Avian Influenza or Avian Flu is a dangerous contagious viral disease of poultry. The disease was used to be called 'fowl plague' due to its severe acute condition and high mortality. This disease in popularly known as bird flue. Chicken, turkeys, pigeons, quails, ducks, geese, guineefowls and other species of birds usually have been shown to be susceptible to this disease. It is assumed that waterfowls act as a reservoir or carrier but usually they are not affected with this disease. All age groups of chickens may be affected with this disease, but chicks and growing chickens are usually more affected. Moreover, it is a zoonotic disease. Human beings may be affected with this disease. When human beings are affected with avian influenza virus then it is usually called 'Bird flu'.
There are 3 types of influenza virus 'A', 'B' & 'C'. Avian Influenza Virus 'A' has been detected in flying birds, poultry, horse, swine, cat and human beings. Influenza viruses 'B' & 'C' only can be found in human beings. At present the influenza virus that has been found in the poultry of different countries belongs to type 'A'. It is an enveloped virus with single stranded segmented RNA genome belongs to group family Orthomyxoviridae. The surface of the virion is covered with two different types of spikes or projections namely haemagglutinin (HA) and neuraminidase (NA) proteins.There are 16 haemagglutinin subtypes (H1 to H16) and 9 neuraminidase subtypes (N1 to N9) of influenza 'A' viruses. At present the influenza virus which has been detected in the poultry of Bangladesh is type H5N1. This virus is usually very active in nature and dangerous for poultry and human beings. Depending upon the disease producing ability the avian influenza viruses have been divided into Low pahtogenic (LPAI) & Highly pathogenic (HPAI). The low pathogenic strains' may be circulated in the nature for certain period of time before they mutate into highly pathogenic strains. Therefore, irrespective of pathogenicity all AI viruses are considered as notifiable avian influenza (NAI) viruses. All types of birds can be infected with AI viruses that may or may not cause clinical disese.
The occurrence of Avian Influenza has been recorded worldwide. It was first recognized as a highly pathogenic viral disease of poultry in 1878 in Italy.
A limited number of outbreaks or epizootics have been reported worldwide since 1959. The majority occurred in Europe, Australia and America, only 5 epizootics resulted in significant spread to numerous farms, and only one was associated with spread to other countries. However, in mid-December 2003, an epizootic of HPAI started in the Far East, quickly spread in many countries of South-East Asia and subsequently across other parts of Asia, Europe and Africa. The disease has affected 66 countries of five continents across the globe. This epizootic was caused by H5N1 sub-type of AI virus. The disease had a disastrous effect on the poultry industry in Asia. More than 250 million birds has either died of the disease or been culled in Asia. In many Southeast Asian countries, HPAI has become endemic with continuous outbreaks; in others there has been resurgence of outbreaks. It is alarming to note that with the exception of Japan, all the previously affected counties in Asia also have had outbreaks in 2006. From 2003 to June 2008 the disease has been confirmed in 66 countries. Among these 386 human beings have been affected in 16 countries including Bangladesh and 243 were died.
Bangladesh first experienced HPAI in early 2007 and the National Reference Laboratory for Avian Influenza (NR-AI) at BLRI diagnosed and confirmed the presence of H5 sub-type virus on 15 March, 2007. Up to 15th' May 2008, NRL-AI has analyzed 399 samples, of which 286 were found H5 and 3 H9 positive. Some of those samples were re-examined and reconfirmed with the same result by International Reference Laboratories in UK and Thailand.
Upto 10 July 2007 the disease has been spread in 55 poultry farms of 17 districts. Among these H5N1 was confirmed in 52 farms and H9N2 in 3 farms. But the incidence began to decline from July to December. In January 2008, the number of incidence rapidly increased. Upto' 21st February 2008, H5 was detected in 72 samples. Upto June 2008, the disease has been spread in 287 farms of 128 upazillas and 14 metropolitan thanas of 47 districts. As a result 16,37,166 chickens of 507 farms were culled. From the reported information it has been observed that the incidence of outbreaks was high in Dhaka (Savar), Dinagpur, Nilphamari, Narsingdhi, Narayangong, Khulna, Borgura and Jessore.
In order to determine the trend, source, spread, phylogenetic relationship with virus isolates from home and abroad as well as socio-economic impact of AI in Bangladesh, a study was undertaken with the help of 17 scientists from different organizations and academic institutions including the department of Livestock Services, Bangladesh Agricultural University, Mymensingh, Chittagong Veterinary and Animal Sciences University, Sylhet Agricultural University and Food and Agriculture Organisation (FAO) of the United Nations with the key initiative of BLRI. The study consists of three parts: Phylogenetic, Epidemiological and Socio-economic analysis which commenced in February 2008 and was completed in June 2008.
The phylogenetic study was undertaken to determine the relationship of Bangladeshi isolates of H5N1-HPAI strains of virus with those of other countries across the world. For this, a total of 47 avian influenza virus samples were sent to FAO/OIE Reference laboratory, the Veterinary Laboratories Agency (VLA), Weybridge, UK. The VLA determined nucleotide sequence of the HA gene segment of 25 isolates which included 15 isolates from 2007 and 10 isolates from 2008. The analysis of gene sequence of the Isolates of 2007 and 2008 demonstrated 99.1 to 100% identical indicating a single introduction of the virus in Bangladesh. The gene sequences of 3 representative Bangladeshi isolates were compared with 32 sequences of 20 different countries. It clearly demonstrated that Bangladeshi isolates belong to the sub-clade 2.2 popularly known as Qinghai lineage or Euro-Asian-African lineage with highest similarities to those from Kuwait, Mongolia, Russia and Afganistan. Three Indian isolates from outbreaks in 2006 fall in the same sub-clade but they do not appear to be the closest neighbours (phylogenic) of Bangladeshi isolates. It may be mentioned that Bangladesh does not have any poultry trade link with Kuwait, Mongolia, Russia and Afganistan. So, the close similarities between the HPAI isolates of these countries with Bangladeshi isolates would suggest that migratory birds might be responsible for initial introduction of HPAI in Bangladesh.
In the epidemiological study, the role of possible risk factors in the spread of HPAI in the country was examined through descriptive as well as case control studies. It was revealed that in most cases mortality in backyard chickens preceded the outbreak in the commercial farm. Poor management and breaches in the biosecurity practices appeared to have significant role in the spread of HPAI. Significant association was observed between the incursion of HPAI in a farm and the risk factors like ad hoc farm workers, visit by feed and medicine suppliers, entry of vehicles within the farm premises, sharing of egg tray, other poultry equipment and vehicles. Most of the important risk factors had a strong link with the market chain. From the epidemiological study, it may be suggested that after introduction of HPAI in Bangladesh virus spread initially to native chickens and then to commercial farms though the market chain because of the breaches in bio-security practices.
Socio-economic analysis shows that the poultry industry of Bangladesh faced a terrific financial loss in 2007 and 2008 due to the incursion of avian influenza, which was estimated to be at Taka 3858 crore (38580 millions). The prices of broiler declined by about 28 per cent while the price of eggs decreased by 26.5 percent. More than one third of consumers refrained from consumption of broiler meat and eggs. As a result of market collapse, many farm owners were compelled to abandon poultry raising because of loss of capital. The affected farmers lost their business and many of them were reluctant to go back to the production system. Nevertheless, after the tension of severe outbreak of avian influenza was over, the consumers increased their consumption of poultry products and the poultry industry went back to the business except in some cases like those of layer farms and also small rural and urban broiler farms.
The study recommends that specific areas be earmarked as sanctuaries for migratory birds. It is recommended further that measures be taken so that native birds (Indigenous birds) do not intermingle with migratory ones. Early detection of HPAI virus, prompt stamping out, efficient surveillance and strict maintenance of bio-security measures both in farms and wet market should be ensured. A decision on vaccination strategy should be made as soon as possible. The owners of affected poultry farms should be rehabilitated promptly and provided with soft bank loans and other incentives. Timely sharing of information and effective coordination between animal and public health personnel be ensured. It is also necessary to share information and undertake studies on HPAI bi-laterally and regionally to minimize the risk of sustained endemic of avian influenza in poultry in the region. [Jahangir Alam]