The level and duration of antibiotic diffusion into milk depends upon several factors including the particular antibiotic, its concentration and method of preparation (aqueous solution, suspending medium). The method of preparation markedly influences retention and can affect adhesion of the antibiotic to equipment and pipelines.
While the decision was pending, the technology was criticized by public interest groups and some scientists who said the drug could increase udder infections in cows, leading to increased use of antibiotics that could end up in milk, and increase pressure on small farmers struggling to compete with larger operations.
The amount of antibiotic excreted into milk may vary from eight to 80%; usually it averages about 50%. It is therefore difficult to know the exact amount of antibiotic residue in milk at different milkings after treatment. Generally, the concentration of antibiotic in milk decreases rapidly with successive milkings, usually at an exponential rate.
The Food and Drug Administration said today that its most reliable test had found no residues of potentially harmful antibiotics in a nationwide survey of milk, contrary to the findings of earlier tests. But critics immediately challenged the latest results. Concern about the safety of milk was raised in March 1988, when the agency reported that more than half of 70 samples of milk taken in 14 cities were contaminated with antibiotics, which are not permitted in milk.
Regulators and veterinarians say that high levels of drugs can persist in an animal’s system because of misuse of medicines on the farm.
That can include exceeding the prescribed dose or injecting a drug into muscle instead of a vein. Problems can also occur if farmers do not follow rules that require them to wait for a specified number of days after administering medication before sending an animal to slaughter or putting it into milk production.
The tests found 788 dairy cows with residue violations in 2008, the most recent year for which data was available. That was a tiny fraction of the 2.6 million dairy cows slaughtered that year, but regulators say the violations are warning signs because the problem persists from year to year and some of the drugs detected are not approved for use in dairy cows.
On a conventional dairy farm, when a cow requires antibiotics, it’s milked separately from the main herd and the milk is not sent to the processing plant. When the antibiotics are cleared from the cow’s system, the cow can reenter the milking herd. On an organic dairy farm, when a cow requires antibiotics, the cow is permanently removed from the herd.
As of 1992, industry is required to 1) screen all bulk milk pickup tankers for beta lactam residues prior to processing, and 2) collect four random samples for FDA testing in separate months during every consecutive six months. Random sampling is at the discretion of the regulatory agency.
Today, every truckload of milk is tested for four to six antibiotics that are commonly used on dairy farms. The list includes drugs like penicillin and ampicillin, which are also prescribed for people. Each year, only a small number of truckloads are found to be “hot milk,” containing trace amounts of antibiotics. In those cases, the milk is destroyed.
Milk should not contain antibiotic residues. Milk production in the UK is regulated by the Dairy Products (Hygiene) Regulations 1995. These regulations include the standards for raw milk. Prior to 1990 milk was deemed to be contaminated if an antibiotic concentration of > 0.01 international units (iu) /ml was present, the standard has now been increased to 0.006 iu/ml.