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Chicken as Reservoir for Extraintestinal Pathogenic E. coli in Humans

13 August 2012, at 12:00am

Research into Extraintestinal pathogenic Escherichia coli (ExPEC) has shown that its potential transmission from food animal sources is likely to be implicated in human infections and that chicken is a major reservoir.

The possibility that ExPEC causing urinary tract infections (UTIs) and other extraintestinal infections in humans could originate from a food animal reservoir raises public health concern, the research shows.

And new interventions may be needed to reduce the level of food contamination and risk for transmission.

Extraintestinal pathogenic Escherichia coli (ExPEC) is the leading cause of community-acquired urinary tract infections (UTIs) in humans, accounting for more than 85 per cent of UTIs.

Each year, 6-8 million UTIs are diagnosed in the United States and 130-175 million are diagnosed worldwide.

Estimated direct health care costs related to uncomplicated UTIs in the United States are $1-$2 billion per year. UTIs also can lead to more severe illnesses, such as pyelonephritis, bacteremia and sepsis.

According to the research team at McGill University, Montréal, University of Guelph, Ontario, the Public Health Agency of Canada and the University of Manitoba, Winnipeg, during the past decade, the emergence of drug-resistant E. coli has dramatically increased.

As a consequence, the management of UTIs, which was previously straightforward, has become more complicated; the risks for treatment failure are higher, and the cost of UTI treatment is increasing, the research team of Catherine Racicot Bergeron, Catharine Prussing, Patrick Boerlin, Danielle Daignault, Lucie Dutil, Richard J. Reid-Smith, George G. Zhanel, and Amee R. Manges wrote.

In the past, extraintestinal E. coli infections have been described as sporadic infections caused by bacteria that originate from the host's intestinal tract.

However, ExPEC strains recently have been associated with a number of possible outbreaks that suggest that ExPEC can be spread to the intestinal tracts of persons in the community by a common source or vehicle.

In an earlier study, the researchers demonstrated the genetic similarities between E. coli isolates recovered from retail meat, particularly chicken, and ExPEC in humans causing community-acquired UTIs.

However, they said that in that study they oversampled isolates from retail chicken because evidence suggested that chicken was likely to be the primary reservoir of ExPEC in humans.

To exclude the possibility that isolates from other retail meat sources (beef and pork) might also be genetically related to UTI isolates from humans, the research team first aimed to characterise additional E. coli isolates recovered from retail beef and pork sources.

These new isolates from retail meat were added to the preexisting collection of retail meat isolates and compared with the same UTI isolates from humans.

The research then aimed to determine whether transmission was primarily human to human through food or whether an animal source was involved.

In the case of human-to-human transmission through food, E. coli strains from humans would be introduced during the meat preparation process by food handlers. In the case of an animal source, the E. coli would derive from the cecal content of the animal itself, and contamination would occur during the slaughtering process.

On the basis of previous findings the team believed that a food animal reservoir exits for ExPEC that cause UTIs in humans and that chicken is the primary source.

The team analysed isolates from animals entering the food chain and E. coli isolates recovered from the cecal contents of slaughtered food animals (beef cattle, chickens, and pigs) were compared with the preexisting geographically and temporally matched collection of isolates from humans with UTIs.

In all 15 clonal groups, comprising 63 isolates were identified.

The 15 groups contained 22 isolates from humans with UTIs and 41 isolates from retail meat.

Of the 41 isolates from retail meat, six (15%) were from retail beef, 29 (71%) from retail chicken, and six (15%) from retail pork.

Considering the sampling proportions (66% beef and pork [484/737] and 34% chicken [253/737]), the fraction of isolates from beef and pork related to isolates from humans with UTIs was significantly lower than expected on the basis of the sampling fraction (29% observed vs. 66% expected; p<0.001, x2 test).

Isolates from chicken were represented in greater numbers among clonal groups.

All clonal group members had the same phylotype.

According to MLST, two of these clonal groups (groups 1 and 3) contained isolates from newly sampled retail beef and pork (EC01DT07-0827-01 and EC01DT06-1559-01) and isolates from humans with UTIs that shared the same STs.

These isolates were further typed: XbaI PFGE patterns differed by >7 bands within the clonal groups associated with retail meat.

The team also identified eight clonal groups containing 46 isolates from humans and from food animals at abattoirs.

These clonal groups comprised 17 isolates from humans with UTIs and 29 from abattoir animals (one [3%] beef cattle, 23 [79%] chickens, five [17%] pigs).

The proportion of chicken was higher than expected with 79% observed against 60% expected, in accordance with the 60% sampling fraction (p = 0.034, x2 test).

The three clonal groups including isolates that were further characterised, and thus more closely related. According to the Tenover criteria, PFGE patterns of all animal strains differed from those of the human strains.

Abattoir clonal group 1 contained 11 isolates (two from humans, eight from chickens, one from a pig). All belonged to the same phylogenetic group (D) and showed the same sequence type (ST117). Two isolates from chickens (EC01AB07-0105-01 and EC01AB07-0425-01) had the same serotype (O180:H4), and the rest of the isolates had unique serotypes.

Abattoir clonal group 2 included six isolates (three from humans, one from a chicken, two from pigs). They all belonged to phylogenetic group A but showed four different MLST profiles. Among them, ST746 was shared by three isolates (one each from a human, chicken, and pig).

However, they did not show the same O:H serotype.

Abattoir clonal group 3 contained 13 isolates (five from humans, six from chickens, two from pigs), which all belonged to phylogenetic group A. The MLST profile ST10 was shared by 7 isolates (four from humans, three from chickens). The six other isolates displayed different MLST profiles. Among the isolates exhibiting ST10, three from chickens (EC01AB05-0765-01, EC01AB07-0005-01, and EC01AB07-1330-01) showed the same serotype (O16:H48).

The study showed in all 12 (29%) of isolates belonging to clonal groups were from beef and pork and 29 (71%) were isolated from chicken (p<0.001).

The research team said that retail beef and pork isolates are much less likely than retail chicken isolates to be clonally related to isolates from humans with UTIs.

Initial screening methods (MLVA and ERIC2) also demonstrated that human samples and cecal samples from food animals in abattoirs can belong to the same clonal groups.

Moreover, within certain abattoir clonal groups, isolates showed the same phylogenetic group and MLST sequence types, indicating that they may have originated from a recent common ancestor.

The three major clonal groups with the highest level of similarity (groups 1, 2, and 3) included isolates from abattoir and retail meat, which suggests that food animals may serve as a reservoir for ExPEC in humans.

The two most common STs (ST10 and ST117), belonging to phylogenetic groups A and D, respectively, have already been reported from human and animal sources.

The results of the study suggest that potential ExPEC transmission from food animal sources is likely to be implicated in human infections and that chicken is a major reservoir.

The possibility that ExPEC causing UTIs and other extraintestinal infections in humans could originate from a food animal reservoir raises public health concern and new interventions may be needed to reduce the level of food contamination and risk for transmission.

Further Reading

- You can view the full report by clicking here.


August 2012