Emerging disease and zoonoses #8--disease and domesticated animals

As I mentioned in previous posts, the emergence of new infectious--especially zoonotic--disease isn't limited to poor countries. Exact numbers vary depending on whose estimates you use, but this source puts the number of pet dogs in the United States alone at roughly 63 million, with over 76 million pet cats. This source is almost 10 years old and thus outdated, but we can use it as a ballpark: it claims almost 100 million cattle and calves in the U.S. Similarly, over 60 million hogs; almost 8 million sheep; 2.5 million horses and ponies; 366 million chickens. We have a lot of animals here, and a good portion of our population in regular contact with them. This brings the potential for a lot of disease transfer.

I mentioned previously a study carried out here that suggested inapparent infection of humans with swine influenza viruses. And of course, we can contract a number of infections from the food we eat every day--Salmonella, E. coli, Campylobacter--even if we aren't in direct contact with it as a living animal.

Farm animals aren't just for eating, either. Petting zoos abound, exposing people to live animals and the microbes they carry. And while you may think that, all these years after Semmelweis and Lister, people would understand the importance of washing their hands, you would be incorrect.

CDC researchers, in a case-control study, identified the direct or indirect risk factors that were the most important for coming down with illness related to Escherichia coli 0157:H7 after a visit to a petting zoo.

A South Carolina observational study found that - despite extensive warning signs -- a large proportion of people visiting a petting zoo at a state fair were engaged in risky behavior.

Another observational study, in Tennessee, showed that visitors to several petting zoos put themselves at risk while visiting the facility and most did not wash their hands when leaving.

Not surprisingly, touching and feeding animals resulted in the highest risks of contracting diarrheal illness (including hemolytic uremic syndrome), while handwashing was protective. In one study, they found that almost a quarter of visitors to the petting zoos were eating or drinking. (Aargh). Knowing this, the stats quoted that the CDC has identified more than 50 outbreaks associated with petting zoos and fairs over the past decade--many of them likely very preventable.

Similarly, bacteria that cause diarrhea aren't the only ones to worry about picking up from animals--or passing along to them. Another study described at the conference noted the increasing prevalence of methicillin-resistant Staphylococcus aureus (MRSA) found in pet dogs and cats, likely transferred to them by their owners. Though we tend to think of disease transmission as flowing from animals to humans, we must keep in mind that it can go both ways.

To deal with all this, we need closer ties between human and veterinary health providers. Indeed, it's been suggested that dogs can play a role as disease sentinels, alerting providers to potential problems looming in human health. (Seed elaborates on this here). This is part of the "one world, one health" model--making it clear that we need a more holistic approach to disease control, inclusive of global human and animal health.

Image from http://www.dog-goes.com/images/home/d_blau.jpg