No temporal trends in the prevalence of atypical scrapie in British sheep, 2002–2006

Background

So-called atypical scrapie was first identified in Great Britain (GB) in 2002 following the introduction of wide-scale scrapie surveillance. In particular, abattoir and fallen stock surveys have been carried out in GB since 2002, with a total of 147 atypical positives identified by the end of 2006. The results of these surveys provide data with which to assess temporal trends in the prevalence of atypical scrapie in sheep in Great Britain between 2002 and 2006.

Results

Using the results of abattoir and fallen stock surveys, the prevalence of atypical scrapie (percentage of samples positive) was estimated. The prevalence in the abattoir and fallen stock surveys, for all years combined, was 0.09% (95% confidence interval (CI): 0.08%–0.11%) and 0.07% (95% CI: 0.05%–0.11%), respectively. There were no significant temporal trends in either survey. Comparing the surveys' results, there were no significant differences in annual prevalence or the prevalence within PrP genotypes. For the abattoir survey, the PrP genotype with the highest prevalence was AHQ/AHQ, which was significantly higher than all other genotypes, except ARR/AHQ, AHQ/ARH and ARH/ARQ.

Conclusion

The estimated prevalence of atypical scrapie was similar in both the abattoir and fallen stock surveys. Our results indicate there was no significant temporal trend in prevalence, adding to evidence that this atypical form of scrapie may be a sporadic condition or, if it is infectious, that the force of infection is very low.

     

Spatial distribution of the active surveillance of sheep scrapie in Great Britain: an exploratory analysis

Background  

This paper explores the spatial distribution of sampling within the active surveillance of sheep scrapie in Great Britain. We investigated the geographic distribution of the birth holdings of sheep sampled for scrapie during 2002 – 2005, including samples taken in abattoir surveys (c. 83,100) and from sheep that died in the field ("fallen stock", c. 14,600). We mapped the birth holdings by county and calculated the sampling rate, defined as the proportion of the holdings in each county sampled by the surveys. The Moran index was used to estimate the global spatial autocorrelation across Great Britain. The contributions of each county to the global Moran index were analysed by a local indicator of spatial autocorrelation (LISA).     

An economic evaluation of preclinical testing strategies compared to the compulsory scrapie flock scheme in the control of classical scrapie

Cost-benefit is rarely combined with nonlinear dynamic models when evaluating control options for infectious diseases. The current strategy for scrapie in Great Britain requires that all genetically susceptible livestock in affected flocks be culled (Compulsory Scrapie Flock Scheme or CSFS). However, this results in the removal of many healthy sheep, and a recently developed pre-clinical test for scrapie now offers a strategy based on disease detection. We explore the flock level cost-effectiveness of scrapie control using a deterministic transmission model and industry estimates of costs associated with genotype testing, pre-clinical tests and the value of a sheep culled. Benefit was measured in terms of the reduction in the number of infected sheep sold on, compared to a baseline strategy of doing nothing, using Incremental Cost Effectiveness analysis to compare across strategies. As market data was not available for pre-clinical testing, a threshold analysis was used to set a unit-cost giving equal costs for CSFS and multiple pre-clinical testing (MT, one test each year for three consecutive years). Assuming a 40% within-flock proportion of susceptible genotypes and a test sensitivity of 90%, a single test (ST) was cheaper but less effective than either the CSFS or MT strategies (30 infected-sales-averted over the lifetime of the average epidemic). The MT strategy was slightly less effective than the CSFS and would be a dominated strategy unless preclinical testing was cheaper than the threshold price of ￡6.28, but may be appropriate for flocks with particularly valuable livestock. Though the ST is not currently recommended, the proportion of susceptible genotypes in the national flock is likely to continue to decrease; this may eventually make it a cost-effective alternative to the MT or CSFS.     

Isolation of Scrapie Agent from the Placenta of Sheep with Natural Scrapie in Japan

A five-month-pregnant Suffolk sheep histologically diagnosed as spontaneous scrapie was studied. Western blot analysis was performed with rabbit serum against the sheep scrapie-associated fibrils (SAF). In the proteinase K (pk)-treated parental brain and spleen samples, three major bands (15 K, 18 K, and 23 K) were detected. These major bands were not detected from the placenta. Infectious agents were isolated in mice from the brain samples but not from the placental homogenates. In another case of a three-month-pregnant Corriedale sheep without any clinical sign of, but histologically diagnosed as scrapie, was also studied in a similar approach. In the parental brain samples, three major bands (15 K, 18 K and 23 K) were detected. SAF protein was not detected in the parental spleen and placenta. No bands reactive with the antiserum were detected in any other samples from the fetal brain and spleen in both cases. However, infectious agents were isolated in mice from both brain and placental homogenates. Since the placenta is an important site of natural infection, it is worthwhile to study these tissues for the epidemiological study of scrapie infection.     

Seroprevalence of ovine paratuberculosis infection in the Northeast of Portugal

A survey to estimate the prevalence of ovine paratuberculosis in sheep flocks was conducted in the Northeast of Portugal. In total, 3900 samples of sheep older than 2 years belonging to 150 sheep flocks from 12 different territorial Livestock Farmers Organizations (OPP) were investigated for the presence of antibodies against Mycobacterium avium subspecies paratuberculosis using a commercial enzyme-linked immunosorbent assay (ELISA) test. Seventy (46.7%) flocks had one or more serologically positive animals. The individual paratuberculosis prevalence was 3.7% (144/3900), with values ranging from 0.0 to 10.3% per flock. Statistically significant differences between OPP, herd size and the presence of clinical signs was observed. According to the test sensitivity and specificity claimed by the manufacturer, the true prevalence in the Northeast of Portugal was calculated as 6.4%. In summary, a small percentage of animals and a high percentage of flocks in the Northeast of Portugal were serologically positive.     

National survey for Salmonella in pigs, cattle and sheep at slaughter in Great Britain (1999-2000)

AIMS: The objective of these surveys was to estimate the prevalence of faecal carriage of Salmonella in healthy pigs, cattle and sheep at slaughter, and of pig carcase contamination with Salmonella. These data can be used as a baseline against which future change in Salmonella prevalence in these species at slaughter can be monitored. METHODS AND RESULTS: In this first randomized National Survey for faecal carriage of Salmonella in slaughter pigs, cattle and sheep in Great Britain, 2509 pigs, 891 cattle and 973 sheep were sampled in 34 pig abattoirs and 117 red meat abattoirs in England, Scotland and Wales. Carriage of Salmonella in 25 g caecal contents was identified in 578 (23.0% pigs) but in only 134 (5.3%) of carcase swabs. The predominant Salmonella serovars found in both types of sample were S. Typhimurium (11.1% caeca, 2.1% carcases) and S. Derby (6.3% caeca, 1.6% carcases). The main definitive phage types (DT) of S. Typhimurium found were DT104 (21.9% of caecal S. Typhimurium isolates), DT193 (18.7%), untypable strains (17.6%), DT208 (13.3%) and U302 (13.3%). Three isolates of S. Enteritidis (PTs 13A and 4) and one enrofloxacin-resistant S. Choleraesuis were also isolated. A positive 'meat-juice ELISA' was obtained from 15.2% of pigs at 40% optical density (O.D.) cut-off level and 35.7% at 10% cut-off. There was poor correlation between positive ELISA results or carcase contamination and the caecal carriage of Salmonella. The ratio of carcase contamination to caecal carriage rates was highest in abattoirs from the midland region of England and in smaller abattoirs. In cattle and sheep 1 g samples of rectal faeces were tested. Two isolates (i.e. 0.2%) were recovered from cattle, one each of S. Typhimurium, DT193 and DT12. One sheep sample (0.1%) contained a Salmonella, S. Typhimurium DT41. In a small subsidiary validation exercise using 25 g of rectal faeces from 174 cattle samples, three (1.7%) isolates of Salmonella (S. Typhimurium DT104, S. Agama, S. Derby) were found. CONCLUSIONS: The carriage rate of Salmonella in prime slaughter cattle and sheep in Great Britain was very low compared with pigs. This suggests that future control measures should be focused on reduction of Salmonella infection on pig farms and minimizing contamination of carcases at slaughter. SIGNIFICANCE AND IMPACT OF THE STUDY: This work has set baseline figures for Salmonella carriage in these species slaughtered for human consumption in Great Britain. These figures were collected in a representative way, which enables them to be used for monitoring trends and setting control targets.     

Revised estimate of numbers of wintering Red-throated Divers Gavia stellata in Great Britain

Capsule An estimated 17 000 Red-throated Divers winter around Great Britain.

Aim To produce an up-to-date assessment of the numbers of Red-throated Divers wintering around Great Britain.

Methods A revised Great Britain (GB) wintering population estimate for Red-throated Divers was compiled using data primarily from systematic line transect surveys by aircraft over marine nearshore areas conducted during 2001–06, supplemented with county bird records and Wetland Bird Survey (WeBS) counts, both from 1995 to 2005. Population estimates for each survey area around GB were calculated as the mean of either annual means (aerial surveys) or annual maxima (WeBS, county bird records) of all data collected within January and February of each year. These were summed to give a revised GB wintering population estimate.

Results A total of 17116 (13198–21034, 95% confidence interval) Red-throated Divers were estimated to winter around GB. Distribution was uneven. By far the greatest numbers were found off southeast and east Britain (59.3% of the total was between Flamborough Head, Yorkshire, and Dungeness, Kent), with large concentrations off the English south coast (10.9%), north Wales and Liverpool Bay (9.8%), and eastern Scotland (6.1%).

Conclusion This is a minimum plausible population estimate and the true population size may be larger. The new 1% of population threshold for statutory conservation work is 170 birds.     

Investigation of a Simple Model for Within-Flock Transmission of Scrapie

Genetic control programs for scrapie in sheep build on solid knowledge of how susceptibility to scrapie is modulated by the prion protein genotype at the level of an individual sheep. In order to satisfactorily analyze the effectivity of control programs at the population level, insight is needed at the flock level, i.e., how the grouping of sheep in flocks affects the population-level transmission risk. In particular, one would like to understand how this risk is affected by between-flock differences in genotype frequency distribution. A first step is to model the scrapie transmission risk within a flock as a function of the flock genotype profile. Here we do so by estimating parameters for a model of within-flock transmission using genotyping data on Dutch flocks affected by scrapie. We show that the data are consistent with a relatively simple transmission model assuming horizontal transmission and homogeneous mixing between animals. The model expresses the basic reproduction number for within-flock scrapie as a weighted average of genotype-specific susceptibilities, multiplied by a single overall transmission parameter. The value of the overall transmission parameter may vary between flocks to account for random between-flock variation in non-genetic determinants such as management practice. Here we provide an estimate of its mean value and variation for Dutch flocks.     

Implications of Conflicting Associations of the Prion Protein (PrP) Gene with Scrapie Susceptibility and Fitness on the Persistence of Scrapie

Background

Existing mathematical models for scrapie dynamics in sheep populations assume that the PrP gene is only associated with scrapie susceptibility and with no other fitness related traits. This assumption contrasts recent findings of PrP gene associations with post-natal lamb survival in scrapie free Scottish Blackface populations. Lambs with scrapie resistant genotypes were found to have significantly lower survival rates than those with susceptible genotypes. The present study aimed to investigate how these conflicting PrP gene associations may affect the dynamic patterns of PrP haplotype frequencies and disease prevalence.

Methodology/Principal Findings

A deterministic mathematical model was developed to explore how the associations between PrP genotype and both scrapie susceptibility and postnatal lamb mortality affect the prevalence of scrapie and the associated change in PrP gene frequencies in a closed flock of sheep. The model incorporates empirical evidence on epidemiological and biological characteristics of scrapie and on mortality rates induced by causes other than scrapie. The model results indicate that unfavorable associations of the scrapie resistant PrP haplotypes with post-natal lamb mortality, if sufficiently strong, can increase scrapie prevalence during an epidemic, and result in scrapie persisting in the population. The range of model parameters, for which such effects were observed, is realistic but relatively narrow.

Conclusions/Significance

The results of the present model suggest that for most parameter combinations an unfavourable association between PrP genotype and post-natal lamb mortality does not greatly alter the dynamics of scrapie and, hence, would not have an adverse impact on a breeding programme. There were, however, a range of scenarios, narrow, but realistic, in which such an unfavourable association resulted in an increased prevalence and in the persistence of infection. Consequently, associations between PrP genotypes and fitness traits should be taken into account when designing future models and breeding programmes.     

Comparative Susceptibility of Sheep of Different Origins,Breeds and PRNP Genotypes to Challenge with Bovine Spongiform Encephalopathy and Scrapie

Sheep are natural hosts of the prion disease, scrapie. They are also susceptible to experimental challenge with various scrapie strains and with bovine spongiform encephalopathy (BSE), which affects cattle and has been accidentally transmitted to a range of other species, including man. Incidence and incubation period of clinical disease in sheep following inoculation is controlled by the PRNP gene, which has different alleles defined on the basis of polymorphisms, particularly at codons 136, 154 and 171, although other codons are associated with survival time, and the exact responses of the sheep may be influenced by other breed-related differences. Here we report the results of a long term single study of experimental scrapie and BSE susceptibility of sheep of Cheviot, Poll Dorset and Suffolk breeds, originating from New Zealand and of a wide range of susceptible and resistant PRNP genotypes. Responses were compared with those of sheep from a closed Cheviot flock of UK origin (Roslin Cheviot flock). The unusually long observation period (6–8 years for most, but up to 12 years for others) allows us to draw robust conclusions about rates of survival of animals previously regarded as resistant to infection, particularly PRNP heterozygotes, and is the most comprehensive such study reported to date. BSE inoculation by an intracerebral route produced disease in all genotype groups with differing incubation periods, although M112T and L141F polymorphisms seemed to give some protection. Scrapie isolate SSBP/1, which has the shortest incubation period in sheep with at least one VRQ PRNP allele, also produced disease following sub-cutaneous inoculation in ARQ/ARQ animals of New Zealand origin, but ARQ/ARQ sheep from the Roslin flock survived the challenge. Our results demonstrate that the links between PRNP genotype and clinical prion disease in sheep are much less secure than previously thought, and may break down when, for example, a different breed of sheep is moved into a new flock.     

Population dynamics of scrapie in a sheep flock.

A detailed analysis of an outbreak of natural scrapie in a flock of Cheviot sheep is described. A total of 137 cases was reported over 13 years among 1307 sheep born into the flock. The epidemiology of scrapie can only be understood with reference to sheep demography, the population genetics of susceptibility to scrapie, pathogenesis during a long incubation period, and the rate of transmission (by both vertical and horizontal routes), all of which interact in complex ways. A mathematical model incorporating these features is described, parameter values and model inputs are derived from available information from the flock and from independent sources, and model outputs are compared with the field data. The model is able to reproduce key features of the outbreak, including its long duration and the ages of cases. The analysis supports earlier work suggesting that many infected sheep do not survive to show clinical signs, that most cases arise through horizontal transmission, and that there is strong selection against susceptible genotypes. However, important aspects of scrapie epidemiology remain poorly understood, including the possible role of carrier genotypes and of an environmental reservoir of infectivity, and the mechanisms maintaining alleles giving susceptibility to scrapie in the sheep population.     

Evidence in Sheep for Pre-Natal Transmission of Scrapie to Lambs from Infected Mothers

Natural scrapie transmission from infected ewes to their lambs is thought to occur by the oral route around the time of birth. However the hypothesis that scrapie transmission can also occur before birth (in utero) is not currently favoured by most researchers. As scrapie is an opportunistic infection with multiple infection routes likely to be functional in sheep, definitive evidence for or against transmission from ewe to her developing fetus has been difficult to achieve. In addition the very early literature on maternal transmission of scrapie in sheep was compromised by lack of knowledge of the role of the PRNP (prion protein) gene in control of susceptibility to scrapie. In this study we experimentally infected pregnant ewes of known PRNP genotype with a distinctive scrapie strain (SSBP/1) and looked for evidence of transmission of SSBP/1 to the offspring. The sheep were from the NPU Cheviot flock, which has endemic natural scrapie from which SSBP/1 can be differentiated on the basis of histology, genetics of disease incidence and strain typing bioassay in mice. We used embryo transfer techniques to allow sheep fetuses of scrapie-susceptible PRNP genotypes to develop in a range of scrapie-resistant and susceptible recipient mothers and challenged the recipients with SSBP/1. Scrapie clinical disease, caused by both natural scrapie and SSBP/1, occurred in the progeny but evidence (including mouse strain typing) of SSBP/1 infection was found only in lambs born to fully susceptible recipient mothers. Progeny were not protected from transmission of natural scrapie or SSBP/1 by washing of embryos to International Embryo Transfer Society standards or by caesarean derivation and complete separation from their birth mothers. Our results strongly suggest that pre-natal (in utero) transmission of scrapie may have occurred in these sheep.     

Potential Benefits of Cattle Vaccination as a Supplementary Control for Bovine Tuberculosis

Vaccination for the control of bovine tuberculosis (bTB) in cattle is not currently used within any international control program, and is illegal within the EU. Candidate vaccines, based upon Mycobacterium bovis bacillus Calmette-Guérin (BCG) all interfere with the action of the tuberculin skin test, which is used to determine if animals, herds and countries are officially bTB-free. New diagnostic tests that Differentiate Infected from Vaccinated Animals (DIVA) offer the potential to introduce vaccination within existing eradication programs. We use within-herd transmission models estimated from historical data from Great Britain (GB) to explore the feasibility of such supplemental use of vaccination. The economic impact of bovine Tuberculosis for farmers is dominated by the costs associated with testing, and associated restrictions on animal movements. Farmers’ willingness to adopt vaccination will require vaccination to not only reduce the burden of infection, but also the risk of restrictions being imposed. We find that, under the intensive sequence of testing in GB, it is the specificity of the DIVA test, rather than the sensitivity, that is the greatest barrier to see a herd level benefit of vaccination. The potential negative effects of vaccination could be mitigated through relaxation of testing. However, this could potentially increase the hidden burden of infection within Officially TB Free herds. Using our models, we explore the range of the DIVA test characteristics necessary to see a protective herd level benefit of vaccination. We estimate that a DIVA specificity of at least 99.85% and sensitivity of >40% is required to see a protective benefit of vaccination with no increase in the risk of missed infection. Data from experimentally infected animals suggest that this target specificity could be achieved in vaccinates using a cocktail of three DIVA antigens while maintaining a sensitivity of 73.3% (95%CI: 61.9, 82.9%) relative to post-mortem detection.     

Epidemiology and control of scrapie within a sheep flock.

Mathematical models of the transmission dynamics of scrapie are used to explore the expected course of an outbreak in a sheep flock, and the potential impacts of different control measures. All models incorporate sheep demography, a long and variable scrapie incubation period, horizontal and vertical routes of transmission and genetic variation in susceptibility. Outputs are compared for models which do and do not incorporate an environmental reservoir of infectivity, and which do and do not incorporate carrier genotypes. Numerical analyses using parameter values consistent with available data indicate that, in a closed flock, scrapie outbreaks may have a duration of several decades, reduce the frequency of susceptible genotypes, and may become endemic if carrier genotypes are present. In an open flock, endemic scrapie is possible even in the absence of carriers. Control measures currently or likely to become available may reduce the incidence of cases but may be fully effective only over a period of several years.     

Preventing experimental vertical transmission of scrapie by embryo transfer

This study investigated whether the transmission of naturally occurring scrapie in sheep can be prevented using embryo transfer. Embryos were collected from 38 donor ewes in a Suffolk sheep flock with a high incidence of naturally occurring scrapie, treated with a sanitary procedure (embryo washing) recommended by the International Embryo Transfer Society and then transferred to 58 scrapie-free recipient ewes. Ninety-four offspring were produced. None of the offspring or the recipient ewes developed scrapie. Furthermore, offspring derived from embryos collected from donor ewes bred to the immunohistochemically positive ram did not develop scrapie. We conclude that scrapie was not transmitted to offspring via the embryo nor was the infective agent transmitted to recipient ewes during embryo transfer procedures.     

Modelling the national scrapie eradication programme in the UK.

In accordance with a policy to eliminate all transmissible spongiform encephalopathies from the food chain, a national untargeted ram breeding programme to eliminate scrapie in the UK is in the final stages of planning. Here we formulate a model of flock-to-flock scrapie transmission, in order to consider the effect of a targeted breeding programme which is in the early stages of consideration. We estimate the size of the susceptible flock population, and discuss implications for potential control programmes. Targeting all rams and ewes in highly susceptible flocks rather than rams in all flocks will eradicate scrapie more quickly, and so is likely to be beneficial as long as suitable penalties or incentives are available to facilitate their identification. A more restricted programme aimed only at highly affected flocks would be much easier to implement and crucially will eradicate scrapie just as quickly. This will leave behind a residue population of susceptible sheep, which could then be gradually removed by a more general breeding programme.     

Quantifying the risk from ovine BSE and the impact of control strategies

Although no naturally infected sheep with bovine spongiform encephalopathy (BSE) has ever been discovered, it remains possible that BSE once infected the UK sheep population, has been transmitted between sheep, and is still present today. We constructed a mathematical model to assess the current maximum theoretical exposure to consumers from BSE-infected ovine material and to estimate the risk reduction that could be achieved by abattoir-based control options if BSE-infected sheep were ever found in the national flock. We predict that, if present, the exposure to consumers from a single BSE-infected sheep would be high: one sheep, close to the end of its incubation period, is likely to contribute 10-1000 times more infectious material than a fully infectious cow. Furthermore, 30% of this exposure comes from infectivity residing in lymphatic and peripheral tissue that cannot be completely removed from a carcass.We are 95% confident that throughout Great Britain, no more than four sheep flocks currently harbour an ongoing BSE epidemic. However, since the exposure from a single infected sheep is high, the annual human exposure from four 'typical' BSE-infected flocks could be considerable. Small reductions in exposure could be achieved by strategies based on tissue testing, a 12-month age restriction or expanded definitions of high-risk tissues. A six-month age restriction is likely to be more effective and genotype-based strategies the most effective.     

Failure to transmit scrapie infection by transferring preimplantation embryos from naturally infected donor sheep

The objective of the study was to examine whether or not the preimplantation embryo can act as a carrier of classic scrapie infection. The study was carried out on quarantined premises with sheep of highly susceptible scrapie genotypes. Uninfected embryos, collected from New Zealand–derived Suffolk ewes, were surgically transferred into recipient ewes that were also of New Zealand origin. Seventeen negative control lambs were born on the study premises from these embryo transfers. Thirty-nine experimental lambs were from embryos collected from naturally infected donor ewes. The experimental lambs were also born on the study premises after their surgical transfer into recipient ewes of New Zealand origin. These embryos had been collected from donor ewes in a scrapie-infected flock where the ewes were clinically sick with scrapie or developed clinical scrapie after embryo collection. All lambs were confirmed as scrapie susceptible of the ARQ/ARQ genotype. Twenty-eight experimental animals survived to the end point of the study at 5 yr of age with a mean survival of 1579 d. In the negative control group, 12 of 17 sheep survived to 5 yr of age with a mean survival of 1508 d. Postmortem examinations were carried out on all animals derived by embryo transfer, and in none was histologic or immunohistochemical evidence of scrapie found. In contrast, in the originating flock the majority of scrapie cases occurred in ARQ/ARQ genotyped animals where a 56% mortality from scrapie had been recorded in animals of this genotype. Thus, the study provides no evidence for transmission of scrapie and reinforces published evidence that vertical transmission of scrapie may be circumvented by embryo transfer procedures.     

Pruritus is a common feature in sheep infected with the BSE agent

Background

The variability in the clinical or pathological presentation of transmissible spongiform encephalopathies (TSEs) in sheep, such as scrapie and bovine spongiform encephalopathy (BSE), has been attributed to prion protein genotype, strain, breed, clinical duration, dose, route and type of inoculum and the age at infection. The study aimed to describe the clinical signs in sheep infected with the BSE agent throughout its clinical course to determine whether the clinical signs were as variable as described for classical scrapie in sheep. The clinical signs were compared to BSE-negative sheep to assess if disease-specific clinical markers exist.

Results

Forty-seven (34%) of 139 sheep, which comprised 123 challenged sheep and 16 undosed controls, were positive for BSE. Affected sheep belonged to five different breeds and three different genotypes (ARQ/ARQ, VRQ/VRQ and AHQ/AHQ). None of the controls or BSE exposed sheep with ARR alleles were positive. Pruritus was present in 41 (87%) BSE positive sheep; the remaining six were judged to be pre-clinically infected. Testing of the response to scratching along the dorsum of a sheep proved to be a good indicator of clinical disease with a test sensitivity of 85% and specificity of 98% and usually coincided with weight loss. Clinical signs that were displayed significantly earlier in BSE positive cases compared to negative cases were behavioural changes, pruritic behaviour, a positive scratch test, alopecia, skin lesions, teeth grinding, tremor, ataxia, loss of weight and loss of body condition. The frequency and severity of each specific clinical sign usually increased with the progression of disease over a period of 16–20 weeks.

Conclusion

Our results suggest that BSE in sheep presents with relatively uniform clinical signs, with pruritus of increased severity and abnormalities in behaviour or movement as the disease progressed. Based on the studied sheep, these clinical features appear to be independent of breed, affected genotype, dose, route of inoculation and whether BSE was passed into sheep from cattle or from other sheep, suggesting that the clinical phenotype of BSE is influenced by the TSE strain more than by other factors. The clinical phenotype of BSE in the genotypes and breed studied was indistinguishable from that described for classical scrapie cases.     

Persistence of Granulocytic Ehrlichia Infection During Wintertime in Two Sheep Flocks in Norway

Granulocytic Ehrlichia infection in sheep is common in Norway in areas with Ixodes ricinus. In this study, 2 sheep flocks that had been grazing on I. ricinus infested pastures the previous season, were blood sampled after being housed indoors for nearly 6 months during wintertime. Thirty animals from each flock were examined for granulocytic Ehrlichia infection in the peripheral blood by blood inoculation studies, stained blood smear evaluation, polymerase chain reaction (PCR) analysis and serology (IFAantibodies). The animals were sampled twice within a three-week period, the first time before and the second time after lambing. Two sheep in one flock were found Ehrlichia positive by both blood smear evaluation and PCR before lambing, and 3 sheep were found positive after lambing; 2 by blood smear examination and 3 by PCR. In the other flock, no sheep was found infected before lambing, but 2 ewes were found positive after lambing by both blood smear evaluation and PCR. In the first flock, 87% of the animals were found seropositive before lambing, and the mean antibody titre (log 10 ± SD) to E. equi was 2.45 ± 0.401. In the second flock, 40% were found seropositive before lambing, and the mean antibody titre was 1.93 ± 0.260. Seroprevalence and mean antibody titre in these 2 flocks were significantly different (p < 0.001). The present study indicates that sheep may be a reservoir host for granulocytic Ehrlichia infection from one grazing season to the next under natural conditions in Norway.