Avian fluBy Dr Ruth Watkins 28 2 06
What is the nature of the highly pathogenic avian influenza A virus H5N1 that has travelled from China across Asia to India Africa and Europe, and if it is not already here, to Britainís borders?
Previously when a highly pathogenic avian influenza virus (HPAI) outbreak occurred in domestic poultry it vanished once the infection was eliminated from domestic flocks. It appears not to have been sustained in wild bird populations - such was the case with the H7N7 outbreak in Dutch poultry in 2003.
An HPAI virus may arise in domestic poultry from a low pathogenicity virus that has been circulating without symptoms, as in Pennsylvania in 1983/4; intensively reared poultry appear to select for pathogenic avian influenza strains.
The current lineage of the current avian HPAI H5N1 virus has been continuously present in China in domestic poultry for a decade. Several different genetic variants have emerged over this time in China, the epicentre of this epidemic, and they have spread locally first recorded in Hong Kong in 1997, and then to SE Asia where for example on 3 occasions a different variant of avian HPAI H5N1 has appeared in Vietnam.
The genetic variant of HPAI H5N1 now in Europe, let us call it Zq, was first identified in spring 2005 at Lake Qinghai in Northern China when some 2000 bar-headed geese and other birds died of it.
In fact this virus is thought to have originated some 1000 miles south. The persistence and spread of this HPAI H5N1 virus strain, Zq, is an extraordinary phenomenon without precedent.
Whilst spread in China and SE Asia, and even into the Middle East, India and Africa has been very largely due to human intervention moving infected poultry and its products, it has also been aided by the FAO approved practice of integrated farming when poultry manure is used to feed farmed fish- in Lake Qinghai also. This allows the spread of the virus to water birds, ducks are particularly significant. Avian influenza viruses infect the gut of ducks, and are spread by the faeco-oral route amongst them. Virus present in the water can be swallowed and establish an enteric infection in a duck. Unlike other bird species, ducks may shed avian influenza viruses for months, remaining persistently infected without producing antibody and remaining well- they are carriers shedding infectious virus from the gut. Other aquatic birds are also considered to be the reservoir host for avian influenza viruses e.g. terns. It is clear that the variant of HPAI H5N1 from Qinghai, Zq, has been spread across Western Europe by wild birds. This has given rise to a trail of dead swans across Europe, as well as other bird species at wetland areas.
Even ducks may become ill with the HPAI H5N1 infection, and have been found dead, infected with the Zq strain. Other wild birds, passerines, geese, quail, pheasants, raptors etc are more likely to fall ill and die as a result of infection than ducks, as are domestic chickens , turkeys etc. The HPAI H5N1 virus gives rise to an acute infection via the respiratory tract in these birds with an incubation period before illness of a few days after exposure. The virus is virulent, spreads outside the respiratory tract and is able to multiply in any tissue including the brain to high titre. The bird usually dies after a short illness. If there is recovery it is accompanied by a good antibody response, the antibody is protective. The virus infection is acute and cleared by the immune response.
The HPAI virus has at least two genetic changes, as compared with the low pathogenicity viruses that are the usual form of avian influenza found in wild birds. The changes allow the virus to replicate to very high levels in infected cells and in all tissues. There is a short sequence of basic amino acids at the cleavage site of the haemagglutinin in HPAI viruses allowing cleavage by protease enzymes present in all tissues. In the low pathogenicity viruses this process is confined to the respiratory tract as the haemagglutinin can only be cleaved by a respiratory tissue specific protease as it has a different sequence of amino acids at the cleavage site. This is an important difference as the newly formed virus can only infect a cell if the haemagglutinin protein has been cleaved to produce a spike that can penetrate the cell membrane to allow entry.
The HPAI form of avian influenza viruses is highly virulent and infectious. By the time one domestic bird in a flock is sick or dead many others will already be infected and incubating the disease. They will also be shedding virus even before they are ill. The mortality rate can be as high as 100%. Thus the infected flock will be decimated or wiped out even if it is not culled.
HPAI virus is shed in every secretion and is in feathers, flesh, manure and if not in eggs then upon their surface. Infection can be spread indirectly between farms or poultry houses by contamination of hands, feet, tyres, straw, feed, water and by infected wild birds or by rodents, infected or contaminated. Other animals can become infected such as felines and pigs. Biosecurity for domestic poultry will be hard to maintain for years in the face of the persistence of this virus in the wild bird population or its repeated reintroduction into wild birds. Amplification of the risk of infection to domestic flocks or wild birds occurs with every outbreak of HPAI in domestic poultry when enormous quantities of the virus are generated. Safe disposal of infected manure is important. A laboratory level of cleanliness is required and may be impossible to maintain when the virus is present for many years in the environment where wild birds act as a reservoir. The source of the outbreak in a commercial turkey farm in France, where all the birds were inside and subject to biosecurity, is being investigated and it is thought it may have been introduced by infected wild bird droppings on straw or a shoe. In Nigeria, Egypt and India infection occurred initially in commercial intensive poultry production units, but was very likely introduced in those countries by the international trade in eggs, birds or products incorporating, or contaminated, with poultry manure. It is said avian influenza virus can remain infectious for up to 2 weeks dried onto fomites and for several months in infected meat and eggs at 4oC.
The influenza virus is an enveloped virus with two viral proteins embedded in the surface, the haemagglutinin, H, and the neuraminidase, N. The H protein binds to cell receptors in order for the virus to gain entrance to and infect a cell (see above), and the N protein acts as an enzyme releasing newly formed budding viruses from the infected cell surface. There are at least 15 different H types and 9 different N types of influenza A viruses and all are found in aquatic birds in various combinations e.g. H5N1, H5N2 etc. Influenza A viruses have also adapted to man, pigs, seals and horses but these species-adapted viruses have limited H and N types e.g. horses have H7N7 (equi 1) and H3N8 (equi 2).
Inside the loose envelope of the influenza virus, the 8 RNA segments that comprise the genome are found. Each codes for a different viral protein, e.g. one for H and one for N and one for the polymerase protein concerned with replication and 5 others coding for proteins concerned with either structure or replication.
The segmented genome allows reassortment of the 8 genes. reassortment has been documented. It occurs when the same cell is infected with two different viruses, a rare event but one that can occur naturally when there is a simultaneous infection with two different influenza viruses e.g. a human is infected with a human influenza A virus and an avian virus. In most instances the reassorted viruses are not shed, or are not competent, or never get the opportunity to establish a chain of infection. However the pandemics in humans of H2N2 in 1957 and H3N2 in 1968 did arise from newly reassorted viruses, with one or more avian genes, from China. The new H genes were derived from avian viruses. These two viruses each caused a pandemic in humans because no one had immunity to H2 or H3 as these haemagglutinin types were not found in the circulating human influenza A virus at the time of their introduction. For mysterious reasons H3 replaced H2 in 1968. In 1977 H1N1 reappeared, a laboratory escapee from the 1950s, and currently H3N2 and H1N1 co-circulate in humans.
The influenza pandemic in humans that followed the First World War, the 1918 and 1919 pandemic, was caused by a virus that had all 8 gene segments derived from an avian H1N1 influenza A virus. A number of point mutations in several of the avian genes had to have taken place to humanise the avian virus sufficiently to pass from human to human. That virus also infected pigs and became host adapted; it still circulates amongst them albeit now distantly related to its ancestor. The H1N1 pig adapted virus is present worldwide in pigs.
There is concern that either of these two courses, reassortment or a number of point mutations, might change the avian HPAI H5N1 virus enabling it to pass efficiently between humans. This has not happened yet. However the emergence of a new human pandemic virus has never been observed under close scrutiny before but has simply appeared as a fait accompli. Humans have been infected by the current avian HPAI H5N1 strains by close contact with an infected bird. The mortality may not be quite as high as 50% if cases present early for treatment and intensive care. Either living with the sick birds or handling them, particularly preparing them for eating such as plucking gutting and so on, has given rise to human infections. There have been clusters but most are thought to be from a point source, an infected bird, and there have been very few instances of spread between humans. Both anti-virals and vaccines are being stockpiled and developed in case of the emergence of a human pandemic strain of H5N1 with, it is feared, virulence similar to that of the 1918-1919 influenza pandemic.
It is not appropriate to treat birds with anti-virals, not wishing to induce resistance, but to reserve these for the treatment of human cases.
What of vaccines should biosecurity fail to protect domestic flocks, whether housed or free range or totally free range pheasants, from infection?
Both Haemagglutinin and Neuraminidase are important in raising immunity but of the two Haemagglutinin is the most important providing protection against infection of the respiratory tract.
The Dutch vaccine is an H5N2 virus and has been inactivated. Thus the vaccine is not infectious and harmless to eat. Two shots two weeks apart are needed to produce a good level of immunity. This is typical of an inactivated vaccine when sufficient quantity of protein must be introduced to the immune system to prime and then boost the antibody response to a protective level. Whatever vaccine is used the H5 in the vaccine must be sufficiently closely related to the H5 in the virus strain, Zq, now present in wild birds in Europe and threatening to infect our UK poultry to provide a protective immunity. All H5 molecules will have an immune cross reactivity but distantly related H5 will not induce a solid protective immunity.
A number of other vaccines have been developed, and will be in development now in response to the world pandemic of avian HPAI H5N1. The HPAI H5N1 virus destroys eggs or tissue culture cells that could be used to grow it. Thus molecular methods must be used to insert the part of H5 that is immunogenic into other viruses that can be grown up in culture and be administered as a vaccine. The H5 can be inserted into a non-pathogenic influenza virus, or as a foreign gene into an avian pox virus or an adenovirus.
The protection given by vaccination may not be complete in all vaccinated animals exposed to a high infective dose. No vaccine is perfect and the influenza vaccines are not very good. Vaccination of horses against equine influenza is recommended for horses taking part in events and travelling abroad and has been very successful in preventing outbreaks. There are instances of a vaccinated horse shedding virus asymptomatically and infecting a non-vaccinated animal. Also the H3N8 vaccine strain has had to be updated because of drift, the accumulation of point mutations over years resulting in the current strain being too distant from the vaccine strain so that protective immunity is not evoked by vaccination, as occurs more rapidly in human influenza when the vaccine is updated annually to ensure its efficacy. Of note is the fact that vaccination of all domestic poultry against the H5N1 strains infecting poultry in Vietnam appears to have been successful in eradication of outbreaks in poultry and put a stop to human cases of infection. Diseased flocks were culled. It is said that 150 million doses of vaccine were given. If a vaccinated bird is infected it is asymptomatic so must therefore have a limited infection, perhaps confined to the respiratory tract, and shed a lesser amount of virus. In the context of a fully vaccinated flock the HPAI infection must peter out. This would mean the overall viral load and contamination of the environment is less, it is safer for humans, and does not allow infection to amplify in local wild birds and reach other vaccinated poultry flocks. An epidemic infection is eliminated when one infected individual cannot infect as much as even one other on average.
Vaccination of domestic poultry may be the only way of eliminating the HPAI H5N1 virus from wild bird populations in the long-term. The annual reproductive rate of wild birds ensures there are large annual populations of young birds susceptible to infection. The use of poultry manure to feed fish in lakes must be prohibited. International movement of poultry and its products such as manure must be controlled and prohibited as much as possible.
Pheasants and free-range poultry would seem to be especially vulnerable to infection from wild birds. In the case of pheasants these are shot in Autumn, for the most part less than one year old, and then plucked and prepared by individuals taking part in the shoot as well as local butchers. Pheasants are reared in Spring and then released saturating the local habitat. There is a significant pheasant sector in Wales and problems with HPAI H5N1 infection could arise in the released birds infecting the local wild birds and poultry, and in the shooting of infected birds when significant contact is made with humans handling the birds and preparing them for the table. Vaccinating young pheasants before release would be the sensible option.
Outbreaks of infection in commercial domestic flocks will result in loss of consumer confidence. Whilst DEFRA have not chosen vaccination as at least part of their strategy to combat this pandemic of HPAI H5N1 at least they have moved forward on diagnosis in contrast to the 2001 FMD outbreak. A bird or flock will not be declared infected unless a positive viral diagnosis has been made. The means of diagnosis is by PCR and the products are probed or sequenced in order to define the type and strain of influenza virus. Progress indeed. Why not vaccinate free- range poultry and pheasants as well as hold vaccine in reserve to ring vaccinate an outbreak in a domestic flock? Vaccination may be found necessary in bio-secure flocks to prevent breakdowns of infection in intensive poultry systems. If we do not order or make any vaccine for birds then we can never use it. Are we heading for another disaster on the scale of 2001?