Educate-Yourself
The Freedom of Knowledge, The Power of Thought ©
Emerging Killer Viruses
ARBOVIRUS DISEASES / AETIOLOGY /
EPIDEMIOLOGY / TRANSMISSION / IMMUNITY / ACTIVE IMMUNITY / PASSIVE IMMUNITY
/ GENERAL CLINICAL FEATURES / ECOLOGICAL FACTORS / HANTAAN VIRUS / SEOUL
VIRUS / "FOUR CORNERS VIRUS" / EBOLA VIRUS / EBOLA SEROLOGY /
TREATMENT OF EBOLA / PREVENTION AND CONTROL OF EBOLA /
By Byron T. Weeks, M.D.
RECENT EXAMPLES OF EMERGING AND POTENTIALLY EMERGENT VIRUSES
Most emergent viruses are zoonotic, with natural animal reservoirs a more
frequent source of new viruses than is the spontaneous evolution of a new
entity. The most frequent factor in emergence is human behaviour that increases
the probability of transfer of viruses from their endogenous animal hosts
to man. Rodents and arthropods are most commonly involved in direct transfer,
and changes in agricultural practices or urban conditions that promote
rodent or vector multiplication favour increased incidence of human disease.
Other animals, especially primates, are important reservoirs for transfer
by arthropods. Because arthropod transmission plays a very large part in
infectious animal disease, specifically potential emergent virus epidemics,
I will dedicate the next part of this essay to a discussion of them.
ARBOVIRUS DISEASES
Approximately 100 of the more than 520 known arthropodborne viruses
(arboviruses) cause human disease. At least 20 of these might fulfill the
criteria for emerging viruses, appearing in epidemic form at generally
unpredictable intervals (Morse and Schluederberg 1990). These viruses are
usually spread by the bites of arthropods, but some can also be transmitted
by other means, for example through milk, excreta or aerosols. The arbovirus
infections are maintained in nature principally, or to an important extent,
through biological transmission between susceptible vertebrate hosts by
blood-sucking insects; they multiply to produce viremia in the vertebrates,
multiply in the tissues of the insects and are passed on to new vertebrates
by the bites of insects after a period of extrinsic incubation. The names
by which these viruses are known are often place names such as West Nile
or Rift Valley, or are based on clinical characteristics like yellow fever.
AETIOLOGY
Most arboviruses are spherical, measuring 17-150 nm or more, a few
are rod-shaped, measuring 70 x 200 nm. All are RNA viruses. Many circulate
in a natural environment and do not infect man. Some infect man only occasionally
or cause only a mild illness; others are of great clinical importance causing
large epidemics and many deaths. Specifically, these belong to the Togaviridae,
the alphaviruses, flaviviruses, the Bunyaviridae, nairoviruses, phleboviruses
and other subgroups. A range of arboviruses are listed in Table 2, while
some patterns of transmission are shown in Figure 1.
EPIDEMIOLOGY
Vertebrate hosts
Maintenance, incidental, link and amplifier hosts are categorized according
to Stickland Hunter's Tropical Medicine (1991) as the following:
Maintenance hosts are essential for the continued existence of the virus,
usually living in symbiosis with the viruses, without actual disease, but
they do develop antibodies. These include birds such as the prairie chicken,
pigeon and wood thrush which transmit Eastern and Western equine encephalitis;
heron and egrets transmitting Japanese encephalitis and migrating birds
which travel over long distances carrying these and other similar viruses;
rodents and insectivores such as rats, hedgehogs, lemmings and chipmunks
are known to carry louping ill and Colorado tick fever; primates such as
monkeys which carry Dengue fever; Leporidae (rabbits and hares) which carry
Californian encephalitis; Ungulates (cattle and deer) which are implicated
in the transmission of European tick-borne encephalitis; bats which carry
Rio Brava virus; and marsupials, reptiles and amphibia such as kangaroos
and snakes which also harbour encephalitis-causing viruses.
Incidental hosts become infected, but transmission from them does not occur
with sufficient regularity for stable maintenance. Man is usually an incidental
host, often, but not always, being a dead end in the chain. These hosts
may or may not show symptoms. Link hosts bridge a gap between maintenance
hosts and man, for example, between small mammals and man by goats (via
milk) in tick-borne encephalitis. Amplifier hosts increase the weight of
infection, as is the case with pigs which act between wild birds and man
in Japanese encephalitis.
The populations and characters of the vertebrate hosts and their threshold
levels of viraemia are important. Small rodents multiply rapidly and have
short lives, thus providing a constant supply of susceptible individuals.
In contrast, monkeys and pigs multiply slowly, and once they have recovered
from an infection, remain immune for life. African monkeys are relatively
resistant to Yellow Fever, but Asian and American monkeys are susceptible,
probably because, unlike the African monkeys, they have not been exposed
continuously for centuries to the infection. Also, possibly related arboviruses
may offer partial immunization.
Invertebrate hosts
Mosquitoes, sandflies and ticks may imbibe virus from a vertebrate
in a state of viraemia, after which the virus undergoes an incubation period
within the arthropod, known as the extrinsic incubation period. In mosquitoes
this period is short: 10 days at 30o C ambient temperature and longer at
lower temperatures. Mosquitoes remain infective for life without any apparent
ill-effects. In fact, their infectivity appears to increase with time after
infection and their effectiveness as transmitters depends upon the frequency
with which they bite. It is also possible that arthropods, whose mouth
parts are contaminated by virus in the act of feeding, could transmit the
virus mechanically if they feed soon afterwards on another animal. For
instance, chikungunya virus can be transmitted mechanically by A. aegypti
for 8 hours after infection. In general, mosquito-borne viruses may not
use ticks as vectors nor can tick-borne viruses reside in mosquitoes.
TRANSMISSION
Arthropod transmission involves several stages:
1) ingestion by the arthropods of virus in the blood (usually) or tissue
fluids of the vertebrate hosts;
2) penetration of the viruses into the tissue of the arthropods, in the
gut wall, or elsewhere after passing through the gut barrier;
3) multiplication of the viruses in the arthropod cells, including those
of the salivary glands.
Stage 2 and part of stage 3 represent the extrinsic incubation period of
the disease (Hunter 1991).
The quantity of blood, and therefore the amount of virus ingested, seems
to be important as each arthropod species must ingest a minimum quantity
of a given virus before multiplication can take place. The same mosquito
species can have two different thresholds for two different viruses and
if one species has a low threshold, other species may have high thresholds
or may be completely resistant. This threshold phenomenon is extremely
important in determining the efficiency of a vector and may also vitally
affect the course of an epidemic. Viruses reportedly persist in overwintering
mosquitoes, while transovarial passage of virus has been seen in some tick
species. For mosquitoes the availability of suitable breeding places (and
therefore rainfall) is a major factor. An efficient vector may have a wide
range of animals on which to feed, but if the arthropod species is abundant,
and even if it bites man only infrequently in the presence of other (and
preferred) animals, the large numbers enable it to maintain transmission
to man. For example, Culex tritaeniorhynchus, which mostly bites birds,
Bovidae, dogs and especially porcines, and only to a limited extent man,
can maintain transmission of Japanese encephalitis from pigs to man by
sheer numbers.
Although transmission of arboviruses usually takes place through the bites
of arthropods, Lassa virus, for example, may be transmitted through contact
with excreta of infective rodents, and others via urine or faeces infecting
the nasopharynx, some through aerosol from a patient or others by one bird
pecking another.
IMMUNITY
After a vertebrate has been infected, the arbovirus probably multiplies
first in the regional lymph glands where the earliest formation of antibodies
also probably takes place. Some do not produce high titres of antibodies
in man and some antibodies are short-lived or appear late. In diagnosis,
haemagglutination-inhibiting and complement-fixing antibodies are important,
but the only protective antibody is of the neutralizing type, which is
also the most specific.
Arboviruses are grouped according to antigenic characters, but after inoculation
of one virus into a fresh animal, not only the homologous antibodies, but
also heterologous antibodies reacting with other viruses of the same group
tend to appear. Recovery from an infection by a member of one group of
arboviruses may provide some degree of resistance to a susbsequent infection
by another member of the same group. For example, infection with West Nile
virus may have modified the Ethiopian epidemic of Yellow Fever in 1962.
Again, the effect of prior infection with Zika, Uganda S and other related
viruses in the forest belt of Nigeria, leading to a high incidence of related
antibodies, is suggested as the explanation of the absence of epidemic
Yellow Fever in man in that area. These related infections probably modify
the disease rather than prevent infection.
ACTIVE IMMUNITY
With Yellow Fever, neutralizing antibodies can be found as early
as a few days after the beginning of the disease and are found constantly
for many years in the sera. The persistence of immunity does not depend
on exogenous reinfection. It is probable that a mosquito infected with
Yellow Fever is not harmed by it, but continues to excrete the virus throughout
life. This means a continuous supply and release of virus, probably from
the epithelial cells of the salivary glands. The virus enters man (or other
animals) and gains the liver and other epithelia, provoking the early antibodies
in the blood, which neutralize circulating viruses. But, as suggested by
Hunter (1991), antibodies which can be detected for so many years in man
must stem from a continuing stimulus, and the sensitive cells and their
progeny probably have a prophase equivalent of the virus incorporated into
their genome, with occasional reversion to productive development which
provides the stimulus for further antibody formation. A degree of immunity
of this kind may possibly be provided when a related virus invades epithelial
cells.
PASSIVE IMMUNITY
Infant rhesus monkeys and human infants born of mothers immune to
Yellow Fever have transient protective antibodies in their sera at birth
which persist for several months. They are probably placentally transferred,
rather than coming from the mother's milk, because antibodies may disappear
from infant sera while they are still suckling. Passive immunity induced
by injection of homologous immune serum, has been used for protection against
tick-borne encephalitis in cases of special risk and similar sera could
be used against other infections, particularly after laboratory or hospital
accidents.
GENERAL CLINICAL FEATURES
Most arbovirus infections are inapparent, that is they produce no
symptoms or often only mild ones (fever and occasional rash). For example,
in an epidemic of Japanese encephalitis it was estimated that for each
case of apparent disease there were 500-1000 inapparent infections. If
clinical manifestations arise after infection they do so after an intrinsic
incubation period lasting from a few days to a week or more. Some arboviruses
damage the endothelial lining of the capillaries increasing permeability
which allows the virus to pass the blood brain barrier causing meningoencephalitis.
Others damage the parenchymatous organs by direct damage to the cells in
which they are situated, while with others damage is caused by the immune
system of the host from the formation of antigen- antibody complexes and
disordered complement formation which damage the renal tubules and alter
the coagulation and fibrinolytic systems of the body causing haemorrhage
(viral hemorrhagic fevers). There is a general pattern of biphasic illness,
the first phase associated with viremia ending when antibodies appear in
the blood and the second phase when the virus is located in organs, such
as the liver or brain.
The onset of clinical manifestations is usually abrupt, generally occurring
after the onset of viraemia. Fever is usual and is sometimes the only sign.
In many cases the clinical manifestations last only while the virus is
disseminated, but in other cases there is remission, short or long. If
long, the disease is biphasic. After this, fever returns with signs indicating
localization of the virus in certain organs. If the period of viraemia
has been symptomless and the virus becomes localized in the central nervous
system, encephalitis appears. In hemorrhagic cases there is a special risk
of shock which can rapidly become irreversible unless promptly treated
(Hunter 1991).
ECOLOGICAL FACTORS
Microorganisms and viruses are adapted to extremely diverse econiches.
One of the most complex sets of adaptive characteristics concern arthropod
transmission of viruses. The arthropod-borne viruses are spectacular examples
of emergence and re-emergence resulting from innocent environmental manipulation
or natural environmental change. Deforestation, amateur irrigation and
the introduction of new species (usually livestock) gives rise to many
virus disease threats of humans and animals. Important aspects of ecological
change and their relation to arbovirus life cycles are:
1) Population movements and the intrusion of humans and domestic animals
into new arthropod habitats, particularly tropical forests;
2) Deforestation, with development of new forest-farmland margins and exposure
of farmers and domestic animals to new arthropods;
3) Irrigation, especially primitive irrigation systems, which are oblivious
to arthropod control;
4) Uncontrolled urbanization, with vector populations breeding in accumulations
of water (tin cans, old tires etc.) and sewage;
5) Increased long distance air travel, with potential for transport of
arthropod vectors;
6) Increased long-distance livestock transportations, with potential for
carriage of viruses and arthropods (especially ticks); and
7) New routing of long-distance bird migration brought about by new man-made
water impoundments (Murphy 1994).
To illustrate the effect ecological change can have on the emergence of
a new disease and the course of it afterwards one can look to dengue, one
of the most rapidly expanding diseases in tropical parts of the world,
with millions of cases occurring each year. For example, Puerto Rico had
five dengue epidemics in the first 75 years of this century, but has had
six epidemics in the past 11 years, at an estimated cost of over $150 million.
Simultaneously, Brazil, Nicaragua and Cuba have had their first major dengue
epidemic in over 50 years, involving multiple virus types. At the lethal
end of the dengue spectrum is dengue haemorrhagic fever, first occurring
in the Americas in 1981. Since then, 11 countries have reported cases,
and since 1990 over 3000 cases have been reported annually. Figure 3 illustrates
the extent of dengue occurrence globally.
The primary reason that dengue is emerging and re-emerging is vector control.
National priority lists are political in nature and tend to emphasize daily
problems, not episodic ones. Expensive mosquito control tends to fall off
the bottom of the list. Meanwhile, as older cheaper chemicals lose effectiveness
or are banned, new and expensive chemicals replace them. Before 1970, A.
aegypti, the vector of dengue and Yellow Fever, was targeted for regional
or even global eradication through the use of DDT (dichlorodiphenyltrichloroethane)
(Murphy 1993). Obviously, this solution is no longer applicable, but nothing
has effectively supplanted it.
CASES IN POINT
HANTAAN VIRUS
The hantavirus (mentioned above) is also the focus of much international
attention. During the Korean War of 1950-1952, thousands of United Nations
troops developed a mysterious disease marked by fever, headache, hemorrhage
and acute renal failure; the mortality rate was 5-10%. Despite much research,
the agent of this disease remained unknown for 28 years, when a new virus,
named Hantaan virus, was isolated in Korea from field mice. Recently, related
viruses have been found in many parts of the world in association with
different rodents and as the cause of human diseases with a variety of
little-known local names. Epidemic haemorrhagic fever, one of the most
important diseases in China, causes more than 100000 cases per year. Transmission
to humans is primarily by inhalation of aerosolized excreta. In May 1993
a cluster of deaths in the southwestern United States set in motion a multiagency
local, state and federal investigation that led to the discovery of a highly
pathogenic hantavirus and to the definition of a new clinical syndrome
(Peters 1994).
SEOUL VIRUS
Another virus of current interest in the USA, Seoul virus, was identified
about 10 years ago in Korea as a Hantaan-like virus whose natural host
is the urban rat. Serologic surveys detect it worldwide, including seroprevalence
rates of 12% in urban rats in Philadelphia and about 64% in Baltimore rats
(Le Duc 1986). Although acute hemorrhagic fever was not identified in inner-city
Baltimore, 1.3% of 1148 local residents were antibody-positive and the
possibility of viral association with chronic renal disease is under study.
"FOUR CORNERS VIRUS"
The disease hantavirus pulmonary syndrome (HPS), is characterized
by an initial fever followed by the abrupt onset of acute pulmonary edema
and shock. After recognition of the initial cases by observant clinicians
in the Southwest, investigations were swiftly mounted by local university
and public health workers but, in spite of efficient and competent studies,
failed to find the cause. By the time the CDC became involved, a number
of possible causative agents had been ruled out, leading most of the investigators
to believe they were dealing with a new entity. This observation led to
a broadly based approach to the field epidemiology and the laboratory study
of the disease. Samples from the field investigations were distributed
among many different laboratories of the National Center for Infectious
Disease (NCID) for analysis by the most sensitive classic and modern molecular
biological tests for a wide range spectrum of infectious agents.
Somewhat surprisingly, successful results were obtained after only a few
days of straightforward serologic tests for hantaviruses. The hemoconcentration,
thrombocytopenia and shock observed in some of the patients had raised
speculation about the involvement of these viral agents; however they had
been previously known as associated with renal syndromes only. The serologic
results came from established techniques such as indirect fluorescent-antibody
assays and enzyme-linked immunosorbent assays. The next steps utilized
reverse transcription and PCR amplification of RNA in postmortem tissue
samples (60% of confirmed cases to date have been fatal), using consensus
primers based on known hantavirus RNA sequences. These yielded products
with sequences typical of hantavirus but clearly different from any known
member of the genus. This provided additional evidence for the hantavirus
etiology and linked the new hantavirus closely to the human disease by
its presence in the tissues of people dying of the infection. Using the
genomic sequences from human tissues, investigators were subsequently able
to implicate the deer mouse as the principle reservoir of the virus.
Hantaviruses have traditionally been difficult to propagate, and this one
was no exception. Thus a full-length cDNA clone of the small RNA segment
of the virus was synthesized. This technique provided a diagnostic reagent
of increased sensitivity that could be made widely available. Eventually,
full length RNA sequences were developed for the medium segment and a partial
sequence was determined for the large segment, permitting the definitive
determination that the new virus, isolated weeks later and registered as
Muerto Canyon virus, was not a reassortant of any known hantavirus.
Immunohistochemical identification of hantavirus antigens and in situ hybridization
with genomic sequences also confirmed the hantavirus etiology of the syndrome.
The extensive presence of antigen in pulmonary capillaries provided an
explanation for the pathophysiology and target organ specificity differing
from that of other known disease-causing hantaviruses. This method, when
applied to paraffin- imbedded tissues, has also served as a retrospective
diagnostic tool, firmly identifying fatal cases from 10 to 15 years ago.
The rapid recognition of the hantavirus etiology of this disease was important
in that it alleviated heightened fear among the general American population,
and saved lives by focusing public health recommendations on the avoidance
of contact with potentially infected rodents. Different hantaviruses have
been isolated in Louisiana, Florida and also Brazil, indicating the uncommon,
yet widespread nature of this disease. Recently (Diglisic 1994), isolation
of a hantavirus from Mus musculus captured in Yugoslavia was reported.
As stated by C.J. Peters, chief of the Special Pathogens Branch of the
Division of Viral and Rickettsial Diseases at NCID, the crucial role of
modern techniques in virology was possible only in a context of past hantavirus
research, and as part of efforts of a multidisciplinary team of clinicians,
epidemiologists, field ecologists and classic microbiologists. The need
for basic research is highlighted by the applied practical success which
resulted from it, as was the case in identifying a new strain of hantavirus.
Future research will need to investigate the molecular mechanisms for induction
of pulmonary edema and an appropriate blocking therapy. The evolutionary
relationships of the hantaviruses and their rodent host specificity must
be understood to predict the future course of transmission, and finally
the basis for the different tropisms of the viruses must be examined at
a molecular level.
EBOLA VIRUS
Ebola virus, a member of the Filoviridae, burst from obscurity with
spectacular outbreaks of severe, haemorrhagic fever. It was first associated
with an outbreak of 318 cases and a case-fatality rate of 90% in Zaire
and caused 150 deaths among 250 cases in Sudan. Smaller outbreaks continue
to appear periodically, particularly in East, Central and southern Africa.
In 1989, a haemorrhagic disease was recognized among cynomolgus macaques
imported into the United States from the Philippines. Strains of Ebola
virus were isolated from these monkeys. Serologic studies in the Philippines
and elsewhere in Southeast Asia indicated that Ebola virus is a prevalent
cause of infection among macaques (Manson 1989).
These threadlike polymorphic viruses are highly variable in length apparently
owing to concatemerization. However, the average length of an infectious
virion appears to be 920 nm. The virions are 80 nm in diameter with a helical
nucleocapsid, a membrane made of 10 nm projections, and host cell membrane.
They contain a unique single-stranded molecule of noninfectious (negative
sense ) RNA. The virus is composed of 7 polypeptides, a nucleoprotein,
a glycoprotein, a polymerase and 4 other undesignated proteins. Proteins
are produced from polyadenylated monocistronic mRNA species transcribed
from virus RNA. The replication in and destruction of the host cell is
rapid and produces a large number of viruses budding from the cell membrane.
Epidemics have resulted from person to person transmission, nosocomial
spread or laboratory infections. The mode of primary infection and the
natural ecology of these viruses are unknown. Association with bats has
been implicated directly in at least 2 episodes when individuals entered
the same bat-filled cave in Eastern Kenya. Ebola infections in Sudan in
1976 and 1979 occurred in workers of a cotton factory containing thousands
of bats in the roof. However, in all instances, study of antibody in bats
failed to detect evidence of infection, and no virus was isolated form
bat tissue.
The index case in 1976 was never identified, but this large outbreak resulted
in 280 deaths of 318 infections. The outbreak was primarily the result
of person to person spread and transmission by contaminated needles in
outpatient and inpatient departments of a hospital and subsequent person
to person spread in surrounding villages. In serosurveys in Zaire, antibody
prevalence to Ebola virus has been 3 to 7%. The incubation period for needle-
transmitted Ebola virus is 5 to 7 days and that for person to person transmitted
disease is 6 to 12 days.
The virus spreads through the blood and is replicated in many organs. The
histopathologic change is focal necrosis in these organs, including the
liver, lymphatic organs, kidneys, ovaries and testes. The central lesions
appear to be those affecting the vascular endothelium and the platelets.
The resulting manifestations are bleeding, especially in the mucosa, abdomen,
pericardium and vagina. Capillary leakage appears to lead to loss of intravascular
volume, bleeding, shock and the acute respiratory disorder seen in fatal
cases. Patients die of intractable shock. Those with severe illness often
have sustained high fevers and are delirious, combative and difficult to
control.
EBOLA SEROLOGY
The serologic method used in the discovery of Ebola was the direct
immunofluorescent assay. The test is performed on a monolayer of infected
and uninfected cells fixed on a microscopic slide. IgG- or IgM-specific
immunoglobulin assays are performed. These tests may then be confirmed
by using western blot or radioimmunoprecipitation. Virus isolation is also
a highly useful diagnostic method, and is performed on suitably preserved
serum, blood or tissue specimens stored at -70oC or freshly collected.
TREATMENT OF EBOLA
No specific antiviral therapy presently exists against Ebola virus,
nor does interferon have any effect. Past recommendations for isolation
of the patient in a plastic isolator have given way to the more moderate
recommendation of strict barrier isolation with body fluid precautions.
This presents no excess risk to the hospital personnel and allows substantially
better patient care, as shown in Table 2. The major factor in nosocomial
transmission is the combination of the unawareness of the possibility of
the disease by a worker who is also inattentive to the requirements of
effective barrier nursing. After diagnosis, the risk of nosocomial transmission
is small.
PREVENTION AND CONTROL OF EBOLA
The basic method of prevention and control is the interruption of
airborn, and contact by person to person spread of the virus. However,
in rural areas, this may be difficult because families are often reluctant
to admit members to the hospital because of limited resources and the culturally
unacceptable separation of sick or dying patients from the care of their
family. Experience with human disease and primate infection suggests that
a vaccine inducing a strong cell- mediated response will be necessary for
virus clearance and adequate protection. Neutralizing antibodies are not
observed in convalescent patients nor do they occur in primates inoculated
with killed vaccine. A vaccine expressing the glycoprotein in vaccinia
is being prepared for laboratory evaluation.
SUMMARY of Symptoms: Fever is prominent. Intially the patient coplains
of severe aching, scratchy throat, headache, fatigue and fever. Physicians
generally diagnose "flu." However, the fever and aching worsen.
Headache rapidly becomes severe, and small hemorrhages called petichiae
appear on the skin, usually where clothing is tight, such as the beltline
and under the bra. As the virus attacks the lining of small blood vessels,
internal bleeding supervenes. Then severe vomiting with black and red blood,
and bloody diarrhea. Due to marked general capillary hemorrhage and seepage
of serum into the body spaces, there will be progressive fall in blood
pressure and shock. The organs are attacked and dissolve rapidly. the kidneys
first usually, with failure of function, then the liver, heart, and brain
with rapid onset of coma, convulsions, shock and death. The body becomes
a blackened sack of skin containing bloody fluid, dissolved muscles and
organs, and bones. TREATMENT: CONSISTS OF ICE BAGS, MORPHINE FOR SEVERE
TOTAL BODY PAIN, INTRAVENOUS FLUIDS, AND BLOOD TRANSFUSIONS. There is no
specific therapy. Antibiotics are ineffective. Death rate in the recent
outbreak in Zaire was 90%.
Byron T. Weeks, MD
All information posted on this web site is
the opinion of the author and is provided for educational purposes only.
It is not to be construed as medical advice. Only a licensed medical doctor
can legally offer medical advice in the United States. Consult the healer
of your choice for medical care and advice.
|