Comparison of commercial enzyme-linked immunosorbent assay kits with agar gel precipitation and hemagglutination-inhibition tests for detecting antibodies to avian influenza viruses.

We evaluated the utility of 5 commercial enzyme-linked immunosorbent assay (ELISA) kits for detecting antibodies to avian influenza viruses. The sensitivities and specificities of the ELISA kits were compared with those of the agar gel precipitation (AGP) and hemagglutination-inhibition (HI) tests. The results suggest that some ELISA kits might not be suitable for monitoring during the early stages of avian influenza virus infections. Therefore, ELISA kits should only be used in conjunction with a profound knowledge about monitoring of avian influenza.

Estimation of the within-herd transmission parameter of bovine leukemia virus.

In this study, we estimated the transmission parameter of bovine leukemia virus (BLV) infection within herds by following up the serological status of cattle in beef and dairy herds. Two consecutive serological tests using ELISA were conducted for cattle at four beef breeding farms and nine dairy farms with an interval of 5 months. Assuming that sero-converted animals were newly infected, transmission parameters were estimated using a hierarchical Bayesian Poisson model. All tested herds had at least one positive animal at the first testing, but sero-prevalence considerably varied between herds. It is estimated that 0.62 animals were infected by one infected animal introduced into a fully susceptible population for both beef breeding and dairy farms during the 5-month interval. The 95% credible interval of the transmission parameter for the combined herds was 0.37-0.89 over this period. Asymptomatically infected animals with BLV can become the source of lifelong infection in herds. These estimated transmission parameters indicated that the early removal of infected animals with periodical tests could reduce the dissemination of BLV infection within herds.

Risk of equine infectious disease transmission by non-race horse movements in Japan.

For determining surveillance programs or infectious disease countermeasures, risk evaluation approaches have been recently undertaken in the field of animal health. In the present study, to help establish efficient and effective surveillance and countermeasures for equine infectious diseases, we evaluated the potential risk of equine infectious disease transmission in non-race horses from the viewpoints of horse movements and health management practices by conducting a survey of non-race horse holdings. From the survey, the non-race horse population was classified into the following five sectors based on their purposes: the equestrian sector, private owner sector, exhibition sector, fattening sector and others. Our survey results showed that the equestrian and private owner sectors had the largest population sizes, and movements between and within these sectors occurred quite frequently, while there was little movement in the other sectors. Qualitative evaluation showed that the equestrian and private owner sectors had relatively high risks of equine infectious disease transmission through horse movements. Therefore, it would be effective to concentrate on these two sectors when implementing surveillance or preventative measures. Special priority should be given to the private owner sector because this sector has not implemented inspection and vaccination well compared with the equestrian sector, which possesses a high compliance rate for these practices. This qualitative risk evaluation focused on horse movements and health management practices could provide a basis for further risk evaluation to establish efficient and effective surveillance and countermeasures for equine infectious diseases.

Duration of maternally derived antibodies against Akabane virus in calves: survival analysis.

To prevent encephalomyelitis caused by Akabane virus, as observed in 2006, vaccination of calves is one of the most effective prophylactic measures. For vaccination of calves, the duration of the maternal antibodies need to be considered because these antibodies are an obstacle to the effectiveness of the vaccine. In order to estimate the age of antibody decay in calves and to find factors influencing the duration of passive immunity, we conducted survival analysis using data from nationwide sentinel surveillance for Akabane disease. The accelerated failure time model based on the presence of interval censored data was used. The best fit model with a log-logistic distribution indicated that the maternal antibodies of beef calves last 1.11 times longer (95% confidence interval [CI]=1.06-1.16) than those of dairy calves. Calves in the western part of Japan and on Kyushu island, Japan, maintained the maternal antibodies 1.17 times (95% CI=1.11-1.23 and 1.10-1.24, respectively) longer than those in the eastern part of Japan. The ages at which beef calves in the eastern part of Japan, western part of Japan and Kyushu loose the antibodies, with 90% probability, were estimated to be 4.1, 4.8 and 4.8 months, respectively, while the ages were 3.7, 4.3 and 4.3 months for dairy calves in the same regions. The duration of maternal immunity to Akabane virus was different for different types of cattle and among different regions. These differences need to be taken into account when a vaccination strategy is adopted for preventing epizootic encephalomyelitis in the future.

Spatial analysis of low pathogenic H5N2 avian influenza outbreaks in Japan in 2005.

We conducted a spatial analysis of low pathogenic H5N2 avian influenza (AI) outbreaks, that affected 41 chicken farms in Japan in 2005. A statistically significant (p=0.001) cluster of AI-positive farms was identified in the central part of Ibaraki Prefecture. Inside the AI cluster, the density was high for both chicken farms and chicken population, the proportion of layer finisher type farms was high and the farm size was large. We considered it important to take precautions for AI outbreaks in densely chicken-populated areas and to implement appropriate movement control around the affected farms to prevent transmission among farms located within small distances in the case of AI outbreaks. Spatial scan statistics are applicable in veterinary epidemiology to detection of high risk areas for animal diseases.

Evaluation of surveillance strategies for bovine brucellosis in Japan using a simulation model.

Bovine brucellosis is caused by Brucella abortus and induces abortions in female cattle, with other cattle at risk of infection from the aborted fetus or contaminated placenta. In Japan, the number of cases has dramatically reduced due to national surveillance and eradication strategies. Bovine brucellosis is now believed to be eradicated in Japan. Here, we examine the surveillance strategies currently in place for early detection of infected cattle in the event of a future reintroduction of the disease. We compared current serological surveillance for the dairy population with bulk-milk surveillance and abortion surveillance, and used time to detection as the main criterion of surveillance efficacy. A stochastic individual-based model (IBM) was developed to simulate disease transmission within and between farms. Using outputs from the transmission model, a comparison of surveillance strategies was simulated. For evaluation of the robustness of the parameter values used in the transmission model, a sensitivity analysis was conducted. For the purpose of evaluating the direct costs of each surveillance strategy, the annual number of samples to be tested and the annual number of farms to be visited were estimated. Our results indicated that current serological surveillance with 60-month test intervals is not effective enough for rapid detection of a brucellosis outbreak. Bulk-milk surveillance appeared the most effective method based on the early detection of infected cows and a reduced number of samples required. The time to detection for abortion surveillance was greater than that of bulk-milk surveillance but varied widely depending on the reported ratio of abortions. Results from the surveillance model were consistent when alternative scenarios were applied to the transmission model. Although our model cannot exactly replicate an actual brucellosis outbreak, or the results of surveillance, our results may help decision-makers to choose the most effective surveillance strategy.

Simulation-based estimation of BSE infection in Japan.

The first case of bovine spongiform encephalopathy (BSE) in Japan was found in September 2001. As a result, national BSE surveillance systems in slaughterhouses and farms were introduced between October 2001 and April 2004. All cattle, with the exception of those under 24 months of age that die at farms, now undergo compulsory testing when they die or are slaughtered. The removal of specified risk material (SRM) from all slaughtered cattle and a ban on the feeding of meat-and-bone meal to all farm animals were implemented in October 2001. However, infected cattle that died or were slaughtered before these measures were put into practice could have been a source of infection to other cattle through the rendering process. The slaughtered cattle could also have been a source of infection to humans via SRM that entered the food chain. The purpose of this study was to estimate the number of BSE-infected cattle that could have been a source of infection to cattle and humans before October 2001. Since all typical cases were dairy cattle, this study focused on the dairy cattle population. We developed a simulation model to obtain the year of death and the final disposition of infected cows born in each year from 1996 to 2001. In this model, the dairy cattle population was divided into birth cohorts, and parameters regarding its population dynamics were assumed to be constant. Using this model, the total number of infected cattle in each birth year was estimated by maximum likelihood estimation using data on the number of detected cases from 2002 to 2006. Finally, the number of infected cattle that died or were slaughtered each year was estimated by Monte-Carlo simulation using the same model with the total number of infected cattle estimated by maximum likelihood estimation. It was estimated that the majority of infected cattle that could have been sources of infection before 2001 were born in 1996. The total number born in 1996 was estimated to be 155 (95% confidence interval: 90-275). Of these 155 cattle, 56 died or were slaughtered before October 2001, after the accumulation of infectious agent in their bodies. Only 5 of these 56 cattle were estimated to have been slaughtered. Therefore, the number of infected cattle that could have served as a source of human infection would appear to have been a very limited subset of the BSE-infected cattle in Japan.

Preliminary evaluation of diagnostic tests for avian influenza using the Markov Chain Monte Carlo (MCMC) Method in an emergency surveillance.

In June 2005, an outbreak of avian influenza (AI) caused by a low pathogenic H5N2 virus was identified in Japan. A serological surveillance was conducted because the infected chickens did not show any clinical signs. The Markov Chain Monte Carlo Method was used to evaluate the performances of serological HI and AGP tests because there was not enough time when the surveillance was initiated to conduct a test evaluation. The sensitivity of the AGP test (0.67) was lower than that of the HI test (0.99), while the specificities were high for both tests (0.96 for AGP and 0.90 for HI). Based on the low sensitivity of the AGP test, the HI test was used for primary screening in later stages of the epidemic.

Epidemiologic indicators associated with within-farm spread of Johne's disease in dairy farms in Japan.

Epidemiologic indicators associated with within-farm infection of Johne's disease in dairy farms in Japan were determined through a nationwide investigation of infected farms. We assumed that subsequent detection of the disease within one year after the first detection could represent the occurrence of within-farm spread occurring before the first detection. Of 594 infected farms, 158 farms (27%) had at least one additional detection. Logistic regression analysis using epidemiologic information obtained from infected farms at the time of the first detection revealed three epidemiologic indicators associated with subsequent detection. Farms at which the first cases included cattle with clinical signs were 3.8 (95% confidence interval: 2.2, 6.8) times more likely to have additional detections than those with cattle without clinical signs. Similarly, farms where two or more cattle were detected at the time of first detection and where cattle were held in a loose housing system were 2.8 (95% CI: 1.8, 4.5) and 2.0 (95% CI: 1.1, 3.6) times more likely to have additional detections than those where only one animal was detected and a tied-up housing system was used, respectively. These epidemiologic indicators are likely important determinants in the selection of farms requiring more intensive on-farm control measures.

A quantitative assessment of the risk of exposure to bovine spongiform encephalopathy via meat-and-bone meal in Japan.

The feeding of meat-and-bone meal (MBM) derived from cattle infected with bovine spongiform encephalopathy (BSE) is a major source of BSE infection. The risks of BSE infection via MBM in Japan were examined quantitatively to estimate infectivity to cattle via MBM derived from a single clinically infected animal being rendered. Three routes of exposure were modeled: (i) feeding cattle concentrates containing MBM as an ingredient, (ii) feeding cattle concentrates contaminated with MBM from non-ruminant feed at feed plants and (iii) directly feeding MBM in supplemental form to cattle on farms. The effectiveness of measures designed to restrict the feeding of ruminants with ruminant MBM (feed restriction) as well as differences in the risk of exposure among regions were examined using the model. The model revealed that the median total infectivity fed to dairy cattle via MBM derived from one infected animal was approximately 0.49 cattle oral ID(50) (5th percentile=0.43ID(50), 95th percentile=0.54ID(50)). This value was reduced by 55% after the addition of MBM to cattle concentrates was restricted in 1996. The risk of exposure in dairy cattle was twice that in beef cattle. Comparisons of regional differences in exposure risk indicated that the risk was highest in a region where 14 of the 20 BSE cases reported to date were born. Our model suggested that the routes of exposure via MBM were unlikely to result in increased propagation of BSE in Japan. Furthermore, despite some regional variation, the risk of exposure declined further after the feed restriction was imposed in 1996.

Cross-sectional survey of ixodid tick species on grazing cattle in Japan.

Ixodid tick species were collected from cattle in 60 grazing fields throughout Japan. Haemaphysalis longicornis was mainly recovered in the western and southern regions, while Ixodes species were collected mainly in the central to northern regions. Other tick species such as Amblyomma testudinarium, Boophilus microplus, H. flava and H. kitaokai were identified from a few fields in the central and southern regions. Haemaphysalis longicornis were recovered in the fields with higher temperatures and annual rainfall, whereas I. ovatus and I. persulcatus were collected in fields with lower temperatures and annual rainfall. Some of these tick species are capable of transmitting pathogens harmful to cattle and humans, so proper control strategies are required.

Evaluation of bovine spongiform encephalopathy (BSE) infection risk of cattle via sewage sludge from wastewater treatment facilities in slaughterhouses in Japan.

Scattered SRM residues from BSE-infected cattle are possible to contaminate sewage during the slaughtering process in slaughterhouses. A proportion of the sludge discharged from wastewater treatment facilities at slaughterhouses has historically been processed into fertilizer. We therefore investigated the associated risk of BSE infection to cattle via sludge-derived fertilizer. Each stage of the process associated with BSE exposure was qualitatively evaluated and quantitative evaluations were subsequently performed using infectious dose as a unit of concern. Results of these qualitative evaluations indicated that installation of filter(s) at the drains to the wastewater treatment facilities has been undertaken by many slaughterhouses and has decreased the likelihood of SRM contamination of sewage. The level of sludge-derived fertilizer ingested by cattle was considered to be very low since the fertilizer is mixed with the ground soil, and the amount of soil ingested by cattle is likely to be small. Results from the quantitative analysis indicated the total infectious dose ingested by cattle in Japan from an infected cow has been estimated to be 5.5 x 10(-3) ID(50). Preventing scattering of SRM during the slaughtering process, installing filters to the drains with the removal of residues from the drain water and preventing the application of sludge-derived fertilizer to pasturelands would be effective to reduce the risk. Although the limited extent of available information, this study should provide useful indication for the development of an inclusive risk assessment for slaughterhouse sludge in the future.