A review has backed the belief that poultry and ruminants are the main sources of Campylobacter infections.

The work found more than half of human campylobacteriosis cases were attributed to poultry. Ruminants such as cattle or sheep were also implicated in a substantial proportion of infections.

Campylobacter is one of the top causes of acute bacterial gastroenteritis in high, low, and middle income countries. The number of confirmed cases has continued to increase across the European Union from 214,000 in 2013 to 246,000 in 2016 and 2017.

Experts at the Statens Serum Insitut in Denmark are dealing with an outbreak of Campylobacter jejuni. Since February, 80 cases have been identified by sequencing. Those ill include 32 women and 48 men aged two to 91 years old. The same type has also been found in individual portions of chicken meat from a chicken slaughterhouse.

Attributing illness to a source
Population genetic analyses are where the genetic diversity of isolates from humans is compared with Campylobacter isolates from possible sources of infection, allowing quantitative attribution to these sources. Multilocus sequence type (MLST) data are used in such analyses.

Researchers reviewed publications from January 2001 using MLST data to attribute human disease isolates to source. They looked at information on samples and isolate datasets used, MLST schemes and attribution algorithms employed. Findings were published in the journal Eurosurveillance.

Of 2,109 studies retrieved worldwide, 25 were included, and poultry, specifically chickens, were identified as the principal source of human infection.

Studies were mainly from Europe and New Zealand and highlight a gap in evidence for low and middle-income countries where Campylobacter may have a large health burden.

Varying approaches
Data sampling and analytical approaches varied, with five different attribution algorithms used. Validation such as self-attribution of isolates from known sources was reported in five publications. None of them reported adjustment for biases identified by validation.

“Common gaps in validation and adjustment highlight opportunities to generate improved estimates in future genomic attribution studies. The consistency of chicken as the main source of human infection, across high income countries, and despite methodological variations, highlights the public health importance of this source,” said the researchers.

Researchers found there was a lack of evolution towards agreed optimal methods which is important to ensure comparability.

Twelve papers only investigated Campylobacter jejuni and three studied only Campylobacter coli. Ten publications considered both. Studies included human clinical isolates from the United Kingdom, New Zealand, Netherlands, Germany, Luxembourg, Switzerland, Austria, Canada, Denmark, France, Italy and Slovenia.

Studies varied in the populations investigated, algorithms used, and approaches to choosing reference datasets for potential sources, but consistently identified the importance of poultry as a source.

“This systematic review brings together compelling evidence for poultry as the major source of human campylobacteriosis, with consistent results across several countries, time periods, and using different analytical algorithms and approaches to assembling isolate data from potential sources,” said researchers.

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