Download Emerging and Zoonotic Diseases

Pathology Symposium, AAHA Annual Meeting, Tampa, FL
Emerging and Zoonotic Diseases
Corrie Brown, DVM, PhD, DACVP
University of Georgia
An emerging disease is defined as a new disease, a new presentation of an old problem, or an
existing disease that shows up in a new geographic area. The term was first used to describe
several new entities in humans that surfaced in the early 1980’s, and since then, new diseases
have continued to emerge and the rate at which they are occurring is steadily increasing. Threequarters of the emerging diseases of humans are zoonotic, which makes veterinary medicine an
important partner in the understanding and control of each of these new problems.
The term “zoonosis” was first coined by Rudolf Virchow, a German physician-scientist who is
probably better known as the “father of modern pathology” because of his elucidation of the
cellular nature of disease. Raised in a family of butchers, Virchow was especially conscious of
the possibilities of disease acquired from agricultural animals. As a junior physician, Virchow
oversaw some aspects of public health, including examination of meat for human consumption.
One of his major tasks was ensuring that pork was free of the parasite Trichinella, a problem he
termed a “zoonosis.” Since Virchow’s first pronouncement of the word over a century ago, more
than a thousand zoonoses have been recognized and studied.
Veterinarians would be well served by increased familiarity with some of the emerging
zoonoses, as they will be queried on these by their clients, and may have to understand the
pathogenesis in order to recognize and/or treat these diseases in the animals they see or counsel
protective measures for their owners.
Below are some of the more prominent emerging zoonoses to be covered:
 Bird flu, the H5N1 strain of highly pathogenic avian influenza, is moving around the world
to cause the most extensive agricultural disease outbreak in history. The transmission to
several hundred people has raised concerns that the virus may mutate to spread more rapidly
among people and cause the next pandemic of human influenza.
 West Nile virus is a flavivirus that has been present in Africa and the Middle East for
centuries, occasionally causing disease in domestic poultry, horses and humans. Its
emergence near the Bronx zoo in 1999 highlighted the ease with which these viruses can
make transoceanic journeys and become established and ineradicable in a very short period
of time. Today West Nile virus occurs throughout the Americas, from Canada to Argentina.
 Rift Valley fever is caused by a bunyavirus, and has been known in eastern and southern
Africa for decades as a cause of febrile illness with abortion and death of neonatal ruminants.
Spread to humans by the vector mosquitoes can lead to a flu-like illness with permanent
deficits due to retinal damage, and some mortality due to hemorrhagic complications. An
outbreak in Kenya in 2007 underscored the ease with which this disease can surface in a new
region and be a serious emerging public health problem.
 Nipah virus emerged in the spring of 1999 when over a hundred people in Malaysia and
Singapore succumbed to an encephalitic condition. All of the victims had had some contact
with pigs. The causative virus, subsequently named Nipah virus, was found to cause a
Pathology Symposium, AAHA Annual Meeting, Tampa, FL
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respiratory illness in pigs, and encephalitis in humans, as well as several other mammalian
species. Mortality rate in humans was approximately 40%. The reservoir is giant fruit bats
and there is concern that these animals could again disseminate Nipah virus.
When SARS emerged in 2003, it caused the largest global public health effort ever
mobilized. We now know that the causative coronavirus came from bats and was altered to
become infectious for humans as it passed through farmed civet cats sold for human
consumption.
Monkeypox occurred for the first time in the western hemisphere when infected imported
Gambian rats mixed with prairie dogs at a holding facility. Subsequent dissemination of
prairie dogs to private homes, pet stores, and veterinary clinics, resulted in human cases of
the disease in four states.
Leptospirosis (Leptospira spp.) is a zoonotic infection acquired from urine of infected
animals. In humans, it causes fever, as well as signs referable to liver, kidneys, and brain.
High risk groups have always been livestock workers and abattoir employees. Within the
last ten years, outbreaks have been associated with flooding and swimming in contaminated
lakes. In addition, increasing cases are being seen in urban hospital emergency rooms,
perhaps due to spread through rat urine.
Capnocytophaga canimorsus is a part of the normal flora of many dogs and cats. It causes
no problem in the animal but when inoculated into a human, usually through a bite wound,
may cause septicemia with disseminated intravascular coagulation and peripheral gangrene.
Individuals with some degree of immunocompromise are especially susceptible but the
disease is not confined to this group. Men over 40 who are alcoholic or who have been
splenectomized are considered to be at highest risk.
Sporothrix schenkii is an environmental saprophyte that causes cutaneous nodules in dogs
and cats in a condition known as sporotrichosis. The organism also infects humans and was
previously thought to be a risk only for gardeners and agricultural workers. However, there
have been several recent cases in veterinarians and veterinary technicians – evidently the
material in the lesions of animals is also infectious. Unfortunately, when the infestation
occurs in humans, the organisms are very hard to find on impression smear or biopsy,
making the diagnosis problematic.
Echinococcus multilocularis is a tapeworm carried by carnivores. Intermediate stage consists
of multiple hydatid cysts, usually in rodents. However, humans can also be infected by
ingesting the tapeworm eggs and will develop alveolar hydatid disease, with multiple
parasitic cysts in liver and lung, primarily. Although initially confined to the arctic regions,
as people move into more northerly regions, and as translocation of wild carnivores occurs,
the disease has moved steadily toward more temperate regions.
Toxoplasmosis is the most costly zoonotic disease in the U.S. Infection during pregnancy
can result in congenital defects, primarily in the CNS. Fresh pork meat is the main source of
infection. More recently this parasite has been incriminated in major die-offs in California
sea otters. Another alarming correlation has been made with the prevalence of schizophrenia
in humans.
E. coli O157:H7 caused a massive countrywide outbreak of foodborne disease, with several
cases of hemolytic uremic syndrome. The source was contaminated spinach from the Salinas
Valley in California, and presumably feces with the organism that made it into the spinach,
but the exact species source has yet to be determined.
Pathology Symposium, AAHA Annual Meeting, Tampa, FL
 Methicillin-resistant Staphylococcus aureus, more commonly known as MRSA, has emerged
as a major public health threat. Originally confined to people in longterm health care
settings, the organism has now found its way into more public settings, such as team sports
and secondary schools. The movement of this organism between animals and humans makes
it difficult to determine if this is a zoonotic agent, an anthroponotic agent. In all probability
it is both.
 Clostridium difficile is also on the rise as a public health threat, and is now being seen outside
of its original venue, the intensive care unit. The emergence of a highly virulent pathotype is
spreading in other settings. The role that animals are playing in the spread of the new
pathotype is only beginning to be revealed.