Dr. N. Sukesan (Rtd. Sr. Vet. Surgeon)
Dr. Sobha S. S (99Batch)
The Flu Virus! One of the most dreadful lethal virus of the Universe. This Influenza virus is infectious to all species of animals and birds: Human Influenza, Swine Influenza, Equine Influenza, Avian Flu etc are the different names.
KNOW THE PAST
Bird flu was first identified in Italy more than 100 years ago. The flu outbreaks are not uncommon. Within the species, when affected they are not so dangerous. Usually they do not spread to humans. But occasionally happens with a sudden change in genotype of the virus which is unfamiliar to human immune system to resist. The history tells the fearful truths which are beyond imagination for the present generation.
|Year of outbreak||Name of outbreak||Casualties|
|1918-19||Spanish Flu Pandemic||40-50 million people died|
|1957||Asian flu||100,000 Americans died|
|1968||Hong Kong Flu||50 million affected of which 70,000 died in United states of America.|
First human case of bird flu infection occurred in May 1997, in a 3 year old boy who died of the disease. In 1999 a different strain of bird flu virus identified as A (H9N2) infected two people in Hong Kong. They recovered by treatments.
In 2003, one person died from bird flu virus A (H5N1) in Asia out of two reported infection. Both belonged to the same family from Hong Kong. In April 2003, a Dutch veterinarian working in a farm infected with bird flu virus of H7 strain died of pneumonia. The Vet did not take any medication against bird flu or human flu. Again in 2003 bird flu virus A (H7N7) infected 80 people in Netherlands killing one.
During this outbreak, there were signs of human–human transmission of the disease – a rare event. So far in 2004 WHO reports 23 casualties of H5N1 outbreaks in Asia. Eight of these in Thailand and 15 in Vietnam. During 2003-05 period the A (H5N1) strain of bird flu virus infected 117 people out of which 64 died.
Europeans are getting worried because the Avian flu is moving westward (through Asia) carried by flocks of waterfowl. UPDATE- 13th October 2005.
Outbreak of bird flu has been reported from Cambodia, China, Indonesia, Japan, Laos, and South Korea during the last few years. None of these infected humans.
During 1997, 1.5 million poultry were culled and destroyed in Hong Kong within 3 days, which would have averted a human pandemic. WHO reports flu pandemics can be expected to occur three or four times every 100 years. Experts say ‘another Influenza pandemic is inevitable and possibly imminent ‘.
Bird flu virus is an RNA virus, which resembles a short rod studded with two kinds of protein spikes such as the Haemagglutinin (H) and Neuraminidase (N).
STRUCTURE OF FLU VIRUS
There are 3 general types of flu viruses distinguished with their capsid (inner membrane) proteins as Type A, B and C. Type A flu virus cause most serious flu epidemics in humans, other mammals and birds. Type B and C viruses are restricted to humans and cause only mild infections.
More than 20 strains of Influenza A viruses are identified, based on their difference in protein spikes. There are 13 distinct H subtypes and 9 distinct N subtypes each of which require a different vaccine to protect against infection. Type A virus that caused the 1968 Hong Kong flu epidemic has type 3 H protein molecules and Type 2 N protein molecules and is called A (H3N2). The bird flu virus, which passed to humans directly from bird, is with Type 5 H protein and type 1 N protein. Hence, it is called A (H5N1). This is identified in chicken in 1961.
Haemagglutinin (H) protein aids the virus in gaining access to the host cell Interior. The Neuraminidase (N) help the daughter virus break free of host cell once the virus Replication is completed. The H molecule has three strands fixed on the surface of virus particle like a tripod with a club like projection on the top. Each has ‘hotspots‘ that display an unusual tendency to change as a result of mutation of the virus RNA during imprecise replication sometimes. These segments of H molecule functions as targets against which the body’s antibodies are directed. New strains of flu virus arise not through mutation but through recombination of genes. This may lead to novel combinations of H and N spikes unrecognizable by the human antibodies which are specific for the old configuration.
Viral recombination of this type seem to have been responsible for the past three major flu pandemics. Public health officials remain concerned that the genes of Influenza A (H5N1) strain virus which is slightly infectious to human beings could yet mix with a human strain to create a new strain that could spread widely in human population. The experts say this could lead to the first flu pandemic of the 21st century.
Influenza viruses are generally thought to evolve via shifting and drifting. Shifting occurs when viruses swap genes and drifting was thought to be due to steady accumulation of mutations.
US Centre for Disease Control points out two chances of greater threats to Humans with the bird flu virus: First, the genetic material of the bird flu virus could evolve by recombination of genes giving rise to new more virulent strains by changing combinations of H and N subtype molecules. (i.e., H1, H2……H 13 and N1, N2…..N9 combinations).
Second chance is the bird flu virus could combine its genetic material with other Influenza viruses that already infect humans. The chances are more in cases where humans are in close contact with poultry.
Both avian and human influenza viruses can infect pigs (Swine influenza). Swine Influenza outbreak was there in USA just after the human flu pandemic of 1918-19. In pigs this create a genetic ‘ melting pot ‘ in which different strains of viruses can swap their genes and acquire each others properties to evolve a new strain.
Rapidly evolving viruses simply recycle old mutations via recombination. This is more efficient. This happens in most type of viruses including influenza, HIV, SARS, Foot and Mouth Disease virus of cattle etc. This necessitates vaccines to be prepared before the viruses emerge.
Fifteen subtypes of influenza viruses are known to infect birds thus providing an extensive reservoir of influenza viruses potentially circulating in bird population.
So far the identified flu viruses from previous outbreaks are Type A(H1N1) ,H2N2, H3N2, H5N1, H9N2 H7N7.
Bird flu or avian flu is thus another zoonotic disease, which can be transmitted to Human beings from birds and animals and vice versa.
ROUTES OF FLU VIRUS
Flu epidemics are often in Asia. New strains originate in the Far East. Most common host for influenza viruses are duck, chickens and pigs which in Asia live in close proximity to each other and to humans. Pigs are subjected to infection by both human and bird strain. Also may be infected with multiple strains. This creates conditions favoring genetic recombination between strains.
The Hong Kong Flu arose from recombination between A (H3N8) from ducks and A(H2N2) from humans. The strain produced was A (H3N2) then passed back to humans creating an epidemic because human population has never experienced that H3N2 combination A before.
EVOLUTION OF NEW STRAINS
Disease can be transmitted to humans from wild birds and chickens. Unless there is strain variation the bird flu virus does not appear to spread easily from person to person. Infected birds spread the virus through saliva, faeces and nasal secretions. Wild and migrating birds act as carriers and spread the disease to very far away places within a short span of time. So far, humans with close contact with sick birds only caught infection with bird flu. There is no evidence till now that the virus is being spread through eating chicken products. Heat kills the virus. Chicken should be cooked thoroughly at temperature at least 70 degree C (158 degree F). People should wash their hands after handling poultry and ensure that poultry carcasses and waste do not contaminate other objects. The Flu virus can survive up to 4 days at 22 degree C (71 degree F) and more than 30 days at 0 degree C (32 degree F). If frozen, it can survive indefinitely.
Affected birds show
Loss of appetite
Excessive thirst and edema
If the strain is virulent mortality rate can range between 50-100 %.
In human beings, symptoms are
Acute respiratory distress and
ANTI VIRAL DRUGS AGAINST BIRD FLU
Recent outbreaks in Asia have been found to be resistant to the oldest and cheapest anti flu drugs Rimantidine and Amantidine. Australian Researchers say Relenza (Zanamivir) and Tamiflu (Oseltamivir) are effective treatments against flu. Anti viral drugs are expensive and in limited supply.
THE DISEASE CAN BE PREVENTED BY
Culling of suspected poultry population and putting infected farms in quarantine .Another method is vaccination of birds against flu virus but this is controversial. In efficient vaccines may allow the virus to replicate without the animal showing any symptoms, a phenomena known as ‘silent epidemic’, which can be dangerous. However, Indonesia has launched a poultry vaccination drive and Thailand is considering the same course of action.
WHO recommends that people at high risk of exposure with bird flu virus, mainly people involving culling operations should be vaccinated with most recent human flu vaccine available. Situations for simultaneous infection with bird flu and human flu virus should be avoided. Wild migrating birds play a role in spreading the bird flu. However, UN Food and Agricultural Organization has recommended that culling wild bird population is not required.
Movement of live poultry should also be strictly regulated. Strict surveillance and monitoring by the concerned state departments is essential.
People working in poultry farms and slaughterhouses, swimming in contaminated rivers and ponds, playing in an area where carcasses were buried, staying near poultry processing plants etc can lead to infection. Disposal of broiler chicken waste in open places must be prohibited. Suitable protective measures and avoiding predisposing conditions are advised. Controlled vast slaughter of suspected birds in an outbreak will contain the virus that spread by air, water and soil.
The arrival of poultry from other states must be inspected thoroughly by qualified persons supporting with clinical laboratory tests. Disease Investigation Wing of the Animal Husbandry Department and Veterinary Colleges should be equipped well to detect and control the bird flu virus infection. Poultry and pig farms must be frequently inspected to detect flu.
Even though the warm climate and cooking habits protect us from bird flu, the flu virus can at any time change their genotype to adapt to new conditions and can lead to a pandemic.
Courtesy: Dr. Jamie Love, Avian Flu, www.yahoo.com; Virus Pictures-Russel Kightely Media: Scientific Illustration.