Chickens have been domesticated for over 4000 years. In the United States, backyard flocks and small-scale chicken farming were common until after World War II. After World War II, modern high-volume poultry farms became widespread. Since about 1950, antibiotics have been added to the diets of chickens because the increase the rate of growth and decrease death loss.
On modern poultry farms, the chickens are raised indoors in order to control heat, light, and feeding. Chickens raised by these modern methods are given antibiotics to help overcome the effects of crowded and unsanitary conditions, or inadequate diets. Antibiotics can be put in food, water, or can be injected. They are used in low levels to aid weight gain. They also help protect against certain diseases such as coccidiosis, bronchitis, fowl pox, and fowl cholera.
Recently, food-borne illnesses caused by eating improperly prepared chicken have received widespread media attention. It has been found that some of the microbes causing these illnesses have developed a resistance to the antibiotics usually used to treat the diseases. Scientists now believe that the antibiotics given to poultry for weeks or months at a time in low doses may cause them to harbor resistant bacteria, which they may then pass along to caregivers and consumers. One such victim was infected with Campylobacter jejuni from eating an undercooked chicken sandwich. He was temporarily paralyzed, and it took a year of rehab to recover fully. Campylobacter jejuni causes as many as seven million sicknesses per year. Of those sicknesses, up to one thousand of these victims die annually. Salmonella can also contaminate chicken. Although it infects less people per year, it is more deadly.
Coccidiosis is one of the many diseases that chickens can contract. This disease is caused by a protozoa called coccidian. Coccidiosis causes intestinal cell destruction, resulting in weight loss or poor weight gain. It can also cause death.
An early coccidiostat, or medicine to fight coccidian, named sulfaquinoxaline, was developed in the 1940s. In the 1950s, another coccidiostat was developed which is still used today: amprolium. This antiprotozoal drug can be put in food, water, or can be injected. Amprolium not only stops the growth of new protozoa, but kills them as well. Amprolium is commonly put in food for broiler (meat) chickens.