What are zoonotic diseases?

Definition

Domestic and wild animals, and their ecosystems, contribute to human health and well-being. Animals provide protein-rich nutrients, transportation, fuel, recreation, and companionship. With the many benefits people derive from animals (including arthropods) comes health risks at this human, animal, and ecosystem interface. This interface can be described as a continuum of direct or indirect human exposure to animals, their products, and their ecosystems.

Zoonotic diseases, also known as zoonoses, are diseases caused by infectious agents (viruses, bacteria, and parasites such as worms and protozoa) transmitted or shared by animals and humans. These diseases are caused by a diverse group of pathogenic microorganisms that ordinarily live among animals. Some zoonotic diseases are transmitted directly from animals to humans, some result from contamination of the environment by animals, and others require a vector, such as a tick or mosquito.

Viral Infections

Arthropod-borne viruses include any of a large group of ribonucleic acid (RNA) viruses that are transmitted primarily by arthropods. There are more than four hundred species of arboviruses. Zoonotic diseases caused by viruses include the following:

Encephalitis. Encephalitis is inflammation of the
brain caused by infection. All arboviral encephalitides are zoonotic. They are
maintained in complex life cycles involving a nonhuman primary vertebrate host and
a primary arthropod vector. Many arboviruses that cause encephalitis have a
variety of vertebrate hosts, and some are transmitted by more than one vector.

Four main flavivirus agents of encephalitis exist in the United States:
eastern equine
encephalitis, western equine encephalitis, St. Louis
encephalitis, and La Crosse encephalitis, all of which are transmitted by
mosquitoes. Most human infections are asymptomatic or may result in nonspecific
flulike symptoms. In some infected persons, infection may progress to full-blown
encephalitis, with permanent neurologic damage or even death. Because the
arboviral encephalitides are viral diseases, antibiotics
are not effective for treatment; no effective antiviral drugs have been developed.
There are no commercially available human vaccines for these diseases, so
treatment is supportive.

West Nile virus. West Nile virus (WNV) is a flavivirus
commonly found in Africa, West Asia, the Middle East, and the United States. The
virus can infect humans, birds, mosquitoes, horses, and other mammals. WNV was
first diagnosed in the United States in 1999.

Most human infections are asymptomatic or may result in a nonspecific flulike syndrome. Approximately 80 percent of people who are infected with WNV will not show any symptoms. Up to 20 percent of infected people develop swollen lymph glands or a skin rash on the chest, stomach, and back. About 1 in 150 infected persons will develop severe illness, with symptoms that can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, and muscle weakness.

There is no specific treatment for WNV infection. In milder cases, symptoms resolve on their own. In more severe cases, infected persons should seek supportive treatment in a hospital.

Hantavirus pulmonary syndrome. Hantavirus pulmonary syndrome
(HPS) is contracted from rodents and has been identified throughout the United
States. Rodent infestation in and around the home remains the primary risk factor
for hantavirus exposure. In the United States, deer mice, cotton
rats, rice rats, and white-footed mice carry hantaviruses that cause HPS.

Although rare, HPS is potentially deadly. Humans can contract the disease when they come into contact with infected rodents or their urine and droppings or when they breathe in the hantavirus from the air. HPS cannot be transmitted from one person to another.

Early symptoms include fatigue, fever, muscle aches, headaches, dizziness, chills, and abdominal problems. Four to ten days later, additional symptoms appear. These symptoms include coughing and shortness of breath, as the lungs fill with fluid. There is no specific treatment or vaccine for hantavirus infection. Supportive care is the basis for therapy.

Lymphocytic choriomeningitis. Lymphocytic choriomeningitis (LCM) is a rodent-borne viral disease that appears as aseptic
meningitis (inflammation of the membrane that surrounds the brain and spinal
cord), encephalitis, or meningoencephalitis (inflammation of the brain and
meninges). LCM is caused by the lymphocytic choriomeningitis virus (LCMV). The
common house mouse, Mus musculus, is the primary host. The virus
is found in the saliva, urine, and feces of infected mice, and people become
infected when exposed to these substances. Other types of rodents, such as
hamsters, can become infected with LCMV in pet stores.

Initial LCM symptoms include fever, malaise, lack of appetite, muscle aches,
headache, nausea, and vomiting. In the second phase of the infection, persons have
symptoms of meningitis (fever, headache, and stiff neck) or
characteristics of encephalitis (drowsiness, confusion, and sensory disturbances),
or have symptoms such as motor abnormalities (for example, paralysis). LCM is
usually not fatal. Aseptic meningitis, encephalitis, or meningoencephalitis
require hospitalization. Anti-inflammatory drugs, such as corticosteroids, may be helpful in treating the disease.

Monkeypox. Monkeypox is a rare viral disease that
usually occurs in central and western Africa. It is caused by the monkeypox virus,
which was first found in 1958 in laboratory monkeys. In June, 2003, several people
in the United States contracted monkeypox after having contact with pet prairie
dogs that were sick with monkeypox. The disease was traced to a shipment of
Gambian rats that were imported to the United States and later kept near prairie
dogs at an Illinois animal vendor.

People can get monkeypox if they are bitten by an infected animal or if they
touch the animal’s blood or body fluids. The disease also can spread from person
to person. After infection, symptoms include fever, headache, muscle aches,
backache, and swollen lymph nodes. A few days later, symptoms include a skin rash
that develops into raised bumps filled with fluid; these bumps will eventually
fall off the skin. The illness usually lasts two to four weeks. There is no
specific treatment for monkeypox.

Rabies. Rabies is a viral disease of mammals
transmitted through the bite of a rabid animal. Transmission is through the
virus-containing saliva of an infected host. The majority of rabies cases occur in
wild animals such as raccoons, skunks, bats, and foxes.

The rabies virus infects the central nervous system of humans, ultimately involving the brain and leading to death. Early symptoms include fever, headache, and general weakness. As the disease progresses, more specific symptoms appear, including insomnia, anxiety, confusion, partial paralysis, hallucinations, hypersalivation (increased saliva), difficulty swallowing, and hydrophobia (fear of water). Once clinical signs of rabies appear, the disease is nearly always fatal, and treatment is mainly supportive.

Several tests are required for the diagnosis of rabies. Thorough wound cleansing has been shown to markedly reduce the likelihood of contracting rabies. A tetanus shot should be given if the infected person has not received one within the previous ten years. A doctor will determine if antibiotics should be used. Persons not previously vaccinated should receive a postexposure vaccination against rabies that includes administration of both passive antibody and vaccine.

Bacterial Infections

Zoonotic diseases caused by bacterial infections include the
following:

Anthrax. Anthrax is an acute infectious disease
caused by the bacterium Bacillus anthracis. It most commonly
occurs in wild and domestic mammals such as cattle, sheep, and goats, but it also
can occur in humans who are exposed to infected animals or to tissue from infected
animals. B. anthracis spores can survive in the soil for many
years. Humans can become infected by handling products from infected animals or by
inhaling anthrax spores in contaminated animal products. Anthrax can also be
contracted by eating undercooked meat from infected animals.

Anthrax infections can be of three types: cutaneous (skin), inhalation, and gastrointestinal. Most cutaneous infections occur when the bacterium enters a cut or abrasion on the skin. About 20 percent of untreated cases of cutaneous anthrax result in death, but death is rare with antimicrobial therapy. The first symptoms of inhalation infection resemble a common cold, but after several days, the symptoms may progress to severe breathing problems. Inhalation anthrax is usually fatal. The gastrointestinal form of anthrax follows the eating of contaminated meat and is followed by an acute inflammation of the intestinal tract. Intestinal anthrax results in death in 25 to 60 percent of cases of infection. Antibiotics are used to treat all three types of anthrax. Early identification and treatment are critical.

Lyme disease. Lyme disease is caused by the bacterium
Borrelia burgdorferi and is transmitted to humans by the bite
of infected blacklegged ticks. The Lyme disease bacterium lives in deer, mice,
squirrels, and other small animals, and ticks become infected by feeding on these
animals. In the northeastern and north-central United States, Lyme disease is
transmitted by the deer tick Ixodes scapularis. In the Pacific
Northwest, the disease is spread by the Western blacklegged tick (I.
pacificus).

In approximately 70 to 80 percent of infected persons, the first sign of infection is usually a circular rash that appears three to thirty days after the tick bite. This “bull’s eye” rash gradually expands in several days, reaching up to twelve inches in diameter. Other early symptoms include fever, chills, headache, fatigue, swollen lymph nodes, and joint and muscle aches. If Lyme disease is left untreated, it can spread to other parts of the body. Symptoms of late-stage Lyme disease include painful, swollen joints; severe headaches and neck stiffness from meningitis; and nervous system problems, such as impaired concentration and memory loss.

Several laboratory tests for Lyme disease are available to measure
antibodies to the infection. These tests may return
false-negative results in persons with early disease, but they are reliable for
diagnosing later stages of disease. Most cases of Lyme disease can be treated and
cured with antibiotics.

Plague. Plague is an infectious disease of
animals and humans caused by the bacterium Yersinia pestis. It is
transmitted from animal to animal and from animal to human by the bites of
infected fleas. Humans usually contract plague from being bitten by a rodent flea
that is carrying the plague bacterium or by handling an infected animal. Plague is
also transmitted by inhaling infected droplets expelled by the coughing of an
infected person or animal, especially domestic cats, which may become infected by
eating infected wild rodents. Fleas become infected by feeding on rodents, such as
chipmunks, prairie dogs, ground squirrels, mice, and other mammals that are
infected with the bacterium. Fleas transmit the plague bacterium to humans and
other mammals during the feeding process.

The characteristic sign of plague is a very painful, swollen lymph node called
a bubo. This sign, accompanied with fever, extreme exhaustion, headache, and a
history of possible exposure to rodent fleas, should lead to suspicion of plague.
The disease progresses rapidly; the bacteria can then invade the bloodstream and
produce severe illness called plague septicemia and lung infection. Once a
human is infected, a progressive and potentially fatal illness generally results
unless specific antibiotic therapy is given. The plague vaccine is no longer
commercially available in the United States.

Rocky Mountain spotted fever. Rocky Mountain spotted
fever (RMSF) is the most severe tickborne illness in the
United States. It is caused by infection with the bacterial organism
Rickettsia rickettsii, which is transmitted by the bite of an
infected tick. The American dog tick (Dermacentor variabilis) and
Rocky Mountain wood tick (D. andersoni) are the primary arthropod
vectors in the United States.

The early symptoms of RMSF are often nonspecific. Initial symptoms may include
severe headache, lack of appetite, muscle pain, nausea, and fever. Later signs and
symptoms include diarrhea, joint pain, abdominal pain, and rash. RMSF can be a
severe illness, and the majority of infected persons are hospitalized. Diagnosis
is based on a combination of clinical signs and symptoms and laboratory tests. It
is best treated using a tetracycline antibiotic, usually
doxycycline.

Salmonellosis. Salmonellosis is an infection with the
bacterium Salmonella, which lives in the intestinal tracts of
humans, animals, and birds. Salmonella is usually transmitted to
humans through foods contaminated with animal feces. Contaminated foods are
usually of animal origin and include beef, poultry, milk, and eggs, but any food,
including vegetables, may become contaminated. Salmonella is
killed by thorough cooking. Salmonella may also be found in the
feces of some pets. Reptiles, such as turtles, lizards, and snakes, and chicks and
young birds, are particularly likely to carry Salmonella in their
feces. People should always wash their hands immediately after handling one of
these animals, even if it appears healthy. Most persons infected with
Salmonella develop diarrhea, fever, and abdominal cramps
twelve to seventy-two hours after infection. The illness usually lasts four to
seven days, and most persons recover without treatment.

Parasitic Infections

Zoonotic diseases caused by parasitic infections include the following:

Cryptosporidiosis. Cryptosporidiosis is a disease caused
by parasites of the genus Cryptosporidium. Both the disease and
the parasite are known as crypto. Many species of crypto infect humans and a wide
range of animals. The parasite is protected by an outer shell that allows it to
survive outside the body for long periods of time. Crypto is one of the most
frequent causes of waterborne disease among humans in the United States and
throughout the world. Crypto lives in the intestines of infected humans or
animals. An infected person or animal passes the parasites in the stool. Crypto is
found in soil, food, water, or surfaces that have been contaminated with the feces
from infected humans or animals. Some people with crypto have no symptoms, but the
most common symptom is watery diarrhea. Other symptoms include stomach cramps,
dehydration, diarrhea, nausea, fever, or weight loss. Diagnosis is made by
examination of stool samples. Most people who have healthy immune systems will
recover without treatment.

Cysticercosis. Cysticercosis is an infection caused by
the pork tapeworm Taenia solium. Infection occurs when the
tapeworm larvae enter the body and form cysticerci (cysts). When cysticerci are
found in the brain, the condition is called neurocysticercosis. The tapeworm that
causes cysticercosis is most often found in rural, developing countries where pigs
are allowed to roam freely and eat human feces.

When pigs swallow pork tapeworm eggs, the eggs are passed through the bowel
movement. The eggs are subsequently spread by people who ingest contaminated food
or water. Once inside the stomach, the tapeworm egg hatches, penetrates the
intestine, travels through the bloodstream, and may develop into larvae in the
muscles, brain, or eyes. Although rare, larvae may float in the eye and cause
swelling or detachment of the retina. Symptoms of neurocysticercosis can include
seizures, headaches, confusion, lack of attention, or difficulty with balance.
Death can occur suddenly with heavy infections.

Diagnosis is usually made by magnetic resonance imaging or computed
tomography brain scans. Infections are generally treated with antiparasitic drugs
in combination with anti-inflammatory drugs.

Trichinellosis. Trichinellosis, also called trichinosis,
is caused by eating the raw or undercooked meat of animals infected with the
larvae of a species of worm called Trichinella. Infection occurs
commonly in domestic pigs. When an animal eats meat that contains
Trichinella cysts (larvae), their stomach acid dissolves the
hard covering of the cyst and releases the worms. The worms migrate into the small
intestine and mature in one to two days. After mating, adult females lay eggs that
develop into immature worms and travel through the arteries to muscles. Inside the
muscles, the worms curl into a ball and become enclosed in a capsule. Infection in
humans occurs when these capsules are consumed in undercooked meat.

The first symptoms of trichinellosis include nausea, diarrhea, vomiting, fatigue, fever, and abdominal discomfort. These first symptoms are later followed by headaches, fevers, chills, cough, eye swelling, aching joints, muscle pains, itchy skin, or diarrhea. In severe cases, death can occur. For mild to moderate infections, most symptoms subside within a few months, although fatigue, weakness, and diarrhea may last for months afterward. Several effective prescription drugs are available to treat trichinellosis.

Impact

Zoonotic diseases have the potential to spread efficiently across international
boundaries, thereby affecting not only human health and well-being but also
international travel and trade. More than 60 percent of the newly identified
infectious agents that have affected people since the mid-twentieth century have
been caused by pathogens originating from animals or animal products. Of
these zoonotic infections, 70 percent originated from wildlife.

Bibliography

Hugh-Jones, Martin E., William T. Hubbert, and Harry V. Hagstad. Zoonoses: Recognition, Control, and Prevention. Ames: Iowa State University Press, 2000. Preceding synopses of parasitic, fungal, and viral agents are sections on the principles and history of zoonoses recognition, newer disease agents, and advances in control and prevention.

Krauss, Hartmut, et al. Zoonoses: Infectious Diseases Transmissible from Animals to Humans. 3d ed. Washington, D.C.: ASM Press, 2003. Discusses the myriad infections introduced by human-animal contact.

Mandell, Gerald L., John E. Bennett, and Raphael Dolin, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. New York: Churchill Livingstone/Elsevier, 2010. This thorough two-volume textbook provides comprehensive coverage of infectious diseases, including zoonotic diseases.

Romich, Janet A. Understanding Zoonotic Diseases. Clifton Park, N.Y.: Thomson Delmar, 2008. A good introduction to zoonotic diseases in humans.

Schlossberg, D., ed. Clinical Infectious Disease. New York: Cambridge University Press, 2008. A detailed presentation of infectious diseases, their causes, epidemiology, symptoms, and treatments.