Advice From the APHA Experts: Animal diseases: How much of a concern are they to our health?

Q&A with Lonnie King, DVM, senior veterinarian at the Centers for Disease Control and Prevention and director of the National Center for Zoonotic, Vector-Borne and Enteric Diseases

The word “zoo” brings to mind a place where we can go to see different kinds of animals. But in public health, it means something else. What are zoonotic diseases?

The word “zoo” does mean animals, but “nosis” is the Greek term for “disease.” So if you put them together, it means animal diseases. Over time, zoonotic has come to mean naturally occurring diseases that can be transmitted from or through animals to people.

Lonnie King, DVMHow far back in recorded history can zoonotic diseases be traced?

The first major group of zoonotic diseases probably occurred between 8,000 and 10,000 years ago, during the time of the advent of agriculture when communities were built because populations were stabilized and diseases were more easily spread. It was also at that time that animals were domesticated, and the first time in history that animals and people were brought together in close quarters, and that was the first group of zoonoses, or zoonotic diseases. In ancient civilizations from Babylon to the Nile Valley to China, even in the Old Testament and in the writings of Hypocrites and Virgil, who wrote centuries ago, there are references to this group of diseases.

What happened during the Middle Ages?

That was a very significant time in history because perhaps 25 percent of the world’s population was wiped out by a zoonotic disease called plague, which is transmitted by an organism named Yersinia pestis. Plague is actually transmitted through a vector, or a carrier, which is the flea, and it resides mostly on rats. As commerce picked up, especially along the silk route from Asia into the populations of Europe, rats actually hitchhiked along the way and transmitted the disease into huge urban populations in Europe.

Is it true that approximately 70 percent of human infectious diseases are animal related?

In the last 20 or 25 years --  and we term this as the age of emerging infectious diseases -- approximately 75 percent of the new human diseases that have emerged are zoonotic, and of the 1,461 human pathogens that we know about today, about 60 percent are what we would term “multihost pathogens.” In other words, they don’t reside just in people by themselves. Contacts through animals or animal products --  even plants -- are actually responsible for multihost diseases.

What are some common zoonotic diseases?

The common ones that people probably think about are rabies of course, which we see worldwide, and a disease called brucellosis, found in cattle and goats, that we are in the process of eliminating from the United States. One of the forms of tuberculosis in cattle, called Mycobacterium bovis, was probably responsible for 25 percent to 30 percent of all the tuberculosis cases in the United States prior to pasteurization. Pasteurization will kill this organism, but in other parts of the world Mycobacterium bovis is actually seen as one of the forms of tuberculosis. Other common (zoonotic) diseases are Lyme disease, leptospirosis, tularemia and West Nile virus. Examples of zoonotic diseases (transmitted) not directly from animals to people but from animal products to people are Salmonella, E. coli 0157:H7, campylobacter and listeria.

What simple measures can we take to reduce the risk of zoonotic disease transmission?

A lot of it is common sense. You need to avoid inappropriate contact when you’re out walking around. In the summer or warm months when there are mosquitoes, you need to worry about West Nile virus or other mosquito-transmitted diseases. You need to use repellants and be careful at dusk. When you’re outside in potential tick-infested areas, wear tight clothing and make frequent checks to make sure ticks aren’t on you. And your pets should be treated for fleas and ticks. By taking them off of our pets we actually eliminate that transmission potentially to us. Vaccinate your pets. We no longer have canine rabies in the United States because of good vaccine. We have rabies in bats, foxes and raccoons but we don’t have them in dogs anymore. But vaccinations for things like leptospirosis is another way of reducing the risk. Take your pets to the veterinarian to make sure they are free of pests and get them dewormed. Roundworms, hookworms and tapeworms are potentially transmitted from our pets.

In the kitchen, handle foods properly so you don’t cross-contaminate. Wash your hands frequently and cook foods to the proper temperatures, such as 165 degrees for meats and hamburgers. And another thing is just handwashing, handwashing, handwashing. It’s good hygiene and probably the single best deterrent for transmission of many of these diseases.

Is it okay for our pets to lick our faces?

We love our pets and the benefits that we get from the human-animal bond helps our psychological well being. And for seniors it really helps with giving them companionship. So there are great benefits to having animals as part of our lives, and here again, you have to use good common sense. Pets do potentially and can potentially transmit diseases like toxoplasmosis; and salmonella through reptiles and turtles or birds — basically small turtles. So pets do carry salmonella and roundworms and hookworms. These kinds of problems with internal parasites or worms used to be fairly commonly transmitted, especially to children who were playing outside. That is one of the things we worry about. And there is cat scratch fever, from an organism called Bartonella.

I think it’s a matter of not being so overly concerned that you miss out on the wonderful benefits of your pets as companions and the richness they add to our lives. Just understand that they are a potential source of threats, not a great or high risk, but you need to use the precautions that we talked about.

What about petting zoos? They are tremendously educational and fun but can we pick up diseases at petting zoos?

I agree they are educational and kids really enjoy them, but I think you have to use common sense. Young children in particular are at the highest risk because they are in there with close exposures. If they touch these animals and they are eating food or putting their hands in their mouth, things like salmonella, cryptosporidium and E. coli — even staph and strep — are concerns with these petting zoos. So I think you have to really use special precautions for young children in particular. Most petting zoos that are set up right have trained operators and transition areas. It’s important that foods not be brought in and that there are immediate opportunities to wash your hands after you go through one of these zoos. CDC has a Web site on Healthy Pets, Healthy People and it is another very good resource for people to gain information about pets, including dogs, cats, hamsters and reptiles and rodents, and how we need to handle them carefully.

We’ve been hearing a lot lately about avian influenza virus, or bird flu, which is found mainly in birds and is highly contagious among poultry. We now know that infections with these viruses can occur in humans. Should we be worried?

We are always concerned, and when people talk about influenza there are actually four different types to be concerned about. One type is what we have circulating around every year — your annual influenzas that people get vaccinated for, and the influenza virus constantly changes. That virus goes from person to person to person to person. Get vaccinated and understand how you might be exposed in public places to this virus.

And then you have a disease called low-pathogenic avian influenza, which is found very commonly in domestic birds and also in wild birds. These viruses are constantly changing and on the move frequently, but they are lowly pathogenic so they are being carried but they’re not causing clinical disease.

And we have high-pathogenic avian influenza, which is what we see today in H5N1. It describes the proteins on the surface of this virus, and highly pathogenic means it is devastating for populations of birds -- mostly domestic chickens and poultry but also for wildlife. We constantly watch this very carefully because there have been 125 to 150 human cases where this high pathogenic influenza is actually passed from birds to people. Fortunately, it has not gained the characteristic to be able to be passed from person to person to person. When that happens, then you have the fourth phase, which we call pandemic influenza, like we saw in 1918 and 1919 when there was a huge amount of deaths and sickness.

There have been two other kinds of circulation of influenzas, pandemic type, not as devastating as the 1918 one, but people have to remember that the origin of these new influenzas are actually probably in southeast Asia or China where there is a constant circulation and exchange of this virus between birds, and oftentimes pigs and people, where you actually have a pool of amplification and change. That’s where the new viruses emerge that can either move around the world as low pathogenic in birds or highly pathogenic, where they are devastating in birds and then into people and can then transfer from person to person so it is a constantly changing group of viruses that drift and shift and we watch them very closely.

Do we need to worry about catching bird flu from eating poultry and eggs?

In areas where the disease may be endemic, where the virus actually may be in meat or eggs, it’s a matter of cooking those products to the right temperatures that will kill influenza. So we really would be at low risk if we handle those products properly. And frankly, with H5N1, we don’t have that in the United Sates. It is just very limited in terms of human disease, but cooking and handling the product properly is the key.

Can our pets catch diseases from us?

They can. You hear a lot now about a disease called methicillin-resistant Staphylococcus aureas, or MRSA, which is a real concern not only to animal populations but to human populations, where this type of staph is building up a resistance in animals and in humans. There are recorded incidences where people will actually transfer this disease to pets. And there have been incidences of human tuberculosis in a person actually being transmitted to household pets as well.

What about dog bites?

Dog bites are a real public health problem in this country and around the world, and while they’re not zoonotic in terms of transferring a virus or bacteria directly to people, they can cause tremendous problems. The last estimate that I saw estimated that there were almost 5 million bites a year and 800,000 of these required medical care. Some of these were emergency visits and hospital visits. Dog bites are quite painful and quite costly and they are a major problem, especially with kids handling pets, and pets that are running without leashes. You can actually get some of the bacterial infections from bites. Cat bites in particular can be very nasty, with a bacteria called Pasteurella. So dog bites are a very significant public health problem that sometimes we don’t think of as zoonoses, but here again it’s the interaction of people and animals that have caused these problems.

— Q&A conducted, edited and condensed by Teddi Dineley Johnson, The Nation's Health, APHA. Animal photos courtesy USDA Agricultural Research Service.

July 2008