Dairy cows, the source of most of the milk consumed in many parts of the world, are susceptible to an infection called mastitis, which is an infection of the mammary glands. This infection is treated with antibiotics, which is an acceptable practice, but antibiotics in milk are regulated in many countries, and any milk from cows that tests positive for antibiotics must be discarded. In the United States, milk must be tested for a number of factors, including certain bacteria, somatic cells which indicate sickness, and other contaminants, such as antibiotics or drug residues. The presence of antibiotics in milk raises several concerns which relate to public health as well as the health of the animals.
The primary concern about the presence of antibiotics in milk is that a positive test for antibiotics could indicate an overuse of antibiotics, which could lead to the emergence of bacterial strains that have resistance to some of the antibiotics that are used to treat human patients. This could create public health problems if such resistant strains spread as they could be difficult to treat and could represent a significant danger to some patients. While the antibiotics that are used in dairy cattle are not used in humans, they are in the same family as some of the human antibiotics, and if a bacteria develops resistance to one antibiotic, it is likely to be resistant to others within the same group. Conversely, antibiotics in milk could prove detrimental to beneficial bacteria that are found in products made from milk, such as yogurt.
Allergic reactions are another problem that could arise if antibiotics are present in milk. Some people are allergic to certain types of antibiotics, and in extreme cases, even a very minute dose can cause serious health problems or even prove fatal. Many people may be unaware of these allergies as they may never have never taken an antibiotic in the same group as those used on cows.
For those who are not allergic to the types of antibiotics used on cows, repeated or prolonged exposure to such drugs could cause the emergence of such an allergy. The acquisition of such an allergy would pose the same health risks as it would for someone who had an inherent allergy to the same kinds of drug. In the United States, the US Food and Drug Administration (FDA) has set acceptable levels for many antibiotics in milk. These levels are set to reflect amounts that the FDA believes to be safe for humans. These levels are extremely low and are on the order of parts per billion.