How to Avoid Zoonoses

by Patricia Gail Burnham

Loving or zoonotic? Not on the lips, so it's probably OK.

It was my mother’s lifelong belief that pets were “dirty” and that made having a pet loving daughter something of a cross for her to bear. She was firmly convinced that I would eventually catch some bizarre and serious disease from a variety of pets that worked their way up through birds, rats and cats to eventually settle into an adult passion for dogs. I generally ignored her concern and stayed remarkably healthy. I never caught distemper from either a cat or dog, and they never seemed to catch my various colds and flu attacks. The reason for that is simple. Most of the common ailments are “Species specific.” Thus canine distemper will affect dogs, wolves and possibly ferrets, but will not affect a human being. Colds, flu, measles, and AIDS affect humans and other primates, but not dogs and cats. We are blessed with what is called “Inter-species immunity.”

There are however a few diseases and parasites that can be transmitted between species. These are called zoonoses or zoonotic diseases. So mother wasn’t entirely wrong. And when she wanted me to wash my hands between handling my pets and preparing lunch, she was entirely right because transmission of most zoonoses can be prevented by simple good hygiene and sanitation.

What we are concerned with here are zoonoses transmitted by pet animals. There are also zoonoses that are transmitted from farm animals, and in developing countries where families and livestock still share living quarters those are common. But in this country the main animal contacts for people are their pets, rodents and wild birds.

What are the zoonoses we should look for and how can we prevent them?

The rarest and scariest one is rabies. An average of one person a year dies of rabies in the U.S. and most of those cases are imported from bites that occur outside the country. However a lot of people receive post animal bite treatment. The best insurance against rabies is to have both your dog and cat vaccinated against rabies. The major reservoir of rabies is in wildlife and a cat’s natural hunting instinct can bring it into contact with a rabid animal. A dog living in a fenced yard is less likely to encounter a rabid animal than is a free roaming cat.

Trans-larval migrans has come to be of concern lately. Most species of animals have their own variety of ascarids (roundworms) living in their intestines. This is even true of human beings. Roundworms tend to have complicated life cycles that involve moving through the body of their host in their larval stage, until they eventually end up in the intestine. One species’ ascarids are sufficiently unique that they are unable to complete their life cycle if they are accidentally ingested by an animal other than their host species.

The problem is that when they are accidentally swallowed by the wrong host, they are not digested and they do not die immediately. They make it from the egg to the larvae stage of their development and then they can go no farther. So they migrate through the hosts’ body, looking for a place to encapsulate in the host’s tissues. Most of the time they do no harm. Antigen tests have shown that as many as 7% of the population may have experienced trans larval migration, most of them with no symptoms. But if the migrating larva enters the eye, it can cause blindness. And encapsulated larvae in other tissues are sometimes mistaken for tumors in x-rays. You wouldn’t want to be operated on for lung cancer, only to find out that the lumps seen in the x-ray were relatively harmless encapsulated larvae. The way to prevent unnecessary surgery is to have an antigen test for larvae whenever the x-rays show a mysterious lump.

Children and Puppies

But a better solution is to keep children from ingesting ascarid eggs. So how do they get them? The adult ascarids in the intestine lay eggs which pass out with the animal’s feces. When the feces decompose the eggs are left in the soil, where they can live for up to a year, just waiting for someone to consume them. Some children actually eat dirt, which is called “Pica.” Even more kids just wear enough dirt on their hands to be a problem. So here is where the hand washing before eating comes in. You don’t want them consuming roundworm eggs along with their peanut butter sandwich.

If you have both dogs and small children it should give you an extra incentive to keep your yard picked up. This year I had a wonderful Thanksgiving dinner at my cousin’s. They had a beautiful new house, a big yard, three small children and a pair of show quality collies. All in all, a delightful household. The lady is a good cook and an immaculate housekeeper. But the outer edges of the yard were littered with old dried dog feces.

I don’t have kids, but my yard is picked up at least once a day. And, if I had young children around I would be even more fanatic about sanitation. Not only do I not want to have to watch where I walk, but I don’t want the soil infected with roundworm eggs. Curiously enough there is little connection between general neatness and yard hygiene. Nobody has ever accused me of being a super housekeeper, but I am a clean yard fanatic. I once had a neighbor who was an intensive care nurse. She had a young daughter and one small corgi dog. When she asked me to dog sit for him, I was astonished to find that there wasn’t a single clean square foot of grass in her yard. I don’t think anyone had picked up after the dog since she had moved in. The solution to trans larval migrans isn’t to get rid of dogs; it is to pick up after them promptly both at home and in public places.

Deworming Necessary

It is also a good idea to have puppies checked for worms and put them on a diligent worming schedule. When I see pictures of kids hugging puppies I cringe a little, because there is one other thing you should know about roundworms. Most dogs past the age of 2 years are not shedding roundworm eggs. Most puppies are. As a young dog matures, the roundworm larvae encapsulate in the tissues where they wait for the hormones of pregnancy to signal them when it is time to migrate to the new puppies and start the next generation. Since nearly all adult bitches had roundworms when they were puppies and have roundworm larvae encapsulated in their tissues, this means that nearly all puppies are born with round worms. They grow and reproduce in the intestines and shed eggs into the feces. The eggs are eaten with the soil, mature into larvae, migrate to the body tissues and encapsulate to wait for the dog to grow up and get pregnant.

It is an endless cycle, and even though puppies are wormed regularly, so far it has not been possible to break the cycle. But it is possible to reduce the level of the infestation. If you worm a puppy diligently she will grow up to be an adult with only a few encapsulated larvae in her tissues. And when she has puppies they will only be lightly infected. And while worming does not get rid of all the worms, and most puppies need to be wormed repeatedly as they grow up, the worming does prevent the remaining worms from producing eggs. So you can have puppies and still keep your yard relatively egg free. When you take a fecal sample to your vet to check for worms, the vet is looking for worm eggs.

If you are a gardener, don’t compost animal waste for your vegetable garden. And you might want to think of some way of discouraging the neighborhood cats from using either your garden, or your kid’s sandbox as a potty area. Cats love both sand and freshly dug earth. And cats have their own ascarids.

The remaining zoonoses are all bacterial infections.

These are Brucellosis, Leptospirosis, Salmonellosis, Tularemia, Plague, Cat Scratch Fever, Pasteurellosis and Psittacosis.

Cat Scratch Fever and Pasteurellosis are transmitted via cat scratches. And pet birds of the parrot family can carry Psittacosis which produces diarrhea in birds and respiratory disease in people.

Tularemia and Plague are generally caught from the bites of infected fleas and ticks that have been feeding on rodents. If somehow one of these diseases was transmitted to your dog there is a remote chance he could, if he bit you, transmit it to you. But the general advice in avoiding Tularemia and Plague is to not handle wild rodents and rabbits.

Salmonella could be transmitted by a dog but it is much more common from pet turtles which is why the shipment of pet turtles was restricted years ago. And these days we don’t even need pet turtles to get exposed to Salmonella with current news reports on Salmonella contamination of our fresh chickens and raw eggs. Your chance of catching a bout of gastrointestinal Salmonella is greater from your local supermarket than from your dog or cat.

Leptospirosis is transmitted to dogs through the urine of infected mice and rats. So the first step in preventing Leptospirosis is to wage war on rats. Fortunately Leptospirosis is one of the diseases for which there is an effective vaccine for dogs. When your vet gives a DHLP shot to your dog the letters stand for Distemper, Hepatitis, Leptospirosis, Parvovirus. There are some combination shots for use on puppies that do not include Leptospirosis so check to make sure that your dog is vaccinated with a combination that does include it. Actually the shots against Leptospirosis are so successful that it is almost never seen in dogs any more. But as long as the rodent reservoir remains it is prudent to keep vaccinating our dogs against it.

Toxoplasmosis is one of the scariest and most destructive of the protozoal zoonoses. You don’t have to worry about dogs for this one. Cats are the hosts. If a pregnant woman is infected there is a one in six chance that her fetus will be infected and if that happens, the result can be birth defects and brain damage to the baby. It is transmitted through cat feces, so pregnant women should not clean litter boxes. Or they should wear rubber gloves and wash thoroughly afterwards.

Every pregnant woman with a cat is not going to get Toxoplasmosis. The incidence is about one case of congenital Toxoplasmosis per one thousand live births. Aside from pregnancy, people get Toxoplasmosis fairly frequently without serious effects. It is estimated that nearly a quarter million people in this country have had it. And cats are not the only source. The most common source is eating undercooked (rare) meat. (Which shouldn’t be a surprise since the cats are infected with it by eating raw prey.)

When people are tested for exposure to Toxoplasmosis, the rate of infection goes up quite dramatically in the late teens and early twenties, the ages at which they start preferring their meat rare. (Along with hand washing, another of mother’s pieces of advice was to never eat anything rare, and she had never heard of Toxoplasmosis.)

General precautions

Fully vaccinate your pets. Pick up your yard often. Wash your hands after handling pets and before handling food. Cook your meat well done whether it is chicken (Salmonella), pork (Trichinosis) or beef (Toxoplasmosis).And you don’t want to know what you can catch from raw fish. It should be sufficient to say that the fish parasitologist that I work with never eats sushi.

Try not to worry about zoonoses as much as my mother did. The rates of infection are really small. The precautions are simple. The pleasure I have had from my dogs and cats has far outweighed any risk they posed to my health. Their companionship and the opportunity they provide to interact with an entirely different species has enriched my life. And over the years the vast majority of my illnesses have been transmitted from the most dangerously infectious species around — other human beings.

Bio – Gail Burnham has written hundreds of magazine articles, plus two dog training books illustrated with greyhounds, Playtraining Your Dog in 1981 and Treats, Play, Love: Make Dog Training Fun For You and Your Best Friend in 2008. She contributed regularly to Celebrating Greyhounds the first five years, most notably the Kira series which was a finalist in the Dog Writer’s Association writing contest. She won a DWAA award for the poem, The Red Bitch’s Hunt, which was part of a series of poems about Coventry and the Red Bitch. Currently she is retired, still writing about dogs, and is is living a very active life with the descendants of the original Tiger.

CG SU 98

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