An Experimental Study of Tick Paralysis in Australia

Abstract

1. Tick paralysis occurs in man, the dog, and other domesticated animals on the East Coast of Australia.2. As far as is known, the disease in Australia is only conveyed by the mature female ofIxodes holocyclus. The immature stages and the male of this species of ticks are non-pathogenic.3. A similar disease, induced in a like manner by females of other species of ticks, has been observed by various authors in South Africa, Canada and Crete.4. A single tick, as shown by experiment, may cause fatal paralysis in man and the dog in Australia, the death-rate in dogs being very high.5. The main symptoms of tick paralysis in Australia do not differ materially from those observed in other countries. They begin with loss of coordination commencing in the hind legs in dogs, the fore legs, head and neck becoming subsequently affected. Death in all cases appears to be due to respiratory paralysis.6. The onset of symptoms has not been observed to occur in less than 5 days from the time the tick or ticks have attached themselves to a dog.7. Some of the ticks do not produce paralysis and others do, as shown repeatedly from my experiments on dogs. This requires explanation. It does not depend on the time that has elapsed between the tick having moulted and its attacking the dog.8. Paralysis has never been seen to occur when ticks were removed from dogs before the time when symptoms usually appear.9. It is impossible to transmit the disease from affected to healthy animals by the inoculation of blood, cerebro-spinal fluid or nervous tissue. Nor can the disease be transmitted by the injection of the body contents of ticks removed from affected animals. The examination of the fluids and organs of affected animals has failed to reveal the presence of pathogenic organisms.10. It is considered that the causal factor in the production of the disease is a tick-derived toxin, this toxin being secreted by the salivary glands.11. During the process of engorgement the salivary glands develop greatly, pouring out a maximum amount of secretion during the 2 days before the female tick drops.12. An emulsion of salivary glands ofI. holocyclusproved to be toxic for when injected into a dog (Experiment V, p. 424) it caused fever, vomiting, etc. The salivary secretion contains anticoagulins. As Nuttall and Strickland found for anticoagulins, individual ticks may vary in toxicity.13. The period which elapses between the attachment of a tick to the host and the first signs of an attack of tick paralysis cannot be regarded in the light of an incubation period; it is dependent on the rate and stage of the tick's engorgement.14. If it be accepted that the causal agent of the disease is a tick-derived toxin, it remains to be shown how such a toxin is produced.15. There is some evidence that immunity to the disease may be acquired after recovering from previous attacks.16. There is no evidence of season having an influence on tick paralysis. It was induced experimentally both in winter (June–August) and summer (November–December) in Sydney.