Emerging zoonoses: frightening interface exists between animal and human health

Over the last few months, it is rarely possible to read through a newspaper, magazine, listen to the radio or watch TV without encountering some item about an emerging zoonotic disease. Ebola virus, SARS, bird flu — these have all been headlines and major stories, underscoring the expanding, intense and often frightening interface of animal and human health.

It is likely that every veterinarian reading this article will have been queried at some point by a client about a new zoonotic disease. Since the discovery of HIV/AIDS in the United States in the early 1980s, a disease of humans believed to be sparked by transfer of a virus from primate species to humans (and therefore a zoonosis), the world has seen approximately 10 new diseases of humans emerge or re-emerge every year. Of all of these new and emerging diseases, it has been shown that 75 percent are zoonotic. As veterinarians, we can serve as marvelous conduits of information to the general public about the facts surrounding these diseases. This article presents a brief overview of four of the more unusual and lethal emerging agents that are associated with and/or affect pet animals: Capnocytophaga canimorsus, raccoon ascarids, alveolar hydatid disease and Cryptococcus gattii.

Capnocytophaga canimorsus

The emerging zoonotic bacteria Capnocytophaga canimorsus is a mouthful, figuratively and literally. This fastidious organism is found in many dog and cat oral cavities and causes no problems for its carnivore hosts. But a major sepsis can occur in humans when dog (or occasionally cat) saliva finds its way into the human body. Usually this is through a bite, but there have been cases of dog saliva entering through a skin wound licked by a dog, or, even transfer across oral mucous membranes (the latter was documented in an article written by British scientists, entitled, "Do not snog the dog," "snog" being the British slang equivalent of the American "smooch"). C. canimorsus can cause a fulminant septicemia, with meningitis, endocarditis and peripheral symmetrical gangrene. Most severe symptoms are seen in those that may have immunocompromise, splenectomy or suffer from chronic alcoholism. Approximately one-third of affected patients will die from the disease. Because of its very narrowly-defined culture requirements, C. canimorsus is difficult to grow and the biochemical tests for identification can lead to confusion with other gram-negative bacteria, which will confound diagnosis until it may be too late to institute adequate therapy. In a study of hospital laboratories, it was found that C. canimorsus infection was accurately diagnosed in only 32 percent of human cases. Veterinarians could benefit from understanding this disease and for helping to make at-risk clients aware of the potential dangers lurking in the spittle of some of man's best friends.

Raccoon ascarids

Visceral larval migrans (VLM), due to dog or cat ascarid larval migration and granuloma formation in a non-definitive host, is probably the most prevalent zoonotic disease, as a high percentage of Americans have antibodies to dog or cat ascarid larvae.

Perhaps the most severe form of VLM is that in which granulomas form in the eye, a disease known as ocular larval migrans (OLM), which accounts for unilateral blindness in at least 700 U.S. children per year. So, ascarid larval migration is a longstanding and serious zoonotic disease that veterinarians must help to prevent through prompt deworming of puppies and kittens. However, a new form of VLM has been seen in recent years, caused by larval migration of eggs of the raccoon ascarid. This nematode, Baylisascaris procyonis, is found in most raccoons in the United States and is a prolific producer of eggs. Because of the suburban nature of raccoons and their tendency to form "latrines," there is often very heavy environmental contamination in close proximity to human domiciles. The eggs are environmentally hardy and also quite sticky, clinging to hands, plastic toys and other items. Those with less than optimal personal hygiene, such as children and the mentally disabled, are at greatest risk of ingesting the eggs. Unlike the VLM, with which we are already familiar, the raccoon ascarid larvae do not die within a short period of time in an unusual host, forming characteristic granulomas, but rather continue to live and wander widely throughout tissues, creating necrotic and hemorrhagic tracts that seriously compromise organ function. The brain, in particular, is often severely affected. Although only a few dozen cases of human infection with raccoon ascarid larvae have been diagnosed in the U.S. to date, they are all associated with permanent neurologic impairment or death. Effective advisement of clients who wish to keep raccoons is warranted and meticulous attention to intestinal parasites is essential.

Alveolar hydatid disease

Echinococcus multilocularis is a tiny cestode, sometimes known as the "Arctic tapeworm" that resides as an adult in the intestines of the definitive hosts, which are foxes, wolves and occasionally dogs and cats in the far reaches of the Northern Hemisphere. The normal cycle consists of eggs being shed in the carnivore's feces, rodents and lagomorphs ingesting the eggs, and then the intermediate form developing into extensive, expanding, multiloculated cysts in the visceral tissues of these small mammals. Disease due to this intermediate cystic form is known as alveolar hydatid disease. When the carnivore ingests an infested mouse, vole or rabbit, the intermediate forms mature into the adult tapeworm within the intestinal tract of the carnivore and the cycle is complete. Unfortunately, humans can also participate in this cycle, and if a person happens to ingest the tapeworm eggs, the parasite behaves in the same manner as in the small mammal. Multilocular cysts will develop in visceral tissues, predominantly liver. These cysts continue to expand and replace the liver, causing hepatic failure. Usually by the time the disease is diagnosed, as much as half the liver has been destroyed. Surgical removal of the cysts is the treatment of choice, a procedure that is fraught with the danger of cyst rupture, which would effectively seed the abdominal cavity with millions of daughter cysts. Human mortality associated with untreated alveolar hydatid disease approaches 80 percent. Because of a variety of factors, including translocation of native Arctic animals to more southerly climes and eradication of rabies in European foxes and subsequent expansion of the fox populations, alveolar hydatid disease is becoming a serious public health problem in many parts of the Northern Hemisphere.

Cryptococcus gattii

Crytococcus neoformans is a well-recognized cause of opportunistic disease in immunocompromised humans. This dimorphic fungus is seen in animal disease as well, and many practitioners will have dealt with skin, nasal or cerebral space-occupying masses that develop in cats when airborne basidiospores from environmental hyphae find their way into the body and the yeast form subsequently replicates to form space-occupying masses. In recent years, a more virulent and novel form of cryptococcosis has emerged to cause skin, neurologic and respiratory disease with high mortality in hundreds of animals and humans in the eastern portion of Vancouver Island. This Cryptococcus, at first thought to be a variant of Cryptococcus neoformans, has now been determined to be a separate species, Cryptococcus gattii. This organism, first recognized in Koalas in Australia more than 30 years ago, is closely associated with the red gum eucalyptus tree, which was imported onto Vancouver Island decades ago and is now well established there. Since 1999, there have been hundreds of cases in humans, dogs, cats and other animals. Mortality associated with the disease is very high. In the climate of Vancouver Island, the Cryptococcus gattii thrives in the red gum eucalyptus but has also spread extensively to infest North American trees. It is thought that infection of various mammalian species arises from exposure to the hyphal form resident in the trees. Humans, cats, dogs and horses all have been infected. Zoonotic disease? Not really, but this may be the world's first recorded botanosis!

Conclusion

There are a number of issues inherent in our globalized society that are allowing for closer connections among animals, humans and plants, and a greater variety of those connections. As international trade expands and the world becomes "flat", it is a certainty that traffic of materials, and biological organisms will only increase.

Along with this globalization and overall improvement of the world economy is a greater potential for new diseases to emerge. Whereas 30 years ago, there was a handful of zoonoses to study, understand, and be able to explain to clients, today, the repertoire has expanded considerably and will continue to do so.

As Hall-of-Famer Yogi Berra, one of the world's most celebrated baseball personalities known for his Yogi-isms, said, "The future ain't what it used to be." Stay tuned.

Dr. Brown is professor and coordinator of international activities for the University of Georgia's College of Veterinary Medicine.