Shubhankar Joshi
5/13/14
Understanding Zoonosis: The Study of Transmissible Diseases and Infections
Zoonosis is a disease or infection that is naturally transmissible from vertebrate animals
to humans, and vice-versa. The disease is caused by many types of pathogenic agents, which
include viruses, parasites, bacteria and fungi. Humans can be infected by zoonotic diseases by
coming into direct contact with infected live poultry, rodents, amphibians etc. Anyone who
comes into contact with animals can get a zoonotic disease, but some people may be more at risk
than others. These include people with a weakened immune system, children aged five years old,
the elderly, and pregnant women. In order to better understand zoonotic viruses, one should
investigate the steps that the virus takes to successfully transition from animal to human hosts.
There are four steps in which a zoonotic infection occurs. The first step in which a
zoonotic infection occurs is the exposure to the virus. The second step is the infection by the
virus. The third step consists of the spread of the virus to other members of the population. The
last step in the development of a zoonotic virus is the adaptations that the virus acquires in order
to infect the host species more effectively. Each of these steps builds on top of the previous in
which the zoonotic infection occurs.
The first step in which a zoonotic infection occurs is the exposure to the virus. There
needs to be contact between the donor and the recipient host in order for the transfer of the virus
to be completed. There are many geographical, ecological, and behavioral factors that allow
humans to stay away from and not be exposed to a virus that currently affects a certain animal
species. An example of a geographical distribution is wildlife trade. An example of ecological
distribution is the hunting of animals. Domestication of animals is an example of behavioral
distribution.
The second step is the infection by the virus. In order to infect a new host, the virus must
be able to effectively infect the appropriate cells of the new host, and that process can be
restricted at many different levels. The hosts often have several responses to the viral infections
that can kill off the virus before it reaches the cell that it intends to infect. Thus, in order to
increase the chance that the virus will infect the cell that it intends to infect, there needs to be a
lot of exposure to the virus. Viruses also infect between closely-related hosts because they can
more easily infect the cell of a host that is similar to that of the previous host. This ability to
cross species is acquired by mutations to the virus’s genome.
The third step consists of the spread of the virus to other members of the population. The
population density of an area plays a key role in the exposure of the virus, as well as the
occurrence of an epidemic. This is due to the virus being able to find many new hosts that are
able to infect in the dense population. The virus uses the hosts for reproduction, even if the host
has already died from infection.
The fourth step in the development of a zoonotic virus is the adaptations that the virus
acquires in order to infect the host species more effectively. Even though a certain virus might be
effective in infecting a host and spreading throughout a population, its survival is not ensured,
especially when the current advances in medicine that can prevent infection and kill the existing
virus in the host are considered. This is why a virus is able to “change” frequently (have a high
mutation rate). These viruses cannot easily be identified and used for vaccinations because their
genome is constantly changing and one vaccination may not be effective for a newer version of
the virus.
We chose to investigate the zoonosis because of the recent attention that it has received in
the news and media, specifically in the cases of Ebola, H5N1 and H1N1. We wanted to explore
the biological mechanisms which would allow these viruses to expand their host range.
Furthermore, some of these examples also illustrate the wide range of consequences these viruses
can inhibit, from mild to extreme.
The first example that we decided to explore was that of the Ebola virus. The Ebola virus
is transmitted from animals to humans via the contact of the lesions in the hands of a human and
an infected animal host. The virus proceeds to infect the blood of the new host. This is then
transmitted from human to human via direct contact of blood or bodily fluids from the infected
individual. The virus can also be spread via contaminated medical equipment such as needles.
There are four known strains of Ebola that are able to infect humans. Most cases of these strains
have developed in remote areas of Africa. There are currently no treatments for those who are
infected by the Ebola virus. The disease would originate from an individual coming into contact
with the excretions or blood from an animal that is infected by the virus. Symptoms of the Ebola
virus include: “a rash, red eyes, hiccups, chest pains and difficulty breathing and
swallowing.” The virus prospers in a large population, so a larger the population makes the effect
of the virus worse. In addition, viruses have emerged to resist more human made prevention
methods such as vaccines and antibodies. Previously, viruses would only infect a small
population and become extinct after the population had died off as it fed off the population for
fuel. Nowadays, with larger populations throughout the world, the spread of the Ebola virus is
extremely dangerous and needs to be tackled in order to prevent the deaths of human lives. Some
treatment methods include: Staying out of areas affected by the virus and maintain proper
sanitation.
The second example that we decided to explore was the Avian Influenza (H5N1 Bird
Flu). The Avian Influenza is a highly pathogenic avian bird flu virus. The H5N1 Bird Flu caused
outbreaks in parts of Asia and the Middle East over domestic poultry. It was first detected in
China in 1996 from a goose. There is currently no proof that the virus can spread from human to
human but possible adaptations to the virus pose threats. There have been about 650 human cases
reported from 15 different countries since 2003. The mortality rate from the Avian Influenza is
about 60% of those infected with the virus. Some symptoms of the Avian Influenza include:
fever, cough, and diarrhea. In terms of treatment and vaccination, two antiviral medications
previously licensed by the FDA have shown resistance to HPAI H5N1. It has been recommended
to use either oseltamivir or zanamivir. There are three different barriers that a virus must
overcome in order to spread from one species to another. These include: cross-species host-host
interaction, virus-host interactions and interspecies host-host interactions. The infection of the
influenza virus depends on the species of the bird and how pathogenic the virus is. The H5N1
virus has a very high pathogenicity. Highly pathogenic viruses are “characterized by an HA
protein with a cleavage site containing multiple basic amino acids, which makes the protein
cleavable by proteases ubiquitously present throughout the body rather than only those present at
mucosal surfaces.”
The third example that we decided to investigate was that of the H1N1 Swine Flu virus.
It is a respiratory illness found in pigs. People with daily exposure to pigs are at a high risk of
swine flu infection, however the transmission from pigs to humans is not common. The H1N1
Swine Flue virus was declared a flu pandemic in 2009 in the U.S. The virus appeared to be a new
strain of H1N1, which resulted when a previous triple re-assortment of bird, swine and human
flu viruses further combined with a Eurasian pig flu virus (swine flu). The initial outbreak of
swine flu was in Mexico and South America. The virus that normally circulated in just pigs,
altered so that humans would be affected with the same virus. This virus had two genes from flu
viruses that normally circulate in pigs, in Europe and Asia, three genes that normally circulate in
North American pigs, and genes from flu viruses from birds and people as well. The method of
transmission from pigs to humans is through air-borne droplets from coughing or sneezing of an
infected animal. While there is no current treatment, there are many measures of preventative
care. Examples of preventative care include getting a flu vaccine. Also, to prevent pig-human
transmission of swine flu, farmers are encouraged to wear facemasks when dealing with the
infected animals and to also wear gloves so the disease can’t be transmitted through hand-eye,
hand-nose, or hand-mouth transmission. Antiviral drugs can also be provided to make the illness
more “bearable.”
In conclusion, in order to be affected by a zoonotic disease, there are four key steps:
exposure to the virus, infection by the virus, transmission of the virus to members of the
population, and the adaptations the virus makes to infect others more effectively. While we have
advanced in understanding the effects of the virus, there are certain aspects that we must
continue to explore. These aspects include the study of when and where the virus forms. In
addition, we must work to understand what it is that causes the spread of the virus, and how we
can limit the virus' natural reservoir, which would in turn, limit the spread of the virus. The
development of a zoonotic disease occurs through the exchange between humans and animals,
and as a first defense against the development of these diseases the exchange of bodily fluids
between humans and animals must be addressed. Not only to be sure that the exchanges are
humane, but also done cleanly in order to deter any more diseases from developing.