Gary Mullen , Renee Anderson , and Paul Nolan

For the past 15 years, wild birds have died in Alabama at surprisingly regular three-year intervals during the winter months due to paralysis induced by feeding ticks. If the pattern continues, there is reason to expect that Alabama residents may see a number of wild birds killed again by ticks this winter.

Tick paralysis of birds is caused by the bird tick (Ixodes brunneus), a common tick that infests a wide range of avian species. 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.

Birds that died from tick paralysis, each with an attached female bird tick.
House finch	Chipping sparrow	Mourning dove

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 (Figure 1). 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.

Figure 1. Seasonal occurrence of tick paralysis of birds in Alabama, based on 85 cases, 1986-1999.

Twenty-one species of birds have been documented as victims of tick paralysis in Alabama (see the table) by AAES researchers at Auburn University. Most of these are song birds (passerines), but mourning doves and other non-passerine birds are known to be hosts of the bird tick. Other species of birds in Alabama that have been found to be parasitized by I. brunneus without exhibiting signs of paralysis include the blue grosbeak, indigo bunting, and rufous-sided towhee.

All 88 cases of avian tick paralysis in Alabama were recorded during the colder months of the year, from mid October to early April (Figure 2). The greatest numbers are documented during January and February. This may reflect the time of year when populations of adult bird tick are highest, in addition to stress on the birds during the coldest winter months and increased activity at feeders where affected birds are most likely to be observed.

Figure 2. Cyclic pattern of tick-induced paralysis of birds at three-year intervals in Alabama.

Of particular interest is what appears to be a cyclic pattern of tick paralysis at three-year intervals in Alabama. Since the winter of 1987-88 when six cases were recorded, the occurrence of avian tick paralysis has followed a consistent pattern with cases every third winter and no cases during the intervening two years. The cause of this remarkably regular cyclic pattern remains unexplained. No cases have been reported in Alabama during the past two years. If the trend continues, residents can expect to see cases of tick paralysis in birds this winter.

To assist AU researchers in monitoring the occurrence of cases, anyone who finds a bird with an attached tick and signs of paralysis is encouraged to contact Dr. Mullen at Auburn University by phone (334-844-2554) or e-mail (gmullen@acesag.auburn. edu). To help document a suspected case, it is important to identify the bird and recover any ticks involved for species verification.