CSI-Dairy: A mysterious ‘positive’ test

Numerous ‘culprits’ exist on dairy farms, robbing herd performance and injuring the dairy’s bottom line. Identifying and arresting the offender isn’t always easy, and often requires a full investigation, gathering and analyzing evidence on the farm and in the lab. This mystery reveals how a small detail can open the door to a ‘hot’ problem.

By Norm Stewart, D.V.M., M.S.

Dr. Norm Stewart is a dairy technical services veterinarian for Intervet/Schering-Plough Animal Health.

Ed Smith, owner of A Plus Dairy, worked diligently with his veterinarian, Dr. Heartly, for more than 20 years, developing and implementing residue prevention and treatment protocols for his dairy. They knew the harm a single positive milk or meat residue test could cause the dairy’s reputation and business, in addition to the dairy industry’s high-level of consumer confidence.

With recent public attention to the issue rising, Ed reminded his employees to adhere to residue prevention and treatment protocols, and all other on-farm protocols to produce wholesome products. Dr. Heartly oversaw all protocol development, and together they conducted on-farm training, so all employees knew and understood the protocols.


Then, one day, a cull cow tested positive for antibiotic residue. Ed called Dr. Heartly in disbelief. He was sure it had to be a mistake, since they always observed the milk and meat withdrawal periods. Dr. Heartly agreed to meet with Ed, review the records and begin his own investigation.

Physical evidence

Dr. Heartly and Ed reviewed their treatment protocols and records for the cows treated that week. In addition to the “positive” cow, they noticed 11 other cows received a particular antibiotic that day, and a total of 18 had received it that week.

A review of the records supported Ed’s belief employees had administered proper dosage according to the product label, and meat and milk withdrawals times were observed.

Drug inventory records confirmed the correct amount of product was used. All product was properly labeled and inventoried. There have been no positive bulk tank milk tests for the antibiotic – or any product – in the last eight years.

Dr. Heartly visited with the two workers who treated the 18 cows. They confirmed the cow in question received the correct dose, outlined in the treatment protocol. The remaining 17 cows treated that week re-entered the milking line following their respective milk withhold periods.

Dr. Heartly observed employees administer antibiotics, as well as other products. He noted that proper restraint for the animals wasn’t always adequate.

Laboratory evidence

Dr. Heartly talked to the USDA Food Safety Inspection Service personnel. They  confirmed the cow, selected randomly, tested positive – greater than 4.0 ppm in the liver – for violative residues of the antibiotic.

Dr. Heartly also contacted the antibiotic manufacturer to ask that pharmacovigilance records be checked to see if they received any complaints of animals testing positive at processing for the antibiotic. No such complaints for tissue or milk residues were on record.

The culprit

Following a thorough review of the physical and laboratory evidence, Dr. Heartly and Ed concluded this one-time antibiotic tissue residue occurred inadvertently, due to improper animal restraint, thereby leading to improper administration of the antibiotic. Dr. Heartly hypothesized the cow was able to have enough movement when restrained, whereby the product was accidentally administered intramuscularly, not subcutaneously, as labeled by the manufacturer. This could have caused a longer meat withdrawal than the labeled prescribed.


Dr. Heartly retrained all employees in animal-restraint protocols for each class of animal, by location, including calves in hutches, calves in large pens, lactating cows, dry cows, and others. The on-hands training helped  employees maintain and build their confidence, in addition to maintaining animal and employee safety by properly and humanely restraining animals.

He also stressed that if an employee believed they incorrectly administered a product, they should tell the manager, so it can be documented and dealt with to avoid an inadvertent milk or meat residue.

The extra attention to detail gave everyone more confidence another drug residue problem could be avoided.


• Dr. Norm Stewart is a dairy technical services veterinarian for Intervet/Schering-Plough Animal Health. Contact him by phone: 815-479-8872 or e-mail: norman.stewart@sp.intervet.com.

Each month, DairyBusiness Communications will check the case files of lead dairy  ‘investigators’ to uncover  another ‘CSI-Dairy’ mystery. Episodes are archived at www.dairybusiness.com.