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Cancer in Wildlife, a Case Study: Beluga from the St. Lawrence Estuary, Qu├ębec, Canada

Figure 2
Figure 2. Age structure of SLE beluga found dead on the shoreline (1983-1997) compared with that of NA beluga obtained through subsistence hunting. We took the data for NA beluga from Alaska Department of Fish and Game's (13) Table 7 (model population of 1,000 beluga), from the column labeled number of deaths per 1,000 beluga. We divided the latter figures by 10 to obtain a model population of 100 beluga.

Primary causes of death and types of cancer. Overall, the three primary causes of death of SLE beluga were metazoan parasites (22%), cancer (18%), and infectious agents (bacterial, viral, or protozoan, 17%) (Figure 3). The major cause of death in adult beluga (n = 79) was cancer. We found 21 cancers in the 100 well-preserved carcasses examined, and of these, excluding the ovarian tumors, 18 were terminal (cancers that led to death).

 

Figure 3
Figure 3. Primary causes of death in the SLE beluga (B) between 1983 and 1999 compared with those of roe deer (A) in Sweden between 1985 and 1995 (32).

Thirty percent (6 of 21) of the cancers affecting SLE beluga originated from the intestine close to the stomach, whereas a seventh intestinal cancer was closer to the anus (14). All other cancers are listed in Table 1.

Table 1

Cancer epidemiology in SLE beluga. The annual rate of cancer in the SLE beluga population was estimated as the number of new cases of cancers per year. The estimated annual rate (AR) was calculated, as in Dorn et al. (15), as an annual rate per 100,000 animals. The actual number of stranded SLE beluga with cancer that were examined in the necropsy room was divided by 17 years (1983-1999) and by the estimated number of SLE beluga, and the result was multiplied by 100,000 animals. A recent index estimate of 650 SLE belugas was used (1,16).

5003ART.Martineau.eq.1 [1]

where AR is the estimated annual rate of cancer in SLE beluga, SLB is the beluga inhabiting the SLE, and t is the study period (1983-1999).

Stranded carcasses are rarely reported in winter (January-March) because of the ice cover, difficult access to most of the shoreline, inclement weather, and the presence of few human observers on the shoreline. To estimate the number of dead animals that strand over a complete year, we assumed that the frequency of death during the winter months is equal to that of other seasons (although it is probably higher for the above reasons), so we used a correction factor of 12 months/9 months (1.33). Assuming that all carcasses have an equal chance of being seen and collected whatever the cause of death, we estimated the minimum number of SLE beluga with cancer (estimated minimum with cancer, EMC) over the last 17 years (1983-1999) as follows:

5003ART.Martineau.eq.2

where EMC is the estimated minimum number of SLE beluga with cancer per year, and Dead SLB is the total number of beluga reported dead and/or examined during the study period (1993-1999). The adjusted estimated annual rate (AAR) of cancer for a complete year (12 months) is:

5003ART.Martineau.eq.3 [3]

where SLB is the beluga inhabiting the SLE and t is the study period (1993-1999).

Discussion

Sample representativeness. This study has been carried out well over a decade. Considering the beluga life span (35-40 years), a sizable proportion of the population died and has been examined over this period. Except for young calves (< 1 year) and for the winter season, it is assumed that all carcasses have equal chances of being recovered and examined whatever the cause of death, for the following reasons: These whales live in a restricted range, as shown by thorough surveys from airplanes and boats (1,17,18); all carcasses have been found within that range or downstream, as a result of drift; and we used no criteria other than reasonable preservation and carcass accessibility to determine whether a given carcass would be examined in the postmortem room. In conclusion, although some deaths may occasionally escape our attention, the sampling of carcasses is most likely representative of the population in terms of causes of mortality.

Comparison of SLE beluga whales with other cetaceans. Cancers in stranded SLE beluga are more numerous than in other cetaceans, where cancer is a rare event and where major causes of death are, excluding recent viral epizootics, entanglement in fishing gear, pneumonia and/or parasitism, and abandonment and starvation of neonates (19-21).

No tumors were observed in 19 carcasses of other species of marine mammals living in the SLE and examined by a veterinary pathologist during the same period, using the same postmortem examination protocol (22). Only 33 cases of cancer have been reported worldwide in captive and wild cetaceans other than SLE beluga (Tables 1-3).

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