Scientific opinion on chronic wasting disease (II)

Are rapid tests and confirmatory western blot used for cattle and small ruminants TSEs reliable tools for the diagnosis of Chronic Wasting Disease in Europe?

The first case of CWD in Europe was detected in a Norwegian reindeer in 2016, followed later by two CWD cases in Norwegian moose. To prevent the potential spread of CWD to the EU, the European Commission (Regulation EU 2017_1972) implemented a CWD surveillance programme in cervids in the six countries having reindeer and or moose (Estonia, Finland, Latvia, Lithuania, Poland, and Sweden). Each country had to test a minimum of 3000 cervids for CWD using diagnostic rapid tests approved by the EC Regulation. Experimental transmission studies in rodents have demonstrated that the CWD strains found in Norwegian reindeer are different from those found in moose and that these European strains are all different from the North American ones. Data on the performances of authorised rapid tests are limited for CWD (from North America) and are currently minimal for CWD from Europe, due to the paucity of positive material. The aim of this study was to evaluate the diagnostic performances of three of the so-called "rapid" tests, commercially available and approved for TSE diagnosis in cattle and small ruminants, to detect the CWD strains circulating in Europe. The performances of these three tests were also compared to two different confirmatory western blot methods. Using parallel testing on the same panel of available samples, we evaluated here the analytical sensitivity of these methods for TSE diagnosis of CWD in Norwegian cervids tissues. Our results show that all the methods applied were able to detect the CWD positive samples even if differences in analytical sensitivity were clearly observed. Although this study could not assess the test accuracy, due to the small number of samples available, it is conceivable that the rapid and confirmatory diagnostic systems applied for CWD surveillance in Northern Europe are reliable tools.

Monitoring of chronic wasting disease (CWD) (IV)

The European Commission requested an analysis of the Chronic Wasting Disease (CWD) monitoring programme in Norway, Sweden, Finland, Iceland, Estonia, Latvia, Lithuania and Poland (9 January 2017-28 February 2022). Thirteen cases were detected in reindeer, 15 in moose and 3 in red deer. They showed two phenotypes, distinguished by the presence or absence of detectable disease-associated normal cellular prion protein (PrP) in lymphoreticular tissues. CWD was detected for the first time in Finland, Sweden and in other areas of Norway. In countries where the disease was not detected, the evidence was insufficient to rule out its presence altogether. Where cases were detected, the prevalence was below 1%. The data also suggest that the high-risk target groups for surveillance should be revised, and 'road kill' removed. Data show that, in addition to differences in age and sex, there are differences in the prion protein gene (PRNP) genotypes between positive and negative wild reindeer. A stepwise framework has been proposed with expanded minimum background surveillance to be implemented in European countries with relevant cervid species. Additional surveillance may include ad hoc surveys for four different objectives, specific to countries with/without cases, focusing on parallel testing of obex and lymph nodes from adult cervids in high-risk target groups, sustained over time, using sampling units and a data-driven design prevalence. Criteria for assessing the probability of CWD presence have been outlined, based on the definition of the geographical area, an annual assessment of risk of introduction, sustained minimum background surveillance, training and engagement of stakeholders and a surveillance programme based on data-driven parameters. All positive cases should be genotyped. Sample sizes for negative samples have been proposed to detect and estimate the frequency of PRNP polymorphisms. Double-strand sequencing of the entire PRNP open reading frame should be undertaken for all selected samples, with data collated in a centralised collection system at EU level.

Scientific report on the analysis of the 2-year compulsory intensified monitoring of atypical scrapie

The European Commission asked EFSA whether the scientific data on the 2-year intensified monitoring in atypical scrapie (AS) outbreaks (2013-2020) provide any evidence on the contagiousness of AS, and whether they added any new knowledge on the epidemiology of AS. An ad hoc data set from intensified monitoring in 22 countries with index case/s of AS in sheep and/or goats (742 flocks from 20 countries, 76 herds from 11 countries) was analysed. No secondary cases were confirmed in goat herds, while 35 secondary cases were confirmed in 28 sheep flocks from eight countries. The results of the calculated design prevalence and of a model simulation indicated that the intensified monitoring had limited ability to detect AS, with no difference between countries with or without secondary cases. A regression model showed an increased, but not statistically significant, prevalence (adjusted by surveillance stream) of secondary cases in infected flocks compared with that of index cases in the non-infected flocks (general population). A simulation model of within-flock transmission, comparing a contagious (i.e. transmissible between animals under natural conditions) with a non-contagious scenario, produced a better fit of the observed data with the non-contagious scenario, in which each sheep in a flock had the same probability of developing AS in the first year of life. Based on the analyses performed, and considering uncertainties and data limitations, it was concluded that there is no new evidence that AS can be transmitted between animals under natural conditions, and it is considered more likely (subjective probability range 50-66%) that AS is a non-contagious, rather than a contagious disease. The analysis of the data of the EU intensified monitoring in atypical scrapie infected flocks/herds confirmed some of the known epidemiological features of AS but identified that major knowledge gaps still remain.

Getting in Front of Chronic Wasting Disease: Model-Informed Proactive Approach for Managing an Emerging Wildlife Disease

Continuing geographic spread of chronic wasting disease (CWD) poses a serious threat to the sustainable future of cervids and hunting in North America. Moreover, CWD has been detected in captive cervids in South Korea and, in recent years, in free-ranging reindeer in Europe (Norway). Management of this disease is limited by logistical, financial, and sociopolitical considerations, and current strategies primarily focus on reducing host densities through hunter harvest and targeted culling. The success of such strategies in mitigating the spread and prevalence of CWD only upon detection is questionable. Here, we propose a proactive approach that emphasizes pre-emptive management through purposeful integration of virtual experiments (simulating alternate interventions as model scenarios) with the aim of evaluating their effectiveness. Here, we have used a published agent-based model that links white-tailed deer demography and behavior with CWD transmission dynamics to first derive a CWD outbreak trajectory and then use the trajectory to highlight issues associated with different phases of the CWD outbreak (pre-establishment/transition/endemic). Specifically, we highlight the practical constraints on surveillance in the pre-establishment phase and recommend that agencies use a realistic detection threshold for their CWD surveillance programs. We further demonstrate that many disease introductions are "dead ends" not leading to a full epidemic due to high stochasticity and harvesting in the pre-establishment phase of CWD. Model evaluated pre-emptive (pre-detection) harvest strategies could increase the resilience of the deer population to CWD spread and establishment. We conclude it is important to adaptively position CWD management ahead of, rather than behind, the CWD front.

Evaluation of public and animal health risks in case of a delayed post-mortem inspection in ungulates

The potential effects of a 24 or 72-h delay in post-mortem inspection (PMI) of ungulates on public health and monitoring of animal health and welfare was evaluated. The assessment used a survey of meat inspectors, expert opinion, literature search and a stochastic model for Salmonella detection sensitivity. Disease detection sensitivity at a delayed PMI is expected to reduce detection sensitivity to a variable extent, depending on the hazard and on the signs/lesions and organs involved. No reduction is expected for Trichinella detection in meat from susceptible animal species and any decrease in detection of transmissible spongiform encephalopathies (TSEs) will not exceed the current tolerance for fallen stock. A 24-h delay in PMI could result in a small reduction in sensitivity of detection for tuberculosis, echinococcosis and cysticercosis. A greater reduction is expected for the detection of pyaemia and Rift valley fever. For the detection of Salmonella, the median model estimates are a reduction of sensitivity of 66.5% (90% probability interval (PI) 0.08-99.75%) after 24-h delay and 94% (90% PI 0.83-100%) after 72-h delay of PMI. Laboratory testing for tuberculosis following a sampling delay of 24-72 h could result in no, or a moderate, decrease in detection depending on the method of confirmation used (PCR, culture, histopathology). For chemical contaminants, a delay in meat inspection of 24 or 72 h is expected to have no impact on the effectiveness of detection of persistent organic pollutants and metals. However, for certain pharmacologically active substances, there will be a reduced effectiveness to detect some of these substances due to potential degradation in the available matrices (tissues and organs) and the non-availability of specific preferred matrices of choice.

Use of different RT-QuIC substrates for detecting CWD prions in the brain of Norwegian cervids

Chronic wasting disease (CWD) is a highly contagious prion disease affecting captive and free-ranging cervid populations. CWD has been detected in United States, Canada, South Korea and, most recently, in Europe (Norway, Finland and Sweden). Animals with CWD release infectious prions in the environment through saliva, urine and feces sustaining disease spreading between cervids but also potentially to other non-cervids ruminants (e.g. sheep, goats and cattle). In the light of these considerations and due to CWD unknown zoonotic potential, it is of utmost importance to follow specific surveillance programs useful to minimize disease spreading and transmission. The European community has already in place specific surveillance measures, but the traditional diagnostic tests performed on nervous or lymphoid tissues lack sensitivity. We have optimized a Real-Time Quaking-Induced Conversion (RT-QuIC) assay for detecting CWD prions with high sensitivity and specificity to try to overcome this problem. In this work, we show that bank vole prion protein (PrP) is an excellent substrate for RT-QuIC reactions, enabling the detection of trace-amounts of CWD prions, regardless of prion strain and cervid species. Beside supporting the traditional diagnostic tests, this technology could be exploited for detecting prions in peripheral tissues from live animals, possibly even at preclinical stages of the disease.