Contamination of raw milk by Listeria monocytogenes on dairy farms

Possible sources of exogenous contamination of raw milk by Listeria monocytogenes were examined on four dairy farms of different size and type of animal housing during morning milking. Feeds, including hay and concentrates, were found to be major sources of both pathogenic and nonpathogenic species of Listeria on the barns. L. innocua was the only species isolated from the grass silage, which was of good quality on all the farms. The numbers of Listeria were below 10(2)/g in all feed samples. Fecal shedding of listeriae was detected in 11.9% of the cows and the prevalences of L. monocytogenes among farms ranged from no detections to 8.7%. The number of Listeria isolations from constructions inside the barns and from the milking environment varied between the farms. Listeriae were detected almost everywhere on one of the farms whereas on another farm the only isolations were from feed passages and floors. 13.6% of the swab samples taken from the teats before washing and drying were Listeria positive, whereas no isolations were made after cleaning the udder. Good milking and barn hygiene is considered important for diminishing the risks of exogenous contamination of raw milk by listeriae.     

Listeriosis in a Holstein cow.

Bovine listeriosis is a sporadie bacterial infection most commonly manifested by encephalitis or meningoencephalitis in adult animals. Cattle fed poorly fermented haylage or corn silage are at increased risk. Mortality is high without early antibiotic and supportive therapy. Listeria monocytogenes can affect many species and is a public health concern.     

A direct plating method for monitoring the contamination of Listeria monocytogenes in silage.

Twenty-two silage samples were analyzed for the presence of L. monocytogenes using five Listeria selective plating media, with and without previous selective enrichment step. L. monocytogenes was recovered from 3 samples by both procedures, but direct plating allowed the quantification of Listeria population. Two of these positive samples were implicated in outbreaks of listeriosis in sheep; the L. monocytogenes population in these samples was about 10(6) cells/g. The L. monocytogenes population in the other positive sample was 10(3) cells/g. Direct isolation of L. monocytogenes was only possible from LPM, PALCAM and LSAMm media. MOX and LSM media were not selective enough to allow direct Listeria isolation. In our hands, LSAMm was the most suitable plating medium for the direct isolation and specific quantification of L. monocytogenes from silage employing a red blood cells overlay technique.     

A survey for Listeria species, motile aeromonads and Yersinia enterocolitica on bovine and ovine carcasses

One hundred ovine and 100 bovine carcasses in two abattoirs were sampled just after dressing for the presence of Yersinia enterocolitica, Listeria monocytogenes and motile aeromonads. Yersinia enterocolitica was not isolated and only two samples were positive for Listeria spp. In both cases, the Listeria species were not normally pathogenic to man. In contrast, motile aeromonads were isolated from 81% of ovine and 35% of bovine carcasses.     

Latest news on listeriosis

Emphasis is given essentially to the presentation of recent data in terms of our total knowledge of Listeria and human and animal listeriosis. This disease of extremely varied origin can be studied in terms of two groups of subjects: females in gestation and all other categories of individuals. There was initially only a single species of Listeria, but at least five are known today, only two of which are pathogenic. Their pathogenic power is related essentially to the presence of listerolysin O, although this is not the only factor involved. Identification of Listeria is easy and can be completed with that of its serovar and lysovar. Epidemiological studies have shown that the great majority of listeriosis are anademic. The contamination of receptive subjects is due to certain forms of food. In the absence of efficient vaccination, disease prevention must be obtained by eliminating Listeria from food.     

Detection of pathogenic Listeria spp. in zoo animal faeces: use of immunomagnetic separation and a chromogenic isolation medium

Faecal samples, collected from 200 healthy animals in Antwerp Zoo, were examined for the presence of pathogenic Listeria spp. A two-stage standard isolation (ISO) method was combined with immunomagnetic separation (IMS). ALOA agar, a chromogenic isolation medium, differentiating Listeria spp. on the basis of beta-glucosidase and phosphatidylinositol-specific phospholipase C (PIPLC) activity, was compared with PALCAM agar. Confirmation of the isolates was based on conventional biochemical tests and a disc test, which detects a specific aminopeptidase produced by all Listeria spp. except Listeria monocytogenes. Listeria spp. were isolated from 42 (21.0%), L. monocytogenes from 14 (7.0%), and Listeria ivanovii from two (1.0%) faecal samples. The application of IMS after primary enrichment detected pathogenic Listeria spp. in 12 (6.0%) samples. The ISO method, combining primary and secondary enrichment, detected pathogenic Listeria spp. in 15 (7.5%) samples. The sensitivity of IMS compared to the ISO method was 73.3% and the specificity was 99.5%. ALOA agar was superior to PALCAM agar for isolation of Listeria spp. The disc test identified all L. monocytogenes isolates. IMS after primary enrichment was a suitable screening method, but secondary enrichment increased the number of positive samples.     

Outbreak of clinical listeriosis in sheep: evaluation from possible contamination routes from feed to raw produce and humans

We report the results of clinical and microbiological investigations on Listeria monocytogenes infections in a flock of 55 sheep and describe the implications for the safety of the raw milk and raw-milk cheeses produced in the on-farm dairy. The outbreak was caused by feeding grass silage, which was contaminated with 5 log10 CFU L. monocytogenes/g. Clinically, although having been fed from the same batch of silage, abortive (nine ewes), encephalitic (one ewe) and septicaemic (four ewes) forms of listeriosis were observed during the outbreak phase. As the starting point of feeding the contaminated silage was known we could calculate an incubation period of 18+/-2 and 26 days for the abortive and the encephalitic form of listeriosis, respectively. Pathologically, the septicaemic cases suffered from Listeria accumulation at comparable numbers in visceral organs but not in the brain. Only a single ewe developed central nervous symptoms and a rhomb-encephalitis was immunohistologically confirmed. In this case the infection proceeded from the nasal mucosa into the brain, with no infections of the liver, spleen and other visceral organs. Sampling of the cheese production chain, the farm environment and the persons living at the farm revealed the exposure of a farm-worker to an isolate genetically indistinguishable from the outbreak clone, obviously through the consumption of faecally contaminated bovine raw milk. The cheese under processing was free of Listeria because, as a result of intensive consultations, the farmer ensured a proper acidification of the cheese. The epidemiological findings suggest that food safety matters should be assessed in any case where infection of food-producing animals with potential human pathogens is observed.     

塑料膜在青贮饲料生产中的应用

青贮是一种保存青绿饲料的重要方法，自塑料膜应用于农业以来，它在提高青贮品质，减少营养物质损失方面起到了很重要的作用。文章介绍了塑料膜在生产实践中常用的几种方法：堆式青贮，真空窖青贮，塑料袋装青贮，拉伸膜裹包青贮，塑料膜隔离分层青贮。还分析了塑料膜在青贮中的发展趋势及在我国的应用前景。     

Listeria monocytogenes in horses in Iceland

Twenty isolates of Listeria monocytogenes associated with five confirmed and four suspected incidents of listeriosis in horses in Iceland were characterised by serotyping, pulsed-field gel electrophoresis and ribotyping. Semiquantitative estimates of the numbers of L monocytogenes were made on faeces from horses with clinical signs of listeriosis and on grass silage fed to them. Large numbers of L monocytogenes were often found in the faeces of horses with severe signs of disease. The 20 isolates could be divided into six genotypes, each incident involving only one genotype. One serovar 1/2a genotype was associated with three confirmed incidents of listeriosis in 1991, 1993 and 1997. In one incident, the same genotype was isolated from the organs of a horse with listeriosis and from the spoiled grass silage fed to it.     

新疆绵羊致病株李氏杆菌鉴别试验

李氏杆菌病是由单核细胞增多症李氏杆菌(LM)引起人与动物的一种共患传染病,其中以绵羊的敏感性最强。近几年,本病对新疆养羊业造成严重危害。本研究采用生化反应,溶血试验,致病力试验,对分离的新疆5株绵羊致病株李氏杆菌90SB_1,90SB_2,90SB_5,90SS_1,125SL 进行了种的鉴别测定,试验结果,五株绵羊致病株(野毒株),在生化反应,培养特性,溶血性,致病性等方面表现高度的一致性,属于同一种 LM。     

Listeriosis in Sheep: Listeria Monocytogenes in Sheep Fed Hay Or Grass Silage During Pregnancy. Immunological State,White Blood Cells,Total Serum Protein and Serum Iron

A comparison was made between a hay fed group, consisting of 23 ewes, and a grass silage fed group of 22 ewes, all pregnant. Excretion of Listeria monocytogenes (Lm) in the faeces and milk, antibody titres in sera and whey and delayed hypersensitivity against Lm, and several blood components were determined. The animals had previously been exposed to Lm, and Lm was isolated from the faeces from several animals when the experiment started.No significant difference in number of excretors between the 2 groups was found during the experimental period. The haemagglutination titres in both sera and whey were low and on the same level in both groups. The titres were higher in animals with 1 foetus than in animals with more than 1 foetus.In the first part of the experimental period the silage group had a reduced number of lymphocytes, lower total serum protein values and higher serum iron values, compared with the hay group.The silage group also had a stronger delayed hypersensitivity reaction against Lm than the hay group, and in the silage group the reaction was significantly stronger in ewes with 3 or more foetuses than in ewes with 1 foetus.In conclusion, the combined effect of some of the changes found in animals fed grass silage may leave them more susceptible to infections.     

Epidemiological studies on the occurrence of Listeria monocytogenes in the feces of dairy cattle

Seasonal variation in the fecal shedding of Listeria spp. in dairy cattle was examined by collecting a total of 3,878 fecal samples during a period of two years. The prevalences of Listeria spp. and L. monocytogenes were higher during the indoor season (12.7% and 9.2%, respectively) than in samples collected from the animals on pasture (5.3% and 3.1%, respectively). The highest frequencies of Listeria spp. (19.4%) and L. monocytogenes (16.1%) were detected in December. Listeriae were isolated from at least one of the dairy cows from 45.8% of the 249 herds examined. 2.9% of the 314 milk samples collected from the farm bulk tanks on 80 dairy farms on four different occasions yielded L. monocytogenes. The seasonal occurrence of these bacteria in milk reflected the frequencies of Listeria in the fecal material but not those in the main roughage used; grass silage and pasture grass. Fecal material is considered to be a potential source of contamination of raw milk by L. monocytogenes. Investigation of the numbers of viable Listeria organisms in different animal fodders is considered essential in further epidemiological studies of these bacteria.     

Septicaemia, encephalitis and abortions in a housed flock of sheep caused by Listeria monocytogenes type 1/2

One hundred and ninety-six housed pregnant ewes were fed poor quality silage for two days. Ewes are reluctant to eat the silage and within 48 hours they became dull and developed diarrhoea and lameness. Despite treatment with antibiotics and calcium borogluconate 19 ewes died, more than 60 developed vaginal discharges and at lambing 94 ewes were barren. Six developed nervous signs and two of these died, one with lesions typical of listeric encephalitis. Post mortem lesions and the isolation of Listeria monocytogenes type 1/2 from lung, liver, spleen and kidney are described. L monocytogenes was also isolated from blood samples from live ewes. The estimated gross financial loss to the farmer was 5130 pounds or, for the flock, 26 pounds per ewe.     

The impact of the quality of silage on animal health and food safety: a review

This paper reviews the microbiological aspects of forage preserved by ensilage. The main principles of preservation by ensilage are a rapid achievement of a low pH by lactic acid fermentation and the maintenance of anaerobic conditions. The silage microflora consists of beneficial micro-organisms, i.e. the lactic acid bacteria responsible for the silage fermentation process, and a number of harmful micro-organisms that are involved in anaerobic or aerobic spoilage processes. Micro-organisms that can cause anaerobic spoilage are enterobacteria and clostridia. Clostridium tyrobutyricum is of particular importance because of its ability to use lactic acid as a substrate. Silage-derived spores of C. tyrobutyricum can cause problems in cheese making. Aerobic spoilage of silage is associated with penetration of oxygen into the silage during storage or feeding. Lactate-oxidizing yeasts are generally responsible for the initiation of aerobic spoilage. The secondary aerobic spoilage flora consists of moulds, bacilli, listeria, and enterobacteria. Mycotoxin-producing moulds, Bacillus cereus, and Listeria monocytogenes in aerobically deteriorated silage form a serious risk to the quality and safety of milk and to animal health.     

产单核细胞李斯特菌毒力因子及免疫预防研究进展

产单核细胞李斯特茵在病原学上已被公认为一种人兽共患病和食源性疾病的致病菌,在各国已引起食品加工部门、公共卫生部门的高度重视和研究者的兴趣.文章就该菌的毒力因子及免疫预防研究做一简述.     

Biochemical differentiation of Listeria from cheese

Listeria spp. isolated from cheese were tested for biochemical characteristics together with reference strains from culture collections. Microtitration plates were used for testing the fermentation patterns. The results were subjected to a numerical cluster analysis based on linkage maps. The variation of the group structures calculating the characteristics with and without the hemolysin reactions is demonstrated. The pathogenic species L. monocytogenes could only be separated from the avirulent species L. innocua by the hemolysin tests. Most of the cheese isolates were identified as L. innocua, some as L. monocytogenes and L. seeligeri. There is a need for an inexpensive commercial test kit to identify the serovars or virulence factors of Listeria spp. in the quality assessment of food. The present study sets up doubts for a sufficiently ensured separation of L. innocua from L. monocytogenes.     

内蒙古青贮饲料质量研究

为探讨内蒙古青贮饲料品质现状,采用抽样方法对内蒙古青贮饲料质量进行研究。结果表明,内蒙古青贮设备主要以窖贮和壕贮为主,青贮时间一般为9和10月,青贮种类主要有全株玉米青贮、玉米秸青贮、甜菜类青贮等;青贮饲料发酵品质总体表现较好,通过对青贮饲料有机酸进行费氏评分,达到优良等级的占总样品的83%,全株玉米青贮和甜菜类青贮经费氏评分均达到优等,但是,全株玉米青贮饲料中的乳酸含量显著高于甜菜类青贮饲料(P＜0.05),乙酸和丁酸含量均显著低于甜菜类青贮饲料(P＜0.05),玉米秸青贮饲料费氏评分为良,内蒙古青贮饲料有机酸费氏评定结果与感官评定结果基本相符;3种青贮饲料中,甜菜类青贮饲料的营养品质最好,全株玉米青贮饲料次之,玉米秸青贮饲料最劣。综合分析结果表明,内蒙古全株玉米青贮饲料和甜菜类青贮饲料营养保存率高,发酵效果好,玉米秸青贮饲料质量较差。     

Incidence of Listeria monocytogenes in wild animals in Japan

Between 1991 and 1993, the intestinal contents and feces of wild animals in Japan were examined for the presence of Listeria. The wild animals examined included 623 mammals (11 species) and 996 birds (18 species). Listeria species were isolated from 38 (6.1%) of the 623 mammalian samples and 133 (13.4%) of 996 bird samples. The highest incidence of Listeria in the mammals was found in Japanese monkeys (20.0%) and that in birds was found in crows (43.2%). The incidence of Listeria in Japanese monkeys varied from 0 to 40.0% depending on the capture area. L. monocytogenes was isolated from II of these positive samples. Serovars 1/2a and 4b predominated in eight serotyped L. monocytogenes isolates.     

Identification of lactic acid bacteria in the feces of dairy cows fed whole crop maize silage to assess the survival of silage bacteria in the gut

In order to assess the survival of lactic acid bacteria (LAB) in whole crop maize silage in the gut of dairy cows, one representative silage sample and three different feces samples were collected from dairy cows on three dairy farms in Hua Bei, China and three dairy farms in Kyushu, Japan. The composition of the bacterial community was examined by denaturing gradient gel electrophoresis and quantitative polymerase chain reaction. Lactobacillus acetotolerans was detected in all bunker‐made maize silage samples, regardless of the dairy farm or sampling region from which they were sourced. A total of eight LAB species were detected in the maize silage samples, of which three (L. acetotolerans, L. pontis and L. casei) appeared to survive digestion. The populations of L. acetotolerans in silage and feces were 10^6–7 and 10^3–4 copies/g, respectively, indicating that, even for the LAB species showing potential survival in the gut, competition in this niche may be harsh and the population may substantially decrease during the digestion process. It may be difficult for silage LAB to survive in the gut of silage‐fed dairy cows, because marked decrease in population can take place during the digestion process, even for surviving species.     

Survival of Listeria monocytogenes in wilted and additive-treated grass silage

Grass was field-dried to 3 different dry matter (DM) levels (200, 430 and 540 g/kg) and inoculated with 10(6)-10(7) cfu/g of a Listeria monocytogenes strain sharing a phagovar occasionally involved in foodborne outbreaks of listeriosis. Formic acid (3 ml/kg) or lactic acid bacteria (8 x 10(5)/g) with cellulolytic enzymes were applied only to forages with low and intermediate DM levels. Forages were ensiled in laboratory silos (1700 ml) and were stored at 25 degrees C for 30 or 90 days. After 90 days of storage, L. monocytogenes could not be detected in any silo, except one with the high dry matter grass without additive. After 30 days of storage, between 10(2) and 10(6) cfu L. monocytogenes/g silage were isolated from the untreated silages. Increasing the DM content from 200 to 540 g/kg did not reduce listeria counts possibly because of the lower production of fermentation acids (higher pH). In silages treated with additives, counts of L. monocytogenes were always lower than in silages without additive. In wet silages (DM 200 g/kg) both additives were effective, but in the wilted silages (DM 430 g/kg) only the bacterial additive reduced listeria counts below detection level. Listeria counts were highly correlated to silage pH (r = 0.92), the concentration of lactic acid (r = -0.80) and the pooled amount of undissociated acids (r = -0.83).