Diagnostic investigation of chronic porcine reproductive and respiratory syndrome virus in a breeding herd of pigs

Forty-five sows and 15 boars were selected at random from a breeding herd known to be chronically infected with porcine reproductive and respiratory syndrome virus (PRRSV) and lymphoid, immune-privileged, and non-lymphoid/non-immune-privileged tissues were tested for the presence of the virus by PCR, virus isolation, and immunohistochemistry. The virus was isolated from the lateral retropharyngeal lymph node of one sow; the isolate was nucleic acid sequenced and determined to be of field origin, and it was inoculated into two PRRSV-naive pregnant sows (A and B) at 95 days of gestation. They were necropsied 14 days later and samples of maternal and fetal tissue and blood samples were collected. Sow A had 10 fresh, six partially autolysed, and two mummified fetuses, and sow B had six fresh and viable fetuses. Viral nucleic acid was detected by PCR in tissue pools from each sow and also from pooled fetal tissues, and the virus was isolated from fetal pools from sow A.     

Rapid pathotyping of Newcastle disease virus (NDV) using fluorogenic probes in a PCR assay

Hybridisation of PCR fragments with fluorogenic probes specific for pathotype allowed an estimation of pathogenicity of Newcastle disease virus (NDV) isolates using a modified TaqMan procedure. Six probes were used, designed to recognise nucleotide sequences in the fusion protein gene sequence corresponding to the precursor protein F0 cleavage site of both virulent and avirulent viruses. Forty-three of the 45 isolates tested, including 18 examined in a blind study were pathotyped successfully and rapidly, with close correlation between cleavage site nucleotide sequences, TaqMan results and intracerebral pathogenicity index (ICPI) values. One isolate, which could not be pathotyped by nucleotide sequencing, was shown using the TaqMan system to be a mixture of virulent and avirulent NDV. The results of this study suggest that using this modified TaqMan protocol, the likely virulence of most ND isolates can be determined rapidly and reproducibly.     

Campylobacter spp. in Irish feedlot cattle: a longitudinal study involving pre-harvest and harvest phases of the food chain

The aim of this study was to investigate faecal shedding and transmission of Campylobacter spp. in cohorts of cattle within a feedlot, to assess subsequent contamination of carcasses with this pathogen and to identify risk factors associated with faecal shedding of Campylobacter spp. A cohort of 133 heifers housed in four adjacent pens was examined over a five and a half month period, from entering the feedlot to slaughter. A parallel investigation of individual rectal faecal samples and pen environmental samples were taken at monthly intervals from November to February. The entire outer and inner surfaces of a carcass side of each animal were swabbed immediately following slaughter. Campylobacter spp. were isolated from 322 (54%) of the 600 rectal faecal samples. Campylobacter jejuni and C. coli accounted for 69 and 29.7% of the isolate recovered, respectively. A total of 159 environmental samples were examined, of these Campylobacter spp. was isolated from 46 samples (29%). Campylobacter jejuni and C. coli accounted for 35 and 59% of these isolates, respectively. Campylobacter spp. was not isolated from any of the dressed carcasses. Logistic regression indicated prevalence of Campylobacter spp. faecal shedding within pens was positively correlated to the pen, the month of sampling and the Campylobacter spp. contamination status of the pen dividing bars and the water trough surface. Campylobacter spp. should be considered as a pathogen shed in the faeces of a substantial proportion of feedlot cattle. However, with good hygienic practice during harvest, a very low level of this pathogen can be achieved on dressed carcasses.     

First-principles theory for the H + H2O, D2O reactions

A full quantum dynamical study of the reactions of a hydrogen atom with water, on an accurate ab initio potential energy surface, is reported. The theoretical results are compared with available experimental data for the exchange and abstraction reactions in H + D2O and H + H2O. Clear agreement between theory and experiment is revealed for available thermal rate coefficients and the effects of vibrational excitation of the reactants. The excellent agreement between experiment and theory on integral cross sections for the exchange reaction is unprecedented beyond atom-diatom reactions. However, the experimental cross sections for abstraction are larger than the theoretical values by more than a factor of 10. Further experiments are required to resolve this.     

Corrigendum to "A linear programming assessment of the profit from strategies to reduce the prevalence of Staphylococcus aureus mastitis" [Prev. Vet. Med. 33 (1998) 183-193

We used a linear programming model to estimate the financial returns to a Staphylococcus aureus testing and control program over a 1-year period for a 100-cow herd, with a 8636kg rolling-herd average. Six tests, which vary in sensitivity from 0.80 to 0.98 and specificity of 0.99, were examined in simulated herds with 10, 20, and 30% prevalence of S. aureus infection. Sensitivity of these results to a range of assumptions regarding rolling-herd average, milk price, somatic cell-count premium, and cost and cure rate of dry treatment were examined to determine the profits from the program. The profits of a control program are most dependent upon prevalence and cell-count premium. In our simulation for a 100-cow herd, a testing and control program results in a profit ranging from US$1.50 to US$20 per cow per year, except under the lowest prevalence and most-adverse conditions (low yield or low SCC premium).     

Conditional dependence between tests affects the diagnosis and surveillance of animal diseases

Dependence between the sensitivities or specificities of pairs of tests affects the sensitivity and specificity of tests when used in combination. Compared with values expected if tests are conditionally independent, a positive dependence in test sensitivity reduces the sensitivity of parallel test interpretation and a positive dependence in test specificity reduces the specificity of serial interpretation. We calculate conditional covariances as a measure of dependence between binary tests and show their relationship to kappa (a chance-corrected measure of test agreement). We use published data for toxoplasmosis and brucellosis in swine, and Johne's disease in cattle to illustrate calculation methods and to indicate the likely magnitude of the dependence between serologic tests used for diagnosis and surveillance of animal diseases.     

Modification of a bovine ELISA to detect camelid antibodies to Mycobacterium paratuberculosis

Mycobacterium avium subsp. paratuberculosis infection, or Johne's disease, reportedly has a low prevalence in South American camelid populations. Recently, however, single cases in the United States as well as an outbreak of the disease in Australian alpacas (Lama pacos) have been described. To provide a rapid and cost-effective method of diagnosing Johne's disease in this species, the bovine Parachek((R)) Johne's Absorbed EIA (CSL, Vic., Australia) was modified to create a camelid-specific serum antibody assay. An anti-llama IgG conjugated to horseradish peroxidase replaced the anti-bovine immunoglobulin. Checkerboard titration of principal reagents was performed using serum from nine tissue and/or fecal culture-positive camelids. Optimal dilutions of key components were determined in order to provide clear discrimination between positive and negative controls. Completion of a kinetic assay determined the optical density at which the enzyme-substrate reaction should be stopped. A herd of 100 camelids with no history of disease or exposure to M. a. paratuberculosis, a subset of which were tissue and/or fecal culture-negative, was tested to establish a cut-off value. Sample results were expressed as a percentage of the results for control sera by transforming optical density values to ELISA values (EV%). A preliminary EV% cut-off of 20 was established. Using this prototype assay, culture-positive animals showed significantly different antibody responses from culture-negative animals. These results indicate that this camelid-specific ELISA, once refined, may be a useful tool for screening camelid herds for M. a. paratuberculosis infection.