Zoonoses: Introduction - The Merck Veterinary Manual

Zoonoses: Introduction

The emergence and reemergence of zoonotic diseases present challenges not only to veterinarians, but also to all professions concerned with public health. Since the 19^th century, the veterinary profession in the USA has been at the forefront of control and eradication of animal diseases, including bovine pleuropneumonia, foot-and-mouth disease, Texas fever, bovine tuberculosis, brucellosis, vesicular exanthema, and classical swine fever (see Table: Global Zoonoses). The early cooperation of veterinarians and public health physicians gave impetus to the eradication of bovine tuberculosis first in Denmark, Sweden, Finland, and Norway and then in the USA and Canada. Unfortunately, bovine tuberculosis has emerged in Mexico along the border with the USA and has caused human disease and dairy cow infections from California to Texas. Bovine tuberculosis and brucellosis remain major problems in the developing world.

People with acquired immune deficiency syndrome (AIDS) are much more susceptible, in general, to zoonotic diseases, including tuberculosis and other related mycobacterial infections; toxoplasmosis; cryptosporidial enteritis, foodborne Salmonella infections, and other enteric organisms; Campylobacter ; Listeria ; and Yersinia . It is possible that other zoonotic diseases that are dormant or infrequent (eg, leptospirosis, plague, glanders, melioidosis, and pseudoglanders) may emerge in individuals with AIDS or other immunocompromising conditions. Many of these are latent or nonpathogenic serovars. AIDS-like infections have been described in lions and the tropical cats of Africa as well as in domesticated cats. None have caused human disease.

In Australia and Malaysia, new diseases have been reported in horses and swine that also affect humans. They are caused by a morbillivirus—a measles-like virus related to canine distemper and rinderpest viruses. Another virus killed many wild felids in a zoo in Egypt. Many emerging viral diseases that have a rodent or unknown animal host have caused devastating fatal diseases in humans in Africa and South America, eg, Lassa fever, an arenavirus serologically related to lymphocytic choriomeningitis, and South American hemorrhagic diseases of Argentina and Bolivia. In Africa, Ebola fever and Marburg disease, the latter a dormant monkey disease, have caused death in medical personnel and in patients. Crimean-Congo hemorrhagic fever has caused death in African travelers and in the Middle East in abattoir workers.

The death of veterinarians in the western USA from plague, and reports of serious illness in veterinary technicians and cat owners, has focused attention on both domestic and feral cats and the larger mountain lion or bobcats as carriers of this ancient disease. Dogs and wild canids are likewise involved in plague regions of the USA. The involvement of cats since the 1970s is evidence of the dynamics of zoonotic diseases in a changing environment. Human populations may be pressuring old habitats, or there may be more subtle changes. The plague bacterium ( Yersinia pestis ) may be more adept at finding new hosts or new foci, as seen in other emerging diseases.

Although Lyme borreliosis has been recognized as an important zoonotic disease in North America, other forms of borreliosis have appeared in the USA and abroad for decades.

The Hantaan virus complex was first noted in 1951 in Korea, where it caused a hemorrhagic disease with renal syndrome. Various forms of the disease exist worldwide, and it is a major public health problem in China. In the USA, 2 fatal forms of the disease have been reported, namely, nephritic and pneumonic (in addition to latent infections). Hantaan virus has caused infections in laboratory rodents, and veterinary technicians have been infected in Asia and Europe.

The naturally occurring oral bacterium of the dog and cat Capnocytophaga canimorsus can cause disease and even death in persons with other debilitating illness, eg, alcoholism. Yersinia enterocolitica in an infected dog can be a hazard to humans, as can Bartonella in infected cats.

Prion diseases have been described in North America, Europe, and Asia and are known to affect sheep, cattle, elk, and deer as well as wild and domestic felids. The bovine prion disease is reported to have caused >140 human cases with 100% mortality in the UK. Although the incidence is <1/1,000,000, the threat of prion diseases to human and animal health is of major concern.

Exposure to animals kept as pets is steadily increasing as the number of pets increases in the USA and other affluent countries. The types of animals kept as pets are also increasing. Examples of these are the “exotic pets” that have become popular in many parts of the world, eg, prairie dogs, that have brought plague, tularemia, and even monkeypox out of the wild into people’s homes. The desire of humans to touch wild animals or have contact with farm animals has resulted in the establishment of “petting zoos.” Contact with farm or wild animals may expose children or other visitors to organisms such as Escherichia coli O157:H4 or even rabies. Public health officials in the USA, Canada, and Britain are trying to control these “zoos” through inspections and rules, including microbicidal handwashing following exposures.

Another source of infection is exemplified by the severe acute respiratory syndrome epidemic caused by a novel coronavirus that appeared in southern China in 2003, first among food preparation workers exposed to civet cats and other “exotic animals” during their preparation as special foods.

The 21^st century holds the threat of even more emerging diseases, nurtured by an ever-increasing human population. Control of zoonotic diseases and protection of the public health will become even more challenging as world population increases. When overpopulation and crowding occur, water shortages occur, hygiene often cannot be maintained, and malnutrition develops, leading to disease and epidemics. Surveillance and reporting of disease is the first line of defense. Knowledge of the epidemiology of the disease organisms is the first step in initiating a control program. The ultimate objective is to protect and preserve both human and animal health.

Zoonoses are generally defined as animal diseases that are transmissible to humans. However, there are several diseases listed below that occur primarily in humans and that may also be transmitted between humans and animals, with some animals serving as reservoirs for human infection (eg, Trichuris trichiura ). The following common bacterial and viral diseases of humans are not found as naturally occurring diseases in animals (ie, animals are not a reservoir): diphtheria ( Corynebacterium diphtheriae ), Legionnaires’ disease ( Legionella pneumoniae , L pneumophila , and related organisms), syphilis ( Treponema pallidum ), trachoma ( Chlamydia trachomatis ), typhoid fever ( Salmonella typhi ), poliomyelitis, hepatitis B, mumps, chickenpox, smallpox, and measles.