The global onslaught of emerging diseases never ceases. The role animals play in the emergence of new diseases and the transmission of those diseases to humans is under constant scrutiny. BSE, SARS and influenza are examples of pathogens that jumped the species barriers over the last decade. While their encroachment on human health was less than spectacular, the economic impact these diseases had on the health-care system, social disruption, and the food industry was incalculable.
In his book Contagion, author Mark Harrison, a professor of medical history at Oxford University, charts how business and government handled, or mishandled, just about every significant outbreak of cross-border disease from the Black Death of the 1300s to more recent real and imagined threats to human and animal life. Vigilance and risk assessment are key ingredients to being prepared. Long before anyone knew there was such a thing as a germ, countries suspected that disease, especially plague, travelled along trade routes. Norovirus fits the definition of a perfect pathogen in this regard in that many outbreaks are associated with human travel, especially confined travel by ship and airplane.
Noroviruses now top the list of foodborne causes of gastroenteritis in humans. They are associated with 30-50 per cent of all foodborne outbreaks. Vomiting and diarrhea are the primary signs and these infections are commonly, and mistakenly, called stomach flu. While most cases last a few days, some can be severe and in rare cases fatal. Hospitalization and mortality associated with norovirus infections occur most frequently among the elderly, young children, and immunocompromised patients. January is peak norovirus season.
Noroviruses are highly infectious. As few as 10 viral particles may be sufficient to infect an individual; people stricken with disease shed billions. Transmission happens through person-to-person contact, fomites, water and food. Noroviruses infect hosts of all ages and can cause very large outbreaks in closed settings. When outbreaks occur, victims often number in the millions; 23 million cases in the U.S. last year. It’s estimated that noroviruses kill 200,000 children under the age of five every year in developing countries.
Until recently, it was thought animals did not carry human noroviruses. It has now been shown that it is indeed possible for animals to carry human strains of norovirus. The first bovine enteric noroviruses were described in Great Britain. At least three genetic clusters of porcine noroviruses have been discovered. Murine (rodent) noroviruses are used as models to study human strains. Furthermore, antibodies against animal noroviruses were detected in humans as well as antibodies against human noroviruses in swine. Experimental infection of germ-free calves and pigs with human noroviruses demonstrated that virus replication and seroconversion could occur.
Recent research has shown that dogs can carry human norovirus and transmit infection to people within the pet’s home environment. Human strains of norovirus were also detected in dogs exposed to people with symptoms and showed mild symptoms themselves.
It is not yet clear how nasty new strains of this bug will be, but the Centers for Disease Control and Prevention (CDC) recently announced, an Aussie newcomer (GII.4 Sydney) first spotted in March 2012, has triggered acute gastroenteritis outbreaks in the United Kingdom and other countries. During the last four months of 2012, GII.4 Sydney accounted for 53 per cent of 266 norovirus outbreaks in the U.S. Roughly half of the Sydney outbreaks resulted from direct person-to-person transmission; another 20 per cent were foodborne. Water has also been incriminated. In general, GII.4 strains are associated with higher rates of hospitalization and death. Questions about the potential role of animals and animal products are starting to surface.
Transmission from animals to humans and vice versa would have far-reaching consequences for epidemiology and food safety. The CDC stated that GII.4 Sydney appears to have replaced a previously dominant norovirus strain. The upshot of this is that these highly contagious organisms change and adapt. Since the norovirus genus comprises viruses that infect humans, pigs, cattle, and mice, the possibility for zoonotic transmission of infection exists. In general, zoonotic transfer could occur either indirectly through the food chain or directly through animal contact. Transfer of animal virus to humans has the potential of being more serious if animal and human strains produce recombinant mutants with altered virulence properties. There is a growing concern of the possibility of recombinant swine/human or bovine/human forms and zoonotic transmission through meat, dairy, or farm products from infected pigs and cattle.
Mechanisms of immunity to norovirus are unclear. It appears immunity is strain specific and lasts only a few months. Given their genetic variability, norovirus infections are likely to reoccur during a lifetime.
According to Harrison, quarantine had become an instrument of statecraft by 1700 and was an excuse for protectionism, sometimes a reason to go to war. The imposition of sanitary measures was often considered an act of aggression. In time, the Europeans had everyone agree on international rules for this sort of thing.
Let’s hope we have things right this time.
— Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).