Development of resistance to tick paralysis in sheep following subclinical exposure to virulent D. andersoni

Although Dermacentor andersoni(Rocky mountain wood tick) occurs throughout northwestern USA and southwestern Canada, only adult ticks within a restricted portion of its geographic range can cause paralysis. This is a costly problem for livestock producers due to treatment of cattle and lost productivity. The symptoms are produced by an unidentified salivary toxin and are characterized by an ascending paralysis that leads to death by asphyxia if the ticks are not removed from the host. Sheep and cattle develop an immune response that renders them resistant to paralysis following exposure to a paralyzing dose of virulent ticks.

The aim of the present study was to determine if a non-paralyzing dose of virulent D. andersoni can induce resistance to a subsequent infestation of a normally paralyzing dose. We infested naïve sheep with a low dose and high dose (i.e. positive control) of virulent ticks. All sheep with the high dose displayed signs of paralysis, while the sheep in the negative control and low dose treatment groups did not. Three weeks later, all sheep were exposed to the high dose. The negative controls were paralyzed within nine days, but only 13% of the positive controls became paralyzed. Half of the sheep that were previously exposed to the non-paralysing dose of virulent ticks did not develop signs of paralysis. The results show that sheep may develop resistance to paralysis with low intensity infestations without producing clinical symptoms.