e Human noroviruses (HuNoVs) are a major cause of viral gastroenteritis, with an estimated 3 million cases per year in the United Kingdom. HuNoVs have recently been isolated from pet dogs in Europe (M. Summa, C.-H. von Bonsdorff, and L. Maunula, J Clin Virol 53:244 –247, 2012, http://dx.doi.org/10.1016/j.jcv.2011.12.014), raising concerns about potential zoonotic infections. With 31% of United Kingdom households owning a dog, this could prove to be an important transmission route. To examine this risk, canine tissues were studied for their ability to bind to HuNoV in vitro. In addition, canine stool samples were analyzed for the presence of viral nucleic acid, and canine serum samples were tested for the presence of anti-HuNoV antibodies. The results showed that seven different genotypes of HuNoV virus-like particles (VLPs) can bind to canine gastrointestinal tissue, suggesting that infection is at least theoretically possible. Although HuNoV RNA was not identified in stool samples from 248 dogs, serological evidence of previous exposure to HuNoV was obtained in 43/325 canine serum samples. Remarkably, canine sero-prevalence for different HuNoV genotypes mirrored the seroprevalence in the human population. Though entry and replication within cells have not been demonstrated, the canine serological data indicate that dogs produce an immune response to HuNoV, implying productive infection. In conclusion, this study reveals zoonotic implications for HuNoV, and to elucidate the significance of this finding, further epidemiological and molecular investigations will be essential. H uman noroviruses (HuNoV) are a major cause of viral gas-troenteritis worldwide, with an estimated 3 million cases each year in the United Kingdom alone (1). HuNoV are members of the Caliciviridae family, which have a single-stranded positive-sense RNA genome and can cause a variety of disease manifestations in a wide range of species. The Norovirus genus itself is divided into at least six different genogroups based on capsid sequences (2, 3). HuNoV strains fall into genogroups I, II, and IV (GI, GII, and GIV). GII strains are responsible for 96% of HuNoV cases worldwide , with GII.4 genotypes being the most prevalent overall (4). In humans, HuNoV typically causes acute diarrhea, vomiting, and abdominal cramps, with the illness lasting on average 28 to 60 h (5). Infection is most common in health care institutions such as hospitals and long-term-care facilities (6), but outbreaks are often reported in association with schools, restaurants, cruise ships, and other settings such as military bases (7). Transmission of HuNoV is via contact with feces or vomit, which occurs predominantly through direct person-to-person contact or contaminated food and water (8). Zoonotic transmission of HuNoV has also been proposed as a hypothetical route of infection (9). Both cattle and pigs have come under scrutiny for their potential role in transmitting HuNoV over the past decade. This has been precipitated by the identification of GII.4 HuNoV RNA in the stools of farmed pigs and cattle (10, 11). Furthermore, over half of the pigs in a U.S. report were seropositive to both GI and GII human noroviruses (12). This finding was supported by a study that demonstrated that human strains can replicate and induce an immune response in gnotobiotic pigs (13). Dogs were first suggested to be potential zoonotic vectors of HuNoV in 1983, following an outbreak of norovirus gastroenteri-tis in an elderly-care home (14). Just prior to development of clinical symptoms in humans, the owner's dog was sick on multiple occasions around the home. Serological testing of the dog later revealed a moderate titer to HuNoV antigen by electron micros-copy, whereas control dogs were all seronegative. Later evidence linking dogs with HuNoV infections in humans followed in an epidemiological study that showed that seropositivity to HuNoV in humans was higher if there was a dog in the household (15), and anti-HuNoV antibodies have recently been identified in dogs across Europe (16). In 2012 it was reported that HuNoV could be detected in stool samples from pet dogs (17). Samples were collected from 92 dogs if the dog or owner had recently suffered from diarrhea or vomiting. Canine stool samples were tested for the presence of GI, GII, and GIV HuNoV, and 4 dogs were found to be positive for GII HuNoV. In one case, the HuNoV strain identified was identical to