GETTING THE BEST OUT OF ANTIBIOTICS BY DR CHRIS PETZEL BVSC – TECHNICAL SERVICES MANAGER, VIRBAC (AUSTRALIA)
Since time immemorial mankind and bacteria have been waging a relentless battle and until the turn of the 20th century the balance seemed tipped in favour of bacteria. Then around the World War II period science lent us a powerful weapon in the war against harmful bacteria: antibiotics. First there was penicillin, a mould that saved thousands during the 1940’s. Sulphur based products followed not long after, and this was only just the beginning. It appeared that finally the balance had swung in our favour. It wasn't long before a range of antibiotic products were being developed and naturally some of these found use in the veterinary profession. Before long a simple ‘shot’ of antibiotics was all that was needed to cure that sick animal. Unfortunately this approach was to lead to some major problems in the future. In the human medical and veterinary professions antibiotics have sometimes been over prescribed, especially with regard to viral infections. In other situations owners have failed to follow veterinary advice and have often administered incorrectly a course of antibiotics when not needed or they dosed too little over too short a time when they were needed. The end result being the development of resistance in bacteria to certain antibiotics. With the advent of resistance we, as veterinarians, are often faced with the difficult task of finding antibiotics to combat bacteria.  The purpose of this article is to hopefully increase our awareness and understanding of what antibiotics do and how they work. With this knowledge we may be better able to use the drugs we now have and delay the development of antibiotic resistance. What is an Antibiotic? An antibiotic by definition is a chemical that kills or disables bacteria. Antibiotics are widely used in the prevention and treatment of infectious diseases. It is important to only use antibiotics where the disease is caused by or involves bacteria. Antibiotics have no effect against viruses or other infectious agents. Antibiotics may be selectively toxic to bacteria but it is also important to remember that they can also affect the animal being treated. As such the selected dose rate is quite often critical. Too much and we may harm our patient, too little and we encourage resistance or a protracted illness. Antibiotics are further classified as ‘Broad’ or ‘Narrow’ spectrum. Bacteria are commonly termed as gram negative or positive and as aerobic or anaerobic. Basically all gram-positive bacteria have simple cell walls and gram negative have complex cell membranes. All aerobic bacteria need oxygen to live whilst anaerobic bacteria die in the presence of oxygen. By identifying a bacterial type we are better able to choose an antibiotic that will be effective against that specific bacteria without harming the ‘good’ bacteria that live in our body. A ‘Broad’ spectrum antibiotics targets a range of bacteria, including ‘good’ bacteria, and is often chosen when we don't know what bacteria are present or if a mixed infection is likely eg abscess or kennel cough. A ‘Narrow’ spectrum antibiotic targets a specific bacterial type without necessarily including ‘good’ bacteria. These bacteria are often identified by culture and sensitivity tests eg salmonella. Also the effectiveness of a particular antibiotic relies heavily on the immune system. The immune system plays a vital role in an animal’s response to invading bacteria. The immune system should be sustained by whatever means possible when treating an infectious disease; otherwise a protracted recovery or relapse is inevitable. These means can include: Minimising stress. ie decrease training, provide warm housing and clean environment. Providing quality nutritional support. Attention should be given to caloric and nutrient intake, especially of protein and vitamins as they play a critical role in immune responsiveness. FeramoÒ Greyhound is an ideal foundation supplement containing vitamins and minerals considered vital for good health. Using products that stimulate the immune system. How do Antibiotics work? Antibiotics affect bacteria in numerous ways and consequently bacteria can protect themselves by a variety of means. The major actions of antibiotics are listed below and provide examples. Inhibit the formation of the bacterial cell wall  – penicillin, amoxicillin and cephalosporins. Impair the function of the bacterial cell membrane. Inhibit protein production in bacteria– tetracyclines, gentamycin and lincosamides. Inhibit production of bacterial DNA – sulphonamides, trimethoprim. Bacteria will utilise numerous strategies to overcome the detrimental effects of antibiotics. This is called antibiotic resistance. It is important to remember bacteria are not smart but simply relentless. A lot of antibiotics originate from compounds produced by microbes to kill competitor microbes eg penicillin. As such, the environment that bacteria live in is a naturally hostile place and they have evolved strategies to overcome these dangers. Resistance is a genetic trait and incorrect antibiotic use will select for resistance more rapidly than would occur naturally. Antibiotic treatment courses may fail for numerous reasons. These can include: Incorrect diagnosis eg viral and not bacterial infection. Incorrect choice of antibiotic   eg bacteria resistant to the antibiotic,eg antibiotic is unable to penetrate infected tissue. Bacteria, though initially susceptible, developed resistance due to incorrect dosing. Poor wound care or self-contamination eg dog licking a wound. Expired or substandard products were used. The prescribed treatment course was not reliably followed. Usually treatments are to be given for 5-7 days (although a response should be apparent in 3-4 days for most infections).  Nutritional levels were not adequate. Supportive, nursing and management factors were not adequate. These can include the use of anti-inflammatory agents, anti-diarrhoeal preparations, urinary acidifiers and fluid/electrolyte replacement. As can be seen, from this brief discussion, the choice and use of antibiotics is a complex procedure. To achieve a successful treatment we need a clinical diagnosis, identification of a specific bacterium, correct choice of drug, dosage and route of administration as well as adequate nursing and nutritional support.    It should also be remembered that antibiotics are prescription only medications and as such should only be used under supervision and supply of a registered veterinary surgeon. Additionally, antibiotics are classified as Category One drugs and will require a minimum of 72 hours cessation prior to racing.