Brainstem removal using compressed air for subsequent bovine spongiform encephalopathy testing

Abstract

The use of compressed air to expel the obex through a hole in the skull generated using a captured bolt stunner. The obex is the part of the brain that is tested for bovine spongiform encephalopathy.

Bovine spongiform encephalopathy (BSE) was recognized as a reportable disease in Canada in 1990 (1). Surveillance to monitor the prevalence of BSE was implemented in 1992 (2); by September 2002, 9101 animals had been tested (1). Because cases of BSE were identified in Canada and the United States in 2003, the Canadian Food Inspection Agency is expanding the number of animals tested per year. The surveillance target for 2004 was exceeded with well over 8000 samples tested. In 2005, 30 000 animals are scheduled to be tested. This number will be further adjusted in succeeding years to reflect the international standards established by the Office International des Épizooties (2).

Samples for testing are collected primarily at federal and provincial slaughterhouses; however, they are also obtained from samples submitted for rabies testing, from brains submitted by private practitioners to provincial veterinary diagnostic laboratories, from rendering companies, and from the dead-on-farm (deadstock) population (1). The target population for testing is focused on animals at higher risk of contracting BSE, including all animals with clinical signs compatible with BSE. As well, animals older than 30 mo that are displaying neurological signs inconsistent with BSE or are otherwise diseased, nonambulatory, subject to emergency slaughter, or found dead are tested (1).

The obex, the portion of the brain submitted for BSE testing, is located where ependymal cells of the choroid plexus join the taenia of the 4th ventricle, just rostral to the opening of the central canal of the medulla oblongata (3,4). To date, the most common method of sampling has been the spatula technique, whereby a spatula is inserted through the foramen magnum and used to retrieve the brainstem (1).

This paper describes the use of compressed air for retrieving the brainstem for BSE testing. When convenient, water pressure can be utilized in the same way, with comparable results. The animal is euthanized with a captive bolt stunner or commercially available firearms and ammunition (5). The head is then partially disarticulated at the atlanto-occipital joint, so that the foramen magnum is accessible. A gloved finger is inserted into the foramen magnum to break down meningeal attachments and the roots of the glossopharyngeal, vagus, accessory, and hypoglossal nerves. All structures within the foramen magnum must be disrupted for success. An air compressor is filled to a pressure of 80 to 120 pounds per square inch (psi). The nozzle of the compressor is inserted as far as possible through the hole in the skull made by the captive bolt to achieve a complete seal around the nozzle, which is important for the compressor to be effective. Air is released from the compressor in a short burst, causing expulsion of the intact medulla oblongata, cerebellum, and pons through the foramen magnum (Figure 1). The portion of the brainstem required for BSE testing can then be excised and submitted to the laboratory.

Figure 1.

Photograph of brainstem after expulsion via compressed air.

Appropriate safety precautions should be taken when pressurizing the compressor, and safety glasses should be worn to prevent eye-related injuries when utilizing the compressor. Anyone in the immediate vicinity should stand back. If desired, a surgical mask may be worn, although BSE is not known to be transmitted via aerosolization.

This method should not be used, however, if rabies is suspected, due to potential risk from aerosolization. As well, rabies testing requires the cerebrum, which is not retrieved by using either this method or the spatula technique. The cost of the equipment, approximately $40.00, is higher than that required for the spatula technique, but the compressed air method is also suitable for the collection of autolyzed or frozen brain samples.

The main advantages of this method are increased speed, efficiency, and practicality of sample collection; also, less skill is required, compared with that for the spatula technique. Sometimes, sampling must be performed under challenging conditions, such as inclement weather or rough terrain, making ease of collection an asset. This method would be particularly beneficial considering the large increase in the number of animals expected to be sampled and tested in the future. CVJ