Recurring Influenza B Virus Infections in Seals

To the Editor: Until 1999, influenza B virus was considered to infect humans only. However, more recent data proved that harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus) also can be infected (1). Since the identification of seals as a novel host, antibodies against human influenza B viruses have been detected in some additional otarid and phocid species in a few relatively small studies (2,3). It has been speculated that seals may be an animal reservoir for human influenza B viruses, although whether influenza B viruses continues to circulate among pinnipeds is unknown. To investigate whether influenza B viruses had continued to circulate in seals, we analyzed serum samples from 615 seals (548 harbor seals [Phoca vitulina] and 67 gray seals [Halichoerus grypus]). The samples had been collected upon the animal’s admission to the Seal Rehabilitation and Research Centre (SRRC) in Pieterburen, the Netherlands, from seals living in Dutch coastal waters during 2002–2012. We tested these samples for influenza B virus–specific antibodies with a previously described hemagglutination inhbition (HI) assay, using the following influenza B virus strains as antigens: B/Seal/Netherlands/1/1999, B/Jiangsu/010/2003, B/Yamanashi/ 166/1998, and B/Malaysia/2506/2004 (4). Influenza B virus–specific antibodies were not detected in serum specimens collected from seals during 2002–2009 and after 2011; however, in 2010, HI serum antibodies against influenza B viruses were detected in 9 of 21 samples, and in 2011, they were detected in 1 of 150 samples from both harbor seals (n = 6) and gray seals (n = 4) (Figure, panel A). Nine of these positive samples were collected from juvenile seals 6–12 months of age with severe respiratory disease, and 1 was collected from a pup of ≈4 weeks of age. In seals >6 months of age, maternal antibodies have declined to undetectable levels (5). Therefore, these 9 juvenile seals must have become infected from late 2009 through early 2010. This suggests that the infection was caused by the novel introduction of an influenza B virus in seals in the coastal waters of the Netherlands, either by seals or by another source. Because most serum samples were collected within 1 day of the animal’s arrival at SRRC, seals must have been infected in the wild and not at the center. Although the 9 positive samples found in 2010 represent 43% of the tested serum samples for that year, this finding does not reflect the proportion of seropositive seals in the population. Only a limited number of seals of the population, most with respiratory problems, are admitted to SRRC, and serum is not collected from all these animals. Although the 9 seropositive seals, all >6 months of age, had been admitted to SRRC with severe respiratory signs, it should be noted that severe respiratory disease in seals has many other causes (6). Because no respiratory samples suitable for diagnostic purposes had been stored, the viral agent could not be determined. Consequently, whether the influenza B virus infection of these seals, as evidenced by serologic test