Megi Resulaj
Dean Andrew Grosovsky
Science and Today’s Headlines
May 16, 2014
Assignment #2
Zoonosis: The Transmission of Viruses from Animals to Humans
Zoonosis is a disease or infection that is naturally transmissible from vertebrate animals
to humans, and vice-versa. The disease is caused by all types of pathogenic agents, including
bacteria, parasites, fungi, and viruses. Humans can get zoonotic diseases by coming into contact
with the infected live poultry, rodents, reptiles, amphibians, insects, and other domestic and wild
animals. Anyone who has contact with animals can get a zoonotic disease, but people may be
more at risk than others. These include people with a weakened immune system, children age 5,
the elderly, and pregnant women. About 75% of recently emerging infectious diseases affecting
humans are diseases of animal origin, and approximately 60% of all human pathogens are
zoonotic, making zoonosis an important issue in today’s increasingly globalized world. As more
This paper aims to explore the recent attention that zoonotic diseases have received in the news
and media, specifically in cases of Ebola, H5N1, and H1N1. It will identify the biological
mechanisms which allow these viruses to expand their host range. In addition, some of our
examples also illustrate the wide range of consequences these viruses can exhibit, from mild to
detrimental.
Zoonotic infection generally occurs in four steps: exposure, infection, spread, and
adaptation. Exposure to the virus requires contact between donor and recipient hosts for transfer
of the virus. This means that there are geographical, ecological, and behavioral factors that allow
humans to stay away from, as well as be exposed to, the virus that currently affects a certain
animal species. An example of a geographical distribution is wildlife trade because traders are
more exposed to a variety of animals that could be infected with a virus that is also compatible
with the human genome. Similarly, hunting animals and domesticating animals also leads to
increased risk of acquiring the “right” virus. The second step, infection by the virus, is more
complex because in order to infect a new host, the virus must be able to efficiently infect the
appropriate cells of the new host, and that process can be restricted at many different levels.
Hosts often have several responses to viral infections that can kill off the virus before it
can reach the cell that it is designated to infect. Therefore, exposure to the virus must be plentiful
in order to increase the chance that the virus will infect the cell that it needs to infect. Viruses
also often cross between closely-related hosts because they can more easily infect a cell of a new
host that is similar to that a previous host; mutations to the virus's genome is how this ability to
cross species is acquired. The third step, the spread of the virus to other members of the
population, implies that population density plays a large role in the exposure to a virus, as well as
the occurrence of epidemics. This is because the virus is able to find many new hosts to infect in
a dense population and can use those hosts for reproduction even if the original host has died
from infection. The final step to a zoonotic infection is the adaptations that the virus acquires in
order to infect more effectively. While a certain virus may effectively infect a host and spread
throughout a population, its survival is not ensured, especially when considering the current
advances in medicine that can prevent infection and kill an existing virus in a host. This is why a
virus that is able to "change" frequently (for example, one with a high mutation rate) has an
advantage over other viruses. 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.
The most deadly zoonotic infection by a virus, and the one receiving the most attention in
the media recently, is Ebola. Ebola virus is transmitted from animals to humans via blood contact
with an infected animal host and lesions in the hands of a human. The virus then affects the
blood of its new host. This then transmits from human to human through direct contact with
blood or bodily fluids from another infected person. Also, the virus can be spread via
contaminated medical equipment such as shots and needles. There are currently four strains of
Ebola known to be able to infect humans and most cases have developed in remote areas of
Africa (WebMD). Ebola is a threat to patients and doctors dealing with the disease because it has
killed 90% of those infected (CNN.com). Besides trying to keep the patient hydrated and well,
there are no known treatments for those infected, which may explain the high rate of death. An
individual coming in contact with the bodily fluid of an infected individual spreads the disease
(USAToday). However the disease may orginate from an individual coming into contact with the
excretions or blood from animals infected with the disease (WebMD). Symptoms of infection
develop relatively quickly and begin to appear “2 to 21 days after infection,” (CNN.com). The
early signs include “a rash, red eyes, hiccups, chest pains and difficulty breathing and
swallowing” (CNN.com). As infection progresses symptoms lead to vomiting, diarrhea, impaired
kidney and liver function and sometimes internal and external bleeding” (CNN.com). The virus
feeds off a large population, so the larger the population the worse the effect of the virus. In
addition, viruses have emerged to resist more human made prevention methods such as vaccines
and antibodies. In the past, viruses would infect a small population and become extinct after the
population had died off as it fed off the population for fuel. Now 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. Symptoms of Ebola include: fever,
headache, muscle aches, sore throat, weakness, diarrhea, bleeding inside and outside the body,
rash, and trouble breathing. The only currently available treatment for infection by the virus is by
staying out of areas affected by the virus and maintaining proper sanitation. Remaining hydrated
and addressing any wounds or rashes that appear due to disease.
The Avian Influenza (H5N1 Bird Flu) is also an issue that has been heavily covered by
the media. It is a highly pathogenic avian bird flu virus caused outbreaks in parts of Asia and the
Middle East over domestic poultry. It was first detected in China in 1996 from a goose and there
is currently no proof that the virus can spread from human to human, but possible adaptations
pose threats. Overall, there have been about 650 human cases reported from 15 different
countries since 2003 with a mortality rate of about 60% of those infected with the virus.
Symptoms include fever and cough, acute respiratory distress, shortness of breath/difficulty
breathing, abdominal pain, diarrhea, pneumonia, respiratory failure, shock, altered mental state,
seizures, failure of multiple organs (e.g. kidney failure), or death. Current treatment and
vaccinations include two antiviral medications previously licensed by the FDA, which have
shown resistance to HPAI H5N1. It has been recommended to use either oseltamivir or
zanamivir, although oseltamivir is more preferred. Infection by the Influenza virus depends on
the species of the bird and how pathogenic the virus is; H5N1, for example, has high
pathogenicity. Highly pathogenic viruses are always of the H5 or H7 subtypes, even though not
all H5 or H7 subtypes are highly pathogenic. 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”
Another zoonotic illness spreading around the world is Swine Flu, caused by the H1N1
virus. Infection by the virus causes a respiratory illness found in pigs. This virus is an
Orthomyxovirus that contains the glycoproteins Haemagglutinin and Neuraminidase, common
throughout pig populations world-wide. The initial outbreak of swine flu was in Mexico, South
America. The virus that normally circulates in just pigs, altered so that humans could 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
(although very rare) is through air-borne droplets from coughing or sneezing of an infected
animal. While there is no treatment, there are many measures of preventative care, such as
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.” The majority of people
with the H1N1 virus make a full recovery without acquiring medical attention or antiviral drugs.
In conclusion, there are four key steps to identify a zoonotic infection by a virus:
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.