|
Fig .1
Clinical features of Campylobacter spp. gastro-enteritis
The incubation period is between 2- 10 days. The infective dose of Campylobacter spp. is unknown, but is believed to be low. One experiment demonstrated that 500 organisms in 500 ml of milk was sufficient to cause acute symptoms in a well-nourished adult male. In children the infective dose may be lower and in a large outbreak in Luton in 1979, 3500 children were unwell. The vehicle in this instance was shown to be improperly pasteurised milk.
The symptoms of Campylobacter infection are predominantly diarrhoea with fever and abdominal pain. Bloody stools are a common feature of the infection in children. There does not seem to be a carrier state and most people infected with the organism become stool negative within 4-6 weeks. In the UK it is not usual to treat people suffering from this condition unless it is thought that the infection has become extra-intestinal and in those cases erythromycin is usually given. Although C.jejuni and C.coli cause a large number of cases, deaths are very rare, but infection is being increasingly recognised as an important predisposing factor for reactive arthritis and Guillam Barre Syndrome.
|
Epidemiology
There is a marked early summer peak of infection with
Campylobacter spp. This is in contrast to Salmonellosis where
most cases are seen in the late summer.
As with other enteric infections, most cases of
infection with Campylobacter spp. are sporadic. Although it is
difficult to positively identify vehicles in cases of this sort, it is
believed that contaminated chicken meat and water are important. In
children, contact with puppies and kittens with diarrhoea have been
shown to be an important additional risk factor. The consumption of milk
from bottles where the tops have been pecked by either magpies or
jackdaws has also been shown to be important in both sporadic cases and
outbreaks in the UK. In some parts of the UK bird-pecked milk has been
thought to be an important contributory factor in the early summer peak
of infection. Outbreak investigation has also revealed, where it has
been possible to discover the vehicle, that raw milk is by far the most
common vehicle of infection, although both chickens and water have been
implicated. In Scotland, where it has been illegal to sell unpasteurised
milk since August 1983, milk-borne outbreaks of Campylobacter
infection have disappeared. |
|
Table 1 |
|
Classification of Campylobacter |
|
(current
information)* |
|
Campylobacter fetus subsp. fetus |
|
Campylobacter fetus subsp.
venerealis |
|
C. hyointestinalis |
|
C. mucosalis |
|
C. concisus |
|
C. sputorum |
|
C. jejuni subsp. jejuni |
|
C. jejuni subsp. doylei |
|
C. coli |
|
C. lari |
|
C. upsaliensis |
|
C. curvus |
|
C. rectus |
|
C. hominis |
*Data from PHLS Campylobacter
Reference Unit,Colindale |
Carriage in food animals and contamination of foodstuffs
Campylobacter jejuni would seem to be part of the normal gut flora of dairy cows and will, therefore, get into milk as a result of faecal contamination. Some outbreaks of infection have also resulted from mastitis caused by Campylobacter spp. The organisms, however, are heat-sensitive and will be killed by proper pasteurisation. All milk-borne outbreaks have been associated with raw or improperly pasteurised milk. Campylobacter spp. can be isolated with high frequency from the natural environment and from a variety of wild birds and animals. It is also found commonly in commercial poultry. Campylobacter jejuni and/or C.coli are present in about 75% of UK broiler flocks. It is not surprising, therefore, that a high percentage of broiler chicken carcases are positive for these organisms. As with dairy cows, Campylobacter spp. must be regarded as being normal gut flora in chickens. Given the importance of contaminated chicken meat as a vehicle for Campylobacter infection a great deal of research has been undertaken on the epidemiology of Campylobacter spp. in chickens. The results of studies carried out in Exeter mirror those from the Veterinary Laboratories Agency in the UK and from Scandinavia and The Netherlands and demonstrate that an important source for the bacteria in broiler chicken flocks is the external environment around the broiler house. Farm workers and visitors will bring the organisms into the flock either on their boots or clothing. Flock colonisation with Campylobacter spp. can be prevented or delayed by stopping the ingress into the house of the bacteria from the external environment. This is maybe best achieved by boot changing or dipping in strong disinfectant and/or the construction of hygiene barriers. Studies have also demonstrated that despite identical feed and management regimens some flocks of broiler chickens are Campylobacter-negative. This may be associated with the presence of antagonistic bacteria in chicken caeca, particularly in young chickens. There is a strong association between demonstrable in vitro inhibition of Campylobacter spp. by bacteria in caecal contents and the Campylobacter status of growing broiler flocks.
The success of C.jejuni as a human pathogen and the strong association between human infection and the consumption of undercooked chicken either from barbecues or take-away outlets may be associated with the fact that the bacterium can be isolated from the muscle of chicken carcases. Three independent studies, in Exeter (UK), Sweden and France, have established this. It is not known how the bacterium comes to be in the muscle and this may be associated either with invasive Campylobacter spp. or contamination from scald tank water during poultry processing.
Growth and survival in foods
Campylobacter spp. are curved/spiral gram negative rods which are highly motile by polar flagella. The species of Campylobacter which are responsible for food poisoning are classed as "thermophilic" as they grow at 42° but not at 25° C. Campylobacter spp. are essentially micro-aerophilic although colonies subcultured onto a variety of agars have been shown to be capable of growth in air if the environment is moist. It is not believed, however, that the bacterium is capable of growth in food stored under normal conditions. This is probably associated with the fact that C.jejuni and C.coli have a growth range of 30-45° C. They are also extremely sensitive to drying, although they are capable of prolonged survival in refrigerated and frozen foodstuffs. There is little information available on the levels of Campylobacter spp. found on foods. Contamination levels on chicken carcases can exceed 10 million cells per carcase. In contrast, levels in raw milk are usually low, often less than 1 cell/ml. High fat content of this product, however, may serve to protect Campylobacter spp. from gastric acidity and permit low numbers of cells to cause infection.
Isolation of Campylobacter spp. From food and environmental samples: the influence of sub-lethal injury
As with the isolation of all other bacteria from mixed populations, the detection of Campylobacter spp. in food and environmental samples requires that the best balance is achieved between suppressing competing flora, which will interfere with the growth/recovery of Campylobacter spp., and encouraging the growth of the target pathogen. This can create difficulties in the examination of non-clinical samples. In common with most other micro-organisms, Campylobacter spp. can be sub-lethally injured by exposure to mild heating, freezing, chilling, or other treatments that are associated with food processing and storage. Sub-lethal injury in Campylobacter spp. has a variety of manifestations, including increased sensitivity to certain antibiotics, particularly rifampicin. This selective agent is particularly important in the examination of samples which can have a large population of other bacteria such as poultry meat and raw milk. Sub-lethally damaged Campylobacter spp. will also show increased sensitivity to hydrogen peroxide and photochemically induced oxygen radicals. Damaged organisms of some strains of Campylobacter spp. also grow poorly at higher incubation temperatures, i.e. 42° C, a temperature commonly used with clinical specimens.
The various manifestations of sub-lethal injury combine to create problems in the isolation of Campylobacter spp. from non-clinical samples. Thus, if food or water samples are inoculated directly into selective/enrichment broth and then incubated at 42° C, many of the Campylobacter spp. present will die and this could lead to false negative results. Given the low infective dose of Campylobacter spp. it is important to use techniques which will maximise isolation rates. Research in Exeter over the last few years has defined protocols, which can be used with a variety of routine samples. The PHLS uses "Exeter" selective medium for the examination of routine food and environmental samples. This medium is a modification of the "Preston" medium and comprises 5 selective agents. Samples are inoculated into the selective broth which is then usually incubated at 37° C for up to 48 hours because incubation at this temperature facilitates the recovery of Campylobacter spp. with relatively mild sub-lethal injury. Provided the broth contains agents which will quench oxygen and its toxic products, and there is small head space above the liquid phase, it is not necessary to incubate broths in a micro-aerobic environment. Following incubation, broths can be streaked onto selective agars such as CCDA and incubated at 37 or 42° C in a micro-aerobic atmosphere for 48 hours. Colonies of Campylobacter spp. are grey and spreading. The usual confirmation tests are a gram stain, where the typical "seagull" morphology should be present, and an oxidase test.
Where the presence of highly damaged cells of Campylobacter spp. is suspected, in water samples, which have been stored, for example, a different approach may be necessary. Current work in Exeter is exploring isolation methodologies that permit the recovery of low numbers of Campylobacter spp. showing extreme sub-lethal injury. Research to date indicates that commercially available base broths differ markedly in their protective effects and that a delay in the addition of some selective agents may increase isolation rates. A delay in the addition of all selective agents, however, may permit overgrowth of competing bacteria with samples such as chicken.
Conclusions
Campylobacter spp. continue to be highly important human pathogens. The most effective forms of control at present are the pasteurisation of milk and the proper handling and cooking of 'high-risk' foods such as poultry meat. Given the difficulties of handling contaminated products in domestic kitchens a longer term aim must be to work towards producing Campylobacter-free poultry.
|
