Endogenous feline retrovirus RD-114 does not elicit neutralizing antibodies in dogs.

At least two retroviruses, endogenous feline leukemia virus (FeLV) and RD-114 virus, are assumed to be integrated in the genome of domestic cats (1). Several feline cell lines have been shown to produce RD-114-related viruses (2). In Japan, feline cell lines are widely used to produce vaccines for dogs, particularly against canine parvovirus. The level of infectious RD-114 virus found in canine virus vaccines has been reported to be as high as 1,800 TCID50/ml (3). In earlier work, we confirmed the contamination of canine vaccines with the infectious RD-114 virus (4). In preliminary studies, in vivo infectivity, acute and sub-acute pathogenicity, and propagation of RD-114 virus by experimental infection in SPF dogs, were assessed. Over the course of these studies (51 days), none of the dogs inoculated with RD-114 virus showed clinical signs, significant increases in rectal temperature or abnormal blood characteristics (4). Importantly, infectious RD-114 virus was not recovered from leukocytes (4). Neutralizing antibodies against feline immunodeficiency virus (FIV) were demonstrated in cats naturally or experimentally infected with FIV, using a focus reduction assay in Crandell-Rees feline kidney cells (5). Neutralizing antibodies are an important component of protection against many viral pathogens, such as FeLV, and their appearance generally correlates with the anti-viral immunity (6, 7). Neutralization assays are often considered the standard, against which