Polio Virus:
Introduction:
Polio is known in full as
poliomyelitis – also called infantile paralysis. It is an acute viral
infectious disease of the nervous system that usually begins with general
symptoms such as fever, headache, nausea, fatigue, and muscle pains and spasms
and is sometimes followed by a more serious and permanent paralysis of muscles
in one or more limbs, the throat, or the chest. More than half of all cases of
polio occur in children under the age of five. The paralysis so commonly
associated with the disease actually affects fewer than 1 percent of persons
infected by the poliovirus.
Between 5 and 10 percent of
infected persons display only the general symptoms outlined above, and more
than 90 percent show no signs of illness at all. For those infected by the
poliovirus, there is no cure, and in the mid-20th century hundreds of thousands
of children were struck by the disease every year. Since the 1960s, thanks to
widespread use of polio vaccines, polio has been eliminated from most of the
world, and it is now endemic only in several countries of Africa and South
Asia. Approximately 1,000–2,000 children are still paralyzed by polio each
year, most of them in India.
History:
A notable year in the history of
polio was 1789, during which Michael Underwood first described a debility of
the lower extremities in children that was recognizable as polio. Polio
outbreaks were first reported in the United States in 1843. A turning point for
the disease occurred in 1955, following the introduction of an inactivated
polio vaccine (IPV). The decline in the incidence of polio continued following
oral polio vaccine (OPV) introduction in 1961.
Signs and Symptoms:
When a person becomes infected
with poliovirus, the virus begins to multiply within the cells that line the
back of the throat, nose, and intestines. Polio symptoms usually appear 7 to 14
days after a person becomes infected with the poliovirus. This period between
polio transmission and the start of symptoms is called the "polio
incubation period." The incubation period for polio can be as short as 4
days or as long as 35 days.
Up to 95 percent of people who
are infected with poliovirus will have no symptoms. However, people who are
infected and do not have polio symptoms can still spread the poliovirus and cause
others to develop polio.
If a person does develop
symptoms, the symptoms can be categorized into one of three groups, which
include:
• Minor
polio symptoms (also known as abortive poliomyelitis)
• Aseptic
meningitis
• Paralytic
poliomyelitis.
Causes:
Poliovirus is a very contagious
virus that can spread easily from person to person. In fact, when a person is
infected with poliovirus, it is expected that polio transmission among
susceptible household contacts will occur in nearly 100 percent of children and
over 90 percent of adults.
Poliovirus is a single-stranded
RNA virus from the family Picornaviridae and genus enterovirus.
Poliovirus only infects humans.
It is more common during summer months in temperate climates. In tropical
climates, there is no seasonal pattern. The poliovirus is rapidly inactivated
by heat, formaldehyde, chlorine, and ultraviolet light.
Prevention:
Polio prevention begins with
polio vaccination. Polio vaccine is highly effective in producing immunity to
the poliovirus and protection from paralytic polio. Approximately 90 percent or
more of polio vaccine recipients develop protective antibodies to all three
poliovirus types after two doses, and at least 99 percent are immune following
three doses. If vaccinations were stopped, polio would return to pre-vaccine
levels in the United States and hundreds of people would die from polio-related
illnesses.
Polio vaccine in the United
States is administered as an inactivated polio vaccine that contains no live
poliovirus. Polio vaccine is highly effective in producing immunity to the
poliovirus and protection from paralytic polio. Approximately 90 percent or
more of polio vaccine recipients develop protective antibodies to all three
poliovirus types after two doses, and at least 99 percent are immune following
three doses.
There is no cure for polio, only
treatment to alleviate the symptoms.
Heat and physical therapy is used to stimulate the muscles and
antispasmodic drugs are given to relax the muscles. While this can improve mobility,
it cannot reverse permanent polio paralysis.
Ebola Virus:
Introduction:
Ebola is a virus of the family
Filoviridae that is responsible for a severe and often fatal viral hemorrhagic
fever; outbreaks in primates such as gorillas and chimpanzees as well as humans
have been recorded. The disease is characterized by extreme fever, rash, and
profuse hemorrhaging. In humans, fatality rates range from 50 to 90 percent.
History:
The virus takes its name from the
Ebola River in the northern Congo basin of central Africa, where it first
emerged in 1976. Outbreaks that year in Zaire (now Congo [Kinshasa]) and The
Sudan resulted in hundreds of deaths, as did another outbreak in Zaire in 1995.
Ebola is closely related to the Marburg virus, which was discovered in 1967,
and the two are the only members of the Filoviridae that cause epidemic human
disease. A third related agent, called Ebola Reston, caused an epidemic in
laboratory monkeys in Reston, Virginia, but apparently is not fatal to humans.
Signs and symptoms:
EVD/EHF is clinically
indistinguishable from Marburg virus disease (MVD), and it can also easily be
confused with many other diseases prevalent in Equatorial Africa, such as other
viral hemorrhagic fevers, falciparum malaria, typhoid fever, shigellosis,
rickettsial diseases, cholera, gram-negative septicemia or EHEC enteritis. The
most detailed studies on the frequency, onset, and duration of EVD clinical
signs and symptoms were performed during the 1995 outbreak in Kikwit, Zaire
(EBOV) and the 2007-2008 outbreak in Bundibugyo, Uganda (BDBV). The mean
incubation period, best calculated currently for EVD outbreaks due to EBOV
infection, is 12.7 days (standard deviation = 4.3 days), but can be as long as
25 days. EVD begins with a sudden onset of an influenza-like stage
characterized by general malaise, fever with chills, arthralgia and myalgia,
and chest pain. Nausea is accompanied by abdominal pain, anorexia, diarrhea,
and vomiting. Respiratory tract involvement is characterized by pharyngitis
with sore throat, cough, dyspnea, and hiccups.
The central nervous system is
affected as judged by the development of severe headaches, agitation,
confusion, fatigue, depression, seizures, and sometimes coma. The circulatory
system is also frequently involved, with the most prominent signs being edema
and conjunctivitis. Hemorrhagic symptoms are infrequent (fewer than 10% of
cases for most serotypes), (the reason why Ebola hemorrhagic fever (EHF) is a
misnomer) and include hematemesis, hemoptysis, melena, and bleeding from mucous
membranes (gastroinestinal tract, nose, vagina and gingiva).
Cutaneous presentation may
include: maculopapular rash, petechiae, purpura, ecchymoses, and hematomas
(especially around needle injection sites). Development of hemorrhagic symptoms
is generally indicative of a negative prognosis. However, contrary to popular
belief, hemorrhage does not lead to hypovolemia and is not the cause of death
(total blood loss is low except during labor). Instead, death occurs due to
multiple organ dysfunction syndrome (MODS) due to fluid redistribution,
hypotension, disseminated intravascular coagulation, and focal tissue necroses
Causes:
EVD is caused by four of five
viruses classified in the genus Ebolavirus, family Filoviridae, order
Mononegavirales: Bundibugyo virus (BDBV), Ebola virus (EBOV), Sudan virus
(SUDV), and Taï Forest virus (TAFV). The fifth virus, Reston virus (RESTV), is
thought to be apathogenic for humans and therefore not discussed here.
Prevention:
Ebola viruses are highly
infectious as well as contagious.
As an outbreak of ebola
progresses, bodily fluids from diarrhea, vomiting, and bleeding represent a
hazard. Due to lack of proper equipment and hygienic practices, large-scale
epidemics occur mostly in poor, isolated areas without modern hospitals or well-educated
medical staff. Many areas where the infectious reservoir exists have just these
characteristics. In such environments, all that can be done is to immediately
cease all needle-sharing or use without adequate sterilization procedures,
isolate patients, and observe strict barrier nursing procedures with the use of
a medical-rated disposable face mask, gloves, goggles, and a gown at all times,
strictly enforced for all medical personnel and visitors.The aim of all of
these techniques is to avoid any person’s contact with the blood or secretions
of any patient, including those who are deceased.
Vaccines have successfully
protected nonhuman primates; however, the six months needed to complete
immunization made it impractical in an epidemic. To resolve this, in 2003, a
vaccine using an adenoviral (ADV) vector carrying the Ebola spike protein was
tested on crab-eating macaques. The monkeys were challenged with the virus 28
days later, and remained resistant. In 2005, a vaccine based on attenuated
recombinant vesicular stomatitis virus (VSV) vector carrying either the Ebola
glycoprotein or Marburg glycoprotein successfully protected nonhuman primates,
opening clinical trials in humans. By October, the study completed the first
human trial; giving three vaccinations over three months showing capability of
safely inducing an immune response. Individuals were followed for a year, and
in 2006, a study testing a faster-acting, single-shot vaccine began. This study
was completed in 2008.
There are currently no Food and
Drug Administration-approved vaccines for the prevention of EVD. Many candidate
vaccines have been developed and tested in various animal models. Of those, the
most promising ones are DNA vaccinesor are based on adenoviruses, vesicular
stomatitis Indiana virus (VSIV) or filovirus-like particles (VLPs) as all of
these candidates could protect nonhuman primates from ebolavirus-induced
disease. DNA vaccines, adenovirus-based vaccines, and VSIV-based vaccines have
entered clinical trials.
Contrary to popular belief,
ebolaviruses are not transmitted by aerosol during natural EVD outbreaks. Due
to the absence of an approved vaccine, prevention of EVD therefore relies
predominantly on behavior modification, proper personal protective equipment,
and sterilization/disinfection.
In 6 December 2011 the
development of a successful vaccine against Ebola for mice were reported.
Unlike the predecessors it can be freeze-dried and thus stored for long periods
in wait for an outbreak. The research will be presented in Proceedings of
National Academy of Sciences.
