1 December 2012

8 minute read

Approximately 265,000 of the estimated 48 million foodborne illness cases each year are caused by Shiga toxigenic Escherichia coli (STEC), with E.coli serogroup O157:H7 (O157) responsible for 36 per cent and non-O157 serogroups for the remainder (CDC, 2011). Symptoms of STEC infections include severe stomach cramps, bloody diarrhea, and vomiting. If fever develops, it rarely exceeds 101°F (38.5°C). Most people recover within five to seven days, but some develop severe or life-threatening complications, including hemolytic uremic syndrome. Young children, the elderly, and people who are immuno-compromised face higher risk from STEC infections than healthy adults.

For beef cattle producers and the meat industry, O157 contamination creates significant economic burden, legal liability and public health concern. Ground beef that tests positive for O157 is considered adulterated, so even a low prevalence of contaminated meat produces a major economic risk for packers. Publicity surrounding recalls has also heightened awareness about bacterial contamination among consumers, with 40 per cent saying they are extremely concerned (NCBA, 2010). In practice, reducing O157 contamination requires vigilance along the entire supply chain from farm to fork.

Currently, postharvest processes, such as low water activity, chilled storage, and carcass wash procedures are well established and on average work well. For example, the national ground beef prevalence of O157 is about 0.2 per cent (USDA FSIS, 2009). Yet occasionally the high prevalence of O157 in cattle at the production stage aligns with high O157 carcass presence at the harvest stage, producing high O157 concentration at the consumption stage. The convergence of these outlier events on a single day (an event day) can produce ground beef production lots with an exceptionally high O157 concentration in the final product. Some say a single event day, with its extra testing requirements, quality control interventions, and internal and/or external recalls, can exact a significant economic toll.

Recently, two O157-specific bacterial extract vaccines for use in feedlot cattle have been granted conditional approval by the US Department of Agriculture (USDA). The vaccines do not entirely prevent infections, but preliminary data demonstrated that vaccination reduced the percentage of animals shedding O157 at slaughter (Thomson et al., 2009; Thornton et al., 2009). This phenomenon could potentially decrease the prevalence of contaminated carcasses.

The value of preharvest O157 vaccination hinges on three questions:

• Will vaccination significantly reduce the number of human illnesses and other relevant outcomes resulting from beef contamination with O157?
• Will those reductions offset the cost of vaccination relative to other interventions?
• Will the system (farm-to-fork) provide a sufficient signal to cattle feeders to vaccinate?

The objective of this article is to provide quantitative analysis of the effect vaccination may have on human health and food safety.

Discussion

The authors say their results demonstrate that preharvest vaccination against O157 could have a significant benefit for the beef industry by reducing the likelihood of hot lots or event days, and the probability of multiple illnesses due to contaminated ground beef servings.

In addition, the model results indicate that a large number of ground-beef-associated human O157 illnesses may be prevented by vaccinating domestic feedlot cattle. Thus, both the packer and the consumer could benefit from routine O157 vaccination of cattle.

In their study, the relationship between the number of human illnesses and vaccinated cattle was quite linear (R²=0.99), similar to the model of Withee et al. (2009). By comparison, they also modelled a reduction in concentration on/in carcasses/ faeces as well as prevalence of shedders due to vaccination. Incorporating a reduction in concentration produced an additional impact on epidemiological outcomes.

Hurd and Malladi propose the reason that vaccine is so effective is its impact on outlier events and the tail-ends of the non-normal distributions. Some have speculated about the possibility of 'super-shedder' cattle (Jacob et al., 2010; Gyles, 2007). Most packers agree the occurrence of event days is sporadic.

The figure below shows the tail of the histogram (segment of the histogram showing highly contaminated production lots) of simulation results for the prevalence of O157 in 325-g samples by comparing vaccinated and unvaccinated production lots. The x-axis is the prevalence of positive samples expressed as a fraction, and the y-axis is the relative frequency (proxy for probability from simulation results). The graph shows that the probabilities of a very highly contaminated production lot can be reduced considerably with vaccination. This is because vaccination is an independent mitigation measure from postharvest interventions. Vaccination seems to impact the few rare cases where the production lot could be highly contaminated due to various outlier events.

Tail-end of histogram showing impact of vaccination on number of production lots with high Escherichia coli O157 prevalence (>5%) in 325-g samples.
Scenario B: Vaccine 60% effective, 100% adoption

Given that a substantial portion of ground beef consumed in the United States is imported and to aid in further economic modelling, Hurd and Malladi modelled ground beef imports and the mixing of domestic and imported ground beef explicitly. This approach is conservative (reducing the observed impact of vaccination) since a smaller proportion of the O157 illnesses are attributed to the consumption of domestic ground beef, reducing the estimates of human illnesses which can be prevented by vaccinating domestic feedlot cattle.

Withee et al. (2009) did not consider ground beef imports and attributed a greater number of human illnesses to the consumption of ground beef from domestic cattle slaughter. Unlike Hurd et al. (2010), the authors assumed a constant probability that contaminated servings of ground beef cause illness, regardless of the specific product streams (e.g. food service) or consumption channel (e.g. retail sale). The differences in risks for various product streams and consumption channels were not considered here, as the focus of the current study was on the impact of vaccination on overall human O157 illnesses due to consumption of all categories of ground beef. Given that only approximately three per cent of all raw ground beef is sold directly to the consumer, this is an area for further research (NCBA, 2004).

Significant uncertainty surrounds several input variables associated with slaughter processes, such as the amount of a carcass surface area represented in ground beef trim or the effectiveness of carcass decontamination treatments. Given these uncertainties, the model results for slaughter outcomes are more appropriate for predicting the relative impact of O157 vaccination, rather than for predicting the absolute levels of these outcomes. The results for slaughter outcome measures are representative of average values for a hypothetical large production plant in the United States and are not applicable for any specific slaughter establishment. However, Hurd and Malladi believe that the modelling approach utilized here would be appropriate for evaluating outcomes for specific slaughter plants, provided that the input parameters are calibrated according to the establishment characteristics.

There is significant uncertainty about the inputs in this model representing the impact of O157 vaccination. Preliminary data indicated that vaccination reduces the O157 prevalence and concentration in faeces (p<0.05 based on sampling at specific times post vaccination as described elsewhere [Thomson et al., 2009; Thornton et al., 2009]).

Hurd and Malladi modelled these parameters as inputs that users can vary to produce different scenarios to consider the impact of the associated uncertainty. Depending on the nature of the biological processes associated with the functioning of the vaccine, it is possible that the reduction in prevalence is correlated with other variables, such as the reduction in fecal concentration. For example, a large reduction in O157 faeces concentration due to vaccination may also be associated with a correspondingly large reduction in O157-measured fecal prevalence.

Further studies are required to examine the potential correlation between the reductions in concentration and reductions in prevalence under different vaccine dosage regimens and field conditions, according to Hurd and Malladi. Such studies will also provide the data required to evaluate the impact of vaccination stochastically, that is with correlated probability distributions for reduction in faeces concentrations and reduction in prevalence.

This stochastic model based on production, slaughter, and consumption factors demonstrated that preharvest O157 vaccination could reduce human illnesses and decrease contaminated ground beef lots, report Hurd and Malladi. The analysis shows that vaccine- associated reduction in the number of shedding animals and the reduced concentration of O157 in faeces both combine to reduce human illnesses. Thus, the benefits of preharvest O157 vaccination of cattle extend to packers as well as consumers.

Further Reading You can view the full report by clicking here.

December 2012