Guidelines for the detection of chronic wasting disease (CWD) in cervids
The use of rapid tests for the detection of CWD, aimed primarily at the potential for the UK to undertake targeted surveillance of cervids.
Background
Several commercial rapid tests are used for routine monitoring of BSE and scrapie within the UK.
There are several peer-reviewed publications that provide evidence of the potential for rapid tests to detect CWD originating from North America and Canada. Additionally, 2 rapid test manufacturers have obtained United States Department of Agriculture (USDA) and Canadian Food Inspection Agency (CFIA) approval for the use of their rapid tests to detect CWD in North America and Canada.
Due to the lack of species-specific CWD control material originating from Europe, it has not been possible to carry out a meaningful rapid test evaluation to date. Moreover, there is a lack of clarity about the influence of factors such as different CWD strains, different cervid species present in the Europe and different cervid genotypes on the capability of the tests to detect CWD.
This document aims to provide guidelines for the use of rapid tests for the detection of CWD and is aimed primarily at the potential for the UK to undertake targeted surveillance of cervids.
These guidelines will be regularly reviewed and amended as required in response to the developing situation within Europe and the proposed response required from the UK to this potential threat.
Points for consideration
Sample type
The UK TSE national reference laboratory (NRL) currently recommends the testing of both obex and retropharyngeal lymph nodes to maximise the sensitivity of surveillance.
When doing rapid tests, you must follow the rapid test kit instructions for use (IFU), as the IFUs are specific to different tissue types.
Rapid tests
In the absence of rapid test data resulting from a full test evaluation of the performance for the detection of CWD in cervids, the UK TSE NRL currently suggests that you use rapid tests that have undergone validation and received USDA or CFIA approval for use with cervid tissue.
The UK TSE NRL also recommends that you use a company that is already marketing TSE rapid tests within Europe. This ensures that the manufacturer already meets the quality requirements of current EU-approved rapid tests. These tests are:
the Bio-Rad TeSeE SAP rapid test, using the CWD addendum
the IDEXX HerdChek Chronic Wasting Disease Antigen Test Kit, EIA
the IDEXX HerdChek Bovine Spongiform Encephalopathy-Scrapie Antigen Test Kit, EIA
Sampling tissues and retention of material for confirmatory testing.
Sampling of Obex tissue
The UK TSE NRL has published guidance on fixation, tissue processing, histology, and immunohistochemistry staining procedures This guidance explains the correct sampling approach for collecting brainstem (obex) tissue for TSE testing in cattle and sheep, which can be applied to cervids. Please note that cerebellum is not a primary sample from these species.
Sampling of lymph nodes of the head of deer
Barrel and Simpson-Morgan (19901) described the lymph nodes and drainage of the tissues of the head of Fallow deer (Dama dama). They stated that the location of nodes was similar to that described for sheep and goats, with the sole exception that the medial retropharyngeal lymphocentre often comprised two nodes on either side (four in total). It therefore seems reasonable to assume that the location of lymph nodes in the head and neck of most species of deer will be more or less similar to those described for the Fallow deer and other ruminants.
Any method of sampling that results in collection of the lymph nodes with minimal contamination between samples is acceptable.
The following guidance and description of lymph node location is based on that provided by Barrel and Simpson-Morgan (1990) and Dyce, Sack and Wensing (2010)2.
Position the carcase or head in dorsal recumbency
Incise and reflect the skin from the ventral and lateral surfaces of the face and neck
The medial retropharyngeal LN lies embedded in fat between the dorsocaudal wall of the pharynx and the muscles overlying the base of the skull; medial to the stylohyoid bone. In fallow deer this lymph node often comprises a pair.
Access to these lymph nodes is facilitated by incising the soft tissues between the tongue and mandibular rami and transecting the hyoid apparatus to reflect the tongue and larynx caudally to expose the soft palate and dorsal pharynx.
Alternatively, once the head has been removed from the spine at the level of the atlanto-occipital joint, these lymph nodes can be assessed after lifting the trachea and dissecting the muscles rostral to the foramen magnum. The lymph nodes are located halfway between each angle of the mandible and the foramen magnum, caudal to the nasopharynx.
Specifically for the testing of lymph nodes, sample from 2 to 3 different areas of the cortex of the lymph node to increase the likelihood of sampling a positive follicle.
Retention of tissue samples for confirmatory testing
When sampling both obex and retropharyngeal lymph nodes for rapid testing, consideration must be given to the retention of sufficient and appropriate obex and retropharyngeal lymph node for confirmatory testing. The approach to brainstem sampling is common to all species, and is covered in Laboratory methods for taking samples for TSE testing and confirmation. For lymphoid tissues, ensure that sufficient lymph node, ideally an anatomically orientable cross-sectional sample with good representation of the cortex, is retained for confirmatory testing in the event of a positive rapid test result.
Confirmatory testing should, as a minimum, be performed on the tissue giving rise to the positive result. All collected tissues should be retained for possible further analysis from any animal test positive in any tissue.
Confirmatory testing
When the result of the rapid test is inconclusive or positive, the sample shall be subjected to confirmatory examinations using at least one of the following methods and protocols as laid down in the latest edition of the Manual for diagnostic tests and vaccines for Terrestrial Animals of the World Organisation for Animal Health (WOAH), namely:
Immunohistochemistry
Western blot
What to do in the event of a positive rapid test result
In the event of a positive rapid testing outcome, please refer to Laboratory methods for taking samples for TSE testing and confirmation.
Characterisation of isolates
In the event of a positive testing outcome, further isolate characterisation should be undertaken, in consultation with the UK TSE NRL.
UKTSE NRL contact details for rapid tests
For TSE rapid test issues please contact the UK TSE NRL general mailbox: