R L Walker

University of California, Davis, Davis, California, United States

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Publications (51)98.56 Total impact

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    ABSTRACT: A deceased 10-yr-old male ostrich was diagnosed with severe necrotizing enteritis and septicemia. The bird was inappetent for 3 wk and had neurologic signs 2 days prior to death. Macroscopically, no significant lesions were noted aside from congestion of the liver, kidneys, and spleen. Histopathology revealed severe fibrinonecrotic enteritis,associated with large numbers of gram-negative bacteria, multifocal fibrinoid necrosis in portal arteries, accumulation of fibrin in hepatic sinusoids, myocardial degeneration, and necrosis. There was also squamous metaplasia in the glands of the esophagus and external ears. A gram-negative rod was isolated in pure culture from intestine, liver, lungs, and trachea and identified as an Aeromonas species. The concentration of vitamin A in the liver was extremely low. The lesions seen in the intestine and liver and the isolation of an Aeromonas sp. from various tissues strongly suggest that this bacterium was the cause of the necrotizing enteritis, septicemia, and death of this ostrich. Vitamin A deficiency might have predisposed the bird to the Aeromonas infection.
    Avian Diseases 07/2009; 53(2):310-6. · 1.73 Impact Factor
  • L V Stamm, R L Walker, D H Read
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    ABSTRACT: The etiology of bovine ulcerative mammary dermatitis (UMD) is poorly characterized. The goal of this study was to genetically analyze spirochetes present in UMD lesions. DNA prepared from UMD lesion biopsies and from spirochetes cultured from the corresponding lesion biopsies was PCR amplified using primers for the 16S rDNA-tRNA(ile) intergenic spacer region (ISR) of Treponema 16S-23S rDNA. Analysis of cloned ISR amplicons from three cultivable UMD-associated spirochetes indicated that two isolates cluster closely with cultivable papillomatous digital dermatitis (PDD)-associated and human-associated Treponema phylotypes, while the remaining isolate is unique. Analysis of ISR amplicons from UMD lesion biopsies identified additional not-yet-cultivable Treponema phylotypes. Our results revealed the presence of a genetically diverse Treponema population in an UMD lesion.
    Veterinary Microbiology 11/2008; 136(1-2):192-6. · 3.13 Impact Factor
  • H L Shivaprasad, R Kokka, R L Walker
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    ABSTRACT: Listeriosis was diagnosed in a 4-yr-old female cockatiel (Nymphicus hollandicus) that died after exhibiting clinical signs that included a fluffed-up appearance, weakness, and loss of weight of several days duration. Grossly, the bird was moderately emaciated, and the liver and spleen were enlarged. Microscopically, there was mild-to-moderate inflammation associated with rod-shaped, gram-positive bacteria in the liver, spleen, kidneys, adrenal glands, bone marrow, and esophagus. Listeria monocytogenes was isolated from the liver, trachea, and intestine. The isolate was identified as type 1 by agglutination with specific antisera, and it further identified as belonging to serovar group 1/2a, 3a by multiplex polymerase chain reaction assay. Listeria monocytogenes also was detected in affected tissues by immunohistochemistry.
    Avian Diseases 10/2007; 51(3):800-4. · 1.73 Impact Factor
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    ABSTRACT: Epizootic bovine abortion (EBA), a tick-transmitted disease of pregnant cattle grazing foothill pastures, is a major cause of reproductive failure in California and adjacent states. Affected fetuses develop a chronic disease, resulting in late-term abortion or premature calving. Despite investigations spanning 50 years, to the authors' knowledge, the etiologic agent of EBA has not yet been isolated from affected fetuses or the tick vector. The diagnosis of EBA is based on gross and microscopic lesions. Recently, documentation that the etiologic agent is susceptible to antibiotics and identification of a unique 16S deltaproteobacterial rDNA gene sequence in 90% of thymus tissues from aborted fetuses have supported the role of a bacterial infection as the cause of EBA. To determine whether bacteria could be detected in the tissues, histochemical staining and immunohistochemical procedures were used on formalin-fixed, paraffin-embedded tissues. Use of a modified Steiner silver stain revealed small numbers of intracytoplasmic bacterial rods in 37 of 42 thymic samples from EBA-affected fetuses. Improved detection was achieved by use of immunohistochemical staining with serum from EBA-affected fetuses that resulted in detection of numerous bacterial rods in the cytoplasm of histiocytic cells in the thymus from all 42 EBA-affected fetuses. Immunohistochemical examination of additional tissues from 21 field and experimental EBA cases revealed positively stained intracytoplasmic bacterial rods in many organs with inflammatory lesions. Use of the modified Steiner stain and immunohistochemical staining of tissues from negative-control fetuses failed to reveal organisms. To the authors' knowledge, this is the first report to document morphologic evidence of a bacterium associated with the lesions of EBA.
    Journal of veterinary diagnostic investigation: official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc 02/2006; 18(1):76-80. · 1.18 Impact Factor
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    ABSTRACT: An immunohistochemistry (IHC) procedure for the detection of Campylobacter fetus antigens using an avidin-biotin complex technique was performed on formalin fixed bovine and ovine fetal tissues from 26 natural cases of Campylobacter spp. abortion (four ovine and 22 bovine). The species of Campylobacter isolated included C. fetus ssp. venerealis from 13 bovine fetuses, C. fetus ssp. fetus from two ovine and one bovine fetus, Campylobacter jejuni from seven bovine fetuses, Campylobacter lari from two ovine fetuses and an unspeciated Campylobacter species in one bovine fetus. Histologic lesions identified in the aborted fetuses included placentitis, serositis, pneumonia, gastroenteritis, hepatitis and encephalitis. Campylobacter fetus antigens were identified by IHC in 13 of 13 bovine fetuses from which C. fetus ssp. venerealis was isolated and in two of two ovine fetuses from which C. fetus ssp. fetus was isolated. The IHC stains were negative in tissues from seven bovine fetuses from which C. jejuni was isolated, one bovine fetus infected with C. fetus ssp. fetus, one bovine fetus infected with the unspeciated Campylobacter and two ovine fetuses infected with C. lari. In positive cases, the IHC stain most frequently identified bacteria in the lung and gastrointestinal tract. The C. fetus IHC procedure performed on formalin fixed tissues is a practical tool for the diagnosis of natural cases of ovine and bovine abortion caused by C. fetus.
    Journal of Veterinary Medicine Series B 05/2005; 52(3):138-41. · 1.48 Impact Factor
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    ABSTRACT: Tritrichomonas foetus (T. foetus) is the causative agent of bovine trichomonosis, a sexually transmitted disease leading to abortion (from 1 to 8 months gestation), infertility, and occasional pyometra. The annual losses to the U.S. beef industry are estimated to be in the hundreds of millions of dollars. Currently, the "gold standard" diagnostic test for trichomonosis in most countries is the cultivation of live organisms from reproductive secretions. The cultured organisms can then be followed by PCR assays with primers that amplify T. foetus to the exclusion of all other trichomonad species. Thus, negative results present as null data, indistinguishable from failed PCR amplification during T. foetus specific amplification. Our newly developed assay improves previously developed PCR based techniques by using diagnostic size variants from within the internal transcribed spacer 1 (ITS1) region that is between the 18S rRNA and 5.8S rRNA subunits. This new PCR assay amplifies trichomonad DNA from a variety of genera and positively identifies the causative agent in the bovine trichomonad infection. This approach eliminates false negatives found in some current assays as well as identifying the causative agent of trichomonad infection. Additionally, our assay incorporates a fluorescently labeled primer enabling high sensitivity and rapid assessment of the specific trichomonad species. Moreover, electrophoretic separation of amplified samples can be outsourced, thus eliminating the need for diagnostic laboratories to purchase expensive analysis equipment.
    Veterinary Parasitology 02/2005; 127(1):33-41. · 2.38 Impact Factor
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    ABSTRACT: Sequence analysis of the 5.8S rRNA gene and the internal transcribed spacer regions (ITSRs) was used to compare trichomonadid protozoa (n = 39) of varying morphologies isolated from the bovine preputial cavity. A multiple sequence alignment was performed with bovine isolate sequences and other trichomonadid protozoa sequences available in GenBank. As a group, Tritrichomonasfoetus isolates (n = 7) had nearly complete homology. A similarity matrix showed low homology between the T. foetus isolates and other trichomonads recovered from cattle (<70%). Two clusters of trichomonads other than T. foetus were identified. Eighteen isolates comprised 1 group. These isolates shared >99% homology among themselves and with Pentatrichomonas hominis. The other non-T. foetus cluster (n = 14) did not exhibit a high degree of homology (<87%) with other bovine isolates or any of the trichomonad sequences available in GenBank. The sequence homology among isolates in that cluster was >99%, except for 1 isolate that varied from the others in both ITSRs (approximately 2% dissimilarity). Sequence analysis of the 5.8S rRNA gene and ITSRs was useful for comparing trichomonadid protozoa isolated from the bovine preputial cavity and demonstrated that 2 distinct types of trichomonads constituted the non-T. foetus isolates recovered from the bovine preputial cavity.
    Journal of veterinary diagnostic investigation: official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc 02/2003; 15(1):14-20. · 1.18 Impact Factor
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    L V Stamm, H L Bergen, R L Walker
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    ABSTRACT: Papillomatous digital dermatitis (PDD), an emerging infectious disease of cattle, is characterized by painful, ulcerative foot lesions. The detection of high numbers of invasive spirochetes in PDD lesions suggests an important role for these organisms in the pathogenesis of PDD. PDD-associated spirochetes have phenotypic characteristics consistent with members of the genus TREPONEMA: Partial 16S ribosomal DNA (rDNA) sequence analysis of clonal isolates from California cattle showed that they comprise three phylotypes which cluster closely with human-associated Treponema spp. of the oral cavity (T. denticola and T. medium/T. vincentii) or genital area (T. phagedenis). The goal of our study was to apply 16S-23S rDNA intergenic spacer region (ISR) sequence analysis to the molecular typing of U.S. PDD-associated Treponema isolates. This methodology has potentially greater discriminatory power for differentiation of closely related bacteria than 16S rDNA analysis. We PCR amplified, cloned, and sequenced the ISRs from six California PDD-associated Treponema isolates and, for comparative purposes, one strain each of T. denticola, T. medium, T. vincentii, and T. phagedenis. Two ISRs that varied in length and composition were present in all the PDD-associated Treponema isolates and in T. denticola, T. medium, and T. phagedenis. ISR1 contained a tRNA(Ala) gene, while ISR2 contained a tRNA(Ile) gene. Only a single ISR (ISR1) was identified in T. vincentii. Comparative analyses of the ISR1 and ISR2 sequences indicated that the California PDD-associated Treponema isolates comprised three phylotypes, in agreement with the results of 16S rDNA analysis. PCR amplification of the 16S-tRNA(Ile) region of ISR2 permitted rapid phylotyping of California and Iowa PDD-associated Treponema isolates based on product length polymorphisms.
    Journal of Clinical Microbiology 10/2002; 40(9):3463-9. · 4.07 Impact Factor
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    ABSTRACT: Direct amplification and sequencing of the 16S rRNA gene and a variable region of the flagellin gene from fetal liver-associated spirochetes belonging to the Borrelia parkeri-B. turicatae tick-borne relapsing fever spirochete group with a late-term abortion in a mare are described.
    Journal of Clinical Microbiology 05/2002; 40(4):1558-62. · 4.07 Impact Factor
  • Journal of the American Veterinary Medical Association 09/2001; 219(3):310-22. · 1.72 Impact Factor
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    ABSTRACT: A comparison of the VIDAS Salmonella (SLM) assay using the manufacturer's recommended sampling method or a Moore swab sampling method demonstrated that the Moore swab method detected a greater number of positive samples (83.0% vs. 67.92%). When results using a conventional culture technique were compared to the VIDAS SLM assay using Moore swab sampling, there was good agreement (97.5%). When the VIDAS SLM assay was used to test in-line milk filters and compared to results from the conventional culture method, there was also good correlation between test results (95.57%). Overall, the VIDAS SLM assay using a modified sampling method compared favorably to the conventional culture method and had the advantages of taking less time to obtain a negative or presumptive positive result, being less technically complicated and requiring less screening of non-lactose fermenting colonies from negative samples.
    International Journal of Food Microbiology 08/2001; 67(1-2):123-9. · 3.43 Impact Factor
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    ABSTRACT: Increased mortality (1.5% per week) and low egg production (5-10% lower than normal) were observed in a flock of domestic breeding Pekin ducks (Anas platyrhynchos). At necropsy, salpingitis and peritonitis were the most significant findings. Histologically, there was accumulation of necrotic debris in the lumen of the oviduct. Numerous bacteria and trichomonads were observed histologically in the lumen of the vagina and occasionally in the shell gland. Escherichia coli and a trichomonad were isolated from the oviduct. The trichomonads were oval (6-8 microm long, 4.5-6 microm wide) and had 4 anterior flagella and an undulating membrane extending over the entire length of the body, finishing in a long posterior flagellum. Morphology was consistent with trichomonads of the genus Tetratrichomonas. Comparative sequence analysis of the 5.8S ribosomal RNA gene and the flanking internal transcribed space regions of the trichomonad isolate did not closely match with available sequences of the same region of other trichomonadid protozoa.
    Journal of veterinary diagnostic investigation: official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc 06/2001; 13(3):240-5. · 1.18 Impact Factor
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    K Sato, T E Carpenter, J T Case, R L Walker
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    ABSTRACT: Electronic records from the California Animal Health and Food Safety Laboratory System between January 1, 1991, and June 30, 1998, were reviewed, and data on 593 Salmonella serotypes isolated from feces or intestinal contents of adult dairy cattle with diarrhea were analyzed to determine spatial and temporal clustering. A statistically significant clustering in space was detected with 10 (Typhimurium, Montevideo, Muenster, Anatum, Give, Menhaden, Kentucky, Agona, Derby, and Newport) of the 13 serotypes examined. Significant temporal clustering was also detected with 10 serotypes (Typhimurium, Montevideo, Muenster, Anatum, Give, Newbrunswick, Menhaden, Kentucky, Derby, and Newport). Six serotypes (Anatum, Menhaden, Montevideo, Muenster, Newbrunswick, and Newport) were significantly spatially and temporally clustered. A difference in temporal and spatial distribution patterns of some serotypes associated with diarrhea in adult dairy in California was found. Knowledge of the specific type of clustering, if present, should improve our understanding of the transmission and control of salmonellosis in the field.
    Journal of veterinary diagnostic investigation: official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc 06/2001; 13(3):206-12. · 1.18 Impact Factor
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    ABSTRACT: A cross-sectional study was conducted in southern Chile between January and March, 1996, to identify risk factors for papillomatous digital dermatitis (PDD) in lactating dairy and dual-purpose cows. A total of 3,265 cows from 22 farms were examined in the milking parlor for PDD lesions. Additional information was collected from dairies' computerized records and by direct interview of managers. Data were analyzed using logistic and logistic-binomial regression (with dairy as a random-effect term). German Red-Pied (dual-purpose) cows were significantly (P < 0.05) less likely (odds ratio (OR) = 0.3) to have PDD lesions than German Black-Pied and Holstein crossbreds. First-parity cows had the highest odds of PDD, and odds diminished, in a dose-effect manner, as parity increased. Odds of PDD increased with increasing days in lactation. Cows that calved during winter were more likely to have PDD (OR = 1.4) than those calving at any other season. Cows on farms that bought heifers in the past 10 years had a 3-fold increase in the odds of PDD compared to those on farms that never bought heifers. Loose-housed cows had a higher risk of PDD (OR = 7), followed by cows in free stalls or in open corrals (OR = 2.8 and 1.3, respectively), compared to cows on pasture all year. Cows on dairies that used a footbath during 1996 were less likely (OR = 0.3) to have PDD than those in dairies not using one. Parlor type was associated with PDD, but this was likely an effect of parlor design on ease of inspection of cows' feet. A policy of trimming all cows' vs. only lame cows' feet and a policy about buying adult cows did not have significant effects on PDD risk.
    Preventive Veterinary Medicine 10/1999; 42(2):87-97. · 2.39 Impact Factor
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    ABSTRACT: Randomly amplified polymorphic DNA (RAPD) analysis was used to investigate the molecular epidemiology of 26 Mycoplasma gallisepticum (MG) isolates obtained from turkeys located in the central valley of California. The MG isolates were recovered from 5 different companies and 13 ranches. Each company had unique MG strains. No evidence of spread of MG between companies was detected. RAPD analysis of MG isolates within a ranch during an outbreak revealed only a single strain involved in each outbreak. RAPD analysis identified an isolate from 1 ranch with a banding pattern identical to that of the 6/85 vaccine strain, which had been used on that particular ranch. Similar RAPD banding patterns of isolates from different ranches within the same company suggested horizontal spread of MG between ranches. The use of 2 primer sets in RAPD analysis was critical to prevent misinterpretation of relationships between different isolates.
    Journal of veterinary diagnostic investigation: official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc 10/1999; 11(5):408-15. · 1.18 Impact Factor
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    ABSTRACT: Randomly amplified polymorphic DNA (RAPD) analysis was used to differentiate 7 strains of Mycoplasma gallisepticum. Six commercially available primers or primer combinations were screened for their ability to differentiate vaccine and type strains. Although major and minor bands were produced with each primer, many of the primers were unsuitable for strain differentiation. The use of primer 6 and combined primers 3 and 4 resulted in complementary RAPD banding patterns for each M. gallisepticum strain. Eleven different isolates representing 7 different strains were segregated into 7 different patterns, corresponding to the 7 strains.
    Journal of veterinary diagnostic investigation: official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc 04/1999; 11(2):158-61. · 1.18 Impact Factor
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    ABSTRACT: To determine whether ampicillin- and tetracycline-resistant strains of Pasteurella multocida and P haemolytica isolated from California cattle with pneumonia were spatially and temporally clustered and to compare overall estimates of percentages of these isolates resistant to these antimicrobials with estimates obtained on the basis of regional and temporal information. Epidemiologic study. Records of P multocida and P haemolytica isolates obtained from lung or tracheal wash samples collected from California cattle with pneumonia between July 1, 1991 and July 31, 1996. Only isolates obtained from samples submitted by dairies and calf ranches were used. Spatial clustering of ampicillin- and tetracycline-resistant isolates was assessed by use of nearest-neighbor and Cuzick and Edwards' analyses. Linear clustering along a north-south line was assessed by use of runs and maximum length of runs tests. Temporal clustering was assessed by use of scan tests. Spatial-temporal clustering was assessed by use of Barton's method. Regional estimates of percentages of P multocida and P haemolytica resistant to ampicillin or tetracycline were calculated. There was significant spatial clustering of resistant isolates and significant linear clustering along a north-south line. Significant differences in regional estimates of percentages of antimicrobial-resistant isolates were found. Results support the hypothesis that antimicrobial-resistant organisms can be clustered at the local level and reinforce the need to establish regional estimates of percentages of bacterial isolates that will be susceptible to commonly used antimicrobials.
    Journal of the American Veterinary Medical Association 05/1998; 212(7):1001-5. · 1.72 Impact Factor
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    D H Read, R L Walker
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    ABSTRACT: Clinical, gross pathologic, and therapeutic studies were performed on a contagious, painful, wart-like digital disease of unknown etiology in California dairy cattle. The disease was called papillomatous digital dermatitis (PDD). Survey indicated that the disease spread geographically throughout southern California over the past few years. In 1991, 31% of herds had papillomatous digital dermatitis, whereas in 1994, 89% were affected. Increased incidence occurred during late spring and summer, 1-3 months after the rainy season. Within-herd morbidity ranged from 0.5% to 12% per month. Study of 93 cows in 10 drylot dairies revealed that 91% had characteristic circumscribed, erosive to papillomatous, intensely painful lesions often surrounded by a ridge of hyperkeratotic skin bearing hypertrophied hairs. Lesions were 2-6 cm across (88%), circular to oval (78%), and raised (59%) and had surfaces that were uniformly erosive and granular (31%), uniformly papillary (28%), or composites of both appearances (41%). Lesions were most frequently seen in lactating heifers (31%) and 3-year-old cows (43%). Clinical signs were characterized by lameness, with walking on toes and clubbing of hooves. Lesions exclusively involved the hind limbs in 82% of cows and the plantar/palmar regions in 84% of cows. Lesions had high (89%) prediliction for plantar/palmar skin bordering the interdigital space. Lesions exclusively involved either the medial or lateral digit in 10% and 28% of cows, respectively. In 50% of cows, both medial and lateral digits of individual limbs were involved; in most cows (31%), lesions apposed each other across the plantar interdigital space, whereas in others (19%), lesions confluently involved the entire plantar/palmar commissural skin folds. In another 12% of cows, lesions were axial. High proportions of lesions showed complete therapeutic responses to antibiotics: parenteral penicillin (9/9) and ceftiofur (41/44), and topical oxytetracycline (4/4). Recurrence or new lesion development occurred in 48% of cows reexamined 7-12 weeks after complete therapeutic response was observed. Overall, the findings indicated that PDD is a distinct disease entity of economic importance in which bacteria may play an important pathogenic role.
    Journal of veterinary diagnostic investigation: official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc 01/1998; 10(1):67-76. · 1.18 Impact Factor
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    ABSTRACT: To determine whether a humoral response against spirochetes isolated from papillomatous digital dermatitis (PDD) lesions is elicited in dairy cattle affected with PDD. 41 cattle with PDD from 8 dairies (study population) and 30 cattle from 2 dairies free of PDD (control population). Additionally evaluated were 32 cattle from a dairy with a past history of PDD but no current disease, and 52 cattle from a dairy with high prevalence of PDD, 25 with and 27 without detectable lesions. ELISA were used to evaluate the humoral response of all cattle to representative isolates from 2 groups of spirochetes of unknown species isolated from PDD lesions. Specificity of the response was evaluated, using immune sera prepared against each of the spirochetes, and by adsorption studies of immune and field sera. The potential for confounding by an antibody response to other spirochetes associated with diseases of cattle was assessed. The antibody response (specific) to both PDD spirochete groups of cows with PDD was significantly increased, compared with that of cows from PDD-free dairies. There was no association between antibody response to PDD-associated spirochetes and antibody response to other spirochetal diseases of cattle. None of the cattle from the dairy with previous history of PDD but without current disease were classified as test positive by either PDD ELISA. There was a significant (P < 0.01) difference in classification results for both PDD ELISA for cattle with PDD from the dairy with a high herd prevalence of PDD, compared with cattle without detectable disease from the same dairy. The humoral response in cattle with PDD lesions was significantly different from that in cattle without detectable lesions, thus providing additional information regarding the potential role of spirochetes isolated from PDD lesions in the etiopathogenesis of PDD.
    American Journal of Veterinary Research 08/1997; 58(7):744-8. · 1.35 Impact Factor
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    ABSTRACT: To identify risk factors associated with Salmonella menhaden associated disease in adult dairy cows during an outbreak in California. Case-control study. 8 case dairies that had > or = 1 adult animal that had clinical signs of salmonellosis and from which S menhaden was isolated and 22 control dairies, 16 of which were matched on the basis of herd size and county and 6 of which were matched on the basis of herd size, county, and breed (Jersey). A questionnaire was developed and reviewed with the herdsman or owner of each dairy. Primary areas of concern were herd management, disease characteristics, and feed-related information. Use of 1 particular feed mill and feeding animal fat were significant risk factors for clinical disease attributable to S menhaden infection. Feed should not be overlooked as a potential source of Salmonella organisms in dairy herds.
    Journal of the American Veterinary Medical Association 02/1997; 210(4):528-30. · 1.72 Impact Factor

Publication Stats

617 Citations
98.56 Total Impact Points

Institutions

  • 1990–2008
    • University of California, Davis
      • • School of Veterinary Medicine
      • • California Animal Health & Food Safety Laboratory System - CAHFS
      Davis, California, United States
  • 2002
    • University of North Carolina at Chapel Hill
      • Department of Epidemiology
      Chapel Hill, NC, United States
  • 2001
    • CSU Mentor
      Long Beach, California, United States
  • 1991
    • North Carolina State University
      • College of Veterinary Medicine
      Raleigh, NC, United States
  • 1988
    • University of Wisconsin–Madison
      Madison, Wisconsin, United States