Anna Ohlson

Application of network analysis parameters in risk-based surveillance - Examples based on cattle trade data and bovine infections in Sweden

Abstract Financial resources may limit the number of samples that can be collected and analysed in disease surveillance programmes. When the aim of surveillance is disease detection and identification of case herds, a risk-based approach can increase the sensitivity of the surveillance system. In this paper, the association between two network analysis measures, i.e. ‘in-degree’ and ‘ingoing infection chain’, and signs of infection is investigated. It is shown that based on regression analysis of combined data from a recent cross-sectional study for endemic viral infections and network analysis of animal movements, a positive serological result for bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) is significantly associated with the purchase of animals. For BCV, this association was significant also when accounting for herd size and regional cattle density, but not for BRSV. Examples are given for different approaches to include cattle movement data in risk-based surveillance by selecting herds based on network analysis measures. Results show that compared to completely random sampling these approaches increase the number of detected positives, both for BCV and BRSV in our study population. It is concluded that network measures for the relevant time period based on updated databases of animal movements can provide a simple and straight forward tool for risk-based sampling.

The relationship between antibody status to bovine corona virus and bovine respiratory syncytial virus and disease incidence, reproduction and herd characteristics in dairy herds.

BackgroundBovine respiratory syncytial virus (BRSV) and bovine corona virus (BCV) affects cattle worldwide. Our objective was to evaluate the effects of these infections on general health and reproduction parameters measurable on herd level and to explore the association between antibody status and some herd characteristics.MethodsWe collected a pooled milk sample from five primiparous cows from 79 Swedish dairy herds in September 2006. The samples were analysed for immunoglobulin G antibodies to BCV and BRSV with indirect enzyme-linked immunosorbent assays. Herd level data from 1 September 2005 to 30 August 2006 were accessed retrospectively. The location of the herds was mapped using a geographical information system.ResultsTen herds were antibody negative to both viruses and were compared with 69 herds positive to BCV or BRSV or both. Positive herds had a higher (P = 0.001) bulk tank milk somatic cell count (BMSCC) compared with negative herds. The medians for all other analyzed health and reproductive parameters were consistently in favour of the herds negative to both viruses although the differences were not statistically significant. A higher proportion (P = 0.01) of herds used professional technicians for artificial insemination, rather than farm personnel, amongst the 33 herds negative to BCV compared with the 46 positive herds.ConclusionsOur result shows that herds that were antibody positive to BCV and/or BRSV had a higher BMSCC compared with herds negative to BCV and BRSV. There was also tendency that negative herds had a better general herd health compared with positive. A higher proportion amongst the BCV negative herds used external technicians for AI instead of farm personnel, indicating that it is possible to avoid infection although having regular visits. Negative herds were located in close proximity to positive herds, indicating that local spread and airborne transmission between herds might not be of great importance and that herds can stay free from these infection transmission although virus is circulating in the area.

A longitudinal study of the dynamics of bovine corona virus and respiratory syncytial virus infections in dairy herds

Abstract The objective of this study was to examine the dynamics of bovine corona virus (BCV) and bovine respiratory syncytial virus (BRSV) infections in dairy herds over a 3-year period. The status of 79 dairy herds located in two Northern and two Southern Regions of Sweden were surveyed by measuring antibody concentrations to BCV and BRSV in pooled milk samples from primiparous cows, and in bulk-tank milk twice annually. In the Southern Regions the percentage of antibody-positive herds remained persistently high (75–100%), whereas in herds based in the Northern Region, the percentage of positive herds for BCV and BRSV was 38–80% and 0–80%, respectively, with antibody levels to BRSV decreasing over time. Pooled milk samples of ‘home-bred’ primiparous animals were found to be most useful in terms of monitoring herd status but could gradually be replaced by bulk-tank sampling once freedom from infection was established.

Phylogenetic analysis of bovine respiratory syncytial viruses from recent outbreaks in feedlot and dairy cattle herds

Bovine respiratory syncytial virus (BRSV) is one of the major causes of bovine respiratory disease worldwide. In order to study the molecular epidemiology of the virus, samples from 30 BRSV outbreaks in cattle herds located in different parts of Sweden were collected from 2007 to 2011. The samples were analyzed by PCR, and the glycoprotein (G) gene was sequenced. BRSV was detected in outbreaks of respiratory disease in both dairy and feedlot herds most often during the winter period but also during the summer months (May to August). This indicates that circulation of the virus between herds occurs throughout the year. Comparative sequence analysis revealed a high degree (more than 94.5%) of sequence identity among the collected strains. Phylogenetic analysis showed that 29 out of the 30 strains formed a unique clade. Identical sequences found in herds sampled within a few months’ time suggested that these herds were part of a common transmission chain. One strain from a single outbreak in a herd in southern Sweden clustered with Danish strains and showed a distant relationship to the rest of the Swedish strains. Further studies are highly warranted to clarify the inter-herd transmission routes of BRSV. Such knowledge is essential for the control of the spread of this virus between herds, regions and even countries.

Tracing the transmission of bovine coronavirus infections in cattle herds based on S gene diversity

Abstract Bovine coronavirus (BCoV) is found worldwide and causes respiratory infections and diarrhoea in calves and adult cattle. In order to investigate the molecular epidemiology of BCoV, 27 reverse transcription polymerase chain reaction (RT-PCR) positive samples from 25 cattle herds in different parts of Sweden were analysed. A 1038-nucleotide fragment was PCR amplified and directly sequenced. The analysed BCoV strains showed a high sequence identity, regardless of whether they were obtained from outbreaks of respiratory disease or diarrhoea or from calves or adult cattle. Circulation of an identical BCoV strain during a 4-month period was demonstrated in calves in one dairy herd. In a regional epizootic of winter dysentery in Northern Sweden, highly similar BCoV strains were detected. In the Southern and Central regions, several genotypes of BCoV circulated contemporaneously, indicating that in these regions, which had a higher density of cattle than the Northern regions, more extensive transmission of the virus was occurring. Identical BCoV sequences supported the epidemiological data that inter-herd contact through purchased calves was important. Swedish BCoV strains unexpectedly showed a high homology with recently detected Italian strains. This study shows that molecular analysis of the spike (S) glycoprotein gene of BCoV can be a useful tool to support or rule out suspected transmission routes.

Bovine respiratory syncytial virus and bovine coronavirus in Swedish organic and conventional dairy herds

BackgroundInfections with bovine respiratory syncytial virus (BRSV) and bovine coronavirus (BoCV) are endemic to the cattle populations in most countries, causing respiratory and/or enteric disease. It has been demonstrated that herds can remain free from these infections for several years also in high prevalence areas. Organically managed (OM) dairy herds have been shown to have lower seroprevalence of both viruses compared to conventionally managed (CM) herds. The objective of this study was to challenge the hypothesis of a lower occurrence of BRSV and BoCV in OM compared to CM dairy herds.In November 2011, May 2012 and May 2013 milk samples from four homebred primiparous cows were collected in 75 to 65 OM and 69 to 62 CM herds. The antibody status regarding BRSV and BoCV was analysed with commercial indirect ELISAs. Herds were classified as positive if at least one individual sample was positive.ResultsThe prevalence of positive herds ranged from 73.4% to 82.3% for BRSV and from 76.8% to 85.3% for BoCV among OM and CM herds, over the three sampling occasions. There was no statistically significant difference between OM and CM herds at any sampling occasion. The incidence risk of newly infected herds did not differ statistically between OM and CM herds at any sampling occasion, neither for BRSV nor for BoCV. The incidence of herds turning sero-negative between samplings corresponded to the incidence of newly infected. Bulk tank milk (BTM) samples were also sampled in the herds and analysed. Several herds were negative on individual samples but positive in BTM. Herd-level data on production, health and reproduction were retrieved from VÄXA Sweden and the study herds were representative of the source population.ConclusionThere was no difference in prevalence of or incidence risk for BRSV or BoCV between Swedish OM and CM herds. Because the incidence of herds becoming seropositive was balanced by herds becoming seronegative it should be possible to lower the prevalence of these two infections among Swedish dairy cattle herds if biosecurity is improved.

Comparison of Bovine coronavirus–specific and Bovine respiratory syncytial virus–specific antibodies in serum versus milk samples detected by enzyme-linked immunosorbent assay

Bovine coronavirus (BCV; Betacoronavirus 1) and Bovine respiratory syncytial virus (BRSV) are significant causes of enteric and respiratory disease in beef and dairy cattle throughout the world. Indirect enzyme-linked immunosorbent assays are widely used to detect serum antibodies for herd monitoring and prevalence studies. In dairy herds, milk is more readily collected than serum. Hence, in order to investigate the test agreement between serum and milk, both serum and milk samples from 105 cows in 27 dairy herds were analyzed in parallel for presence of immunoglobulin G antibodies to BCV and BRSV. The Bland–Altman analyses of data demonstrated good agreement between serum and milk antibody titers for both viruses. The results indicate milk samples are sufficient for surveillance of antibodies to BCV and BRSV.

Exploring milk shipment data for their potential for disease monitoring and for assessing resilience in dairy farms

Abstract The use of routinely recorded data for research purposes and disease surveillance is an attractive proposition. However, this requires that the validity and reliability of the data be evaluated for the purpose for which they are to be used. This manuscript reports an evaluation of milk shipment data for evaluating their usefulness in disease monitoring and the resilience of organic and conventional dairy herds in Sweden. A large number of inconsistencies were observed in the data, necessitating substantial efforts to “clean” the data. Given that the selection of rules used in the cleaning process was subjective in nature, a sensitivity analysis was carried out to determine if different cleaning routines produced substantially different results. Despite the cleaning efforts we observed far more large residuals at the shipment level than expected. Thus, it was concluded that the data were too “noisy” to be used for identification of short term impacts on milk production. Resilience was evaluated by examining the residual variance in milk shipped per cow per day under the assumption that herds with high resilience would have lower residual variance. The effects on residual variance of organic status or whether or not the herd used an automatic milking system were evaluated in models in which the residual variance was stratified or not by these factors. We did not find consistent evidence to suggest that organic herds had higher resilience than conventional herds, but this could be partly due to using residual variance as the measure indicating resilience.