S.P. Oliver

Food safety hazards associated with consumption of raw milk.

An increasing number of people are consuming raw unpasteurized milk. Enhanced nutritional qualities, taste, and health benefits have all been advocated as reasons for increased interest in raw milk consumption. However, science-based data to substantiate these claims are limited. People continue to consume raw milk even though numerous epidemiological studies have shown clearly that raw milk can be contaminated by a variety of pathogens, some of which are associated with human illness and disease. Several documented milkborne disease outbreaks occurred from 2000-2008 and were traced back to consumption of raw unpasteurized milk. Numerous people were found to have infections, some were hospitalized, and a few died. In the majority of these outbreaks, the organism associated with the milkborne outbreak was isolated from the implicated product(s) or from subsequent products made at the suspected dairy or source. In contrast, fewer milkborne disease outbreaks were associated with consumption of pasteurized milk during this same time period. Twenty nine states allow the sale of raw milk by some means. Direct purchase, cow-share or leasing programs, and the sale of raw milk as pet food have been used as means for consumers to obtain raw milk. Where raw milk is offered for sale, strategies to reduce risks associated with raw milk and products made from raw milk are needed. Developing uniform regulations including microbial standards for raw milk to be sold for human consumption, labeling of raw milk, improving sanitation during milking, and enhancing and targeting educational efforts are potential approaches to this issue. Development of pre- and postharvest control measures to effectively reduce contamination is critical to the control of pathogens in raw milk. One sure way to prevent raw milk-associated foodborne illness is for consumers to refrain from drinking raw milk and from consuming dairy products manufactured using raw milk.

Clinical, epidemiological and molecular characteristics of Streptococcus uberis infections in dairy herds.

A longitudinal observational study (18 months) was carried out in two Dutch dairy herds to explore clinical, epidemiological and molecular characteristics of Streptococcus uberis mastitis. Infections (n = 84) were detected in 70 quarters of 46 cows. Bacterial isolates were characterized at strain level by random amplified polymorphic DNA (RAPD) fingerprinting. Persistent infections were usually attributable to one strain, while recurrent infections could be caused by different strains. When multiple quarters of a cow were infected, infections were mostly caused by one strain. In each herd, multiple strains were identified yet one strain predominated. The majority of all infections were subclinical, and infections attributed to predominant strains were more chronic than infections attributed to other strains. Epidemiological and molecular data suggest infection from environmental sources with a variety of S. uberis strains as well as within-cow and between-cow transmission of a limited number of S. uberis strains, with possible transfer of bacteria via the milking machine.

Antimicrobial susceptibility of coagulase-negative Staphylococcus species isolated from bovine milk

Coagulase-negative Staphylococcus (CNS) isolates (n=168) obtained from milk from heifers and dairy cows were screened for minimum inhibitory concentration (MIC) to antimicrobials used commonly for mastitis therapy. Of the 10 CNS species included in the study, the predominant species were Staphylococcus chromogenes (n=61), Staphylococcus epidermidis (n=37), Staphylococcus hyicus (n=37), and Staphylococcus simulans (n=16). The majority of CNS was susceptible to ampicillin, oxacillin, cephalothin, and ceftiofur. Erythromycin and pirlimycin were also very effective in vitro inhibitors of CNS. The only exception was observed with S. epidermidis. Of 37 S. epidermidis evaluated, 13 (35%) exhibited efflux-based resistance to erythromycin (> or =16 microg/ml) encoded by msrA and one isolate carried ermC encoding ribosomal methylase-based resistance to both erythromycin (> or =64 microg/ml) and pirlimycin (> or =64 microg/ml). A total of 17 S. epidermidis, 11 S. chromogenes, and one S. hyicus exhibited phenotypic resistance to ampicillin (> or =0.5 microg/ml). Constitutive beta-lactamase production was observed in all ampicillin resistant isolates except 4 S. epidermidis that exhibited inducible beta-lactamase production. Induced beta-lactamase production was also observed in 13 S. epidermidis that were phenotypically susceptible to the entire MIC panel. All isolates that produced beta-lactamase either constitutively or by induction carried blaZ. S. epidermidis (n=12, 32%) that were resistant to methicillin (oxacillin > or =0.5 microg/ml) carried low affinity penicillin-binding protein encoded by mecA. Most multi-drug resistant (MDR) S. epidermidis (> or =2 resistance genes) were resistant to ampicillin, erythromycin and methicillin. All except one MDR S. epidermidis had icaAB, which encodes for polysaccharide intercellular adhesion. Based on pulsed field gel electrophoresis, MDR S. epidermidis were closely related genotypically, and were isolated from different cows on the same farm suggesting clonal dissemination. Bovine S. epidermidis share antimicrobial resistance patterns and virulence determinants of strains observed in human infections. Studying CNS at the species level can provide valuable information about species-specific differences that can be vital data for effective mastitis therapy and management.

Endocrine profiles of dairy cows following experimentally induced clinical mastitis during early lactation

Concentrations of LH, cortisol, estradiol-17beta (E(2)), prolactin and 13,14-dihydro-15-keto-prostaglandin F(2alpha) (PGFM) were determined in cows with experimentally induced clinical mastitis during early lactation. Cows free of intramammary infection (IMI) and in the luteal phase of the estrous cycle were balanced by lactation number and days in milk and assigned to either control (n=5) or treatment (n=5) groups. Treated cows were infected experimentally (day 0), in two mammary quarters, with Streptococcus uberis and developed clinical mastitis within 60 h after inoculation as evidenced by increased mastitis scores, elevated rectal temperatures, mammary swelling and isolation of S. uberis pathogen. Four days following bacterial challenge, blood samples were collected every 20 min for 8 h for determination of PGFM and LH following administration of oxytocin and GnRH, respectively. Blood samples were also collected on days 0, 4 and 7 of the experiment to determine concentrations of E(2), prolactin and cortisol. Four days after bacterial challenge, concentrations of cortisol were higher (P=0.04) in experimentally infected cows than controls. Experimentally challenged cows had increased (P=0.02) concentrations of cortisol on days 4 and 7 compared with day 0. Control cows had no significant increase in blood cortisol during the experimental period. Baseline concentrations of PGFM did not differ between groups; however, peak concentrations of PGFM following oxytocin challenge were elevated (P=0.006) in cows with clinical mastitis compared with control animals. Prolactin, E(2) and LH did not differ between cows with clinical mastitis or controls. Experimentally induced mastitis during early lactation elevated concentrations of cortisol during the luteal phase of the estrous cycle. Furthermore, mastitic cows demonstrated an increased PGFM response following oxytocin administration. Altered reproductive efficiency in cows with clinical mastitis caused by Gram-positive pathogens may be the result of increased uterine sensitivity to prostaglandin F(2alpha) (PGF(2alpha)).

SECRETION COMPOSITION DURING BOVINE MAMMARY INVOLUTION AND THE RELATIONSHIP WITH MASTITIS

1. Bacteriological analysis revealed that 30% of quarters contained coagulase-negative staphylococci, Staphylococcus aureus, Corynebacterium bovis, or streptococci. 2. As involution progressed, somatic cell counts, percent protein, pH, and concentrations of serum albumin, lactoferrin, and immunoglobulin G increased while percent fat, concentrations of citrate, and the citrate to lactoferrin molar ratio decreased. 3. Mammary secretion from infected quarters had significantly higher numbers of somatic cells, percent polymorphonuclear leukocytes, and pH, but lower percentage lymphocytes, fat, and lactoferrin concentrations compared to uninfected quarters. 4. Results suggest intramammary infection altered normal secretion composition during bovine involution and lactogenesis. 5. Lower levels of antibacterial components in bovine mammary secretion during the peripartum period may have reduced the natural defense potential of the gland.

Prevalence and persistence of coagulase-negative Staphylococcus species in three dairy research herds.

Coagulase-negative Staphylococcus species (CNS) were isolated from 11.3% (1407 of 12,412) of mammary quarter milk samples obtained from cows in three dairy research herds in 2005. Approximately 27% (383/1407) of CNS was identified to the species level. The species distribution among those CNS identified from all herds was Staphylococcus chromogenes (48%), Staphylococcus hyicus (26%), Staphylococcus epidermidis (10%), Staphylococcus simulans (7%), Staphylococcus warneri (2%), Staphylococcus hominis (2%), Staphylococcus saprophyticus (1%), Staphylococcus xylosus (1%), Staphylococcus haemolyticus (<1%), Staphylococcus sciuri (<1%), and Staphylococcus intermedius (<1%). Staphylococcuschromogenes was the predominant CNS isolated from all three herds; however, differences were seen in the prevalence of other CNS species. A total of 158 CNS (S. chromogenesn=66, S. hyicusn=38, S. epidermidisn=37, S. simulans n=10, and S. warneri n=7) were analyzed by pulsed-field gel electrophoresis (PFGE). The majority (33/41) of CNS isolated from the same mammary quarter on more than one occasion had the same PFGE pattern indicating persistence of the same infection over time. When all PFGE patterns for each CNS were analyzed, no common pulsotype was seen among the three herds indicating that CNS are quite diverse. Composite milk somatic cell count (SCC) data were obtained +/-14d of when CNS were isolated. Average milk SCC (5.32 log(10)/ml) for cows in which CNS was the only bacteria isolated was significantly higher than the average milk SCC (4.90 log(10)/ml) from cows with quarter milk samples that were bacteriologically negative.

Antimicrobial Resistance of Mastitis Pathogens

Antibiotics are used extensively in the dairy industry to combat disease and to improve animal performance. Antibiotics such as penicillin, cephalosporin, streptomycin, and tetracycline are used for the treatment and prevention of diseases affecting dairy cows caused by a variety of gram-positive and gram-negative bacteria. Antibiotics are often administrated routinely to entire herds to prevent mastitis during the dry period. An increase in the incidence of disease in a herd generally results in increased use of antimicrobials, which in turn increases the potential for antibiotic residues in milk and the potential for increased bacterial resistance to antimicrobials. Continued use of antibiotics in the treatment and prevention of diseases of dairy cows will continue to be scrutinized. It is clear that strategies employing the prudent use of antimicrobials are needed. This clearly illustrates the importance of effective herd disease prevention and control programs. Based on studies published to date, scientific evidence does not support widespread, emerging resistance among mastitis pathogens to antibacterial drugs even though many of these antibiotics have been used in the dairy industry for treatment and prevention of disease for several decades. However, it is clear that use of antibiotics in dairy cows can contribute to increased antimicrobial resistance. While antimicrobial resistance does occur, we are of the opinion that the advantages of using antibiotics for the treatment of mastitis far outweigh the disadvantages. The clinical consequences of antimicrobial resistance of dairy pathogens affecting humans appear small. Antimicrobial resistance among dairy pathogens, particularly those found in milk, is likely not a human health concern as long as the milk is pasteurized. However, there are an increasing number of people who choose to consume raw milk. Transmission of an antimicrobial-resistant mastitis pathogen and/or foodborne pathogen to humans could occur if contaminated unpasteurized milk is consumed, which is another important reason why people should not consume raw milk. Likewise, resistant bacteria contaminating meat from dairy cows should not be a significant human health concern if the meat is cooked properly. Prudent use of antibiotics in the dairy industry is important, worthwhile, and necessary. Use of antibiotics at times when animals are susceptible to new infection such as the dry period is a sound management decision and a prudent use of antibiotics on the farm. Strategies involving prudent use of antibiotics for treatment encompass identification of the pathogen causing the infection, determining the susceptibility/resistance of the pathogen to assess the most appropriate antibiotic to use for treatment, and a sufficient treatment duration to ensure effective concentrations of the antibiotic to eliminate the pathogen. As the debate on the use of antibiotics in animal agriculture continues, we need to consider the consequences of, “What would happen if antibiotics are banned for use in the dairy industry and in other food-producing animals?” The implications of this question are far reaching and include such aspects as animal welfare, health, and well-being and impacts on food quantity, quality, and food costs. This question should be an important aspect in this ongoing and controversial debate!

Adhesion and invasion of Escherichia coli from single and recurrent clinical cases of bovine mastitis in vitro.

Seven strains of Escherichia coli, originating from clinical cases of bovine mastitis, and one Salmonella typhimurium control strain were tested for their ability to adhere to, and invade, bovine mammary epithelial cells (MAC-T cells) in vitro. Four of the seven strains were isolated from cows with chronic intramammary infections with recurrent episodes of clinical mastitis and three strains were isolated from single cases of clinical mastitis. Both adhesion and invasion of all strains were dose and time dependent. The four E. coli strains isolated from recurrent cases of clinical mastitis invaded twice as frequently as and three times faster than the strains isolated from single cases of clinical mastitis. By contrast, there was no difference in the amount or speed of adhesion between the two types of strains of E. coli. Adhesion and invasion curves of E. coli resembled a two-step chain reaction, where invasion was the rate-limiting step. Although adhesion and invasion of E. coli has not been demonstrated in vivo yet, the results of the present study may contribute to an understanding of the pathogenesis of chronic intramammary infections caused by E. coli.

ASAS Centennial Paper: Developments and future outlook for preharvest food safety

The last century of food animal agriculture is a remarkable triumph of scientific research. Knowledge derived through research has resulted in the development and use of new technologies that have increased the efficiency of food production and created a huge animal production and food manufacturing industry capable of feeding the US population while also providing significant quantities of high-quality food for export to other countries. Although the US food supply is among the safest in the world, the US Center for Disease Prevention and Control estimates that 76 million people get sick, more than 300,000 are hospitalized, and 5,000 die each year from foodborne illness. Consequently, preventing foodborne illness and death remains a major public health concern. Challenges to providing a safe, abundant, and nutritious food supply are complex because all aspects of food production, from farm to fork, must be considered. Given the national and international demand and expectations for food safety as well as the formidable challenges of producing and maintaining a safe food supply, food safety research and educational programs have taken on a new urgency. Remarkable progress has been made during the last century. Wisdom from a century of animal agriculture research now includes the realization that on-farm pathogens are intricately associated with animal health and well-being, the production of high-quality food, and profitability. In this review, some of the developments that have occurred over the last few decades are summarized, including types, sources, and concentrations of disease-causing pathogens encountered in food-producing animal environments and their association with food safety; current and future methods to control or reduce foodborne pathogens on the farm; and present and future preharvest food safety research directions. Future scientific breakthroughs will no doubt have a profound impact on animal agriculture and the production of high-quality food, but we will also be faced with moral, ethical, and societal dilemmas that must be reconciled. A strong, science-based approach that addresses all the complex issues involved in continuing to improve food safety and public health is necessary to prevent foodborne illnesses. Not only must research be conducted to solve complex food safety issues, but results of that research must also be communicated effectively to producers and consumers.

Potential virulence factors of Streptococcus dysgalactiae associated with bovine mastitis

Mastitis caused by environmental pathogens is a major problem that affects many well-managed dairy herds. Among the environmental pathogens, Streptococcus dysgalactiae is isolated frequently from intramammary infections during lactation and during the nonlactating period. In spite of its high prevalence, little is known about factors that contribute to the virulence of S. dysgalactiae. During the last decade, several cell-associated and extracellular factors of S. dysgalactiae have been identified; yet, the relative importance of these factors in the transmission and pathogenesis of mastitis caused by S. dysgalactiae has not been defined. Streptococcus dysgalactiae can interact with several plasma and extracellular host-derived proteins such as immunoglobulin G, albumin, fibronectin, fibrinogen, collagen, vitronectin, plasminogen, and alpha 2-macroglobulin. These interactions are mediated by bacterial surface proteins. This organism also produces hyaluronidase and fibrinolysin which may be involved in promoting dissemination of the organism into host tissue. Streptococcus dysgalactiae adheres to and is internalized by bovine mammary epithelial cells in vitro. Involvement of host cell kinases, intact microfilaments and de novo eukaryotic protein synthesis are required for internalization of S. dysgalactiae into bovine mammary epithelial cells; a process that appeared to occur by a receptor-mediated endocytosis mechanism. However, de novo bacterial protein synthesis was not required for epithelial cell internalization. Furthermore, S. dysgalactiae survived within mammary epithelial cells for extended periods of time without losing viability or damaging the eukaryotic cell. Further research on characterization of host-pathogen interactions that take place during the early stages of mammary gland infection will enhance our understanding of pathogenesis of intramammary infection which may contribute to development of methods to minimize production losses due to mastitis.