Seldom are all factors considered when vaccination programs are initiated. Ordinarily some protective immunity is produced when poultry are vaccinated, although the vaccine in itself cannot guarantee it. When vaccination failures occur, the total vaccination program should be reviewed. Although vaccination offers a method of preventing many poultry diseases from establishing themselves, certain drugs and antibiotics are needed to help alleviate the symptoms of a vast number of diseases. The drugs make up an unassociated list of chemicals, and a great many are specific for a certain disease or for a group of similar diseases.
New ones come on the market regularly and many others are in the process of experimentation.
Drugs must reach the pathogenic microscopic organisms in the bird to be effective. Most drugs kill on contact, weaken the organism, or upset its life cycle. Drugs may be administered through the feed, in water, or by injection. Most drugs are chemicals that disrupt the life cycle of the organism involved. As most of the drugs are quantitative, their activity is determined by the amount administered to the bird.