Tick (Ixodes holocyclus) paralysis in the dog – quantitative studies on immunity following artificial infestation with the tick

SUMMARY Beagles have been immunised against the paralysing effects of the Australian paralysis tick Ixodes holycyclus by allowing female ticks to feed on these dogs. Complete immunity to the toxic effects of lethal numbers of feeding ticks has persisted in these beagles for at least 53 weeks and in similarly-immunised foxhounds for at least 102 weeks, during which periods the beagles and foxhounds were kept free of ticks. Serum antitoxin titres increased to a maximum value (hyperimmunity) of 46 antitoxin units/mL with a minimum number of 11 ticks feeding simultaneously. Titres declined to a low level after 12 to 14 weeks of freedom from tick infestation but increased again on reinfestation with ticks. Large numbers of ticks (up to 60) appeared to be required after several stimulation/relaxation cycles to obtain hyperimmune levels of antitoxin in serum. The serum antitoxin titre appeared to be a good indicator of effective immunity to tick paralysis during the initial development of hyperimmunity but was less indicative thereafter, as dogs whose serum antitoxin titres had reached an apparent maximum remained immune to tick paralysis after titres had decreased to low levels. There was no evidence of cutaneous hypersensitivity or any other tick rejection mechanism that could account for this effect.