WHICH ANTIVIRAL DRUGS WORK ON SWINE FLU AND ITS SYMPTOMS

H1N1 Flu Virus (Swine Flu)

Looking for an effective flu treatment and wondering if antibiotics will work? Antibiotics are medications that fight infections caused by bacteria, but the flu is cause by a virus.

Taking antibiotics when you have a virus may do more harm than good. Taking antibiotics when they are not needed increases your risk of getting an infection later that may resist antibiotic treatment.

Why Won't Antibiotics Cure Cold or Flu?

Antibiotics only cure certain infections due to bacteria -- and if taken carelessly, you may get more serious health problems than you bargained for.

With any illness, it is critical to address the underlying cause of the illness, whether it's bacterial or viral. Antibiotics will not kill cold or flu viruses.

Should I Avoid Antibiotics Altogether?

Not at all. Antibiotics can save people's lives, and if you need them, you should get them as quickly as you can. Since only a doctor can prescribe antibiotics, this means that you should talk to your doctor if you think you might need them (as opposed to taking your friend's leftover antibiotics from last winter's illness, for example).

However, it is the grave over-reliance and inappropriate use of antibiotics that have contributed to the global antibiotic resistance crisis that we face.

A study by the CDC showed that many adults believe that if they are sick enough to see a doctor for a cold, they should get an antibiotic treatment. The study also showed that patients are not aware of the consequences of taking the drugs if they are not needed. And when antibiotics are misused, bacteria can become resistant.

What Are Antivirals?

Antivirals are medications that reduce the ability of flu viruses to multiply. The CDC considers antiviral drugs as a "second line of defense against the flu" after getting the flu vaccine. When taken at the onset of flu, these drugs help decrease the severity and duration of flu symptoms. They can also be used in cases to help prevent the flu, but they are not a replacement for getting the flu vaccine.

Which Antivirals Does the CDC Recommend?

The CDC recommends oseltamivir (Tamiflu), peramivir (Rapivab), and zanamivir (Relenza). Antiviral drugs for flu are most effective when given within 48 hours after symptoms start to appear. These flu drugs can decrease the duration of the flu by one to two days if used within this early time period. These antivirals are usually given for a period of five days for the treatment of flu. For prevention of flu, antiviral drugs may be given for at least 7 days. In some cases, antivirals may be given for longer periods of time. 

Tamiflu is approved for treatment in those over 2 weeks of age and for prevention in people 1 year old and older. 

Rapivad, given in one intravenous dose, is approved for people age 18 years or older. 

Relenza is approved for treatment of people 7 years old and older and for prevention in people 5 years old and older.

Are There Side Effects With Antiviral Drugs?

Side effects of antiviral drugs may include nervousness, poor concentration, nausea, and vomiting. Relenza is not recommended for people with a history of breathing problems, such as asthma, because it may cause a worsening of breathing problems. Discuss side effects with your doctor.

What Does Antibiotic Resistance Mean?

According to the CDC, antibiotic resistance happens when bacteria changes in some way to reduce or eliminate the effectiveness of the antibiotic.

When bacteria are exposed to antibiotics repeatedly, such as when you take the medication needlessly or too frequently, the germs in your body start to evolve. These changes can make the germs stronger than before so they completely resist the antibiotic. Your illness may linger with no signs of improvement. Or your illness may suddenly take a turn for the worse, requiring you to seek emergency medical care. You may have to be admitted to the hospital and get several different antibiotics administered in your veins. Sadly, those around you may get the resistant bacteria and come down with a similar illness that is very difficult to treat.

But Aren't Antibiotics Quick Cures for Illnesses?

Unfortunately, demand for a "quick fix" for what ails us has fueled this resistance crisis. In face, more than two-thirds of the 150 million antibiotic prescriptions written each year for patients outside of hospitals are unnecessary, according to a CDC study.

How Can I Protect my Family and Myself From Antibiotic Resistance?

There is a way to protect yourself and others from resistant bacteria, and that is to respect antibiotics and take them only when necessary for a bacterial infection. Here are some useful tips:

1. When you see a doctor, don't demand antibiotics. Understand that antibiotics are used for bacterial infections, not symptoms of a cold or flu virus.
2. If a doctor prescribes antibiotics, use them as prescribed. Take all of the antibiotics as directed and don't save some for future use.
3. Don't share antibiotics with others.

Preventing the flu in the first place may help you avoid getting sick altogether. Get a flu shot annually. Also, make sure you wash your hands frequently and thoroughly to prevent spreading germs.

H1N1 flu is also known as swine flu. It's called swine flu because in the past, the people who caught it had direct contact with pigs. That changed several years ago, when a new virus emerged that spread among people who hadn't been near pigs.

In 2009, H1N1 was spreading fast around the world, so the World Health Organization called it a pandemic. Since then, people have continued to get sick from swine flu, but not as many.

While swine flu isn't as scary as it seemed a few years ago, it's still important to protect yourself from getting it. Like seasonal flu, it can cause more serious health problems for some people. The best protection is to get a flu vaccine, or flu shot, every year. Swine flu is one of the viruses included in the vaccine.

Slideshow: Is It a Cold or Is It the Flu?

Causes of Swine Flu

Swine flu is contagious, and it spreads in the same way as the seasonal flu. When people who have it cough or sneeze, they spray tiny drops of the virus into the air. If you come in contact with these drops or touch a surface (such as a doorknob or sink) that an infected person has recently touched, you can catch H1N1 swine flu.

Despite the name, you can't catch swine flu from eating bacon, ham, or any other pork product.

Swine Flu Symptoms

People who have swine flu can be contagious one day before they have any symptoms, and as many as 7 days after they get sick. Kids can be contagious for as long as 10 days.

Most symptoms are the same as seasonal flu. They can include:

cough

fever

sore throat

stuffy or runny nose

body aches

headache

chills

fatigue

Like seasonal flu, swine flu can lead to more serious complications, including pneumonia and respiratory failure. And it can make conditions like diabetes or asthma worse. If you have symptoms like shortness of breath, severe vomiting, abdominal pain, dizziness, or confusion, call your doctor or 911 right away.

Tests for Swine Flu

It's hard to tell whether you have swine flu or seasonal flu, because most symptoms are the same. People with swine flu may be more likely to feel nauseous and throw up than people who have seasonal flu. But a lab test is the only way to know for sure. Even a rapid flu test you can get in your doctor's office won't tell you for sure.

To test for swine flu, your doctor takes a sample from your nose or throat. You may not need to be tested. The CDC says the people who need to be tested are those in the hospital or those at high risk for getting life-threatening problems from the flu, such as:

Children under 5 years old
People 65 or older
Children and teens (under age 18) who are getting long-term aspirin therapy, and who might be at risk for Reye's syndrome after being infected with swine flu. Reye's syndrome is a life-threatening illness linked to aspirin use in children.
Pregnant women
Adults and children who have chronic lung, heart, liver, blood, nervous system, neuromuscular, or metabolic problems
Adults and children who have suppressed immune systems (including those who take medications to suppress their immune systems or who have HIV)
People in nursing homes and other long-term care facilities


How Is Swine Flu Treated?
Some of the same antiviral drugs that are used to treat seasonal flu also work against H1N1 swine flu. Oseltamivir (Tamiflu) and zanamivir (Relenza) seem to work best, although some kinds of swine flu are resistant to Tamiflu.

These drugs can help you get over swine flu faster. They can also help keep it from being too severe. They work best when taken within 48 hours of the first flu symptoms, but they can help when taken later.

Antibiotics won't help, because flu is caused by a virus, not bacteria.

Over-the-counter pain remedies and cold and flu medications can help relieve aches, pains, and fever. Don't give aspirin to children under age 18 because of the risk for Reye’s syndrome. Check to make sure that over-the-counter cold medications do not have aspirin before giving them to children.

Vaccine for Swine Flu
 The same flu vaccine that protects against seasonal flu also protects against the H1N1 swine flu strain. You can get it as a shot or as a nasal spray. Either way, it "teaches" your immune system to attack the real virus.

Besides a flu shot, there are other things you can do to stay healthy:

Wash your hands throughout the day with soap and water. Sing the "Happy Birthday" song twice to make sure you've washed long enough. Or use an alcohol-based hand sanitizer.
Don't touch your eyes, nose, or mouth.

Avoid people who are sick.

Did you know this about swine flu?

What is the swine flu?

Swine flu (swine influenza) is a respiratory disease caused by viruses (influenza viruses) that infect the respiratory tract of pigs, resulting in nasal secretions, a barking cough, decreased appetite, and listless behavior. Swine flu produces most of the same symptoms in pigs as human flu produces in people. Swine flu can last about one to two weeks in pigs that survive. Swine influenza virus was first isolated from pigs in 1930 in the U.S. and has been recognized by pork producers and veterinarians to cause infections in pigs worldwide. In a number of instances, people have developed the swine flu infection when they are closely associated with pigs (for example, farmers, pork processors), and likewise, pig populations have occasionally been infected with the human flu infection. In most instances, the cross-species infections (swine virus to man; human flu virus to pigs) have remained in local areas and have not caused national or worldwide infections in either pigs or humans. Unfortunately, this cross-species situation with influenza viruses has had the potential to change. Investigators decided the 2009 so-called "swine flu" strain, first seen in Mexico, should be termed novel H1N1 flu since it was mainly found infecting people and exhibits two main surface antigens, H1 (hemagglutinin type 1) and N1 (neuraminidase type1). The eight RNA strands from novel H1N1 flu have one strand derived from human flu strains, two from avian (bird) strains, and five from swine strains.

Swine flu is transmitted from person to person by inhalation or ingestion of droplets containing virus from people sneezing or coughing; it is not transmitted by eating cooked pork products. The newest swine flu virus that has caused swine flu is influenza A H3N2v (commonly termed H3N2v) that began as an outbreak in 2011. The "v" in the name means the virus is a variant that normally infects only pigs but has begun to infect humans. There have been small outbreaks of H1N1 since the pandemic; a recent one is in India where at least three people have died.

Swine flu (H1N1 and H3N2v influenza virus) facts

Swine flu is a respiratory disease caused by influenza viruses that infect the respiratory tract of pigs and result in a barking cough, decreased appetite, nasal secretions, and listless behavior; the virus can be transmitted to humans.

Swine flu viruses may mutate (change) so that they are easily transmissible among humans.

The 2009 swine flu outbreak (pandemic) was due to infection with the H1N1 virus and was first observed in Mexico.

Symptoms of swine flu in humans are similar to most influenza infections: fever (100 F or greater), cough, nasal secretions, fatigue, and headache.

Vaccination is the best way to prevent or reduce the chances of becoming infected with influenza viruses.

Two antiviral agents, zanamivir (Relenza) and oseltamivir (Tamiflu), have been reported to help prevent or reduce the effects of swine flu if taken within 48 hours of the onset of symptoms.

There are various methods listed in this article to help individuals from getting the flu.

The most serious complication of the flu is pneumonia.

What causes swine flu? & Why is swine flu now infecting humans?

What causes swine flu?

The cause of the 2009 swine flu was an influenza A virus type designated as H1N1. In 2011, a new swine flu virus was detected. The new strain was named influenza A (H3N2)v. Only a few people (mainly children) were first infected, but officials from the U.S. Centers for Disease Control and Prevention (CDC) reported increased numbers of people infected in the 2012-2013 flu season. Currently, there are not large numbers of people infected with H3N2v. Unfortunately, another virus termed H3N2 (note no "v" in its name) has been detected and caused flu, but this strain is different from H3N2v. In general, all of the influenza A viruses have a structure similar to the H1N1 virus; each type has a somewhat different H and/or N structure.

Why is swine flu now infecting humans?

Many researchers now consider that two main series of events can lead to swine flu (and also avian or bird flu) becoming a major cause for influenza illness in humans.

First, the influenza viruses (types A, B, C) are enveloped RNA viruses with a segmented genome; this means the viral RNA genetic code is not a single strand of RNA but exists as eight different RNA segments in the influenza viruses. A human (or bird) influenza virus can infect a pig respiratory cell at the same time as a swine influenza virus; some of the replicating RNA strands from the human virus can get mistakenly enclosed inside the enveloped swine influenza virus. For example, one cell could contain eight swine flu and eight human flu RNA segments. The total number of RNA types in one cell would be 16; four swine and four human flu RNA segments could be incorporated into one particle, making a viable eight RNA-segmented flu virus from the 16 available segment types. Various combinations of RNA segments can result in a new subtype of virus (this process is known as antigenic shift) that may have the ability to preferentially infect humans but still show characteristics unique to the swine influenza virus (see Figure 1). It is even possible to include RNA strands from birds, swine, and human influenza viruses into one virus if a single cell becomes infected with all three types of influenza (for example, two bird flu, three swine flu, and three human flu RNA segments to produce a viable eight-segment new type of flu viral genome). Formation of a new viral type is considered to be antigenic shift; small changes within an individual RNA segment in flu viruses are termed antigenic drift (see figure 1) and result in minor changes in the virus. However, these small genetic changes can accumulate over time to produce enough minor changes that cumulatively alter the virus' makeup over time (usually years).

Second, pigs can play a unique role as an intermediary host to new flu types because pig respiratory cells can be infected directly with bird, human, and other mammalian flu viruses. Consequently, pig respiratory cells are able to be infected with many types of flu and can function as a "mixing pot" for flu RNA segments (see figure 1). Bird flu viruses, which usually infect the gastrointestinal cells of many bird species, are shed in bird feces. Pigs can pick these viruses up from the environment, and this seems to be the major way that bird flu virus RNA segments enter the mammalian flu virus population. Figure 1 shows this process in H1N1, but the figure represents the genetic process for all flu viruses, including human, swine, and avian strains.

WHAT IS SWINE FLU

What is the swine flu?

Swine flu (swine influenza) is a respiratory disease caused by viruses (influenza viruses) that infect the respiratory tract of pigs, resulting in nasal secretions, a barking cough, decreased appetite, and listless behavior. Swine flu produces most of the same symptoms in pigs as human flu produces in people. Swine flu can last about one to two weeks in pigs that survive. Swine influenza virus was first isolated from pigs in 1930 in the U.S. and has been recognized by pork producers and veterinarians to cause infections in pigs worldwide. In a number of instances, people have developed the swine flu infection when they are closely associated with pigs (for example, farmers, pork processors), and likewise, pig populations have occasionally been infected with the human flu infection. In most instances, the cross-species infections (swine virus to man; human flu virus to pigs) have remained in local areas and have not caused national or worldwide infections in either pigs or humans. Unfortunately, this cross-species situation with influenza viruses has had the potential to change. Investigators decided the 2009 so-called "swine flu" strain, first seen in Mexico, should be termed novel H1N1 flu since it was mainly found infecting people and exhibits two main surface antigens, H1 (hemagglutinin type 1) and N1 (neuraminidase type1). The eight RNA strands from novel H1N1 flu have one strand derived from human flu strains, two from avian (bird) strains, and five from swine strains.

Swine flu is transmitted from person to person by inhalation or ingestion of droplets containing virus from people sneezing or coughing; it is not transmitted by eating cooked pork products. The newest swine flu virus that has caused swine flu is influenza A H3N2v (commonly termed H3N2v) that began as an outbreak in 2011. The "v" in the name means the virus is a variant that normally infects only pigs but has begun to infect humans. There have been small outbreaks of H1N1 since the pandemic; a recent one is in India where at least three people have died.

What is the history of swine flu in humans?

In 1976, there was an outbreak of swine flu at Fort Dix. This virus was not the same as the 2009 H1N1 outbreak, but it was similar insofar as it was an influenza A virus that had similarities to the swine flu virus. There was one death at Fort Dix. The government decided to produce a vaccine against this virus, but the vaccine was associated with rare instances of neurological complications (Guillain-Barré syndrome) and was discontinued. Some individuals speculate that formalin, used to inactivate the virus, may have played a role in the development of this complication in 1976. One of the reasons it takes a few months to develop a new vaccine is to test the vaccine for safety to avoid the complications seen in the 1976 vaccine. Individuals with active infections or diseases of the nervous system are also not recommended to get flu vaccines.

Early in the spring of 2009, H1N1 flu virus was first detected in Mexico, causing some deaths among a "younger" population. It began increasing during the summer 2009 and rapidly spread to the U.S. and to Europe and eventually worldwide. The WHO declared it first fit their criteria for an epidemic and then, in June 2009, the WHO declared the first flu pandemic in 41 years. There was a worldwide concern and people began to improve in hand washing and other prevention methods while they awaited vaccine development. The trivalent vaccine made for the 2009-2010 flu season offered virtually no protection from H1N1. New vaccines were developed (both live and killed virus) and started to become available in Sept. 2009-Oct. 2009. The CDC established a protocol guideline for those who should get the vaccine first. By late December to January, a vaccine against H1N1 was available in moderate supply worldwide. The numbers of infected patients began to recede and the pandemic ended. However, a strain of H1N1 was incorporated into the yearly trivalent vaccine for the 2010-2011 flu season because the virus was present in the world populations.

As stated in the first section of this article, a new strain of swine flu, (H3N2)v, was detected in 2011; it has not affected any large numbers of people in the current flu season. However, another antigenically distinct virus with the same H and N components (termed H3N2 (note no "v") has caused flu in humans; viral antigens were incorporated into the 2013-2014 seasonal flu shots and nasal spray vaccines

Swine influenza, also called pig influenza, swine flu, hog flu and pig flu, is an infection caused by any one of several types of swine influenza viruses. Swine influenza virus (SIV) or swine-origin influenza virus (S-OIV) is any strain of the influenza family of viruses that is endemic in pigs.[2] As of 2009, the known SIV strains include influenza C and the subtypes of influenza A known as H1N1, H1N2, H2N1, H3N1, H3N2, and H2N3.

Swine influenza virus is common throughout pig populations worldwide. Transmission of the virus from pigs to humans is not common and does not always lead to human flu, often resulting only in the production of antibodies in the blood. If transmission does cause human flu, it is called zoonotic swine flu. People with regular exposure to pigs are at increased risk of swine flu infection.

Around the mid-20th century, identification of influenza subtypes became possible, allowing accurate diagnosis of transmission to humans. Since then, only 50 such transmissions have been confirmed. These strains of swine flu rarely pass from human to human. Symptoms of zoonotic swine flu in humans are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.

In August 2010, the World Health Organization declared the swine flu pandemic officially over.

Cases of swine flu have been reported in India, with over 25000 positive test cases and 1370 deaths till March 2015.

Ebola Virus Pathogenesis: Implications for Vaccines and Therapies

Ebola virus is an aggressive pathogen that causes a highly lethal hemorrhagic fever syndrome in humans and nonhuman primates. First recognized near the Ebola River valley during an outbreak in Zaire in 1976 (6, 20), outbreaks have occurred in Africa in the ensuing 27 years, with mortality rates ranging from 50 to 90% (26, 28). Outbreaks have been identified yearly for the past 3 years in central Africa, the most recent of which continues in the Republic of the Congo, with more than 125 fatalities to date according to the World Health Organization (http://www.who.int/csr/don/2003_05_07/en/, accessed 7 May 2003). The natural host for Ebola virus is unknown, so it has not been possible to implement programs to control or eliminate viral reservoirs of transmission to human populations. The rapid progression of Ebola virus infection has further complicated the control of this disease, affording little opportunity to develop acquired immunity. There is currently no antiviral therapy or vaccine that is effective against Ebola virus infection in humans.

Although its clinical course is well known, the specific mechanisms underlying the pathogenicity of Ebola virus have not been clearly delineated. This is due, in part, to the difficulty in obtaining samples and studying the disease in the relatively remote areas in which the outbreaks occur. In addition, a high degree of biohazard containment is required for laboratory studies and clinical analysis. Isolation of the viral cDNAs and the development of expression systems have allowed the study of Ebola virus gene products under less restrictive conditions and facilitated an understanding of the mechanisms underlying virally induced cell damage.

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EBOLA VIRUS DISEASE PROGRESSION

Typically, Ebola virus infection runs its course within 14 to 21 days. Infection initially presents with nonspecific flu-like symptoms such as fever, myalgia, and malaise. As the infection progresses, patients exhibit severe bleeding and coagulation abnormalities, including gastrointestinal bleeding, rash, and a range of hematological irregularities, such as lymphopenia and neutrophilia. Cytokines are released when reticuloendothelial cells encounter virus, which can contribute to exaggerated inflammatory responses that are not protective. Damage to the liver, combined with massive viremia, leads to disseminated intravascular coagulopathy. The virus eventually infects microvascular endothelial cells and compromises vascular integrity. The terminal stages of Ebola virus infection usually include diffuse bleeding, and hypotensive shock accounts for many Ebola virus fatalities (for reviews, see references 9 and 28).

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STRUCTURE AND CLASSIFICATION OF THE EBOLA VIRUS

Ebola virus and the related Marburg virus are members of the Filovirus family, which are pleomorphic, negative-sense RNA viruses whose genome organization is most similar to the Paramyxoviridae. Of the four identified strains of Ebola virus, three—the Zaire, Ivory Coast, and Sudan strains—have been shown to cause disease in both humans and nonhuman primates, with the Zaire strain exhibiting the highest lethality rate (13, 29). The only documented outbreaks of Ebola virus infection in the United States were fortunately restricted to nonhuman primates at holding facilities in Virginia and Texas, caused by the Reston strain, which has not yet caused fatal disease in humans (19).

The Ebola virus genome is 19 kb long, with seven open reading frames encoding structural proteins, including the virion envelope glycoprotein (GP), nucleoprotein (NP), and matrix proteins VP24 and VP40; nonstructural proteins, including VP30 and VP35; and the viral polymerase (reviewed in reference 28). Unlike that of Marburg virus, the GP open reading frame of Ebola virus gives rise to two gene products, a soluble 60- to 70-kDa protein (sGP) and a full-length 150- to 170-kDa protein (GP) that inserts into the viral membrane (29, 41), through transcriptional editing.

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EBOLA VIRUS GP AND VIRAL PATHOGENESIS

The Ebola virus GP is synthesized in a secreted (sGP) or full-length transmembrane form, and each gene product has distinct biochemical and biological properties. For example, GP appears to form a trimeric complex (30) and binds preferentially to endothelial cells, whereas sGP does not (49). Preferential binding of Ebola virus GP to the endothelium was demonstrated by use of two independent methodologies as follows: direct binding was assessed by fluorescence-activated cell sorter analysis, and pseudotyping experiments were performed in which virus titers, cell numbers, and confluence were carefully determined so that the multiplicity of infection was controlled and equal in all cell types. Another study failed to demonstrate this preferential binding (17), but direct binding of GP to endothelial cells was not measured and neither the multiplicity of infection, target cell numbers, nor cell confluence was reported in that study. The receptors required for cell binding and infection are not completely understood. A folate-related receptor can serve as a cofactor to facilitate infection (8), but whether it serves as a receptor remains unclear. The cell surface lectin DC-SIGN can also facilitate GP binding to cells through viral carbohydrate determinants, but it does not appear to mediate entry by itself (1, 32). In contrast to GP, sGP gives rise to a dimeric protein (30) that interacts with neutrophils (49). sGP mediates neutrophil binding, directly or indirectly, through CD16b, the neutrophil-specific form of the Fcγ receptor III (49). After the initial description of the neutrophil binding of sGP, it was shown that immunoglobulin G (IgG), but not an Fab fragment, against sGP was needed to detect neutrophil binding (T. Maruyama, M. J. Buchmeier, P. W. H. I. Parren, and D. R. Burton, Technical Comment, Science 282:843-844, 1998). A subsequent study showed that the binding could also be seen if an irrelevant IgG was used with the Fab fragment against sGP (Z.-Y. Yang, R. Delgado, L. Xu, R. F. Todd, E. G. Nabel, A. Sanchez, and G. J. Nabel, Author's Reply, Science 282:844-846, 1998). Though such binding could potentially arise from binding of immune complexes, additional studies using resonance energy transfer showed that neutrophils incubated with sGP showed a significant reduction in the CR3-Fcγ RIIIB RET signal (22), demonstrating that sGP alters the physical and functional interaction between Fcγ RIIIB and CR3. Through this interaction, sGP may contribute to immune evasion by inhibiting early steps in neutrophil activation (as measured by the down-modulation of l-selectin) that would ordinarily assist in virus clearance (49).

Several lines of evidence suggest that the viral GP plays a key role in the manifestations of Ebola virus infection. The transmembrane form of GP targets the Ebola virus to cells that are relevant to its pathogenesis. Specifically, GP allows the virus to introduce its contents into monocytes and/or macrophages, where cell damage or exposure to viral particles may cause the release of cytokines (34) associated with inflammation and fever, and into endothelial cells, which damages vascular integrity (48) (Fig. ​(Fig.1).1). Thus, sGP may alter the immune response by inhibiting neutrophil activation, while the transmembrane GP may contribute to the hemorrhagic fever symptoms by targeting virus to cells of the reticuloendothelial network and the lining of blood vessels.

FIG. 1.

Host immune responses to Ebola virus and cell damage due to direct infection of monocytes and macrophages cause the release of cytokines associated with inflammation and fever (A). Infection of endothelial cells also induces a cytopathic effect and damage ...

GP expression in cultured human endothelial and epithelial cells causes cell rounding and detachment (48). GP is the only one of the seven Ebola virus gene products to exert this effect, and though GP from all four documented Ebola virus strains acts similarly, the highly pathogenic Zaire strain has the most potent activity in this cell culture assay (33). These effects require the presence of the mucin-like, serine-and-threonine-rich domain of GP and correspond with the down-regulation of specific molecules on the cell surface (48). Cytotoxicity appears to be precisely controlled by a mechanism involving down-regulation of GP expression through a transcriptional RNA editing event by the viral polymerase. The importance of this phenomenon was shown by use of a reverse genetics system for replicating Ebola virus in which a mutation that increases the level of full-length GP expression is significantly more cytotoxic than the wild-type virus (42).

The in vivo relevance of GP-induced endothelial cell toxicity was explored in blood vessel explants (48) in which human saphenous veins were infected with replication-defective adenoviral vectors carrying the gene for GP or sGP. Staining with horseradish peroxidase and scanning electron microscopy were used to observe severe damage to the endothelial cell lining in vessels that received the virus encoding full-length Ebola virus GP but not sGP or vectors in which the mucin domain of GP was removed. Cell damage in explant cultures paralleled the species specificity of different Ebola virus strains: no toxicity was observed when Reston strain GP was introduced into human vascular explants, whereas significant tissue damage was observed in vascular explants from nonhuman primates.

Further in vitro analyses have begun to elucidate the molecular mechanisms underlying GP-induced cytotoxicity. Critical mediators of cell adhesion to the matrix and immune signaling (e.g., integrins and major histocompatibility complex class I cell surface proteins) are among the cell surface molecules that are dysregulated (33, 37). Transient expression of Ebola virus GP in human kidney 293T cells caused a reduction of specific integrins (primary molecules responsible for cell adhesion to the extracellular matrix) on the cell surface. GP mutants lacking the membrane-spanning region of the ectodomain did not cause this down-regulation, suggesting that anchorage of GP to the cell membrane is required for this effect. Disruption of major histocompatibility complex class I expression on the cell surface is a mechanism for evading host immune responses that is shared by several pathogens, including cytomegalovirus, human immunodeficiency virus (HIV), and herpesviruses (27). It is not known whether GP affects integrin levels by altering intracellular trafficking or by modulation of protein synthesis or degradation, but preliminary experiments suggest a role for cellular protein transport machinery in GP-mediated cytotoxicity (N. Sullivan, unpublished observations). In any event, the biologic effects of GP alone may account largely for the features of Ebola virus infection that lead to fatal disease, including inflammatory dysregulation, immune suppression, and loss of vascular integrity.

Structural analyses of GP have revealed features in common with other viral envelope proteins. The crystal structure of the GP ectodomain revealed a coiled-coil domain resembling a trimer of helical hairpin-like loops (23, 44). The hairpin structure is adjacent to the fusion-peptide region (16) hypothesized to insert directly into the target cell membrane. Analogous coiled-coil regions have been defined for GPs of influenza virus, murine retroviruses, HIV, and simian immunodeficiency virus (SIV) as well as for some cellular proteins, called SNARES, that function in intracellular vesicle fusion (44). For HIV gp160, it has been possible to identify peptides that bind to a transient intermediate form that precedes hairpin formation. Because of their potent inhibition of viral entry, these reagents have shown considerable promise in clinical trials (21). The Ebola virus GP contains a homologous hairpin structure for which a possible inhibitory peptide has been identified (43), a region that remains a potential therapeutic target.

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IMMUNE RESPONSE TO EBOLA VIRUS INFECTION

Ebola virus replicates at an unusually high rate that overwhelms the protein synthesis apparatus of infected cells and host immune defenses (28). Both the adaptive immune and inflammatory systems respond to infection at the same time that some cell types, specifically monocytes and macrophages, are targets relevant to disease pathogenesis. This feature of the infection was initially suggested by the immunohistochemical localization of Ebola virus in vivo: endothelial cells, mononuclear phagocytes, and hepatocytes are the main targets of infection (3, 4, 12, 50).

The components of the immune system that may protect against Ebola virus infection have not been defined. Antibody titers against Ebola virus GPs are readily detectable in patients who recover from Ebola virus infection; however, anecdotal reports have indicated that serum from recovered patients did not consistently protect against infection or exhibit neutralization of virus replication in cell culture. Furthermore, passive transfer of antibodies in animal models only delays the onset of symptoms and does not alter overall survival (18). More recently, the neutralization of virus replication by selected monoclonal antibodies isolated from the bone marrow of recovered patients was demonstrated in vitro (24), and monoclonal antibodies that recognize specific epitopes of Ebola virus GP have been shown to confer immune protection in a murine model of Ebola virus infection (15,45) and in guinea pigs (25). However, it is relatively easy to protect against infection in the mouse model, and protection of guinea pigs required a high dose of antibody administered very close to the time of virus challenge. Taken together, these results suggest that antibodies alone do not provide protective immunity in a natural context and that cellular immunity is likely to play a significant role in virus clearance. Whether hyperimmune serum from surviving vaccinated animals or certain infrequently occurring antibodies are capable of attenuating infection remains unknown, but such antibodies could potentially contribute to therapy if they can be identified and optimized.

A comparison of immune parameters in survivors and nonsurvivors of infection has provided clues into the constituents of an effective immune response. Baize et al. (2) characterized the immune responses of patients in two large Ebola virus outbreaks in Gabon in 1996. There was no significant difference in viral antigen load between survivors and nonsurvivors, but immune responses varied, suggesting that survival is dependent on the initial or innate immune response to infection. Survivors exhibited more significant IgM responses, clearance of viral antigen, and sustained T-cell cytokine responses, as indicated by high levels of T-cell-related mRNA in the peripheral blood. In contrast, antibodies specific for the virus were nearly undetectable in fatal cases, and while gamma interferon (IFN-γ) was detected early after infection, T-cell cytokine RNA levels were more indicative of a failure to develop adaptive immunity in the days preceding death.

During infection, there is evidence that both host and viral proteins contribute to the pathogenesis of Ebola virus. Increases in the levels of inflammatory cytokines IFN-γ, IFN-α, interleukin-2 (IL-2), IL-10, and tumor necrosis factor alpha were associated with fatality from Ebola hemorrhagic fever (40). Moreover, in vitro experiments demonstrated that tumor necrosis factor released from filovirus-infected monocytes and macrophages increased the permeability of cultured human endothelial cell monolayers (12). However, other reports have observed an association between elevated levels of IFN-γ mRNA and protection from infection (2), and a protective effect of IFN-α and -γ is suggested by the fact that the virus has evolved at least one protein, VP35, that acts as an IFN-α/β antagonist (5). Whether the effects of cytokines are protective or damaging may depend not only on the cytokine profile but also may represent a delicate balance influenced by the route and titer of incoming virus as well as factors specific to the individual host immune response.

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VACCINE DEVELOPMENT

Several animal models have been developed to study the pathogenesis of Ebola virus infection and to assess the efficacy of various vaccine approaches. Guinea pigs and nonhuman primates represent the primary animal models for vaccine development because the progression and pathogenesis most closely resemble those of the human disease (10,46, 47). A murine model was later developed by serial passage of virus in mice (7). Though the model allows the use of knockout and inbred strains to evaluate genetic determinants of disease, it is considered less predictive of human disease because it relies on a serially passaged, attenuated virus. While symptoms and time course of disease in guinea pigs parallel those in humans, nonhuman primate infection is considered the most predictive and useful for vaccine development (14).

Live attenuated viruses and recombinant proteins have been used successfully in a variety of vaccines, but the safety and immunogenicity of gene-based vaccines have proven increasingly attractive. Among the gene-based approaches, naked plasmid DNA has been used successfully in animal models to direct the synthesis of immunogens within the host cells and has proven helpful in a variety of infectious diseases (reviewed in references 11and 38).

Genetic immunization with plasmid DNA was developed in the guinea pig and was the first successful vaccine for Ebola virus (47). In this model, NP elicited a primarily humoral response and was less efficacious, while sGP and GP elicited T-cell proliferative and cytotoxic responses as well as a humoral response. Protection against lethal challenge was conferred by each of these immunogens when animals were infected within 1 month of the last immunization, but only GP or sGP provided long-lasting protection. The degree of protection correlated with antibody titer and antigen-specific T-cell responses. Subsequent studies of NP and GP plasmids conferred protective immunity in mice (39), but it is uncertain whether the attenuated murine virus is more sensitive to neutralization than the wild-type virus. Thus, the relative potency of NP, or its requirement as an immunogen for providing long-term protection, remains uncertain.

While DNA vaccines have been highly effective in rodents, their efficacy in nonhuman primates or humans has been less impressive. Priming-boosting immunization protocols that use DNA immunization followed by boosting with poxvirus vectors carrying the genes for pathogen proteins have yielded dramatically enhanced immune responses in animal studies, with 30-fold or greater increases in antibody titer from the booster (31). A different priming-boosting strategy using replication-defective adenovirus for an Ebola virus vaccine was tested in cynomolgus macaques (36). This study demonstrated the superior immunologic efficacy of this priming-boosting combination for both cellular and humoral responses. These animals displayed complete immune protection against a lethal challenge of virus, providing the first demonstration of an Ebola virus vaccine approach that protects primates against infection. Recently, an accelerated vaccination has been developed that confers protection against a lethal virus challenge in nonhuman primates after a single immunization (36a). If this vaccine works similarly in humans, it may be useful in the containment of acute outbreaks by ring vaccination.

In summary, an understanding of the mechanisms underlying Ebola virus-induced cytopathic effects has facilitated the process of vaccine and antiviral therapy development, which has in turn provided new information about pathogenesis and the immune response. Ebola virus does not exhibit the high degree of variability that other enveloped viruses may employ to evade host immunity, but Ebola virus GP alters target-cell function and exemplifies a novel strategy for immune evasion that may have arisen through the evolution of Ebola virus with its natural host. The cytotoxic effects of GP on macrophage and endothelial cell function disrupt inflammatory cell function and the integrity of the vasculature. In addition, by altering the cell surface expression of adhesion proteins and immune recognition molecules, Ebola virus may disrupt processes critical to immune activation and cytolytic-T-cell function. These phenomena likely account for the dysregulation of the inflammatory response and the vascular dysfunction characteristic of lethal Ebola virus infection, providing a rationale for focusing on GP as a target for a preventative vaccine and providing leads for other clinical interventions.

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