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Salmonella causes greatest health burden

By Joe Whitworth+

17-Mar-2014

Salmonella causes the greatest overall burden annually in the US according to a metric to measure health-related quality of life.

Quality-adjusted life years (QALYs) help define which pathogens cause the most disease burden per case of illness.

Salmonella was estimated to cause a total annual QALY loss of 16,782, compared to Campylobacter spp. (13,256) and Toxoplasma gondii (10,964).

Listeria monocytogenes had the highest average QALY loss per case of illness, but there are fewer cases per year, according to research findings.  

Help shape policy

Measuring the disease burden in this way can help food safety prioritization and policy, Michael Batz, a researcher on the study told FoodQualityNews.com.  

“For example, how might one compare Norovirus, which causes millions of generally mild illness each year, to Listeria monocytogenes, which causes orders of magnitude fewer cases of greater individual severity? QALYs allow us to do that,” he said.

“They give us a systematic way to measure all these different diseases using the same yardstick.”

The 14 pathogens studied represent more than 95% of foodborne illnesses, hospitalizations, and deaths due to specified agents in the US.

The researchers present quantitative disease-outcome trees characterizing severity and outcomes.

They report EuroQoL 5D (EQ-5D) domain scores, associated population-based health-related quality of life (HRQL) preference weights, and QALY losses for each health state in each tree.

Disease outcome tree

The disease outcome trees for each pathogen are created to characterize different severities, like what fraction of cases of salmonellosis are hospitalized and which are mild.

“For each disease state in each tree, we characterize the health state from 0 (death) to 1 (perfect health), and we do this using the EQ5D instrument (we score each health state along 5 domains of the EQ5D),” said Batz.

“When all’s said and done, we can then sum across health states (using the aforementioned likelihoods of different severities) to get average QALY loss per case of illness, which we multiply by the number of cases of illness we expect to see in a given year.”

Batz et al used CDC studies, the Nationwide Inpatient Sample (a US database) and peer-reviewed research.

Estimation of per-case QALY loss involved creation of pathogen-specific disease-outcome trees with health state descriptions; characterization of health states using EQ-5D scores; and calculation of QALY losses based on EQ-5D HRQL weights and health-state duration.

Batz said the estimates can be improved as they are built upon a lot of assumptions – meaning they are more confident in saying which are the top five pathogens, than which is first, second and third.

“There is a lot of underreporting in disease surveillance systems, and we could really use an improved sense of what fraction of human illnesses from pathogens like Salmonella and Campylobacter are actually foodborne vs. other exposure pathways,” he said.

“Improved data might change some of our estimates up or down, and could even impact our rankings, but our overall findings are likely to stand.”

Pathogen disease-outcome trees partition illnesses by severity, with branches describing disease progression resulting in recovery, death, or long-term condition.

Our trees have three primary branches: severe (hospitalized), moderate (visit a physician), and mild (do not seek care) cases,” said the researchers.

QALY loss

The researchers estimated more than 5,800 QALYs lost per 1,000 cases of L. monocytogenes and V. vulnificus, compared to 125 QALYs lost per 1,000 cases of T. gondii.

There are 26 QALYs lost for E. coli O157:H7, 16 for Salmonella and Campylobacter, and 14 for Y. enterocolitica.

The remaining seven pathogens are estimated to cause less than five QALYs lost per 1000 cases.

About 65% of total QALY loss is due to premature mortality; around 25% is due to congenital disease and chronic sequelae of acute infection.

“For example, fully eliminating foodborne disease from the seven lowest-ranked pathogens would have about the same public health impact as reducing the rate of salmonellosis by 15%,” said the researchers.

“Preventing a single case of V. vulnificus infection reduces public health burden as much as preventing about 370 cases of campylobacteriosis.”  

The study adds to a growing research using integrated health metrics, such as QALYs, disability adjusted life years (DALYs), or cost of illness, to estimate the annual burden of foodborne illnesses.

The pathogens studied were Campylobacter spp., Clostridium perfringens, Cryptosporidium parvum, Cyclospora cayetanensis, E.coli O157:H7, Shiga toxin–producing E. coli non-O157, Listeria monocytogenes, nontyphoidal Salmonella enterica, Shigella, Toxoplasma gondii, Vibrio vulnificus, Vibrio parahaemolyticus and other noncholera Vibrio, and Yersinia enterocolitica.

Source: Foodborne Pathogens and Disease

Online ahead of print, DOI: 10.1089/fpd.2013.1658

Disease-Outcome Trees, EQ-5D Scores, and Estimated Annual Losses of Quality-Adjusted Life Years (QALYs) for 14 Foodborne Pathogens in the United States”

Authors: Michael Batz, Sandra Hoffmann, and J. Glenn Morris, Jr

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