Comparison of Salmonella species recovered from Irish liquid milk production holdings with temporal clinical veterinary isolates

https://doi.org/10.1016/j.ijheh.2007.05.008Get rights and content

Abstract

The presence of microbiological hazards in foodstuffs including, Salmonella, form a major source of food-borne diseases in humans. In-line milk filters from 97 liquid milk production holdings in Cork, the largest dairy region in Ireland, were surveyed for the presence of Salmonella species at herd level over a 2-year period (September 2001–September 2003). Each dairy farm was visited 6 times at 4 monthly intervals (denoted by cycles A–F). Six of the 97 herds (6%) were positive. Ten isolates were detected based on culture methods. These included five (5%) Salmonella Typhimurium DT104, 4 (4%) Salmonella Dublin, and 1 (1%) Salmonella Agona from a total of 556 filters. During cycle C, in addition to the milk filters, a bulk tank milk (BTM) sample was procured from each dairy holding and analysed but no Salmonella were isolated. For comparison purposes a further 26 temporal veterinary clinical isolates (21 S. Typhimurium of varying phage type, and 5 S. Dublin) were procured from the Cork Veterinary Clinical Diagnostic Laboratory, Cork. The study collection showed resistance to one or more antimicrobial agents. During the study, Salmonella spp. were isolated from five of the herds prior to any clinical signs in the farm animals. Pulsed-field gel electrophoresis (PFGE) profiles indicated clonality among the isolates pre- and post-clinical illness. A phenotypic and genotypic database for Salmonella spp. has been developed and used for comparative purposes.

Introduction

Salmonellosis is an important disease in cattle and calves, Salmonella enterica subspecies enterica serovars Typhimurium and Dublin are considered to be the primary serotypes isolated although other species have been detected in this reservoir (Veling et al., 2001). Environmental contamination of a herd may occur through, feed, wildlife, pasture or water. Globally a prevalence of Salmonella of between 2.6 and 25.3% from retail meat, bulk tank milk and cull dairy cow fecal samples has been reported (Duffy et al., 1999; Jayarao and Henning, 2001; Zhao et al., 2001; Murinda et al., 2002; Zhao et al., 2002). Infection may be introduced into Salmonella-free herds by purchase of either adult cattle as replacements or calves for intensive rearing (Wray and Davies, 2003).

Humans can acquire Salmonella-related illnesses by consuming foods of animal origin (Crilly et al., 2001; Jayarao and Henning, 2001; Murinda et al., 2002). One route of Salmonella transmission is via raw/unpasteurised milk i.e. milk in its natural state (Cody 1999). Pasteurisation destroys disease-producing bacteria and therefore reduces the potential risk to human health. Even though it is illegal to sell raw cow's milk in Europe, the practice of the consuming raw milk is ongoing at farm level (Murphy et al., 2005).

The pathogenesis of Salmonella is generally the same in animals as it is in humans, intestinal colonisation, penetration and passage of the organism from gut lumen into the epithelium of the small intestine where inflammation occurs followed by enterotoxin production within the enterocyte (Patterson and Isaacson, 2003). Typical clinical signs, in humans and animals, of acute Salmonella enteritis include: fever, severe watery diarrhoea with subsequent onset of dehydration (D’Aoust, 1997). If sufficient damage occurs to the intestinal lining, the bacteria may enter the bloodstream, resulting in systemic infection. Adult cattle may be asymptomatic or latent carriers that harbour the bacteria in lymph nodes or tonsils and excrete the organism in their faeces, milk, genital discharges and urine, particularly in the case of cows when stressed during parturition (Wray and Davies, 2003). Surviving cattle may shed Salmonella for several months and therefore be a source of infection not only to other animals but also to humans (McDonough et al., 1999). A further risk to public health is the possible dissemination of antimicrobial resistant Salmonella along the food chain.

Several studies have reported on antimicrobial resistance in Salmonella (Nastasi and Mammina, 2001; Liebana et al., 2002; Chen et al., 2004; McDermott et al., 2004). Of particular note is Salmonella enterica serovar Typhimurium Definitive Type 104 (DT104), a multi-drug-resistant strain commonly resistant to ampicillin, chloramphenicol, streptomycin, sulphonamides, tetracycline (denoted R-type ACSSuT) and the possible emergence of S. enterica serovar Typhimurium DT104b (Daly et al., 2004; Gorman and Adley, 2004). Along with its multi-drug resistant phenotype DT104 strain has a second property, hypervirulence. The basis for hypervirulence is multifactorial, and can be attributed to acquisition of virulence and antibiotic resistance genes, the infectious dose, hyperinvasiveness and the role of undefined host factors . Clearly clinical outcomes of DT104 infections can be more problematic than infections from other Salmonella serotypes (Institute of Food Technologists Expert Report (IFT), 2000; Carlson et al., 2003). The presence of these pathogens in milk is a potential veterinary and public health risk (Tamada et al., 2001; Zhao et al., 2002).

In-line milk filters are a simple, cost-effective and convenient tool to screen a cohort of animals for pathogenic bacteria. The in-line milk filters are used routinely on dairy farms in Ireland and function to trap debris e.g. fecal material, soil, grass and hair, preventing entry to the bulk tank milk (BTM) (McEwen et al., 1988; Hancock et al., 1994; Heuvelink et al., 1998; Hassan et al., 2000; Mac Millan, 2002; Murphy et al., 2005). Milk filters used were spun from non-woven cotton. The sock-like filters have a pore size in the range 60–300 μm, (5.2 cm wide and of varying lengths typically 23.2, 36.4, or 56 cm). Each milk filter has the capacity to filter milk from approximately 150 cows (Mac Millan, 2002).

The objectives of the study were to determine the prevalence of Salmonella spp. using milk filters from a sub-set of milk producing commercial dairy herds in the southern region of Ireland, to investigate the associated antibiotic resistance profiles. Using pulsed-field gel electrophoresis (PFGE) a strain database was created for comparative purposes in the event of human or animal outbreaks.

Section snippets

Bacterial isolates

Twenty-one S. Typhimurium (of varying phage type) and 5 S. Dublin veterinary clinical isolates (2000–2004) were obtained from the Cork Veterinary Clinical Diagnostic Laboratory, Model Farm Road, Cork. The selected Salmonella isolates from clinical cases were temporal isolates recovered in the same geographical region as the milk filter isolates and were used in this study for comparative purposes. Serovar identification and phage typing was performed at the Central Veterinary Research

Results

A total of 556 in-line milk filters from 97 liquid milk production holdings in the Cork region of Ireland were examined for the presence of Salmonella spp. Ten (1.8%) samples were Salmonella positive. Five S. Typhimurium DT104 multidrug resistant strains were isolated. Four S. Dublin isolates were also cultured (2 from the same farm over a period of 2 months), along with a S. Agona.

Two DT104 isolates were cultured from 2 milk filters over a 2-week period during a sporadic S. Typhimurium DT104

Discussion

The study highlighted the potential public health risk of the presence of Salmonella spp. at herd level. Cattle may be asymptomatic carriers of this recognised zoonotic hazard, and surveillance for this pathogen has now been incorporated in the microbiological criteria for food producing premises under the new European General Food Law No. 2073/2005.

All herd owners approached agreed to participate in this study with one exception. The only criteria stipulated was that the herd had to be

Acknowledgements

The authors acknowledge the financial support provided by safe Food—the Food Safety Promotion Board in respect of Grant no. 02-RES-001. In addition the authors thank Mr. Eugene Power, Cork Veterinary Clinical Diagnostic Laboratory for permission to work on the veterinary clinical isolates. The authors also thank Mary Murphy, Veterinary Food Safety Laboratory, Cork County Council for her technical advice.

References (39)

  • S. Chen et al.

    Characterisation of multiple antimicrobial-resistant Salmonella serovars isolated from retail meats

    Appl. Environ. Microbiol.

    (2004)
  • S.H. Cody

    Raw-milk products may contain Salmonella

    J. Am. Med. Assoc.

    (1999)
  • J. Crilly et al.

    Epidemiology of Salmonella infection in the south of Ireland

    Irish J. Agri. Food. Res.

    (2001)
  • M. Daly et al.

    Molecular characterization of Irish Salmonella enterica serotype Typhimurium: detection of class 1 integrons and assessment of genetic relationships by DNA amplification fingerprinting

    Appl. Environ. Microbiol.

    (2000)
  • M. Daly et al.

    Evidence for a chromosomally located third integron in Salmonella enterica serovar Typhimurium DT104b

    Antimicrob. Agents Chemother.

    (2004)
  • J.Y. D’Aoust

    Salmonella species

  • R. Gorman et al.

    Characterisation of Salmonella enterica serotype Typhimurium isolates from human, food, and animal sources in the Republic of Ireland

    J. Clin. Microbiol.

    (2004)
  • D.D. Hancock et al.

    The prevalence of Escherichia coli O157:H7 in dairy and beef cattle in Washington State

    Epidemiol. Infect.

    (1994)
  • L. Hassan et al.

    A Cross-sectional study of the prevalence of Listeria monocytogenes and Salmonella in New York dairy herds

    J. Dairy. Sci.

    (2000)
  • Cited by (0)

    View full text