The use of aqueous cetylpyridinium chloride as a transport medium for suspected tuberculous tissues from badgers

Deterioration of tuberculous tissue specimens from badgers during transit in the post was simulated in the laboratory, and it was found that unpreserved tissues showing miliary lesions and initially producing a confluent growth of Mycobacterium bovis became culture-negative over a period of 4 days at ambient summer temperatures (>20 degrees C). These findings implied that false negative results may be obtained from specimens containing only small numbers of bacilli following delays of 2 days between collection and culture. A bench trial of cetyl pyridinium chloride and bromide as preservatives showed viable bacillary numbers were maintained well. A field trial of a 1% aqueous solution of the chloride salt as a transport medium was carried out on 666 split tissue submissions and a significantly improved isolation rate was found from the preserved samples, with 40 (93%) of the total of 43 isolates of M. bovis compared with 27 isolates (63%) when no preservative was used. Isolation using the guinea pig test was attempted on 354 of these samples and yielded 38 (93%) of the 41 isolates, compared with culture of preserved samples which yielded 39 isolates (95%), an insignificant difference. The use of this transport medium maximised cultural isolations, such that no significant gain was achieved by the continued use of the animal test.