Susanna Sternberg Lewerin

Network analysis of cattle and pig movements in Sweden : Measures relevant for disease control and risk based surveillance

Registration of cattle and pig movements is mandatory in Sweden and all registered movements between farms in the years 2006-2008 were investigated using network analysis. The networks were analysed as monthly and yearly networks, separately per species and with the two species together. Measures that have been previously discussed in relation to outbreaks and disease control were calculated; moreover a measure of the ingoing infection chain was constructed. The ingoing infection chain captures ingoing contacts through other holdings, taking the temporal aspect and sequence of the movements into account. The distribution of the contacts among the holdings was skewed. Many farms had few or no contacts, while others had many, a pattern which has also been described from other countries. The cattle network and the combined network showed a recurring seasonal pattern, while this was not seen in the pig network. The in-degree was not equivalent to the ingoing infection chain; there were holdings with limited direct contacts, but a large number of indirect contacts. The ingoing infection chain could be a useful measure when setting up strategies for disease control and for risk based surveillance as it identifies holdings with many contacts through live animal movements and thus at potentially higher risk for introduction of contagious diseases.

Outbreak of Mycobacterium tuberculosis infection among captive Asian elephants in a Swedish zoo

Between 2001 and 2003, there was an outbreak of tuberculosis in a Swedish zoo which involved elephants, giraffes, rhinoceroses and buffaloes. Cultures of trunk lavages were used to detect infected elephants, tuberculin testing was used in the giraffes and buffaloes, and tracheal lavage and tuberculin testing were used in the rhinoceroses. The bacteria isolated were investigated by spoligotyping and restriction fragment length polymorphism. Five elephants and one giraffe were found to have been infected by four different strains of Mycobacterium tuberculosis.

Application of routines that contribute to on-farm biosecurity as reported by Swedish livestock farmers.

On-farm biosecurity is important for preventing the spread of several contagious animal diseases. In this study, biosecurity routines among Swedish farmers with livestock (cattle, pigs, sheep or goats) were examined through questionnaires posted by mail. Moreover, the use of protective clothing among professionals visiting farms, such as animal transporters and veterinarians, were investigated through assessments made by the farmers. Questionnaires were completed, partly or fully, by 518 farmers (overall response rate 34%). Possible associations between biosecurity routines and livestock species, geographic location and herd size were analysed. Large variations in biosecurity routines were found, both within and between groups, and some farms appeared to have a relatively high level of biosecurity. However, <40% of the farmers reported that they provide protective clothing for visitors, and 50% of farmers buying live animals introduced these directly into the herd without prior isolation. In general, a higher level of biosecurity was reported by farmers with herds with only pigs, when compared to farmers with cattle, sheep/goats or mixed species. A higher level of biosecurity was also reported by larger farms compared to hobby farms. Inconsistent biosecurity routines were reported, which was interpreted as a lack of knowledge of how different infections can spread and how this can be prevented. Furthermore, some replies indicated that the farmers perceived the risk of introduction of disease as low. According to the farmers assessments, the use of protective clothing among professionals visiting farms varied considerably, both among different professions and within the same profession. On average, veterinarians and artificial insemination (AI) technicians got high scores in this assessment, while salesmen, repairmen and animal transporters were reported to seldom use protective clothing. Based on the findings, there is room for improvement of on-farm biosecurity. There is also a need to further investigate the motivators and constraints for altered routines among both Swedish livestock farmers and professionals visiting farms.

Feed-borne Outbreak of Salmonella Cubana in Swedish Pig Farms: Risk Factors and Factors Affecting the Restriction Period in Infected Farms

In 2003, a feed-borne outbreak of Salmonella Cubana occurred in Sweden as a result of contamination in a feed plant. Salmonella Cubana was detected in 49 out of 77 pig farms having received possibly contaminated feed. In this study, potential risk factors for farms being salmonella positive were examined, and a survival analysis was performed to investigate risk factors affecting the restriction period for salmonella positive farms.The median restriction time for all 49 farms was 17 weeks. An increased risk for farms being salmonella infected (positive in feed and/or faeces) was seen for fattening farms and farms feeding soy. The survival analysis showed that herds with a low level of infection and farms with a high hygiene level had shorter restriction times.This study is unique as it investigates a real outbreak of feed-borne salmonella, where the source of infection was reliably identified, the period of exposure could be defined and data were collected from all exposed farms.SammanfattningSommaren 2003 påvisades en kontamination med Salmonella Cubana i en svensk foderfabrik. Vidare utredning visade att potentiellt smittat foder hade levererats till 77 svinbesättningar.I 49 av de 77 besättningarna kunde S. Cubana isoleras vid provtagningar av svin och/eller fodersystem. Dessa besättningar spärrförklarades och sanerades i enlighet med det svenska salmonellakontrollprogrammet. Denna studie omfattar en riskfaktoranalys och en överlevnadsanalys. Riskfaktorer för att en besättning som mottagit foder från den kontaminerade fabriken infekterats med salmonella undersöktes, respektive faktorer som kan ha påverkat spärrtidens längd på de gårdar som befanns salmonellapositiva. För de 49 drabbade besättningar var medianvärdet för spärrtidens längd 17 veckor. En ökad risk för att ha blivit infekterad med salmonella sågs hos gårdar med slaktsvinsproduktion och hos gårdar som köpt soja från den kontaminerade foderfabriken. Det visades även att gårdar med ett lågt smittryck, samt de med en hög hygiennivå, hade kortare spärrtid. Denna studie är unik då den undersöker ett verkligt utbrott av foderburen salmonella där smittkällan identifierats, exponeringsperioden definierats och data samlats in från samtliga gårdar som fått det potentiellt smittade fodret. .

Disease awareness, information retrieval and change in biosecurity routines among pig farmers in association with the first PRRS outbreak in Sweden

Reaching farmers with information is important when eradicating outbreaks of contagious diseases, the actions they take related to contacts and biosecurity, as well as early notification of disease can have a significant effect on limiting the spread of disease. The aim of this study was to investigate Swedish pig farmers disease awareness, information retrieval and if they change their biosecurity routines during an outbreak of an exotic infectious disease, using the experience from the first outbreak of PRRS in Sweden in 2007. Data were collected through a questionnaire to 153 farmers. Our findings indicate that written information which was sent to all farmers was not sufficient. Herd size, as an indicator for the type of farmer, was significantly associated with awareness. Farmers with medium or large herds were more aware there had been an outbreak (OR 32.3, p=0.001), of the means of spread and the signs of disease, and they were more active in information search compared to farmers with small herds. Closeness to the outbreak was important for motivating farmers to actively search for information. The results from this study could be useful when planning information campaigns during future outbreaks and when modelling disease outbreaks.

Spatial and temporal investigations of reported movements, births and deaths of cattle and pigs in Sweden

BackgroundLivestock movements can affect the spread and control of contagious diseases and new data recording systems enable analysis of these movements. The results can be used for contingency planning, modelling of disease spread and design of disease control programs.MethodsData on the Swedish cattle and pig populations during the period July 2005 until June 2006 were obtained from databases held by the Swedish Board of Agriculture. Movements of cattle and pigs were investigated from geographical and temporal perspectives, births and deaths of cattle were investigated from a temporal perspective and the geographical distribution of holdings was also investigated.ResultsMost movements of cattle and pigs were to holdings within 100 km, but movements up to 1200 km occurred. Consequently, the majority of movements occurred within the same county or to adjacent counties. Approximately 54% of the cattle holdings and 45% of the pig holdings did not purchase any live animals. Seasonal variations in births and deaths of cattle were identified, with peaks in spring. Cattle movements peaked in spring and autumn. The maximum number of holdings within a 3 km radius of one holding was 45 for cattle and 23 for pigs, with large variations among counties. Missing data and reporting bias (digit preference) were detected in the data.ConclusionThe databases are valuable tools in contact tracing. However since movements can be reported up to a week after the event and some data are missing they cannot replace other methods in the acute phase of an outbreak. We identified long distance transports of cattle and pigs, and these findings support an implementation of a total standstill in the country in the case of an outbreak of foot-and-mouth disease. The databases contain valuable information and improvements in data quality would make them even more useful.

Probability of freedom from disease after the first detection and eradication of PRRS in Sweden: Scenario-tree modelling of the surveillance system

In July 2007, PRRS was detected for the first time in Sweden. A total of eight positive herds were identified and various measures were taken to eradicate the disease, including restrictions and slaughter of infected herds. Subsequently, both active and passive surveillance activities were undertaken. This study describes stochastic scenario-tree modelling of all the various surveillance system components, to estimate the current probability that Sweden is free from PRRS. The model includes all actions taken after the first positive herd was detected. The surveillance system components included in the model were as follows: investigations undertaken in association with the outbreak, a serological study based on samples collected at slaughter, samples collected in the national PRRS surveillance programme and passive clinical surveillance. The probability of freedom was estimated in time steps of 1 month, from July to December 2007. After each time step, the calculated posterior probability of freedom from the previous month, combined with the probability of introduction, was used as a prior probability for the next month. The result from the model showed a 99.8% probability that Sweden was free from PRRS at the end of December 2007. The estimated total sensitivity of the surveillance system varied between 81.2% and 94.3% and was highest during the first months after the outbreak. For sensitivity analysis purposes, the model was also applied using higher risks of introduction. However, this did not make considerable difference to the final estimates.

Possible means of introduction of bluetongue virus serotype 8 (BTV-8) to Sweden in August 2008: comparison of results from two models for atmospheric transport of the Culicoides vector.

In September 2008, bluetongue virus serotype 8 (BTV-8) infection was detected for the first time in Sweden, in a dairy herd on the west coast. Two different previously published operational atmospheric dispersion models indicate that midges from infected regions in Europe are likely to have reached Sweden by atmospheric transport during an estimated infection window. Both models indicated that the likely dates for the incursion of midges were overnight on August 6 to 7 and August 14 to 15; however, the less constrained model indicated a number of additional possible dates. The distribution of infected herds detected by active surveillance coincides with the regions that were indicated by the models to have been reached by midges from regions in Denmark and Germany with infected herds. It is likely that several points of introduction of infected midges occurred, possibly on different occasions. No alternative routes for introduction of the infection to Sweden were identified, supporting the theory that BTV-8 was introduced by infected midges carried by the wind.

Anthrax outbreak in a Swedish beef cattle herd - 1st case in 27 years: Case report

After 27 years with no detected cases, an outbreak of anthrax occurred in a beef cattle herd in the south of Sweden. The outbreak was unusual as it occurred in winter, in animals not exposed to meat-and-bone meal, in a non-endemic country.The affected herd consisted of 90 animals, including calves and young stock. The animals were kept in a barn on deep straw bedding and fed only roughage. Seven animals died during 10 days, with no typical previous clinical signs except fever. The carcasses were reportedly normal in appearance, particularly as regards rigor mortis, bleeding and coagulation of the blood. Subsequently, three more animals died and anthrax was suspected at necropsy and confirmed by culture and PCR on blood samples.The isolated strain was susceptible to tetracycline, ciprofloxacin and ampicillin. Subtyping by MLVA showed the strain to cluster with isolates in the A lineage of Bacillus anthracis.Environmental samples from the holding were all negative except for two soil samples taken from a spot where infected carcasses had been kept until they were picked up for transport.The most likely source of the infection was concluded to be contaminated roughage, although this could not be substantiated by laboratory analysis. The suspected feed was mixed with soil and dust and originated from fields where flooding occurred the previous year, followed by a dry summer with a very low water level in the river allowing for the harvesting on soil usually not exposed. In the early 1900s, animal carcasses are said to have been dumped in this river during anthrax outbreaks and it is most likely that some anthrax spores could remain in the area.The case indicates that untypical cases in non-endemic areas may be missed to a larger extent than previously thought. Field tests allowing a preliminary risk assessment of animal carcasses would be helpful for increased sensitivity of detection and prevention of further exposure to the causative agent.

Infection with bluetongue virus serotype 8 in Sweden in 2008

On September 6, 2008, bluetongue was detected for the first time in Sweden. Intensified active surveillance in cattle and vector surveillance, prompted by the situation in northern Europe, preceded the detection. A vaccination campaign and intensive surveillance activities were initiated nationally, but with a focus on the southern part of the country. Measures included both active and passive surveillance by serology and PCR in ruminants, along with vector surveillance. The investigations revealed that the infection occurred over a large area of southern Sweden during September and October 2008, despite comparatively low vector activity, an apparently low viral load and no clinical disease. Transplacental infection was detected in one case.