlortetracycline).
- Intermediate-acting tetracyclines (demethylchlortetracycline and methacycline).
- Long-acting tetracyclines (doxycycline and minocycline).
The tetracyclines are stable as dry powders but not in an aqueous solution, particularly at higher pH ranges (7 – 8,5). Preparations for parenteral administration must be carefully formulated to provide stable solutions.
Doxycycline and minocycline exhibit the greatest liposolubility and better penetration of bacteria such as Staphylococcus aureus than does the group as a whole. This may contribute to their efficacy in the treatment of gingival (gum) diseases that may be associated with bacterial glycocalyx.
The tetracyclines are generally bacteriostatic, and a responsive host-defence system is essential for their successful use. At high concentrations, as may be attained in urine, they become bactericidal because the organisms seem to lose the functional integrity of the cytoplasmic membrane. Tetracyclines are more effective against multiplying microorganisms and tend to be more active at a pH of 6 – 6.5. Antibacterial efficacy is described as time-dependent.
Penicillin
Penicillin is a group of antibiotics originally obtained from Penicillium moulds, principally P. chrysogenum and P. rubens. Most penicillins in clinical use are synthesised by P. chrysogenum using deep tank fermentation and then purification. Routes of administration include IV, IM and Per os. Penicillins are metabolized by the liver and excreted from the body in urine through filtration by the kidneys.
Penicillin is generally considered safe for use in dogs, cats, horses, livestock, and many exotic pets. However, it can cause a disruption of the normal bacterial population within the gut of some species. Penicillin was approved by the FDA many years ago to treat bacterial pneumonia in cattle and sheep and is still a useful tool for treating animal diseases. As an over-the-counter (OTC) drug, penicillin is readily available at farm stores without a prescription from a veterinarian.
MSD Animal Health antibiotics.
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MSD Animal Health Antibiotics |
Description |
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Disulfox L.A.
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· Treats footrot, navel-ill, joint-ill and pneumonia in stock. · Treats coccidiosis and bacterial scours in calves and lambs. · Treats strangles in horses. Composition: Contains sodium sulfadimethoxine 40 % m/v. |
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Engemycin® 10%
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· Cattle – Treats tick-borne gallsickness (anaplasmosis), · Horses – Treats strangles, bacterial pneumonia and enteritis. · Pigs – Treats bacterial pneumonia, mastitis, bacterial enteritis, · Sheep and Goats – Treats heartwater, bacterial pneumonia, Composition: Each 1 mL of Engemycin® 10% contains 100 mg oxytetracycline |
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Engemycin® Spray
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· Treats topical infections such as lacerations, abrasions, gaping Composition: Each 200 mL contains oxytetracycline hydrochloride, 5 g |
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Reverin 135
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· Treats heartwater, tick-borne gall sickness (anaplasmosis), · Treats strangles in horses. Composition: A sterile, stable solution of oxytetracycline. It contains the |
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Reverin LA 230
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· Long-acting oxytetracycline for the treatment and control of Composition: Each mL contains 230 mg oxytetracycline hydrochloride |
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MSD has several other antibiotics; however, they are all scheduled products that require veterinary consultation. |
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