Public health risks of emerging zoonotic viruses

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A Canadian team has published a review about the evidence for public health risks of three emerging potentially zoonotic viruses (Hepatitis E virus, Norovirus and Rotavirus). The lead author of the research article told us what threats we can expect for the future

Zoonoses

According to a recent scientific publication, zoonoses cause globally 26% of total Disability-Adjusted Life Year (DALYs – The years lost to disability or death) being results of infectious diseases. The most serious effects occur in the low-income countries (where the diseases transmitted to humans from animals cause 10% of all DALYs), however, zoonoses can be also burdensome in the advanced economies, especially in the event of an epidemic. For example, the costs for preventive and control measures required to contain the BSE, have been estimated at over 250 million of English pounds (almost €350 millions) in the UK and between 1.8 and 2 billion euro in Germany. The projections concerning prospective spread of diseases transmitted by animals are not comforting: the researcher Akio Yamada, in the introduction to the book “Confronting Emerging Zoonoses”, points out that “the direct and indirect interactions between different species of animals, including humans, are anticipated to increase, leading to the emergence of more zoonotic pathogens”. According to the UN report “World Population Prospects: the 2012 Revision”, indeed, the world population is expected to reach 9.6 billion by 2050, with 8.2 billion being inhabitants of developing countries. Moreover, the global livestock production is expected to increase by 92%, significantly raising the chances of human-animal interactions and the spread of the zoonoses. “Around 60% of current human infectious diseases are zoonotic”, confirms W. Peter Horby in his article on the “Drivers of Emerging Zoonotic Infectious Diseases”. He adds that “around 60% of recent emerging infectious diseases of humans have arisen from animals”. Many scientists prioritize studying the new forms of zoonoses and the risks they pose to public health. Barbara Wilhelm, as part of her PhD research at the University of Guelph, Canada, has conducted a review of existing studies on three potentially zoonotic RNA-viruses: Hepatitis E virus, Norovirus and Rotavirus. We interviewed the researcher, also author of the research article published in the magazine “Preventive Veterinary Medicine” (January 2015), about the state of the art of scientific research on animal diseases.

Based on the data available to you, may the new zoonoses pose a serious danger to public health in the future?

It’s important to remember, overall, that illness resulting from these viruses is currently quite rare. That said, the group most at risk for becoming ill after exposure to emerging viruses, is a group popularly described by the acronym YOPI – the Young, Old, Pregnant, or Immuno-compromised (e.g. people on cancer treatment who are taking drugs that suppress their immune system). One recent study I read on foodborne zoonoses estimated that 20% of the Canadian population could fall into this YOPI category. It is important for people in these higher risk categories to follow the advice of their health care providers regarding, for example, food preparation, to limit their chances of exposure to these viruses.

It’s also important to realize, for the three viruses we studied in our review, that globally many more people acquire their infection with these viruses from other people, than zoonotically (via contact with animals).

What are the most important conclusions of your review?

An important issue, which experts are still investigating, is the question of how to decide if someone’s illness is most likely caused by a zoonotic virus, or not, and if so- where was that person exposed to that virus. It’s more complicated than you might think, and it is a big topic of discussion. That said, using the criteria we chose in our review, we were somewhat surprised to find more published cases of zoonotic rotavirus, relative to zoonotic hepatitis E virus, which is a topic of a lot of study and publicity over the past decade.

According to the results of your analysis what are the biggest threats?

We studied three potentially zoonotic viruses, and they all behave differently. Hepatitis E virus can cause severe and sometimes fatal hepatitis (liver infection). Both norovirus (which we found may have the genetic potential to be acquired as a zoonosis, but currently we have not found any reports of this) and rotavirus, can cause severe gastro-intestinal illness. From the perspective of the ill patient, any of these can be a big threat!

However, my own opinion is that, in the future, zoonotic rotavirus could potentially have a big public health impact, since the current vaccines designed for immunizing young children against rotavirus do not contain these animal-derived strains.

Did you encounter particular difficulties during the risks assessment?

That’s a good question. I’ve mentioned that the question of attribution (establishing where and how the patient was exposed to the virus) is a difficult one, so identifying the critical places to intervene in the transmission pathways for these viruses is still the subject of a lot of research. The source of exposure for the zoonotic rotavirus cases which we found in our review, for example, was not known, for any of the published cases.

That said, zoonotic hepatitis E virus illness has been repeatedly linked to consumption of undercooked meat, so cooking meat thoroughly (in pork to an internal temperature of 71C, which many people would consider high) is important, particularly for people in the YOPI (or high risk) groups.

What other pathogens are likely to constitute a risk of potential zoonoses in addition to those you analyzed?

They fall into a couple of categories. Some have been identified fairly recently; examples would include avian influenza virus and Nipah virus. Some have been known for decades but are making a re-appearance in some geographic areas, such as dog rabies.

A big area of research globally involves the potential impact of climate change on zoonotic diseases, particularly vector-borne diseases, which require an intermediate host in the transmission pathway. In North America and Europe, one example of this is Lyme disease, or the disease caused by Borrelia burgdorferi. This microbe is carried in the blood of some wild animals, and the intermediate host is a tick. As the overall climate of North America becomes warmer, the geographic range of the ticks may change, and with it- possibly the level of human illness caused by this microbe. The research community and the World Health Organization are pretty confident that climate change will have an effect on human health for this and other reasons, but we are still uncertain of the full impact.

Based on your analysis, what is your impression about the level of general knowledge of the phenomenon?

Well, I work now in the area of research synthesis, or summarizing research and disseminating research findings to policy-makers and the public- so I am likely biased! I think that awareness of zoonosis as a potential differential diagnosis of illness, is improving. My impression is that, of the people in the YOPI category, the immune-compromised in particular are usually advised by their physicians, at least in Canada, to take measures to limit their chances of exposure.

In contrast, my personal impression is that people are often less well-informed of potential occupational exposures to zoonotic diseases. For example, I am also a food animal veterinarian, and as a group we seem to have a higher chance of being exposed to hepatitis E virus, as well as some other microbial pathogens, relative to the general population.

Do you think there is a need for dedicated studies for this issue?

Governments and international agencies try to maintain the right balance of research efforts, with limited resources. Personally I think we need to use surveillance, (meaning public health systems designed for early detection of specific problems), for emergence of animal or animal-human hybrid strains of rotavirus: rotavirus, in general, is an important cause of childhood disease, globally. That said, there are a number of other childhood diseases that public health workers have been battling for a long time, and they, too, require resources. For example, childhood tetanus is still an important cause of death in very young children globally, which is heart-breaking since it is preventable with vaccination.

The three viruses which we investigated, and emerging zoonotic diseases in general, are in fact receiving a lot of study. Researcher, medical and public health workers, and decision-makers, as a community, try to stay on top of issues that might become important in the future- while dealing with the ones that are a problem right now.

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