The duration of fecal Salmonella shedding following clinical disease among dairy cattle in the northeastern USA

https://doi.org/10.1016/j.prevetmed.2009.07.002Get rights and content

Abstract

The objectives of this study were to determine the duration of fecal Salmonella shedding among dairy cattle in the northeastern United States following laboratory-confirmed clinical disease and to evaluate whether age group or serotype was associated with either shedding period or mortality. Study farms included 22 dairy herds that had at least two previous salmonellosis cases confirmed by fecal culture. Veterinarians continued to submit culture samples from clinical suspects following herd enrollment, and fecal samples from positive cattle were collected monthly until three sequential negative results were obtained or until loss to follow-up. There were 357 culture-positive clinical cases that each involved a single serotype during the shedding period. The Kaplan–Meier median duration of fecal Salmonella shedding was 50 days, and the maximum was 391 days. S. Newport was the predominant serotype, accounting for 51% of the cases. Age group and serotype were not significant predictors of Salmonella shedding duration in a Cox proportional hazards model, when stratifying by herd. However, the proportion of adult cows shedding for at least two consecutive monthly samples was significantly greater than the proportion of female calves shedding for this duration (Fisher's exact test p-value <0.01). Age group was also associated with mortality in this study; calves with salmonellosis were more likely to die than cows as estimated by a logistic regression model which controlled for herd as a random effect (p-value = 0.04).

Introduction

Salmonella is a zoonotic enteric pathogen with significant public health implications, resulting in approximately 1.4 million illnesses, 16,000 hospitalizations, and between 400 and 600 deaths annually in the U.S. alone (Mead et al., 1999, Voetsch et al., 2004). Though primarily a cause of self-limiting acute enteritis (diarrhea, abdominal pain, and fever, with a typical duration of 4–7 days), Salmonella can produce invasive infections that lead to sepsis and death. People generally acquire salmonellosis through foodborne exposure, although direct contact with infected animals is another possible route (Mead et al., 1999, L Plym and Wierup, 2006). Preliminary CDC FoodNet data from 2007 show that Salmonella accounted for 38% of all laboratory-confirmed cases of foodborne infections, based on surveillance in 10 states (Centers for Disease Control and Prevention (CDC), 2008b). Dairy cattle are considered an important source of Salmonella serotypes that are a threat to human health. Fecal contamination of beef carcasses at the time of slaughter is thought to represent the predominant route of transmission. According to the 1996 USDA National Animal Health Monitoring System (NAHMS) report, 15% of culled dairy cows were shedding Salmonella at livestock markets, and 66% of markets had at least one cow shedding Salmonella (Wells et al., 2001). Contamination of crops by manure used as fertilizer, as well as water contamination by manure run-off, are additional sources of transmission (Islam et al., 2004, Sivapalasingam et al., 2004). Those who work or otherwise interact with livestock are also at risk of infection via direct exposure when cattle are shedding Salmonella.

Introduction of Salmonella onto a dairy farm can occur through a variety of routes, including purchased cattle, contaminated feed or water, wild animals such as rodents and birds, and human traffic (Bender, 1994, Evans and Davies, 1996, Sanchez et al., 2002, Nielsen et al., 2007). Thus, the presence of Salmonella on a farm is not an unexpected finding. In fact, one study involving 110 dairy farms in four states found that over 90% of the farms had at least one Salmonella-positive culture obtained (fecal and/or environmental) during the course of five sampling visits over a 1 year period (Fossler et al., 2004). The NAHMS Dairy 2002 study, based on a single sampling visit to five herds in each of 21 states, found that 31% of herds yielded at least one Salmonella-positive fecal culture (Blau et al., 2005). Considering these high herd-level prevalence values, it would seem logical that fecal shedders within a herd represent a potential point of intervention to mitigate public health risk.

Clinical signs of salmonellosis in cattle may include diarrhea, fever lasting 1–7 days, anorexia, dehydration, decreased milk production, abortion, and evidence of endotoxemia (Divers and Peek, 2008). Infected cattle can shed the organism while ill and following clinical recovery, and asymptomatic shedders never show signs at all. Once a cow has become infected, the duration and magnitude of fecal shedding are important determinants of public health risk. Widespread environmental contamination can result from Salmonella shedding, and the organism can survive for prolonged periods in suitable conditions outside a host (Wray and Wray, 2000, You et al., 2006). Fecal shedding also increases the risk of within-herd transmission, potentially serving as a source of infection for other animals in the herd. Finally, the shedding of Salmonella on one farm increases the probability of inadvertent transmission to other herds, perpetuating the cycle of infection. All of the above lead to an increased risk of zoonotic transmission. In addition to its public health consequences, salmonellosis can be a costly disease for dairy producers on account of mortality, treatment expenses, reduced milk yield, and weight loss within the herd (Peters, 1985, Huston et al., 2002).

The objectives of this study were to determine the duration of fecal Salmonella shedding among dairy cattle in the northeastern United States following laboratory-confirmed clinical disease and to ascertain whether age group or serotype was predictive of either shedding period or mortality.

Section snippets

Study design

This study was based on data collected prospectively as part of a larger project to estimate the incidence of salmonellosis among dairy cattle from 831 herds in the northeastern USA (Cummings et al., 2009). In that project, Salmonella culture was performed on fecal samples obtained from dairy cattle with compatible clinical signs (including diarrhea with blood, mucus, or a foul odor, fever of at least 103 °F, depression, and decreased appetite) between February, 2004 and September, 2005. The

Descriptive statistics

Median herd size was 553 adult cattle (range: 245–1516). The median number of culture-positive clinical cases per herd was eight (range: 2–121). There were 357 clinical cases that each involved a single serotype during the period of shedding. Only eight clinical cases were culture-positive for multiple serotypes during the shedding period, and these were excluded from the statistical analysis. Of the cattle that yielded at least two positive samples, 92% (86/94) had the same serotype isolated

Discussion

A number of studies have described the prevalence and/or risk factors for fecal Salmonella shedding among dairy cattle (Kabagambe et al., 2000, Warnick et al., 2001, Huston et al., 2002, Fossler et al., 2004), but few have examined the duration of shedding in either subclinical or clinical cases. This study had the particular advantage of involving a large number of clinically affected animals from multiple herds. Over 350 cattle with laboratory-confirmed salmonellosis were tested via fecal

Conclusion

In this study, adult cattle with clinical salmonellosis tended to shed the organism in their feces longer than calves did, partly because calves often died early in the course of disease. Newport was the predominant serotype observed, accounting for over half the cases. The duration of fecal Salmonella shedding may exceed 1 year in some animals, and shedding frequently persists well beyond the typical length of clinical signs in cattle with salmonellosis. Additional work is needed to determine

Acknowledgements

The authors thank the veterinarians and dairy herd owners who participated in this study. This project was supported in part by the Cornell University Zoonosis Research Unit of the Food and Waterborne Diseases Integrated Research Network, funded by the National Institute of Allergy and Infectious Diseases, National Institutes of Health, under contract number N01-AI-30054.

References (31)

  • Centers for Disease Control Prevention (CDC)

    Preliminary FoodNet data on the incidence of infection with pathogens transmitted commonly through food – 10 states, 2006

    Morb. Mortal. Wkly Rep.

    (2007)
  • Centers for Disease Control Prevention (CDC)

    Preliminary FoodNet data on the incidence of infection with pathogens transmitted commonly through food – 10 states, 2007

    Morb. Mortal. Wkly Rep.

    (2008)
  • F.G. Clegg et al.

    Outbreaks of Salmonella Newport infection in dairy herds and their relationship to management and contamination of the environment

    Vet. Rec.

    (1983)
  • R.N. Cobbold et al.

    Long-term persistence of multi-drug-resistant Salmonella enterica serovar Newport in two dairy herds

    J. Am. Vet. Med. Assoc.

    (2006)
  • T.J. Divers et al.

    Rebhun's Diseases of Dairy Cattle

    (2008)
  • Cited by (0)

    View full text