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About Avian Flu
Avian influenza, also referred to as avian flu or bird flu, is a disease that affects birds, is caused by influenzavirus A, and is characterized primarily by respiratory problems but with a wide range of other symptoms. Much like human flu, there are many different strains of avian flu due to the different strains of virus causing the flu. The danger and lethality of avian flu ranges wildly due to the different strains of virus, from causing mild symptoms and killing very few of its victims to causing severe respiratory difficulties and killing a very high percentage of infected birds.
While avian flu generally infects birds, it is has been known to jump the species barrier from birds to humans. Thus, avian flu poses the threat of a deadly pandemic due to the fact that humansÕ immune systems are generally not accustomed to strains of avian flu, and thus are at great risk should that particular strain of flu become dangerous. In fact, history and research has shown that avian flu has been the cause of dozens of pandemics, including one of the most deadly pandemics in recent history, the Spanish Flu Pandemic of 1918.
Influenzavirus A
Influenzavirus A, first isolated and identified in Italy in 1878, is a genus of the family of viruses known as Orthomyxoviridae. There is only one species of this virus Ð ÒInfluenza A virusÓ. Type B influenzaviruses affect mainly humans and can result in illness ranging from mild to lethal symptoms. Type C influenzaviruses cause mild illness in humans.
In virus terminology, the genus is the highest group. After the genus comes the family, then the type, then the subtype, and then the strain. Influenzavirus A causes avian influenza. Avian flu is hosted by birds, but may infect several species of mammals at a low transmission rate, although after jumping the species barrier, it may be just as dangerous as other types of flu. Strains of avian flu are generally categorized as low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI), according to their infection rates and danger.
There are 16 known groups of influenzavirus A, all of which originated in water fowl. The more stable forms of the virus are nonpathogenic in those water fowl, so those fowl constitute the silent reservoir in which those strains of the virus travel as a vector. A silent reservoir is a species that may play host to a virus, while not being strongly affected in a negative way, in that the species may not become sick or display severe symptoms of sickness.
Human Pathogenicity
Influenza viruses usually target a single species, due to a so-called species barrier. This barrier is due to the fact that each strain of virus relies upon certain proteins on its coat to invade and commandeer a host cell, and that cells vary widely from species to species, so some proteins that work on one species will not necessarily work on the next. However, some influenzavirus A strains have managed to jump the species barrier by a number of genetic mutations that allow them to infect not only birds, but also swine and humans.
Four major subtypes of the virus in current circulation - H5N1, H7N3, H7N7, and H9N2 Ð are capable of human infection. Typically, human infection from these strains is followed by mild symptoms and is generally not lethally threatening. However, H5 and H7 strains have historically been capable of high pathogenicity among birds and humans. Currently, the most dangerous of these strains is the H5N1 virus, which does cause very severe symptoms and poses the threat of beginning a truly deadly epidemic.
Symptoms
Note: Symptoms of avian flu vary depending on the virus subtype
Human influenza-like symptoms: fever, cough, sore throat, muscle aches
Eye infections (conjunctivitis)
Viral pneumonia
Respiratory diseases like acute respiratory distress (ARDs) and severe acute respiratory distress (SARDs)
Treatment
Studies show that some prescription medicines for treatment of human influenza viruses may be effective in treatment of avian flu. However, this has not been satisfactorily confirmed, given that studies on the efficacy of these medications are only observational, and that most patients are treated well into the mid- to late stages of the disease. Four influenza antivirals are approved by the USÕs Food and Drug Administration for influenza treatment: amantadine, rimantadine, oseltamivir, and zanamivir. Isolated strains from the 2004 outbreaks in Asia showed that H5N1 is resistant to amantadine and rimantadine.
Influenzavirus A is particularly adept at mutating and recombining genomes from multiple strains. These rapid genetic changes lead to an ability to evolve from selective pressure quickly, which can render vaccines and drugs useless. Thus, the Center for Disease Control and the World Health Organization have both issued warnings to strictly regulate the dosages of medicine and to begin medication as quickly as possible.
Low pathogenic avian influenza (LPAI) spreads primarily from bird to bird among waterfowl, and is endemic in that population. However, high pathogenic avian influenza (HPAI) transmits in both wild and domesticated bird populations. In fact, HPAI is more active among domesticated bird populations, due to the fact that these birds are kept in close quarters together, share food and water sources, and are subject to unsanitary conditions, all of which aid in transmission rates. HPAI is particularly noticeable among domesticated birds, largely because the death rate is so high that hundreds of thousands of birds will die in a very short time span. On an individual level, transmission occurs primarily through sharing of food and water with infected birds, physical contact with infected birds, and contact with feces from infected birds. Of note is the fact that the birds do not necessarily have to display symptoms of avian flu to be capable of transmitting the influenzavirus; they can be silent carriers of the virus.
Up until recently, scientists had largely assumed that one wild bird would provide the flu to infect a flock of domesticated birds, which would then retransmit the flu to another flock when transported commercially to meat distributers or other bird farms. However, as of recent years, researchers began to notice that genetically similiar strains of the H5N1 virus would appear almost simultaneously in geographically separate areas of the world, between which there was no commercial transportation of birds. In one well-noted example, an outbreak of avian flu, caused by H5N1, began among migratory birds at Qinghai Lake nature reserve in China. In the following months, the same strain of virus was tracked along migratory routes, going as far as Turkey, where two people died from the avian flu. This discovery of migratory carriers was particularly surprising due to the fact that this implied that H5N1 was being carried by wild, migratory waterfowl and distributed by interaction between those wild birds and domesticated poultry that the wild birds encountered during their migration.
Threat of Pandemic
Current Spread of the Avian Flu
According to the CDC, pandemic avian flu spread occurs among human due to the fulfillment of three
conditions:
- “A new subtype of influenza A virus is introduced into the human population.”
- “The virus causes serious illness in humans.”
- “The virus can spread easily from person to person in a sustained manner.”
Given that there is a natural reservoir for avian flu – all water fowl – it is unrealistic and dangerous to attempt a complete eradication of the flu altogether, as has been proposed in previous decades. In fact, such an effort would most likely result in an accelerated rate of mutation of IVA, resulting in novel strains that would be more likely to be able to cross the species barrier quickly and sustain itself among other species populations. Therefore, the fight against avian flu must be led by countermeasures, rather than more active approaches. Prevention and speedy and proper management of outbreaks must be stressed above all else.