|Credit J. Vet. Sci. 2017; 18(S1): 269-280|
Although wild and migratory (particularly aquatic) birds are the primary reservoir for LPAI (Low Pathogenic Avian Influenza) viruses - until 1996, highly pathogenic (HPAI) viruses had only been detected on poultry farms.
Mutated HPAI viruses would sometimes appear on farms after LPAI H7 and H5 viruses circulated unchecked among gallinacious birds (chickens, turkeys, quail, etc.) for several weeks. Hence the OIE regulation to quarantine, cull, and report any H5 or H7 outbreak, even if its pathogenicity is low.Also called `fowl plague', HPAI viruses were never detected in wild or migratory birds until 1996, when a previously unknown H5N1 virus (clade 0) was detected in a domestic goose in Guangdong Province, China (A/Goose/Guangdong/1/1996 (GsGD)).
The following year, H5N1 appeared in Hong Kong's chickens, and sparked the first human epidemic, hospitalizing 18 and killing six (see Outbreak of avian influenza A(H5N1) virus infection in Hong Kong in 1997). A bold decision to cull every chicken in Hong Kong, and the immediate isolation of cases, are credited with stopping this first outbreak.
The virus appeared to go to ground for five years, but re-emerged - albeit as a new clade - in Hong Kong when a two members of a family fell ill (one fatally) after returning from Fujian Province on the Mainland. A third relative died (untested) on the mainland.
Within a year, variants of this H5N1 virus caused outbreaks in poultry, wild birds, humans, and even captive animals from Beijing and Korea to Thailand and Vietnam. Two years later, H5N1 had made it out of Asia and was impacting Europe, Africa, and the Middle East.
And as H5N1 expanded geographically, it expanded genetically as well (see chart below), forming new clades, subclades, and genotypes. Many of these variants were less `fit' than their competition, and died out along the way, but others thrived and survived, and became the dominant clade in different parts of the world.
The most recent WHO report finds H5N1 clade 22.214.171.124 viruses circulating in Egypt, clade 126.96.36.199c viruses in China, West Africa, and Southeast Asia, and clade 188.8.131.52a viruses in Bangladesh, Bhutan and India.
While there were other H5 viruses in circulation (primarily in China) - including HPAI H5N2, H5N5, and H5N8 - until 2014, H5N1 was the HPAI H5 virus of greatest concern. That year two new clade 184.108.40.206. subtypes emerged; H5N8 in South Korea, and H5N6 in China/Laos/Vietnam.H5N8 is the first HPAI H5 virus to make the jump to North America sparking the United State's largest avian flu outbreak, and caused Europe's greatest epizootic on record last winter. H5N6 has shown a limited ability to infect - and kill - humans, and has expanded its range in eastern Asia (Japan, South Korea, Philippines, Taiwan) over the past year.
Not only have both have proven themselves to be unusually well adapted for carriage by wild and migratory birds, they are both evolving at a rapid rate. All of which brings us to an excellent, and highly detailed look at this growing threat.I've included a link, the abstract, and few small excerpts, but you'll definitely want to download, read, and save the full report.
J. Vet. Sci. 2017; 18(S1): 269-280
Evolution, global spread, and pathogenicity of highly pathogenic avian influenza H5Nx clade 220.127.116.11
Dong-Hun Lee1, Kateri Bertran1, Jung-Hoon Kwon2, David E. Swayne1,*
Received: June 30, 2017; Accepted: July 22, 2017; Published online: August 31, 2017.
Novel subtypes of Asian-origin (Goose/Guangdong lineage) H5 highly pathogenic avian influenza (HPAI) viruses belonging to clade 2.3.4, such as H5N2, H5N5, H5N6, and H5N8, have been identified in China since 2008 and have since evolved into four genetically distinct clade 18.104.22.168 groups (A–D).
Since 2014, HPAI clade 22.214.171.124 viruses have spread rapidly via migratory wild aquatic birds and have evolved through reassortment with prevailing local low pathogenicity avian influenza viruses. Group A H5N8 viruses and its reassortant viruses caused outbreaks in wide geographic regions (Asia, Europe, and North America) during 2014–2015.
Novel reassortant Group B H5N8 viruses caused outbreaks in Asia, Europe, and Africa during 2016–2017. Novel reassortant Group C H5N6 viruses caused outbreaks in Korea and Japan during the 2016–2017 winter season. Group D H5N6 viruses caused outbreaks in China and Vietnam. A wide range of avian species, including wild and domestic waterfowl, domestic poultry, and even zoo birds, seem to be permissive for infection by and/or transmission of clade 126.96.36.199 HPAI viruses. Further, compared to previous H5N1 HPAI viruses, these reassortant viruses show altered pathogenicity in birds.
In this review, we discuss the evolution, global spread, and pathogenicity of H5 clade 188.8.131.52 HPAI viruses.
Since 2014, HPAI clade 184.108.40.206 viruses have spread rapidly and globally through wild birds and have evolved through reassortment with prevailing local LPAI viruses. A wide range of avian species, including wild and domestic waterfowl, domestic poultry, and even zoo birds, appear to be permissive for infection by and/or transmission of clade 220.127.116.11 viruses.
In general, clade 18.104.22.168-infected birds exhibit clinical disease, mortality, and pathological features that are indicative of a typical HPAI virus infection, although several studies have pointed out that these H5 reassortants have reduced virulence compared to the parental Gs/GD H5N1 virus. Interestingly, clade 22.214.171.124-infected waterfowl may either succumb to infection or act as asymptomatic spreaders, even with viruses that are optimally adapted to domestic poultry.
The wide host range of clade 126.96.36.199 viruses, which can enable unrecognized virus spread, might explain the successful recent globalization of this lineage. The emergence of novel reassortant viruses and their circulation among bird populations are of great concern because of the potential for virus dissemination by intercontinental wild bird migration.
Therefore, in places where HPAI has become endemic in domestic poultry, explicit efforts should be taken to eradicate HPAI by using a combination of preventive and control measures that include stamping out, appropriate vaccination with post-vaccination monitoring, movement control, and enhanced biosecurity. In addition, intensified surveillance and further characterization of HPAI viruses can provide insight into preventing future outbreaks and assessing zoonotic potential.
Keywords: epidemiology, influenza in birds, poultry, transmission, virulenceAfter the dramatic changes in range and behavior that came with both H5N6 and H5N8 last winter, there is a great deal of interest in what might return this fall. While no study can predict that with any accuracy, this report does provides us with a detailed benchmark for comparison.
© 2017 The Korean Society of Veterinary Science.
It also reminds us how quickly things can change, particularly when dealing with a rapidly evolving influenza virus.