Bird Flu

In fact, one strain of bird flu, the H5N1 virus, is endemic to much of Asia and has now spread into parts of Europe and Africa also. Avian H5N1 infections have recently killed poultry and other birds in a number of countries. Strains of avian H5N1 influenza may infect birds, pigs, tigers and humans also. Although the symptoms vary, the virulent strains can cause death within a few days.

Human H5N1 influenza infection was first recognized in 1997 when this virus infected 18 people in Hong Kong, causing 6 deaths. Close contact with infected poultry is known to be the primary source for human infection. Though rare, there have been isolated reports of human-to-human transmission of the virus also.

Avian influenza in birds

Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus. The disease occurs worldwide. While almost all birds are thought to be prone to infection with bird flu viruses, many wild bird species carry these viruses with no perceptible signs of harm.

Other bird species, including domestic poultry, develop the disease when infected with avian influenza viruses. These viruses cause two distinctly different forms of the disease – one common and mild and the other rare and highly lethal. In its mild form, bird flu shows symptoms like ruffled feathers, reduced egg production, or mild effects on the respiratory system. Sometimes the mild outbreaks can go completely undetected.

In comparison, the highly pathogenic form is characterized by sudden onset of severe disease, rapid contagion, and a mortality rate that can approach 100% within 48 hours. Easily noticeable, in this form of the disease, the virus affects the respiratory tract as well as invades the multiple organs and tissues. This results in massive internal hemorrhaging. In layman’s terms this form of the disease is also called “chicken Ebola”.

All subtypes of influenza viruses are known to infect wild waterfowl, thus providing an extensive reservoir of influenza viruses perpetually circulating in bird populations. In wild birds, routine testing will nearly always find some influenza viruses. The vast majority of these viruses cause no harm.

So far, all outbreaks of the highly pathogenic form of bird flu have been caused by viruses of the H5 and H7 subtypes. The highly pathogenic viruses possess a tell-tale genetic “trade mark,” that is, a distinctive set of basic amino acids. This helps distinguish them from all other bird flu viruses. Latest research has shown that H5 and H7 viruses of low pathogenicity can also mutate into highly pathogenic viruses.

Being highly contagious among poultry, avian influenza viruses are readily transmitted. Highly pathogenic viruses can survive for long periods in the environment, especially when temperatures are low.

In order o terminate the spread of the highly pathogenic disease, the most important control measures are rapid culling of all infected or exposed birds, proper disposal of carcasses, quarantining, rigorous disinfection of farms and rigorous restrictions on poultry movement. Vaccination of poultry in a high-risk area can be used as a supplementary emergency measure.

Apart from being difficult to control, outbreaks in backyard flocks are associated with a heightened risk of human exposure and infection. Slaughtering, defeathering, butchering, and preparation of poultry meat for cooking carries a high risk of exposure to the virus.

Should the new role of migratory birds be scientifically confirmed, it will mark a change in a long-standing stable relationship between the H5N1 virus and its natural wild-bird reservoir.

Human infection with avian influenza viruses

Genetic studies have confirmed that the influenza A virus H5N1 can mutate rapidly. Of the hundreds of strains of avian influenza A viruses, only four are known to have caused human infections: H5N1, H7N3, H7N7, and H9N2. In general, human infection with these viruses has resulted in mild symptoms and very little severe illness, with one notable exception: the highly pathogenic H5N1 virus. This virus is of greatest present concern for human health.

The H5N1 virus has caused by far the greatest number of human cases of very severe disease and the greatest number of deaths. Now what is of greater concern is the risk that the H5N1 virus, if given enough opportunities, will develop the characteristics it needs to start the next influenza pandemic. This has not happened thus far because the virus has yet to meet the prerequisite characteristic that is the ability to spread efficiently and sustainably among humans.

The virus can improve its transmissibility among humans via two principal mechanisms. The first is through the method of “reassortment”, in which the genetic material is exchanged between human and avian viruses. The second mechanism is a more gradual process of adaptive mutation, whereby the capability of the virus to bind to human cells increases during subsequent infections of humans. Understanding this adaptive mutation will probably give the world some time to take defensive action, if detected sufficiently early.

During the first documented outbreak of human infections with H5N1, which occurred in Hong Kong in 1997, 18 human cases coincided with an outbreak of highly pathogenic avian influenza, caused by a virtually identical virus found in poultry farms and live markets. It was determined that direct contact with diseased poultry was the source of infection. Human infections ceased following the rapid destruction of poultry. This drastic action, it is believed, may have averted an influenza pandemic.

All evidence to date indicates that close contact with dead or sick birds is the principal source of human infection with the H5N1 virus. This is worsened by employing risky behaviour like slaughtering, defeathering, butchering and preparing infected birds for consumption. Exposure to chicken faeces and use of infected water bodies can also be a major source of infection.

At present, H5N1 avian influenza virus remains largely a disease of birds. Despite the infection of tens of millions of poultry over large geographical areas since mid-2003, fewer than 200 human cases have been laboratory confirmed. For reasons unknown, very few cases have been detected in presumed high-risk groups, such as commercial poultry workers, workers at live poultry markets, cullers, veterinarians, and health staff caring for patients without adequate protective equipment.

Treatment and vaccination for avian influenza virus

Bird flu symptoms are very similar to normal human influenza and the disease cannot be diagnosed from bird flu symptoms alone. An oral or throat swab is required to check for signs of the avian influenza virus. Human symptoms of bird flu include fever, cough, sore throat, muscle aches, headache and conjunctivitis. Avian influenza ultimately causes breathing difficulties, pneumonia and is potentially fatal.

There are two ways to protect people from the effects of a disease like avian flu: treat the symptoms of the disease after infection, or prevent infection in the first place. In the case of the flu, the prevention takes the form of bird flu vaccine, and the treatment the form of antiviral drugs.

Bird flu vaccine causes the production of antibodies against a virus prior to being infected. When an immunized person is exposed to the flu virus, the antibodies generated in response to the vaccine readily attack the virus as it enters the body and prevents illness.

But, as avian flu is not a disease that is generally passed directly from one person to another, a bird flu vaccine would be of limited use. However, getting a flu shot keeps you from getting sick.

On the other hand, antiviral drugs work by reducing the ability of the virus to replicate, once a person is already infected. Antiviral drugs like oseltamivir and zanamivir work to block the flu virus from escaping an infected cell and spreading further. This helps to ease flu symptoms and reduce the duration of the illness.

But, the antiviral drug must be taken within 48 hours of the onset of disease. However, there is growing concern now that extensive use of drugs can result in gene mutation of the viruses causing resistance to existing medication.

There are four antiviral drugs on the market currently – oseltamivir and zanamivir which are neuraminidase inhibitors – and amantadine and rimantadine, which are called adamantine drugs. Both amantadine and rimantadine are older drugs that new influenzas are more likely to have developed a resistance to.

Tamiflu to treat bird flu Evidence suggests that some antiviral drugs, notably oseltamivir, can reduce the duration of viral replication and improve prospects of survival, provided this bird flu treatment is administered within 48 hours following symptom onset. However, clinical data on the effectiveness of oseltamivir is limited. In suspected cases, Tamiflu should be prescribed as bird flu treatment as soon as possible to maximize its therapeutic benefits. However, given the significant mortality currently associated with H5N1 infection and evidence of prolonged viral replication in this disease, administration of the drug should also be considered in patients presenting later in the course of illness. The recommended dose of oseltamivir for bird flu treatment, in adults and adolescents 13 years of age and older, is 150mg per day, given as 75mg twice a day for five days. Oseltamivir is not indicated for the treatment of children younger than one year of age.

Oseltamivir can be used as a treatment when someone is sick and as a preventive to keep people who have likely been infected, from getting sick.

If the duration of viral replication is prolonged, clinicians should consider increasing the duration of treatment from seven to ten days. In cases of severe infection, clinicians may need to consider increasing the recommended daily dose or the duration of treatment. However it must be noted that doses above 300mg per day are associated with increased side effects.