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Use of antimicrobial drugs in veterinary practice

British Medical Journal, Sept 5, 1998 by A.M. Johnson

Shortly after antimicrobial drugs were developed they were used in veterinary medicine to treat mastitis in dairy cows.[1 2] Disease is inevitable in all animals, whether Farm animals or pets, and healthy animals may also be carriers and asymptomatic excreters of pathogens. The use of antimicrobial drugs varies between species and may be influenced by husbandry and the pattern of trade in a particular class of animal.[3] Antimicrobial drugs are given to animals by injection (intravenously, intramuscularly, and subcutaneously), orally in food or water, topically on the skin, and by intramammary and intrauterine infusions. Antibiotics are easier to give to animals by injection, and the digestive system of ruminants often renders oral antimicrobial drugs ineffective.

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Treatment of farm animals

The prevalence of pathogens on farms depends on many factors, not least the type of husbandry, the environmental pressure on a farm, and the standard of stockmanship. The most commonly used antimicrobial drugs in animals reared for food are from five major classes: [Beta] lactams, tetracyclines, aminoglycosides, macrolides, and sulphonamides. In addition, quinolones have been available in some European countries for more than 20 years.[4] The discovery of third generation fluoroquinolones with a broader spectrum of activity has led to interest in their use in animals.[5 6]

Whereas the treatment of bacterial disease in humans and their pet animals is invariably directed at a patient, the treatment of animals reared for food, especially pigs and poultry, is generally directed at groups or herds of animals.[3] The main reasons to use antimicrobial drugs in animals are for treatment and prophylaxis or strategic treatment. In farm annals antimicrobial agents are also used to enhance performance by increasing feed conversion, growth rate, or yield. Treatment is usually of individual animals or a group of diseased animals with identified disease.

Prophylaxis

Prophylaxis or strategic treatment is usually to contain the spread of infection and prevent illness before the development of clinical signs. It entails treating a herd or group of animals after illness has been diagnosed in one or more animals in the group or on the basis of previous experience. A special form of prevention is called metaphylaxis, in which all animals in the herd or group are treated when the proportion of animals that has become diseased during a defined period and the probability of most or all of them getting infected are high.[7] Diseases requiting the most extensive use of antimicrobial drugs for treatment or prophylaxis are respiratory and enteric diseases in pigs and cattle and mastiffs in dairy cattle.[7 8] Fish that are reared intensively have special requirements as any infection may lead quickly to an acute outbreak of disease in a cage or pond.

The large groups of animals in, for example, pig herds and poultry flocks may cause logistical problems for antimicrobial treatment. However, therapeutic or prophylactic antimicrobial drugs can be given in food or water. The object of therapeutic treatment is to produce certain drug concentrations in tissues and body fluids and to maintain these until the infection has been overcome. Ill animals may have a poor appetite, in which case the drug would be better given in water rather than food as animals tend to continue to drink, but parenteral treatment may become inevitable. When drugs are given in food or water, the quantity of food or water consumed, the weight of the individual animals, and the pharmacodynamics and kinetics of the drugs to be used must be fully considered in calculating inclusion rates of the antimicrobial drug to ensure adequate treatment. Other factors may also need to be taken into consideration--for example, animals with enteritis will have an increased water intake so the administration rate of a drug will need to be adjusted.[8]

Performance enhancement

The performance enhancement properties of antimicrobial drugs are used to improve the productivity of healthy animals by improving growth rate, feed conversion, or yield. The growth promoting properties of these agents (also known as digestive enhancement) in farm animals were discovered in the late 1940s. The practice of feeding subtherapeutic doses of antibiotics was readily adopted and soon became an integral part of the animal industry. The antimicrobial drugs are given continuously at low doses, usually as feed additives.

Legal requirements

The legal requirements covering the distribution of animal medicines differ according to the legal classification of each product, which also determines who may sell the product and under what restraint or control. Under the Medicines (Restrictions on the Administration of Veterinary Medicinal Products) Regulations 1994 no person is allowed to administer any veterinary medicinal product to an animal unless the product has been granted a marketing authorisation (product licence) for treating a particular condition in the given species. Under the regulations veterinary surgeons are the primary prescribers of medicines, and in the United Kingdom veterinarians usually prescribe and dispense medicines for animals. This applies to both animals reared for food and to pet animals. For animals reared for food, veterinarians or people acting under their direction may administer only a product that contains substances licensed for use in such animals, and they must keep records. The above regulations also allow veterinarians or people under their direction to supply a drug licensed for use in humans for use in a particular animal, provided that it is not being reared for food.

Health Care Industry

Use of antimicrobial drugs in veterinary practice

British Medical Journal, Sept 5, 1998 by A.M. Johnson

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