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Francesca Corsini
Science Gateway Seminar
Dean Grosovsky
May 12, 2014
Zoonosis: Newly Emerging Viruses
Zoonosis is a disease or infection that is naturally transmissible from vertebrate animals
to humans, and vice-versa (WHO). Humans can get zoonotic diseases by coming into contact
with the infected live poultry, rodents, reptiles, amphibians, insects, and other domestic and wild
animals. The disease may be caused by different types of pathogenic agents, including bacteria,
parasites, fungi, and viruses. 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 under the age of 5; the elderly; and pregnant women (WHO). In order to better
understand zoonotic viruses the steps that the virus takes to successfully transition from animal
to human hosts must be established.
The first of the four fundamental steps in the development of a zoonotic infection is the
host species exposure to the virus. Contact between donor and recipient hosts is required 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.
The second step is the initial infection by the virus. 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 between closely related hosts because they can more easily
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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 to the development of a zoonotic virus is the spread of the virus to other
members of the host population. 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 in the development of a zoonotic virus is the adaptations that the virus
acquires in order to infect the host species 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.
Once these zoonotic viruses are able to infect humans the effects that each particular
virus has can be profound and vary greatly from virus to virus. One example of a zoonotic
disease that has recently developed is the Avian Influenza (H5N1 Bird Flu). H5N1 is a highly
pathogenic avian bird flu virus, which was first detected in China in 1996 from a goose.
Outbreaks in parts of Asia and the Middle East were caused over interactions between humans
and domestic poultry. Currently, there is no proof that the virus can spread from human to human
but possible adaptations pose threats. There have been about 650 human cases reported from 15
different countries since 2003 (Flu.gov). Mortality rate is about 60% of those infected with the
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virus (Flu.Gov).
Common symptoms of those infected with H5N1 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); and death” (Flu.Gov). The only two antiviral medications, which were
previously licensed by the FDA to treat those infected, have shown resistance to H5N1.
There are three different barriers that a virus must overcome in order to spread from one
species to another. The first is cross-species host-host interaction. The species from which the
virus originated and the species that received the virus would have to interact first. The second
obstacle is virus-host interaction. This depends upon the virus’ ability to infect the next host
species. The third barrier is the intra-species host-host interactions. This consists of transferring
the virus between infected and non-infected hosts within the species.
Infection of Influenza virus depends on the species of the bird and how pathogenic the
virus is. For example, H5N1 has 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” (Parrish, et. all).
A third newly emerged zoonotic virus is the H1N1 Virus, commonly known as Swine
Flu. This is a respiratory illness and it was found in pigs (CDC). People with daily exposure to
pigs are at high risk of swine flu infection, however transmission from pigs to humans is not
common. The flu pandemic was first within the United States in 2009. 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).
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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 (Centers for Disease Control and Prevention). 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 for H1N1, 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 tolerable for the patient. The majority of
people with the H1N1 virus make a full recovery without acquiring medical attention or antiviral
drugs.
A final example of a zoonotic disease, which is extremely lethal, is Ebola. The 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,
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which may explain the high rate of death.
An individual coming in contact with the bodily fluid of an infected person spreads the
disease (USAToday). However the disease may originate 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. There are no vaccines or
antiviral medications currently given to those infected with the virus. The only defense against
infection consists of: remaining out of areas affected by the virus; maintaining proper sanitation;
as well as remaining hydrated; and addressing any wounds or rashes that appear if infected with
the disease.
While we have advanced in understanding the affects 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. Often, in order to be affected by a zoonotic disease, there are four key steps: exposure
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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. 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.
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Bibliography:
CNN.com. "What Is Ebola and Why Does It Kill?" CNN. Cable News Network, 01 Jan. 1970.
Web. 13 May 2014.
Dur, Jessica. "All You Need to Know about Ebola." USA Today. Gannett, 07 Apr. 2014. Web.
13 May 2014.
"Ebola Virus: Symptoms, Treatment, and Prevention." WebMD. WebMD, n.d. Web. 12 May
2014.
"H5N1 Avian Flu (H5N1 Bird Flu)." Flu.Gov. H5N1 (Avian Flu). N.p., n.d. Web. 16 May 2014.
"Nairaland Forum." No Outbreak Of Ebola Virus In Nigeria- FG. N.p., n.d. Web. 13 May 2014.
Nyabbisan, Cameroon. CNN. Cable News Network, n.d. Web. 13 May 2014.
Parrish, Colin, Edward Holmes, David Morens, Eun-Chung Park, Donald Burke, Charles
Calisher, Catherine Laughlin, Linda Saif, and Peter Daszak. "Cross-Species Virus
Transmission and the Emergence of New Epidemic Diseases." Microbiology and
Molecular Biology Reviews (n.d.): n. pag. American Society for Microbiology. Web. 13
May 2014.
"Use of Antivirals." Centers for Disease Control and Prevention. CDC, 13 Mar. 2014. Web. 16
May 2014.
"The 2009 H1N1 Pandemic: Summary Highlights, April 2009-April 2010." Centers for Disease
Control and Prevention. Centers for Disease Control and Prevention, n.d. Web. 16 May
2014.
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"Zoonoses and the Human-Animal-Ecosystems Interface." WHO. N.p., n.d. Web. 13 May 2014.