While feeding on a bird host, the adult female tick introduces a chemical substance in her saliva that induces a progressive, ascending paralysis of the muscles that can ultimately kill the bird if the tick is not removed. Paralysis begins in the feet and legs, progressing upward to affect the wings and thoracic muscles. Death is usually attributed to respiratory failure following paralysis of the diaphragm muscles. Hemorrhaging of the lungs and gastrointestinal tract are contributing factors. If the ticks are removed before the paralysis progresses too far, birds generally recover, often with dramatic results, within a few hours.
Affected birds become lethargic, often exhibiting lameness and a staggering gait.
As the paralysis progresses, they are unable to perch, have difficulty flying, and are seen crouching or lying on the ground. The birds typically show no external signs of injury, and are usually recognized by their impaired mobility and the presence of one or more fully engorged ticks attached to the head or neck area. Although most cases involve only a single tick, up to 36 ticks per host have been observed. Cases often are reported around bird feeders where abnormal behavior of parasitized birds is likely to be noticed. Twenty-one species of birds have been documented as victims of tick paralysis in Alabama by Alabama Agricultural Experiment Station researchers at Auburn University.