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A survey was conducted (July 2001 to June 2002) on antibiotic usage of 113 dairy herds from 13 counties in Pennsylvania. Fifty percent of dairy farms surveyed maintained antibiotic treatment records. Only 21% of dairy producers had written plans for treating sick animals. Thirty-two percent of dairy producers sought veterinarian advice before administering antibiotics and on most farms (93%), antibiotics were administered by the owner/manager or designated herdsman. Twenty-four percent of the dairy producers said they always completed the course of antibiotic treatment. Any extra-label use of antibiotics was administered only on the guidelines of a veterinarian on majority of the farms. Comprehensive records from 33 dairy farms indicated that antibiotic usage was largest for calves with enteritis (36%) followed by pneumonia in calves (25%) and foot rot in cattle (16%). Twenty-four antibiotics including beta-lactams, spectinomycin, florfenicol, and tetracyclines were used on these farms. Beta-lactam antibiotics were used mostly for dry cow therapy, clinical mastitis, and on some farms for pneumonia and metritis. On 18% of the dairy herds surveyed, ceftiofur was used in an extra-label manner to treat mastitis in lactating cattle. On 70% of farms, calves were fed medicated milk replacers containing oxytetracycline and neomycin. The results of this study suggest that antibiotics are used extensively on dairy herds for both therapeutic and prophylactic purposes. Beta-lactams and tetracyclines were the most widely used antibiotics. There is considerable variation in the management practices associated with antibiotic use on dairy farms. It is anticipated that the findings of this survey will permit developing new strategies for prudent use of antibiotics on dairy herds.

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A 2-part study was conducted to determine the risk of exposure to human pathogens from raw milk. The first part of the study focused on determining raw milk consumption habits of dairy producers. A total of 248 dairy producers from 16 counties in Pennsylvania were surveyed. Overall, 105 (42.3%) of the 248 dairy producers consumed raw milk and 170 (68.5%) of the 248 dairy producers were aware of foodborne pathogens in raw milk. Dairy producers who were not aware of foodborne pathogens in raw milk were 2-fold more likely to consume raw milk compared with dairy producers who were aware of foodborne pathogens. The majority of dairy producers who consumed raw milk indicated that taste (72%) and convenience (60%) were the primary factors for consuming raw milk. Dairy producers who resided on the dairy farm were nearly 3-fold more likely to consume raw milk compared with those who lived elsewhere. In the second part of the study, bulk tank milk from the 248 participating dairy herds was examined for foodborne pathogens. Campylobacter jejuni (2%), Shiga toxin-producing Escherichia coli (2.4%), Listeria monocytogenes (2.8%), Salmonella (6%), and Yersinia enterocolitica (1.2%) were detected in the milk samples. Salmonella isolates were identified as S. enterica serotype Typhimurium (n = 10) and S. enterica serotype Newport (n = 5). Of the 248 bulk tank milk samples, 32 (13%) contained > or = 1 species of bacterial pathogens. The findings of the study could assist in developing farm community-based educational programs on the risks of consuming raw milk.

A study was conducted to understand the descriptive and molecular epidemiology of antimicrobial-resistant gram-negative enteric bacteria in the feces of healthy lactating dairy cattle. Gram-negative enteric bacteria resistant to ampicillin, florfenicol, spectinomycin, and tetracycline were isolated from the feces of 35, 8, 5, and 42% of 213 lactating cattle on 74, 39, 9, 26, and 82% of 23 farms surveyed, respectively. Antimicrobial-resistant gram-negative bacteria accounted for 5 (florfenicol) to 14% (tetracycline) of total gram-negative enteric microflora. Nine bacterial species were isolated, of which Escherichia coli (87%) was the most predominant species. MICs showing reduced susceptibility to ampicillin, ceftiofur, chloramphenicol, florfenicol, spectinomycin, streptomycin, and tetracycline were observed in E. coli isolates. Isolates exhibited resistance to ampicillin (48%), ceftiofur (11%), chloramphenicol (20%), florfenicol (78%), spectinomycin (18%), and tetracycline (93%). Multidrug resistance (>3 to 6 antimicrobials) was seen in 40% of E. coli isolates from healthy lactating cattle. Of 113 tetracycline-resistant E. coli isolates, tet(B) was the predominant resistance determinant and was detected in 93% of isolates, while the remaining 7% isolates carried the tet(A) determinant. DNA-DNA hybridization assays revealed that tet determinants were located on the chromosome. Pulsed-field gel electrophoresis revealed that tetracycline-resistant E. coli isolates (n ‫؍‬ 99 isolates) belonged to 60 subtypes, which is suggestive of a highly diverse population of tetracycline-resistant organisms. On most occasions, E. coli subtypes, although shared between cows within the herd, were confined mostly to a dairy herd. The findings of this study suggest that commensal enteric E. coli from healthy lactating cattle can be an important reservoir for tetracycline and perhaps other antimicrobial resistance determinants.