Background

White-tailed deer are the most numerous big game species in North America and their distribution extends essentially continuously from eastern portions of Wyoming east to the Atlantic Ocean. No studies have been conducted to examine movement patterns and habitat use of CWD infected white-tailed deer populations. White-tailed deer are expanding their ranges and increasing their population densities in many areas in Wyoming (Pauley and Lindzey, 1993). White-tailed deer in southeast Wyoming are more mobile than in other areas- a higher percentage of animals are migratory and they move greater distances than previously documented. Furthermore, in contrast to many other areas in the west, white-tailed deer migrations in southeast Wyoming are not limited to riparian corridors; on the other hand, these deer use riparian and agricultural habitats more than other habitat types (Sawyer and Lindzey, 2000). 

Chronic wasting disease is laterally transmitted among deer and elk in captivity (Williams and Young, 1992; Miller et al., 1998). Modeling suggests that lateral transmission in the wild is necessary to maintain the disease at detected prevalences (Miller et al., 2000; Gross and Miller, 2001). Chronic wasting disease is probably transmitted among deer by direct contact and/or environmental contamination.

Diagnosis of the transmissible spongiform encephalopathies (TSEs) is problematic because typical tests for evidence of infection by pathogens, such as culture or serology for antibodies in the blood, are not possible. Immunohistochemistry for the abnormal protein (PrPCWD), which serves as a marker of infectivity, has been used successfully in tonsil and lymph nodes, in subclinically and/or clinically affected deer with CWD (Sigurdson et al., 1999; Miller and Williams, 2002; Wolfe et al., 2002). The very early (42 days post-inoculation) appearance of PrPCWD in lymphoid tissue of mule deer following experimental exposure (Sigurdson et al., 1999) suggests that biopsy of lymphoid tissues could provide a useful ante-mortem diagnostic test for CWD infection in white-tailed deer.

The stated objectives for this work were: The primary goals of this study are: 1. to describe patterns of white-tailed deer movement in the Platte River corridor of Wyoming, and to determine how these deer interact with livestock, 2. to determine whether CWD infected white-tailed deer movements, habitat associations, and livestock interactions differ from those of healthy deer, 3. to evaluate the tonsillar biopsy as an ante-mortem CWD diagnostic test in white-tailed deer.

Methodology

Thirty-three adult deer (22 female, 11 male) were captured between 29 January and 18 April 2003, tonsils biopsied, and fitted with radio transmitters (Advanced Telemetry Systems, Isanti, MN). Eight adult deer were captured using netted box traps (Clover Traps), while 25 were captured using a helicopter net-gun. Additionally, 17 fawns were trapped. 

Deer were located throughout the diurnal period using ground or aerial telemetry. Universal Transverse Mercator (UTM) coordinates were recorded for each location. If cattle were present, distance to cattle was recorded using a range-finder (ground locations only). Only locations < 1km from cattle were used in the analysis.

Deer that died within 21 days of capture (n=5) were assumed to be capture-related mortalities, and were not used in the analysis. 

Findings

Of the 5 capture related mortalities, 4 tested positive for CWD. Only the surviving animals testing positive or negative for CWD were used for analysis, those with unknown status were excluded.

Six hundred ninety-two telemetry locations were recorded from 28 deer. Locations within 1km of cattle totaled 52. Twelve deer had at least one location < 1km from cattle (Table 1). Ten deer had two or more locations near cattle. Nine of the 10 having two or more locations close to cattle tested negative for CWD. Mean distance from cattle for the 9 testing negative was 137meters, SD=119, range 10-328. The one deer that tested positive had 6 locations near cattle with a mean distance of 217meters, SD=160, range 100-500.

There were 4 deer that tested positive for CWD, but only one had any locations near cattle, and that deer was not located closer than 100meters to cattle. Caution should be used when making inferences with such small sample sizes, but it would appear that deer testing positive for CWD avoided cattle.

No deer testing positive moved more than 5km over the duration of our monitoring period. Major movements or migrations were recorded for 8 deer testing negative. Five of the migrants moved approximately 10km, while 3 deer had movements > 25km with one deer moving 72km. Only one of the migrants on their summer range was found near cattle. 

Our results indicate that white-tailed deer with pre-clinical CWD (most of the deer trapped appeared healthy) are at greater risk of dying from the stress associated with capture than are uninfected deer. We have anecdotal evidence that deer with pre-clinical CWD are more likely to die after stressful weather events (blizzards, cold snaps, or heat spells; W. Schultz, personal communication). This combined data seems to indicate that something is occurring physiologically to deer with CWD before they show signs of the disease, which makes them more susceptible to stress.

Our preliminary data seems to indicate that white-tailed deer with CWD are less likely to migrate than are healthy deer; in any case, they are not any more likely to do so. This has implications for the role that infected white-tailed deer play in the epidemiology of the disease. If infected deer do not migrate, they will not contribute to the spread of the disease. On the other hand, if their movements are limited, they may contribute to the prevalence of the area.

Our preliminary research shows no detectable difference in habitat use or interaction with cattle between infected and healthy deer. If anything, infected white-tailed deer interact less with cattle than do uninfected deer. In general, our radio collared deer did not interact with cattle very much. This limits the likelihood of white-tailed deer as a source for disease to cattle. At this point in time, it does not appear that any livestock species are susceptible to CWD; our data imply that even if they are, they are unlikely to interact with infected white-tailed deer. 

Implications

This study documented the tonsil biopsy technique as a valid test for chronic wasting disease (CWD) in live white-tailed deer. The tonsil biopsy technique is the only technique currently able to diagnose CWD in live animals. Other results are preliminary, but suggest that white-tailed deer with CWD are no more likely to be migratory than are healthy deer. Based on a small sample size, our data indicate that, if anything, white-tailed deer with CWD are apt to be less migratory than are healthy deer. If this is verified, it would minimize the importance that migration of CWD infected white-tailed deer play in the spread of this disease. Results also indicate that white-tailed deer with early stages of CWD are more likely to die from stressors than are unaffected deer. We found that the stress of capture (regardless of capture method) was much more likely to cause death in CWD infected white-tailed deer than uninfected deer. Finally, this study provided no evidence that CWD 

4infected white-tailed deer are more likely than uninfected deer to interact with cattle. Indeed, we found very little interaction of white-tailed deer with cattle regardless of their disease status. This indicates that white-tailed deer pose little threat to the health of cattle. Note: at this point, it appears that domestic livestock are not susceptible to CWD.