Specific Drug Treatment

If the disease under consideration is caused by an infectious agent, then it is likely that a drug is available to kill, or at least neutralise, that agent. For example:

• bacteria - use antibiotics

• viruses - no specific drug therapy is available

• protozoa - use specific antiprotozoal drugs

• fungi (yeasts and moulds) - use specific anti-fungal drugs

• worms - use anthelmintics

• ectoparasites - organo-phosphorus or pyrethroid products are often used

Antibiotics

Antibiotics are chemicals which destroy bacteria but have little or no adverse effect on the animal.

Bacteriocidal and bacteriostatic antibiotics should not be used simultaneously in an animal, since one counteracts the effects of the other. This is because the bacteriocidals work best against rapidly growing and dividing bacteria, whereas bacteriostatics actually inhibit bacterial growth and multiplication. In addition, if the animal’s immune defence mechanisms are likely to be severely impaired and unable to destroy bacteria, for example after calving or as a result of toxaemia, then it is probably best to use a bacteriocidal rather than a bacteriostatic drug. Bacteriocidal antibiotics are also used in the treatment of endocarditis. However, the distinction is not that precise, in that some antibiotics are bacteriostatic when used at low doses, but are bacteriocidal when given in large amounts.

Different antibiotics are effective against different types of bacteria.

If antibiotics are used indiscriminately, and especially for prolonged periods, there is a danger of bacteria mutating into forms which are resistant to the particular antibiotic. Sometimes this resistance is ‘infectious’ and can spread extremely rapidly in the form of genetic material to other strains of bacteria which have not been exposed to the antibiotic. The particular genetic material is unusual in that it is not part of the nucleus of the cell. Chloramphenicol is currently one of the drugs of choice in the treatment of human typhoid and certain other enteric infections and, to maintain its effectiveness, it has been requested that its use in the veterinary field is restricted to only essential cases, thus decreasing the risk of bacterial resistance developing.

There are many other factors which must be taken into account when using antibiotics and these few examples were given merely as an illustration of the complexity of the subject.

There are usually milk- and meat-withholding periods following the administration of antibiotics and these need to be carefully observed. Some of the newer antibiotics, for example ceftiofur, are interesting.

They are so effective against bacteria that they can be used at extremely low concentrations, so low that they have no adverse effect on man but are still high enough to kill the bacteria. This means that they have no milk withholding period and only a short meat withdrawal period prior to slaughter.

Antiprotozoals

There is a range of drugs specifically aimed at protozoal infections. These include monensin, sulphonamides and amprolium, which are used against coccidiosis, and imidocarb, which is effective against the blood parasites Babesia and Anaplasma.

Antifungals

Antifungal drugs include griseofulvin, which is given by mouth, and nystatin, which is applied to the skin.

Anthelmintics

These are drugs which destroy helminths, that is intestinal worms, lungworms or liver fluke. As with antibiotics, each drug has its own spectrum of activity, some (thiabendazole) being effective against adults only, others (levamisole) being effective against adults and mature larvae, while the avermectin group (ivermectin, doramectin and moxidectin) can be used against adults and larvae. Specific products (e.g. rafoxanide or nitroxynil) are needed for liver fluke. The avermectin group persists in the animal to provide protection against reinfection for three to six weeks, depending on the type of worm and the type of product in use. This is discussed in greater detail in Chapter 4.

Insecticides for ectoparasites

Once again, there is a wide selection of products available.

The avermectin group of chemicals (ivermectin, doramectin and moxidectin) have good activity against warbles, mange and sucking lice, but are less effective against biting lice. They also give an extended period of cover.

Pyrethroids are used as fly repellents and for lice treatment only. They produce a rapid action by a ‘knock-down’ effect, and are sometimes supplied as a combination with piperonyl butoxide.

Organo-phosphorus compounds produce death by an excessive stimulation of the insect’s ner­vous system. In high doses they can be toxic to animals. They are often available as pour-on prepa­rations, having been combined with a chemical which carries the drug through the skin of the ani­mal and throughout its body via the blood. They are a common treatment for lice, mange and war­bles, and are particularly effective because they give whole-body cover. Phosmet is a commonly used example.

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