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Zoonotic Diseases
In November 2002, a near pandemic of SARS (Severe Acute Respiratory
Syndrome) began. Within weeks, the respiratory disease which is
caused by a coronavirus spread from one province in China to 37
different countries. Between November 2002 and July 2003, over 8,000
individuals experienced documented cases of SARS and about ten
percent of those infected died from the illness.
Coronaviruses are diseases that affect mammals and birds. China and
other countries began to look for a cause for the coronavirus. Some of
the tests revealed the coronavirus in wild animals being sold for food at
a local market in the province that the pandemic started in. Some palm
civets were shown to carry the virus, although the animals were not
always showing symptoms. Further tests showed that raccoons, dogs,
ferrets, badgers, and domestic cats in the area also carried the virus.
Scientists believe that either the first few humans contracted the virus
from eating these animals or through infected bats.
The near pandemic of SARS that occurred is just one of a number of
diseases that humans can become infected with through animal carriers.
Zooneses are diseases that can be transmitted from animals to
humans. In most cases, the diseases are considered animal diseases
primarily rather than human diseases, although humans can suffer from
the diseases. Reverse zoonosis or anthroponosis is a disease that
can be transferred from humans to wild or domestic animals. By nature,
zoonotic diseases are infectious, as they transfer from animal to animal.
Currently, there are more than 150 diseases that health officials
consider to be zoonotic diseases. Some parasites like hookworms can
also be transmitted between animals and people as well.
Zoonotic diseases are not a new problem, although we are becoming
more aware of them through veterinary science and public health
studies. Documents from ancient times mention zoonotic diseases like
the plague. The 1919 influenza epidemic that killed more individuals
than the casualties in World War I is believed to have been a zoonotic
disease. Some scientists argue that the influenza strain, which caused
the problems, began in swine two years earlier in Kansas. This example
is just one of the past health issues that have been caused by infectious
diseases that originated in animals.
Zooneses present a growing concern for veterinarians and public health
officials. There are several reasons for this. First, the human population
is growing rapidly, particularly in some parts of the world. This can
create situations of overcrowding and overpopulation, which can
increase the risk of an infectious disease spreading. These factors can
also increase the risk of water shortages, malnutrition, and other
situations that can both weaken human resistance and put humans into
greater contact with possibly contaminated animals. As humans spread
outward in urban areas, they may also increasingly come into contact
with wild animals, as wild habitat is developed.
Another factor in the concern of zooneses is that more and more
humans are keeping animals as pets. This includes not only traditional
pets like dogs and cats, but also more exotic pets. Some of these exotic
pets have already caused outbreaks of diseases like the plague and
monkeypox in humans. Humans may also experience closer contact with
animals at zoos, farms, and wildlife sanctuaries. Agricultural tourism
often includes contact with animals through petting zoos, feeding
programs, and other programs. Zoos and wildlife sanctuaries may
sponsor programs where individuals feed animals, participate in shows
with them, or gain greater access to some animals. All of this increases
the likelihood that diseases may be transferred to humans. The “wild” is
increasingly a part of our lives.
Finally, zoonotic diseases are often unrecognized diseases in humans.
There have been several examples within the last decade of animal
diseases infecting humans. In these cases, the symptoms or effects of
the disease may not be known for humans. Humans may also lack
immunity to the diseases, which can result in a number of infected
individuals. For example, West Nile virus first appeared in North America
in 1999. By the summer of 2002, the virus was moving across the
United States, causing illnesses and various symptoms.
A number of modern diseases began as diseases in animals, before
making the jump to infecting humans as well. Some of the diseases that
are thought to have come from animals include HIV, smallpox, measles,
and influenza. Some scientists think that the common cold may also
have been a zoonotic disease. The area of conservation medicine is
an evolving field that takes, integrates and builds upon research and
information from both human and veterinary medicine.
In the sections below, we will examine several different zoonotic
diseases, including Hantavirus and the plague. The diseases that we’ll
discuss are just a few of the zoonotic diseases that exist and can infect
humans. In addition to these, we have already discussed some other
zoonotic diseases in previous units, such as rabies, trichinosis, and avian
Hantaviruses are a group of viruses that are found in rodents and can
be transmitted to humans. The rodents carry the viruses, but do not
become ill from the viruses themselves. The Hantaviruses are
transmitted through the bites of rodents or contact with their excreta,
including feces.
The Hantaviruses are relatively recent discoveries. They were first
identified in the 1950s by a Korean doctor Ho-Wang Lee. The name for
the virus came from infected striped field mice that were found by the
Hantaan River in Korea. The viruses have been identified as causing a
number of different illnesses around the world since that time. In 1993,
scientists discovered that a previously unknown Hantavirus species had
caused an outbreak in the United States. Hantavirus
cardiopulmonary syndrome (HCPS or HPS) is a serious illness that
affects the lungs after exposure to the Hantavirus. Although HPS is rare,
it can be a life-threatening illness, creating flu-like symptoms. Another
Hantavirus causes hemmorragic fever with renal syndrome (HFRS)
which also produces flu-like symptoms but also affects the kidneys.
HFRS has been documented around the world in the last fifty years,
particularly in eastern Asia. It has been researched by countries such as
the United States for its potential use as a biological weapon.
Hantavirus can be transmitted from rodents to humans in several
different ways. Dust or other materials that contain the urine, feces, or
saliva of an infected rodent can infect a human if the human breathes in
the dust or if the contaminated material comes into contact with an
open wound or a mucus membrane (such as the eye) of a human.
Humans can also contract a Hantavirus if they are bitten by an infected
rodent. Scientists do not believe that HCPS can be transmitted between
humans so contact with a contaminated rodent or materials that have
been contaminated by an infected rodent are the only ways that a
human can contract the virus. In extremely rare cases, HFRS may be
transmitted from person to person, but most cases occur from rodent to
human contact.
In the United States, HCPS was first documented in 1993 in New Mexico
and several other southwestern states. By 2007, states like New Mexico,
California, Texas, and Colorado had documented over 30 cases of HCPS.
Other states have also reported HCPS cases. The Center for Disease
Control has found that contaminated rodents were present in at least
twenty national parks in the United States. At least one person has died
in the United States from HCPS.
Veterinary scientists and others studying the Hantavirus suggest that
those at the greatest risk for HCPS are individuals who spend time in
rural areas and those who camp. Campers, for example, may lay down
sleeping bags on forest floors or on the floors of cabins that might
contain contaminated dust or other materials. This can increase the risk
that the virus will be breathed in. Rural areas may be more likely to
contain buildings, barns, wood piles, or other areas that are popular
with rodents. Dry and dusty areas of these places have a higher risk of
being contaminated with Hantavirus.
Preventing Hantavirus or reducing the risk of transmission to humans
includes making buildings and other areas less hospitable to rodents.
Potential nesting areas should be removed from around buildings.
Scientists recommend that individuals working in dusty environments
like cabins or barns wear masks and disinfect anything that might
contain the Hantavirus.
Zoonotic Diseases
Anthrax is a disease caused by a spore forming bacteria. The illness is
acute, coming on suddenly and most forms of anthrax are fatal,
although vaccines and antibiotics can prevent and treat some cases. The
bacteria, Bacillius anthracis, which causes anthrax, can survive in a
dormant state for decades or even centuries and the spores are found
on all of the earth’s continents, including Antarctica.
Ancient records show that anthrax was a problem for grazing animals
long before the modern era. Thousands of animals and humans died
each year before the twentieth century from the disease. With
prevention programs and more knowledge about the disease, there are
only a few cases each year globally today. However, the illness may be
increasing in some developing countries that have not implemented
these prevention programs or who do not have many veterinary
services. Anthrax has been used as a bioterrorism weapon in the past,
although many countries have now destroyed their stores of the
In general, animals that are herbivores (such as cows that eat plant
materials) are at a greater risk that other animals. Animals that are
scavengers (feeding on dead animals) or carnivores (eating meat) do
not generally become infected with anthrax. Cows and other herbivores
become infected when they ingest or inhale the spores of the anthrax
bacteria while eating or when the spore enters an open wound. Once
inside the body, the bacteria multiply and two toxins in the bacteria
typically kill the animal within several weeks. Although the bacteria die
quickly after the death of the animal, spores can be released in any
blood or fluids that may come from the animal.
When the anthrax spores are inhaled by an animal, the spores are
brought into the lungs. There, they are picked up by cells and brought
to the lymph nodes. The damage caused by the bacteria in the central
chest lymph nodes causes a great deal of pain and the animal will have
trouble breathing. The spores multiply and move into the bloodstream,
from where they infect the rest of the body. The toxins that the spores
release in the body cause bleeding, tissue destruction, and death.
Humans are exposed to and infected by the bacteria in a number of
different ways. Generally, exposure comes through the meat, wool, skin,
or other products of the infected animal. Individuals who work with wool
or animal hides are often exposed to low levels of the spores, but the
levels are generally not high enough to make the individuals sick.
Anthrax bacteria can enter humans in the same way that it can enter
other animals: through eating it, inhaling it, or having it come into
contact with the skin. The bacteria do not spread from human to human,
although the spores inside a human can infect another person after
death, during an autopsy, for example.
The three types of anthrax in humans correspond to the way that the
person is infected. Cutaneous anthrax occurs after the skin comes in
contact with the bacteria. A papule forms on the skin, eventually it
ruptures, and dead tissue or a “black wound” is found at the site. About
twenty percent of individuals who develop this type of anthrax and are
not treated for it die. Inhalation anthrax occurs when the spores are
inhaled, causing severe respiratory issues. The mortality rate can reach
100 percent for this type, particularly when the bacteria enter the
bloodstream. Gastro-intestinal anthrax occurs when contaminated
meat is ingested, causing vomiting, nausea, and abdominal pain. 25 to
75 percent of individuals infected with this type of anthrax will die.
To prevent the anthrax disease, vaccines are available for both animals
and humans. However, human use has often been restricted to just
high-risk groups, such as military personnel that might face a
bioterrorist attack of anthrax. If anthrax is known to be present,
decontamination through antimicrobial soaps or bleach can help
eliminate the bacteria
Zoonotic Diseases
While the diseases discussed above are serious zoonotic illnesses that
often result in death, not all zoonotic diseases are life-threatening. One
common zoonotic disease that affects humans is ringworm. The name of
this disease is a bit misleading, as it is not caused by worms at all, but
by fungus. The name ringworm comes from the often red, raised circle
that appears at the site of the fungal infection.
Ringworm comes from fungi that have adapted to living on animals.
Dermatophytosis is the scientific term used to distinguish fungal
infections on the skin or other areas of animals. Fungi are organisms
that feed on plant and animal materials and include yeast, mushrooms,
and molds. These organisms tend to do best in warm, wet
environments, which explains why many people get a fungal infection
after walking around in public locker rooms. Some scientists estimate
that about twenty percent of the world’s population has ringworm.
However, not everyone who comes into contact with the ringworm fungi
will contract the infection. Young children, individuals with weak immune
systems, and elderly individuals are at a greater risk. Most of us carry
some forms of helpful bacteria and fungi on our bodies, so the type of
fungi that causes ringworm comes from different types of fungi than
that which normally inhabits our bodies without problems.
The fungi that cause ringworm are spread through direct contact with
infected skin, soil, or hair. In cattle, the number of cases increase in the
fall and winter as animals may be housed indoors in close quarters with
each other. Many different types of animals can be carriers for the
infection, including cats, dogs, horses, goats, cows, and other domestic
pets and livestock. Cats are believed to be a common carrier of
ringworm, although many cats do not show signs of ringworm
Infected animals showing signs of ringworm typically have circular
patches of hair loss. The area may also be scaly, although not usually
inflamed. In humans, the ringworm lesions on the skin are often raised,
red, circular, scaly, and itchy, although the appearance can differ
depending on the type of fungus and the area of the body that the
lesion is found on.
To diagnose ringworm, veterinarians may do a fungal culture, which
uses skin and hair tests to see what fungi grows on the culture.
However, this test can take up to several weeks. About 50 percent of
ringworm infections will glow under black light (or ultraviolet light) in
what is called the Wood’s lamp test. A culture is the only definitive test
for ringworm, however. Some cases of ringworm will clear up on their
own within several weeks. If treatment is sought, it often depends on
the severity of the case. In some cases, even mild cases of ringworm
may be treated to prevent further infections in additional animals or to
relieve the itchiness in humans. Anti-fungal shampoos or creams may be
used to cure the infection. In severe cases, oral anti-fungal medications
may also be given.
As the fungi that cause ringworm can live for about 18 months in the
environment, if the conditions are right, it can be difficult to prevent the
infections in animals and the transfer to humans. Disinfecting surfaces
where the fungi might be can help reduce the risk of infection. If a pet
has ringworm, veterinary scientists recommend washing and disinfecting
all bedding and toys and confining the pet to one room of the home until
the infection is gone. Although ringworm is not a deadly disease in most
cases, it is highly contagious and can be spread to humans. As such,
ringworm is a concern as a zoonotic disease.
In this unit, we have learned more about zoonotic diseases, or diseases
that can be transferred from animals to humans. Some of these
diseases can be deadly for both animals and humans and as such they
are of great concern to veterinary scientists and public health officials.
Today, scientists are not only researching how to treat zoonotic
diseases, but also how to prevent these diseases in the future. With the
closer proximity between animals and humans, the threat of zoonotic
diseases will continue and veterinary science will need to be at the
forefront of research to help keep these diseases from becoming public
health issues or epidemics.
Unit Seven: Lab Questions
The Jungle Search for Viruses
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