Outbreak alert on H5N1 Bird flu

H5N1 bird flu has now become widespread in many wild birds, eventually infecting many domestic poultry and farms. Recently, the Center for Disease Control, US, also highlighted the same situation. This bird flu is caused by an avian influenza Type A virus, which was thought to infect wild birds primarily later in all the other animals. This virus almost infected 100 different species of wild avians, including wild aquatic birds. This infection has over time been transmitted to many domestic poultry birds and infected a few mammals too. It is known that most of the infected birds behaved asymptomatic or had mild symptoms that were not initially identified. Based on the pathogenic behavior, many studies have classified it into two types: low-pathogenic avian influenza (LPAI) and high-pathogenic avian influenza (HPAI). In LPAI, there is no risk of death and only mild symptoms are observed, whereas in HPAI, severe symptoms and a high mortality rate are observed. The structure of the avian influenza virus is an orthomyxovirus, 100-mm spherical-shaped virion enveloped with a nucleocapsid consisting of 8 single-stranded strands of RNA. A matrix protein lines the inside of the envelope, while glycoprotein peplomers, rod-shaped haemagglutinin (HA), homotrimers of class I membrane glycoproteins, and mushroom-shaped neuraminidase (NA), tetramers of a class II membrane protein, line the outside. Based on the HA and NA variations on the virion molecule, there are a total of 15 HA subtypes and 9 NA subtypes, which leads to different combinations of both glycoproteins that form a specific strain. Apart from all other strains, there are five major strains that can infect humans: H5N1, H7N3, H7N7, H7N9, and H9N2. Comparatively with others, the H5N1 virulent strain is considered an extensive infective strain. During 2020, the number of HPAI virus subtypes started increasing, which showed a global outbreak in poultry and wild birds. Multiple HPAI A (H5N8) outbreaks began in 2020 in Europe, Africa, and Southeast Asia, and HPAI A (H5N1) and A (H5N5) virus outbreaks began in 2021 in Europe. In 2021, almost 54 countries reported the outbreak of Influenza A (H5N1) viruses, and many more strains with different N subunit moieties were found infecting animals.H5N1禽流感目前已在多种野生鸟类中广泛传播，最终波及大量家养家禽与养殖场。近期，美国疾病控制中心（Center for Disease Control, US）也通报了相同的疫情态势。这种禽流感由A型禽流感病毒引发，此前学界认为该病毒主要感染野生鸟类，后续才会感染其他动物。该病毒已感染近100种野生禽鸟，包括野生水禽。随着时间推移，该病毒已传播至众多家养家禽，还感染了少量哺乳动物。 已知多数受感染禽鸟无明显症状，或仅表现出轻微症状，初期未被察觉。根据致病特性，多项研究将其分为两类：低致病性禽流感（low-pathogenic avian influenza, LPAI）与高致病性禽流感（high-pathogenic avian influenza, HPAI）。低致病性禽流感不会导致禽只死亡，仅引发轻微症状；而高致病性禽流感则会造成严重症状与极高的死亡率。 禽流感病毒属于正黏病毒（orthomyxovirus），其病毒颗粒呈直径100纳米的球形，外有包膜，内部包含由8条单链RNA组成的核衣壳。包膜内侧衬有基质蛋白，外侧则分布有两类糖蛋白刺突：棒状的血凝素（haemagglutinin, HA）——属于I型膜蛋白同源三聚体，以及蘑菇状的神经氨酸酶（neuraminidase, NA）——属于II型膜蛋白四聚体。 根据病毒颗粒表面HA和NA的差异，目前已发现15种HA亚型与9种NA亚型，二者的不同组合可形成特定的毒株。除各类毒株外，共有5种主要毒株可感染人类：H5N1、H7N3、H7N7、H7N9及H9N2。与其他毒株相比，H5N1强毒株被认为是传染性极强的毒株。 2020年，高致病性禽流感病毒亚型的数量开始攀升，引发了家禽与野生鸟类的全球暴发疫情。2020年，欧洲、非洲及东南亚地区先后出现多起H5N8型高致病性禽流感病毒暴发事件；2021年，欧洲地区则出现了H5N1及H5N5型高致病性禽流感病毒的暴发疫情。2021年，全球共有近54个国家报告了甲型流感（H5N1）病毒的暴发疫情，同时还发现了多种携带不同N亚基的毒株感染动物。