Zootecnica International - World Poultry Journal

Diane G. Newell

Project Director Med-Vet-Net
Veterinary Laboratories Agency,
New Haw, Addlestone, Surrey,
United Kingdom

Campylobacter jejuni and C. coli are common bacterial commensals of the avian gut but are major food-borne causes of human acute enteritis. Reduction of the risk to human health from Campylobacter contaminated poultry is a priority in many countries. Unfortunately, the biosecurity measures currently used for broilers have been largely ineffective for Campylobacters. However, molecular epidemiological studies are now beginning to accurately identify the sources of those Campylobacters colonizing flocks. The evidence to date suggests that intervention measures, at least initially, need to be targeted at horizontal transmission from the external broiler house environment.

Introduction

Campylobacter jejuni, and its close relative C. coli, are Gram-negative, highly motile, microaerophilic and thermophilic bacteria, now recognised as major causes of human acute bacterial enteritis world-wide (1). C. jejuni in particular is ubiquitous in the environment and colonizes the gastrointestinal tracts of most mammals and birds. Although there are multiple potential sources of infection for man, epidemiological studies implicate the consumption or handling of raw or undercooked poultry meat as a major route.
C.jejuni/coli commonly colonize the intestinal tracts of most poultry at slaughter including broilers, laying hens, ducks, turkeys, and game birds (2). This colonization is generally asymptomatic. Although hepatitis in chickens, associated with Campylobacters recoverable from the liver, has been reported, the disease is not experimentally reproducible, and, therefore, these bacteria are unlikely to be the cause.
The epidemiology of Campylobacter colonization in broiler flocks, though well investigated, remains unclear (3). For the majority of flocks, colonization is not detectable until the birds are two to three weeks of age. In the initial stages of flock colonization the within-flock prevalence can be low but shedding is rapid and bird-to-bird transmission, even within flocks of penned birds (4), is so efficient that within two to three days up to 100% of birds become colonized. Thus, once the first bird becomes colonized then infection of the whole flock seems unavoidable. This is because Campylobacters are remarkably efficient colonizers of the avian gut. Cecal levels of up to 109 cfu per gram of cecal contents are common. Consequently the potential bacterial pathogen load entering a poultry abattoir with each infected flock can be as high as 1012 cfu. The subsequent faecal contamination of poultry meat products during processing is inevitable, thereby constituting a significant risk to human health as confirmed by quantitative risk analysis (5).
Controlling Campylobacters in the food chain is now a primary objective of many food safety authorities. However, the development and implementation of intervention strategies requires knowledge of the extent of the problem. Structured national surveys are difficult and expensive to undertake in such a complex and fragmented industry. Nevertheless, ongoing surveillance in some European countries indicates that the prevalence of broiler flock colonization at slaughter varies between countries, and with season and management practices. In countries like Denmark, the Netherlands, and Great Britain the overall prevalence is about 50%; but in northern European countries, such as Sweden, this prevalence is down to about 10% (6). Prevalence usually peaks in the summer months and is dependent on the type of production system, for example reaching 100 % in organic and free-range flocks (7). Variation in prevalence between national flocks may be, at least in part, a consequence of different husbandry or environmental factors but may also be a reflection of differences in sampling frames and detection methodologies. Standard detection procedures are urgently needed and in support of the European Zoonoses Directive (2003), a collaborative group is establishing what the minimum requirements for national surveillance should be.
Many potential intervention strategies at the farm level have been proposed (3). In general these either aim to prevent the birds becoming colonized with Campylobacters or to modify the gut environment thereby reducing the extent of colonization. The latter strategy includes measures such as vaccination, probiotic treatments and the breeding of genetic resistance. Such measures generally remain in early research phases. The remainder of this review will focus on measures to identify and control the potential sources of flock colonization.

Sources of broiler flock colonization and associated control measures

The possible sources of Campylobacter colonization in poultry have already been reviewed (3). All available data suggest that the general measures taken to control Salmonellae in poultry have little, if any, effect on the prevalence of Campylobacter colonization in flocks. This is presumably a reflection of the considerable differences in ecology and physiology between these two organisms and suggests that a more targeted approach to the identification and subsequent control of potential Campylobacter sources is required. The development and use of molecular typing tools for Campylobacters has begun to enable such approaches to be adopted (8). The problem of Campylobacter genetic instability, which has generally hampered the use of such techniques to study the epidemiology of human campylobacteriosis (9), appears to be far less important in the acute outbreak events associated with poultry flock colonization, provided that a layered strategy for typing is adopted. However, unfortunately, Campylobacters in and around the broiler house environment tend to be difficult to recover and maintain in vitro, which to date has constrained available data. A combination of random sampling and molecular typing procedures has been developed to attempt to overcome some of these problems. In this approach the flock within, and the surrounding environment of, individual broiler houses are sampled from chick placement until the flock becomes positive. The environmental samples are enriched for Campylobacter recovery and initial growth then stored frozen. The strain from the flock is isolated and the short variable region of the flaA gene sequenced so that an oligonucleotide sequence can be designed unique to the colonizing strain. This is used to generate a labelled probe for incorporation into a lightcycler assay (10). The assay is then used to survey all the environmental samples to determine the presence of potential sources of that specific strain. This novel approach is now allowing the retrospective identification of the potential environmental sources of those strains colonizing the broiler flock.
In such molecular epidemiological investigations, most broiler flocks in Europe are colonized by only a limited number (1-2) of strains suggesting a point source outbreak (6, 11). The role of vertical transmission as a potential source is debatable. Organisms are recoverable from the urogenital tracts of laying hens (12, 13), and even from the semen of cockerels (14), and by PCR Campylobacter DNA is associated with newly hatched chicks (15). Nevertheless, on the basis of the presence of the lag phase, the lack of recoverable organisms from chicks and lack of similarity between strains colonizing parent and broiler flocks, vertical transmission is generally considered a relatively unimportant route of flock colonization (3).
In contrast, horizontal transmission appears to have a major role. These organisms are ubiquitous in rural environments, where they are regularly shed from most domestic and wild animals and birds and can contaminate soil, concrete, equipment, and surface water. In addition, broiler house environments, internal and external, are heavily contaminated when colonized flocks are in residence. Survival, but not growth, of Campylobacters in such environments can occur for months especially in temperate, moist, and dark conditions. However, because these organisms survive poorly in dry conditions, poultry feedstuffs and fresh litter have been largely eliminated as potential sources.
Molecular epidemiological investigations in European broiler houses that are routinely cleared between flocks, suggest that house cleansing and disinfection is generally adequate, even after occupation with a colonized flock, (16) and consequently carry-over from one flock to a subsequent flock in the same house is relatively infrequent (17). Largely by elimination this suggests that Campylobacters from contaminated external environments are the major source. This is generally supported by typing studies demonstrating that strains recovered from environmental samples, like wild birds faeces and puddles, can be recovered from subsequently colonized flocks (18, 19). The major route of transmission of such environmental organisms into a house appears to be via farm staff. Interestingly, flock thinning (the planned partial depopulation of a flock) (20), with associated increases in human traffic, is a significant risk factor for flock positivity.
Other significant risk factors indicated in at least some epidemiological studies include unchlorinated water supplies, the presence of vermin or dung beetles, and other livestock on the same site (3).
Finally, even if a flock remains Campylobacter-negative at slaughter age, the crates in which the birds are collected, and subsequently transported to the abattoir, are demonstrably contaminated with Campylobacters, despite washing (21, 22) and the strains on these crates, prior to loading, have been recovered subsequently on the carcasses of the birds (21).
Many measures have been recommended to reduce the risk of flock colonization (23) including:
•    All-in all out-policy;
•    Remove litter between flocks and disinfect buildings between flocks
•   Maintain buildings in good repair and with intact concrete apron
•   Clean water and/or effective water treatment
•   Dispose of dead birds properly
•   Change and disinfect boots
•   Change outer protective clothing
•   Avoid thinning during production cycle
•   Restrict visitor access
•   Do not keep other domestic animals on the same site

However, to date in experimental intervention studies such measures only delay, rather than prevent, the onset of colonization (24). Thus it seems likely that biosecurity alone will be insufficient to consistently produce negative flocks and that complementary measures, such as vaccination or probiotics, will be required.

Conclusions

Worldwide, much of the poultry meat presented at retail is contaminated with C. jejuni and/or C. coli. This contamination is largely a result of the preferential colonization of the avian tract by these organisms. The role of poultry-associated strains in human infection remains to be established, but there is clearly a need for the control and prevention of Campylobacters in poultry and poultry meat products. In the future this will require the development of innovative and sequential intervention strategies throughout the food production chain. In the meantime, improved knowledge of the biology of the organism and its interaction with its various hosts are essential. The increasing availability of Campylobacter genome sequence data, and subsequent post-genomics studies, will hopefully enable rapid progress towards targeted approaches for intervention.

References

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From Proceedings of “53rd Western Poultry Disease Conference”, Sacramento, California.