Antibiotic Usage in Dairy Herds in Pennsylvania

Antibiotics are used in livestock production as therapeutics, growth promoters, and prophylactics. Therapeutic use of antibiotics is often required to manage clinically apparent diseases. The therapeutic regimen is dictated by label instructions from the manufacturer, or in accordance with extralabel instructions from a veterinarian. Antibiotics are used as growth promoters in livestock via administration at low doses for extended periods of time. As prophylactics, antibiotics are used at low doses for a period of 2 weeks, to prevent disease. Although the duration of antimicrobial use differs for growth promotion and prophylaxis, the dosage for both is typically less than 200 gm/ton, and is considered subtherapeutic.

The earliest evidence of growth-promoting effects of antibiotics became apparent when chickens exposed to small doses of chlortetracycline grew more rapidly than non-exposed chickens. At subtherapeutic levels, antibiotics are helpful in: (1) improving growth, (2) reducing risk of disease, (3) improving digestion, (4) improving weight gain, and (5) decreasing time and the amount of feed needed to reach slaughter weight. Almost 90% of all antibiotics used in farm animals and poultry are administered in subtherapeutic concentrations. About 70% of all antibiotics used in subtherapeutic concentrations in animal feeds are given for the purpose of disease prevention (prophylaxis), while 30% is used for growth promotion. Zwald and co-workers conducted a study on antibiotic usage on conventional dairy farms in Michigan, Minnesota, New York, and Wisconsin. They reported that use of newer antibiotics like ceftiofur was a common practice on farms. They also observed high use of dry cow therapy on these farms and approximately half of farms fed medicated milk replacers to calves. Some of the farms also reported using antibiotics that were prohibited for use in dairy cattle.

A study conducted in Kenya's prolific milk producing districts showed that small dairy producers produced milk with beta-lactam residues exceeding the established maximum residue levels. Use of antibiotics in Sweden and Norway for mastitis treatment has been influenced by policies and recommendations. In these countries the preference for beta-lactams, including procaine, benzyl penicillin and combinations with dihydrostreptomycin, was based on the withdrawal period. Dairy producers in Sweden use long acting drug treatment for subclinical mastitis and dry cow therapy, whereas the same formulations are not accepted in Norway. These examples reveal that antibiotic usage varies from country to country, within a country, and between farms, depending on policies and desired results.

The extensive use of antibiotics has led bacteria to adapt defenses against antibiotics. This has resulted in the evolution, spread, and persistence of antimicrobial resistance in bacterial populations in animal agriculture. The emergence and spread of antibiotic resistant genes is enhanced through the intensive use of antimicrobials in over-crowded populations of production animals. This phenomenon is known as "selective pressure." Selective pressure is a general concept that refers to the many factors that create an environmental landscape which allows organisms with novel mutations or newly acquired characteristics to survive and proliferate. The classic example of continuous selective pressure in the animal production industry is the use of antibiotics in feed at subtherapeutic concentrations. Usage of antibiotics is known to leave residues in farm products. Several antibiotics enter the aquatic and terrestrial ecosystems through the discharge of effluents from farms. When applied to the land, farm wastes containing bioactive veterinary drug residues and antimicrobial resistant bacteria are susceptible to runoff into bodies of water and can potentially create reservoirs in the environment for antibiotic resistant bacteria.

The Veterinary Public Health Research and Training Center of the Department of Veterinary Science conducted a survey (July 2001- June 2002) on 113 dairy herds, from 28 counties in Pennsylvania, for use of antimicrobial agents. Fifty-six of 113 (50%) dairy farms surveyed maintained written or computerized antibiotic treatment records on the farm. Only 21% of dairy producers had written plans for treating sick animals. On most occasions (93%) antibiotics were administered by the owner/manager or designated herdsman, but only 32% of farms sought a veterinarian's advice before administering antibiotics. However, no more than 24% of the dairy producers followed the instructions for antibiotic usage and completed the course of antibiotic treatment. The majority of dairy producers used extra labeled antibiotics, with guidelines from a veterinarian; separated and visibly marked treated cows; and milked treated cows last with a separate milking unit. About 3% of dairy producers surveyed had antibiotic residue violations in the past 6 months.

An in-depth analysis of the records of 33 of the 113 dairy farms was done to determine the incidence of disease conditions for which antibiotics were administered. Pneumonia and enteritis were recorded on 88 and 100% of the farms, respectively. Nearly 79, 100, and 100% of farms had lactating cattle with metritis, foot rot, and clinical mastitis, respectively. Use of antibiotics was the largest in calves for enteritis (36%), followed by pneumonia in calves (25%) and foot rot in cattle (16%). The most widely observed health problems in dry cows on farms were clinical mastitis (27%) and pneumonia (24%). A small percent of dry cows were treated with antibiotics for clinical mastitis (8%) and pneumonia (10%). Twenty four antibiotics including beta-lactams (penicillins and cephalosporins), spectinomycin, and florfenicol were widely used for therapeutic purposes, while oxytetracycline and neomycin was used in milk replacers for prophylaxis. Feeding medicated milk replacers to calves (70%) for prevention of calf scours was widely practiced. Beta-lactam antibiotics were mostly used for dry cow therapy, clinical mastitis, and on some farms for pneumonia and metritis. Ceftiofur, a third generation cephalosporin, was not only used for the prescribed conditions, but also as an extralabel drug on 18% farms for treating mastitis in lactating cattle. Current practices related to the antimicrobial usage on farms could contribute to the development of antibiotic resistant bacteria.

The results of this study suggest antibiotics are extensively used on dairy herds for both therapeutic and prophylactic purposes and its use can be considerably reduced. Based on the findings of this survey it can be inferred that dairy producers should consider antibiotic as a non renewable "valuable resource" that needs to be used prudently in consultation with a veterinarian. Antibiotics usage needs to be done more efficiently that includes maintaining records, avoiding extra label use of antibiotics unless prescribed, and completing the prescribed dosage as indicated on the label. Importantly, dairy producers should be aware of the fact that if antibiotics are improperly used, in the long run it could result in the emergence of antibiotic resistant bacteria in the dairy herd rendering the antibiotic in question ineffective and complicate other treatment protocols.

Ashish Sawant and Bhushan Jayarao
Veterinary Science Extension