Newcastle disease and other causes of mortality in double-crested cormorants, phalacrocorax auritus
Since 1990, Newcastle disease (ND) caused repeated epidemics in juvenile double-crested cormorants ('Phalacrocorax auritus'; DCC). This disease is important because it may: (1) spread to poultry, (2) spread to wild birds cohabiting with DCC, and (3) modify population dynamics of DCC. This study had three parts: (1) Mortality of DCC in a breeding colony on Dore Lake (Saskatchewan, Canada) was monitored from 1994 to 1996. Birds were observed every third day from inside a tunnel-and-blind system, causing minimal investigator disturbance. The most important causes of mortality, not induced by humans, were ND (21% of hatched chicks in 1995), starvation (4 to 12% per year), and coyote predation (2% in 1994). Newcastle disease only affected juvenile DCC, which had wing and leg paralysis. Affected DCC examined histologically (n = 25) had non-suppurative encephalomyelitis, with significantly more (P < 0.001) neuronal necrosis, gliosis, perivascular infiltration with mononuclear cells, and endothelial hypertrophy than in control DCC (n = 18). Immunohistochemically, Newcastle disease virus (NDV) antigen was limited to central nervous system and kidney, and velogenic NDV was isolated most frequently and in the highest concentration from the kidney. The predicted amino acid sequence of the fusion protein cleavage site was identical to those from previous ND epidemics in DCC, indicating that the same virus has been circulating in DCC since 1990. (2) Twelve 16-week-old captive-raised DCC were infected experimentally with NDV. No birds died, possibly due to age-related resistance. Duration of NDV excretion from the cloaca was 15 ± 6.2 days post infection, with a maximum of 28 days post infection, suggesting that DCC populations may maintain velogenic NDV year-round through transmission between susceptible individuals. (3) A reverse transcriptase-polymerase chain reaction test (RT-PCR) was developed to detect NDV in diagnostic samples, as an alternative to virus isolation. The RT-PCR test was performed on allantoic fluid samples containing different strains of hemagglutinating viruses, and on tissues of chickens and DCC infected with NDV. The test cross-reacted with other paramyxovirus strains and with avian influenzavirus strains, and its detection limit was higher than virus isolation, so that it was not considered a suitable alternative.