Abstract

Stranded beluga whales (Delphinapterus leucas) from the St. Lawrence estuary were examined for the cranial sinus nematode, Pharurus pallasii (van Beneden, 1870) Arnold and Gaskin 1975. Analyses indicated that 87% of all stranded adult beluga (>7 years old, N=30) are infected (mean intensity=340, range" 1-2042), 67% of stranded juvenile beluga (1 to 7 years old, N=3) are infected (mean intensity=15, range: 2-28) and none of the young of the year (<1 year old, N=5). The prevalence of infection was similar in male and female adult beluga, namely, 87%. There is a statistically significant difference in mean intensity of infections between male adult beluga (mean intensity=577, N=15) and female adult beluga (mean intensity=199, N=I 5).

Fifth-stage worms were found in the pterygoid sinuses and peribullar (ear) sinuses. The worms were equally distributed between the left and right sinuses (worms in pterygoid and peribullar sinuses together on each side compared to that on opposite side). However, parasites were four times more numerous in the peribullar sinuses compared to the pterygoid sinuses. Female P. pallasii were twice as numerous as males and significantly larger (mean female length: 27.9 + 3.8 mm; mean male length: 21.2 + 2.9 mm). No mucosal or bony macroscopic lesions were observed in association with P. pallasii even in beluga with heavy infections (i.e. >2000 worms). The lungs were systematically examined for adult P. pallasii - none was observed (N=25).

This study demonstrates that the cranial sinuses are the principal niche of P. pallasii. We had hypothesized that host sex had no effect on the intensity of infection. Our preliminary data, however, indicates that the sex of the host influences the intensity of infection (the mean intensity of infection of P. pallasii in adult male beluga was almost three times greater than that in adult female beluga). Absence of this parasite in nursing calves, the low intensity of infection in juveniles and high intensities of infections in adults suggest a heteroxenous life cycle for P. pallasii (i.e. more than one host is required for the transmission and development of this parasite). These observations lead to the testable hypothesis that beluga acquire infections of P. pallasii through the intake of infected food. There is no evidence of transplacental transmission of P. pallasii as suggested for some pseudaliids in odontocetes.