380 MULTIPLE SCLEROSIS AND CANINE DISTEMPER IN ICELAND

SIR,-Dr Nathanson and colleagues’ have attempted to test hypothesis that multiple sclerosis (M.S.) may be an unusual complication of an infection transmitted from animal to man, canine distemper virus (c.D.v.) or a closely related virus being the prime aetiological candidate,’-’ by examining epizootics of

our

in Iceland and comparing these data with M.S. 1946 to 1965.9-11 Nathanson et al. concluded that M. s. had occurred in high prevalence in Iceland in the virtual absence of distemper, and that neither dogs nor canine distemper were a sine qua non for the occurrence of tvts. We agree that the data from Iceland as presented do not support the animal-distemper/ht.s. association. We believe, however, that Nathanson et al. have not considered certain aspects of Gudmundsson’s data which do, indeed, give suggestive evidence in its favour. Nathanson et al. rely on "period prevalence" of M.S. for 1946-65, but some of Gudmundssons’ 129 patients had onset of the disease as far back as 1904. Further, since Gudmundsson considered only M.S. patients who survived at least to 1946, only the 78 cases with onset during 1946 through 1965 can properly be used to study time trends in incidence. It is incidence, not period prevalence, which the hypothesis relates to epizootics of canine distemper. Also, Nathanson et al. cite Icelandic distemper epizootics in 1921-22, 1941-42, and 1966-67. However, since no case of M.S. considered had onset later than 1965, the last of these epizootics is irrelevant. Concerning our own analysis of the data, we wish to make the following three points: (1) The 1941-42 distemper epidemic best tests the animaldistemper/M.s. hypothesis. We focused on this epidemic because Gudmundsson has provided us with the annual inci-

"distemper"

prevalence from

1. Nathanson, N , Palsson, P A., Gudmundsson, G. Lancet, 1978, n, 1127. 2. Cook, S. D , Dowling, P C. ibid. 1977, i, 980. 3. Chan, W. W.-C. ibid. p. 487. 4. Cook, S. D., Natelson, B. H., Levin, B. E., Chavis, P. S., Dowling, P. C. Ann. Neurol. 1978, 3, 141. 5 Cook, S. D., Dowling, P. C , Russell, W. C. Lancet, 1978, i, 605. 6. Cook, S. D., Menonna, J., Dowling, P. C. Neurology, 1978, 28, 129. 7. Cook, S. D., Dowling, P. C., Russell, W. C. Clin. Res. 1978, 26, 608. 8. Cook, S. D., Dowling, P. C., Russell, W. C. J. neurol. Sci. (in the press). 9. Gudmundsson, K. R.. Gudmundsson, G. Acta neurol, scand. 1962, 38,

suppl. 2, p. 1. 10. Gudmundsson, K. ibid 1971, 47, suppl. 48, p. 5. 11. Gudmundsson, K., Bergmann, S., Bjornsson, O. J. neurol. Sci. 1974, 21, 47. NEW M.S. CASES IN ICELAND

1946-55

J., Ellertsson,

COMPARED WITH

A. B.

1956-65

Fig. 1-Age-specific prevalence of M.S. in Iceland 1945-65, based on definite and probable cases. dence of M.S. in Iceland (78 of his 129 cases) for a 20-year period closely following this distemper outbreak, whereas such clinical information is unavailable for the 1921-22 or 1966-67 epizootics. Gudmundsson’s data show that the annual incidence of M.S. declined from 3.0per 100000 in 1946-55 (44 cases) to 1.9 in 1956-65 (34 cases), a significant difference (P

Multiple sclerosis and canine distemper in Iceland.

380 MULTIPLE SCLEROSIS AND CANINE DISTEMPER IN ICELAND SIR,-Dr Nathanson and colleagues’ have attempted to test hypothesis that multiple sclerosis (M...
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