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1.
The backyard chicken (BYC) movement in the USA has increased human contact with poultry and subsequently, human contact with the pathogen Salmonella. However, to date, there have been few studies assessing prevalence of Salmonella in backyard flocks, despite the known public health risk this zoonotic bacterium poses. The objective of this study was to characterize human‐BYC interactions and assess the prevalence of Salmonella among BYC flocks. We interviewed 50 BYC owners using a structured questionnaire to determine flock and household characteristics that facilitate contact with BYC and that may be associated with Salmonella in the BYC environment. Composite faecal material, cloacal swabs and dust samples from 53 flocks housed on 50 residential properties in the Greater Boston, Massachusetts area were tested for Salmonella using standard culture techniques and confirmed using Matrix‐Assisted Laser Desorption/Ionization‐Time of Flight Mass Spectrometer. Microbroth dilution and whole genome sequencing were used to determine phenotypic and genotypic resistance profiles, respectively, and sequence results were used to determine multilocus sequencing type. No owners self‐reported a diagnosis of salmonellosis in the household. Over 75% of a subset of owners reported that they and their children consider BYC pets. This perception is evident in how owners reported interacting with their birds. Salmonella enterica subspecies enterica serotype Kentucky ST152 (serogroup C)—a strain not commonly associated with human infection—was confirmed in one flock, or 2% of tested flocks, and demonstrated resistance to tetracycline and streptomycin. We detected Salmonella at low prevalence in BYC. Further study of the health effects of exposure to zoonotic gastrointestinal pathogens such as Salmonella among families with BYC is warranted.  相似文献   

2.
Owners of small backyard poultry and swine operations may be at higher risk of zoonotic diseases due to husbandry inexperience and/or a lack of knowledge. Backyard poultry and swine owners in southcentral Pennsylvania were surveyed regarding their knowledge and attitudes towards zoonotic disease prevention. One hundred and six backyard poultry and/or swine owners completed the survey (74 poultry, 15 swine, 17 both), which included questions on demographics, flock/herd characteristics, recognition of selected zoonotic diseases and clinical signs in animals, and biosecurity practices for visitors and owners. Most responded that they were aware of avian (92.2%) and swine (84.4%) influenza, and were less aware of other zoonotic diseases such as salmonellosis and brucellosis. The majority of backyard poultry and swine owners combined (62.9%) reported allowing visitors freely around their animals and did not require any special precautions. Backyard poultry and swine owners most commonly reported rarely (32.7%) or never (28.9%) wearing work gloves and never (57.1%) wearing nose/mouth coverings, such as a respirator mask, while handling animals or manure. The study findings indicated that veterinarians (61.5%) and the Internet (50.0%) are the main sources where small‐scale farm producers seek animal disease information. Approximately one‐third (34.9%) of the respondents reported receiving seasonal influenza vaccine. The findings of this study will be utilized to provide targeted veterinary and public health education for the prevention of zoonotic diseases in backyard farm animal settings in Pennsylvania.  相似文献   

3.
Worldwide, foodborne illness is a significant public health issue in both developed and developing countries. Salmonellosis, campylobacteriosis and shigellosis are common foodborne gastrointestinal illnesses caused by the bacteria Salmonella spp., Campylobacter spp. and Shigella spp. respectively. These zoonotic diseases are frequently linked to eggs and poultry products. The aim of this study was to investigate the presence of these pathogens in Australian backyard poultry flocks and to determine risk factors for these pathogens. Poultry faeces samples were collected from 82 backyards and screened for Salmonella spp., Campylobacter spp. and Shigella spp. using qPCR. A questionnaire was administered to the backyard poultry owners to assess their knowledge regarding management of poultry and eggs and to identify potential risk factors that may contribute to the presence of zoonotic pathogens in the flocks. One composite faecal sample was collected from each backyard (82 samples). Composite sampling here means taking one or more grab samples from a backyard to make up approximately 10 grams. Four per cent of samples, that is 4% backyards tested, were positive for Salmonella spp., 10% were positive for Campylobacter spp. and none were positive for Shigella spp. A higher infection rate was seen in multi-aged flocks (24%) compared with the single-aged flocks (3%). The survey found that many participants were engaging in risky food safety behaviours with 46% of participants responding that they washed their eggs with running water or still water instead of wiping the dirt off with a damp cloth to clean the eggs and 19% stored their eggs at room temperature. This study demonstrated that backyard poultry may pose a potential risk for salmonellosis and campylobacteriosis. Additionally, Australian public health and food safety regulations should be modified and effectively implemented to address the risks associated with backyard poultry husbandry.  相似文献   

4.
In law, backyard poultry are “food‐producing animals” and “farmed animals” and are subject to regulations regarding welfare, prescribing, banned procedures, disposal of carcases, feeding bans, notifiable diseases and disease surveillance in addition to those applying to most other pets. Many owners and some veterinary surgeons are unclear about the requirements of these regulations. Backyard poultry are also associated with some different zoonotic disease risks to mammalian pets. Because a high proportion of poultry morbidity and mortality relates to infectious diseases, the health of backyard poultry is amenable to improvement through basic husbandry, biosecurity, hygiene and preventive medicine measures that can be incorporated into a simple “flock‐health plan”. This article reviews these topics.  相似文献   

5.
Salmonella enterica is a common zoonotic pathogen in humans. Transmission typically occurs through consumption of contaminated food products or contact with infected animals, including poultry or their environment. The objective of this study was to estimate the frequency of Salmonella contamination in the environment in poultry exhibits at agricultural fairs. Samples were collected from cages, feed, floors and tables in the exhibit and cultured for Salmonella. At least one environmental sample was positive for Salmonella in 10 of 11 fairs (91%), and Salmonella was isolated from 28 of 55 environmental samples (50.9%). Eleven different serotypes were detected. Results of this study demonstrate that environmental surfaces at agricultural fairs can be contaminated with Salmonella and could potentially serve as a route of transmission to bird owners and the general public. Poultry owners and the general public should be educated about the risks of Salmonella infection from the poultry exhibit environment. Agricultural fairs should consider instituting policies and practices to improve hygiene and mitigate the risk of zoonotic salmonellosis.  相似文献   

6.
Low‐pathogenicity avian influenza (LPAI) viruses have caused illness in poultry and humans with poultry contact. To determine whether there is evidence of exposure to avian influenza viruses (AIV) among backyard poultry in Minnesota and their human caretakers, 150 flocks of backyard birds were sampled for antibodies to AIV from August 2007 through December 2008. One hundred flocks were tested through routine slaughter surveillance by the Minnesota Board of Animal Health and an additional 50 flocks were contacted and sampled by study investigators. Blood was collected from 10 to 13 birds from each flock and a survey of biosecurity and management practices was administered to the flock owner. Blood samples were tested by agar gel immunodiffusion (AGID) for influenza A antibodies. Tested flocks had a median flock size of 100 birds (range: 12–800 birds), and were most commonly owned for meat for personal use (81% of respondents), fun or hobby (58%) and eggs for personal use (56%). Although 7% of flock owners reported that their birds had shown respiratory signs in the previous 3 months, only 1 of 150 flocks tested positive for influenza by AGID. Antibodies to LPAI H6N1 were detected in the positive flock. The owner of the positive flock did not have antibodies to H6 or other common AIV. Based on the findings of this study, the risk of transmission of LPAI viruses from backyard poultry to owners in Minnesota appears to be low under current conditions and management practices.  相似文献   

7.
Increase in the number of small‐scale backyard poultry flocks in the USA has substantially increased human‐to‐live poultry contact, leading to increased public health risks of the transmission of multi‐drug resistant (MDR) zoonotic and food‐borne bacteria. The objective of this study was to detect the occurrence of Salmonella and MDR Gram‐negative bacteria (GNB) in the backyard poultry flock environment. A total of 34 backyard poultry flocks in Washington State (WA) were sampled. From each flock, one composite coop sample and three drag swabs from nest floor, waterer‐feeder, and a random site with visible faecal smearing, respectively, were collected. The samples were processed for isolation of Salmonella and other fermenting and non‐fermenting GNB under ceftiofur selection. Each isolate was identified to species level using MALDI‐TOFF and tested for resistance against 16 antibiotics belonging to eight antibiotic classes. Salmonella serovar 1,4,[5],12:i:‐ was isolated from one (3%) out of 34 flocks. Additionally, a total of 133 ceftiofur resistant (CefR) GNB including Escherichia coli (53), Acinetobacter spp. (45), Pseudomonas spp. (22), Achromobacter spp. (8), Bordetella trematum (1), Hafnia alvei (1), Ochrobactrum intermedium (1), Raoultella ornithinolytica (1), and Stenotrophomonas maltophilia (1) were isolated. Of these, 110 (82%) isolates displayed MDR. Each flock was found positive for the presence of one or more CefR GNB. Several MDR E. coli (n = 15) were identified as extended‐spectrum β‐lactamase (ESBL) positive. Carbapenem resistance was detected in non‐fermenting GNB including Acinetobacter spp. (n = 20), Pseudomonas spp. (n = 11) and Stenotrophomonas maltophila (n = 1). ESBL positive E. coli and carbapenem resistant non‐fermenting GNB are widespread in the backyard poultry flock environment in WA State. These GNB are known to cause opportunistic infections, especially in immunocompromised hosts. Better understanding of the ecology and epidemiology of these GNB in the backyard poultry flock settings is needed to identify potential risks of transmission to people in proximity.  相似文献   

8.
Backyard gallinaceous bird flocks may play an important role in the spread of infectious diseases within poultry populations as well as the transmission of zoonotic diseases to humans. An epidemiologic characterization was conducted of Colorado backyard flocks to gather information on general flock characteristics, human movement of birds, human-bird interaction, biosecurity practices, and flock health. Our results suggest that backyard poultry flocks in Colorado are small-sized flocks (68.6% of flocks had < 50 birds); consist primarily of layer chickens (85.49% of flocks), show chickens (32.18% of flocks), and waterfowl (34.07% of flocks); and are primarily owned for food (meat or egg) production for the family (86.44%) or as pet or hobby birds (42.27%). The backyard flock environment may promote bird-to-bird transmission as well as bird-to-human transmission of infectious disease. Birds are primarily housed with free access to the outside (96.85%), and many are moved from the home premises (46.06% within 1 yr). Human contact with backyard flocks is high, biosecurity practices are minimal, and bird health is negatively impacted by increased movement events. Increased knowledge of backyard bird characteristics and associated management practices can provide guidelines for the development of measures to decrease disease transmission between bird populations, decrease disease transmission from birds to humans, and increase the overall health of backyard birds.  相似文献   

9.
10.
This study was conducted to estimate the apparent prevalence of Salmonella spp. in birds kept under backyard system in Entre Ríos, Argentina, and determine the performance of two selective plating media used for Salmonella isolation, and the antimicrobial resistance of the isolated. Also, the association of farms characteristics with Salmonella presence was evaluated. A total of 657 backyard chickens and 15 gooses were sampled one time by cloacal swab, belonging to 51 and one family farms, respectively, and four counties in Entre Rios state from April 2014 to May 2015. Only four samples from backyard chickens belonged to three family farms from Uruguay County were positive to Salmonella spp., so the apparent prevalence was 0.6% for this kind of chicken. Four serovars were isolated (Salmonella ser. Lille, S. ser. Newport, S. ser. Enteritidis and S. ser. Rissen), which were susceptible to all antibiotics tested with the exception of erythromycin. For Hektoen enteric agar and brilliant green agar, relative specificity and positive predictive value were 1, and the relative sensitivity and negative predictive value did not show any difference between them. The agreement was very good between these two plating media. None of the variables studied could be selected to calculate the risk factors associated with Salmonella isolation because p > .15. Although the prevalence of Salmonella spp. is low in backyard birds in Entre Rios, the presence of S. ser. Enteritidis should not be discounted, because it is found in the county that concentrates a large population of intensive poultry production in the state.  相似文献   

11.
Abstract

AIMS: To determine the proportion of residential land parcels with backyard poultry in an urban and urban-rural fringe area of provincial New Zealand. To document key husbandry and biosecurity practices of owners of backyard poultry, and to identify factors that might assist animal health authorities in locating backyard poultry flocks in the event of an infectious disease emergency.

METHODS: A cross-sectional survey was undertaken, in which residents of 449 land parcels in an urban and urban-rural fringe area within and adjacent to the city of Palmerston North, respectively, were visited between February and November 2006. Residents were asked if backyard poultry were kept on the premises. Details recorded for those that kept poultry included the type and number of birds kept, and details of management and biosecurity practices. The geographical distribution of poultry-positive land parcels was assessed for evidence of spatial clustering.

RESULTS: Backyard poultry were kept on 3.5 (95% CI=2.15.0)% of land parcels; 1.6 (95% CI=0.7-3.4)% in the urban area and 18.9 (95% CI= 11.6–29.3)% in the urban-rural fringe area. There were no significant clusters of poultry-positive land parcels in either area. On all poultry-positive parcels birds were allowed, for at least a portion of the day, to range freely over the property. Three poultry-positive land parcels were within a distance of 1 km of a commercial poultry enterprise in the urban-rural fringe area. Most owners of backyard poultry used feed prepared commercially.

CONCLUSIONS: The prevalence of ownership of backyard poultry in this area of New Zealand was low, and varied according to classification of the land, viz urban, or urban-rural fringe. The close proximity of backyard flocks to the single commercial enterprise in the urban-rural fringe area reiterates the importance of strict biosecurity measures on commercial farms. In the event of an infectious disease emergency, it is proposed that a sampling frame of owners of backyard poultry might be rapidly obtained by contacting suppliers of commercial feed.  相似文献   

12.

Background

Although modern commercial poultry production today is based on large farms and intensive husbandry, keeping backyard poultry has regained popularity in industrialized countries. However, the health status of backyard flocks is still relatively poorly documented. A questionnaire was sent to the owners of 376 backyard poultry flocks (<500 birds) in order to study health management procedures and characterize backyard poultry populations in Finland. Information was also collected on the postmortem findings from non-commercial flocks using necropsy data from the Finnish Food Safety Authority (Evira).

Results

Backyard flocks in Finland are small in size (<50 birds), comprising mainly chickens. Based on the results of the questionnaire, the health of such flocks is good, mortality low and vaccinations are not commonly used. Most of the flocks were registered in the national poultry register. The standard biosecurity practices are not generally applied and contact with wild birds, pets and farm animals is frequent, which can make the flocks more prone to infectious diseases. We conducted an 11-year retrospective study of the postmortem necropsy findings of the Evira in order to document the diseases, which caused mortality in backyard chickens in Finland. Necropsy was performed on a total of 132 non-commercial laying hens during 2000 – 2011. The most common postmortem findings were Marek’s disease (27%) and colibacillosis (17%).

Conclusions

This study is the first to report data on characteristics of and management practices for backyard chicken flocks in Finland. Close connections with commercial flocks are rare and farms are usually distantly located suggesting that the risk that these backyard flocks pose to commercial poultry is low.  相似文献   

13.
According to the European Food Safety Authority, salmonellosis is still one of the main causes of infectious foodborne gastroenteritis in humans. Broilers are an important source of salmonellosis after eggs and pork. Between 1987 and 1999 the trend of human salmonellosis incidence in Belgium increased constantly. However, from 2000 until 2005 a decrease in human cases was observed, probably following the sanitary measures implemented in the poultry breeder and laying sector. In order to decrease human infections it is essential to tackle the problem at the farm level to minimize cross-contamination from farm to fork. This paper seeks to answer two questions: (i) given the Salmonella status of the farm at a certain occasion (equal to the sampling time of the flock), what are the risk factors that the farm will be Salmonella positive at a following occasion? And (ii) what are the risk factors for a farm to be persistently positive for two consecutive flocks? We used surveillance data on 6824 broiler flocks studied for Salmonella infectivity from 2005 to 2006 in Belgium. The farms were tested regularly (3 weeks before slaughter of each broiler flock) for the presence of Salmonella based on multiple faecal samples per flock on a farm yielding clustered data. Generalized estimating equations, alternating logistic regression models, and random-intercept logistic regression models were employed to analyse these correlated binary data. Our results indicated that there are many factors that influence Salmonella risk in broiler flocks, and that they interact. Accounting for interactions between risk factors leads to an improved determination of those risk factors that increase infection with Salmonella. For the conditional analysis, the risk factors found to increase the risk of Salmonella infection on a farm at a current occasion given the previous Salmonella status included: Salmonella infection of day-old chicks (of the current flock); a previously infected flock even though the farm was equipped with a hygiene place to change clothes prior to entering the broiler house; having temporary workmen when there was a separation between birds of different species; and separating birds of different species in the Walloon region relative to the Flanders region. Sanitary measures such as a cleaning and disinfecting procedure conducted by an external cleaning firm, applying the all-in all-out procedure, and hand washing decreased the risk despite their interaction with other factors. From the joint analysis, the most important factors identified for increased risk for persistent Salmonella on a farm involved the interaction between having temporary workmen when there were poultry or farmers in contact with foreign poultry or persons, and the interaction between having temporary workmen when there were poultry or farmers in contact with external poultry or persons.  相似文献   

14.
Salmonella is, after Campylobacter, the most reported zoonotic pathogen in the EU. Poultry are a common source of infection to humans, and turkey flocks are commonly colonized with the organism. We investigated the prevalence and risk factors of Salmonella infection in 179 houses in 60 holdings representative of turkey meat and breeder production in Great Britain. From each holding, up to four houses were chosen, and two consecutive flocks per house were sampled/tested for Salmonella to investigate the persistence, elimination and introduction of Salmonella in consecutive crops. At the first sampling, the overall flock‐level Salmonella prevalence was 32.8% and 8.9% for meat and breeding flocks respectively. There was a higher prevalence of Salmonella in flocks in the rearing stage than in the fattening and breeding stages. At the first sampling, the flock‐level prevalence of Salmonella was 26.8% (95% CI: 20.7–33.7%), while the prevalence level in the subsequent flock was 20.5% (95% CI: 13.6–29.7%). No houses were positive for any of the EU‐regulated serovars. The most commonly encountered serovars were S. Kottbus and S. Kedougou. Carry‐over of infection was observed in 44.8% of the positive houses, and introduction of new infection occurred in 8.4% of houses. Data from the questionnaires and auditing of all holdings and houses were combined and used to calculate adjusted farm‐ and house‐adjusted risk factors. Significant risk factors were feed from a source other than a national compounder (OR = 2.4), feeder type other than pan feeders (OR = 2.4) and hygiene practices other than terminal cleaning and disinfection using power‐washing with sanitizer and anteroom with boot change (OR = 2.8). The study discusses the main challenges currently faced by the industry to control Salmonella in turkey production.  相似文献   

15.
Backyard poultry has become increasingly popular in industrialized countries. In addition to keeping chickens for eggs and meat, owners often treat the birds as pets. However, several pathogenic enteric bacteria have the potential for zoonotic transmission from poultry to humans but very little is known about the occurrence of zoonotic pathogens in backyard flocks. The occurrence and the antimicrobial resistance of Salmonella enterica, Campylobacter spp., Listeria monocytogenes and enteropathogenic Yersinia spp. was studied in 51 voluntary backyard chicken farms in Finland during October 2012 and January 2013. Campylobacter isolates were further characterized by pulsed‐field gel electrophoresis (PFGE), and the occurrence of ESBL/AmpC‐producing E. coli was investigated. The findings from this study indicate that backyard chickens are a reservoir of Campylobacter jejuni strains and a potential source of C. jejuni infection for humans. Backyard chickens can also carry L. monocytogenes, although their role as a primary reservoir is questionable. Campylobacter coli, Yersinia pseudotuberculosis and Salmonella enterica were only found sporadically in the faecal and environmental samples of backyard poultry in Finland. No Yersinia enterocolitica carrying the virulence plasmid was isolated. All pathogens were highly susceptible to most of the antimicrobials studied. Only a few AmpC‐ and no ESBL‐producing E. coli were found.  相似文献   

16.
1. The aim of the study was to collect baseline data on welfare, biosecurity and diseases of backyard chickens kept in the Greater London Urban Area (GLUA), United Kingdom (UK).

2. A total of 65 backyard chicken flock-keepers were recruited from May to July 2010 through adverts on websites, at City farms, veterinary practices and pet feed stores and surveyed by means of a questionnaire. A total of 30 responses were suitable for analysis.

3. Information on keepers’ and flocks’ characteristics, housing and husbandry practices and owners’ knowledge of health problems in chickens and zoonotic diseases was collected. A welfare assessment protocol was developed and the flocks assessed accordingly.

4. Results showed that chickens were generally provided with living conditions that allowed them to perform their natural behaviours.

5. Most of the flock owners did not comply with the regulations of the Department for Environment, Food and Rural Affairs (Defra) on the feeding of catering waste.

6. Disease prevention measures such as vaccination and biosecurity, including limiting the access of human visitors, wild birds and rodents to the flocks were rare.

7. A lack of avian and zoonotic disease knowledge and awareness among the owners has implications for disease control and highlights the need for improved communication between owners, authorities and veterinarians.  相似文献   


17.
Keeping backyard poultry in urban areas is a burgeoning trend in the United States. As such, we believe urban pet poultry owners are increasingly likely to seek veterinary services from urban companion-animal practitioners. Traditionally, poultry species have been classified as production animals. Most small-animal practitioners have limited experience or knowledge of these species and hesitate to accept these animals at their practices. We developed a one-day course to train veterinarians in pet poultry (as opposed to commercial poultry) medicine. The course covers poultry examination, diseases, and treatments and provides an introduction to poultry breeds and behavior and the basics of nutrition and husbandry. We believe this type of continuing education program is important for veterinarians because they are often on the front line of human public health issues. In addition, courses of this type increase the number of veterinarians trained to spot serious avian diseases, including foreign diseases and diseases with zoonotic potential. Most important, veterinarians with this training develop the knowledge to contribute to the health and well-being of pet poultry along with their clients' other companion animals.  相似文献   

18.
A survey was conducted to characterize domestic and exotic bird populations, estimate seroprevalence to selected disease agents, and describe health management practices on 62 premises containing "backyard" flocks located within one mile of 22 commercial California meat-turkey flocks participating in National Animal Health Monitoring System (NAHMS). Chickens were present on 56 backyard premises and turkeys on seven. Antibodies were identified against Mycoplasma gallisepticum, M. synoviae, M. meleagridis, Salmonella pullorum, Newcastle disease virus, avian encephalomyelitis virus, Bordetella avium, hemorrhagic enteritis virus, infectious bronchitis virus, and infectious bursal disease virus in 367 blood samples from 32 backyard premises. Twenty-two owners of backyard premises said they restricted visitor contact with their birds, and two required visitors to wear rubber boots and use boot disinfectant. Owners of seven premises used biologics and/or pharmaceutics for disease prevention. One family member worked on a commercial turkey ranch, but no other contact between owners, relatives, or employees and commercial poultry was reported.  相似文献   

19.
Animal antimicrobial use and husbandry practices increase risk of emerging zoonotic disease and antibiotic resistance. We surveyed 700 households to elicit information on human and animal medicine use and husbandry practices. Households that owned livestock (n = 265/459, 57.7%) reported using animal treatments 630 times during the previous 6 months; 57.6% obtained medicines, including antibiotics, from drug sellers. Government animal healthcare providers were rarely visited (9.7%), and respondents more often sought animal health care from pharmacies and village doctors (70.6% and 11.9%, respectively), citing the latter two as less costly and more successful based on past performance. Animal husbandry practices that could promote the transmission of microbes from animals to humans included the following: the proximity of chickens to humans (50.1% of households reported that the chickens slept in the bedroom); the shared use of natural bodies of water for human and animal bathing (78.3%); the use of livestock waste as fertilizer (60.9%); and gender roles that dictate that females are the primary caretakers of poultry and children (62.8%). In the absence of an effective animal healthcare system, villagers must depend on informal healthcare providers for treatment of their animals. Suboptimal use of antimicrobials coupled with unhygienic animal husbandry practices is an important risk factor for emerging zoonotic disease and resistant pathogens.  相似文献   

20.
This systematic review considers the relationship between arthropods commonly found in and around livestock premises and zoonotic bacteria. The principal focus is upon insects and arachnids on poultry units, where houses, litter and manure provide good conditions for the growth, multiplication and protection of flies, beetles and mites, and where zoonotic pathogens such as Salmonella and Campylobacter are prevalent. Other members of the Enterobacteriaceae and the taxa Clostridium, Helicobacter, Erysipelas and Chlamydiaceae are also discussed. Salmonella is widely distributed in the flies of affected livestock units and is detectable to a lesser degree in beetles and mites. Persistent carriage appears to be common and there is some field and experimental evidence to support arthropod‐mediated transmission between poultry flocks, particularly carry‐over from one flock to the next. Campylobacter may readily be isolated from arthropods in contact with affected poultry flocks, although carriage is short‐lived. There appears to be a role for flies, at least, in the breaching of biosecurity around Campylobacter‐negative flocks. The carriage of other zoonotic bacteria by arthropods has been documented, but the duration and significance of such associations remain uncertain in the context of livestock production.  相似文献   

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