| Most of the following is a synopsis from various peer-reviewed publications, but I’ve also included notes from a recent presentation by Robert Webster. Thoughts or speculations on my part are indicated as they appear.
One thing to remember is that H5N1 is not the only threat. It heads the ‘pandemic hit list’ for sure, but after that, the other potential pandemic viruses would be H7N7, H9N2, then H2N2 from laboratory samples.
In 1996, an outbreak of HPAI in geese was reported in China, with >40% of geese dead from infection with an H5N1 virus (Gs/GD/96).
In 1997, 18 people were infected in Hong Kong, 6 of whom died. The precursor of this (HK/156/97) virus is thought to be from a Gs/GD/96-like H5N1 virus, while the internal genes are thought to have come from H9N2 (A/quail/Hong Kong/G1/97). At the time, poultry market samples in Hong Kong showed 20% positive for H5N1, 5% positive for H9N2. It is likely that the virus causing human infections might have been the result of reassortment between these two viruses, since the H9N2 was circulating in quails and H5N1 was circulating in geese, and the various birds were sold in close proximity in live markets.
After extensive culling by the authorities in Hong Kong, this particular strain HK/156/97 disappeared from circulation.
In December 2002, wild waterfowl started dying in 2 different parks in Hong Kong. The H5N1 isolated was different from the 97 samples, but had retained HPAI characteristics. This was the first time that HPAI H5 was observed to be highly pathogenic to wild waterfowl, which was considered to be its natural host.
In February 2003, a family from Hong Kong visited Fujian in China. While there, their 8 year old daughter died of a febrile illness. The father and another son also became ill, returned to Hong Kong, and were diagnosed to be infected with H5N1. This cluster represents the first human cases of the current outbreak of H5N1 infections. The virus from these human cases as well as those from the outbreak in waterfowl the preceding year had the same precursor (Gs/GD/96) as the 97 virus, but the internal genes were from a different (unknown) avian source. This virus did not have some of the characteristics associated with replication in land-based poultry, showing that this is not a pre-requisite for transmission to human hosts.
Between 2002-05, HPAI H5N1 continued to spread over China and Asia, forming multiple regional sublineages. At the same time, surveys in southern China showed continuous formation of new strains of H5N1 from reassortment with unknown avian donor viruses. In addition, studies showed that these viruses were becoming increasingly virulent to mice in laboratory conditions. A third trend was the increasing isolation of the virus from tracheal rather than cloacal samples in waterfowl, signally some change in transmission mechanisms.
In May 2005, large numbers of wild waterfowl died in Qinghai in China. Subsequent to that, this particular sublineage of viruses spread to Europe, Middle East, and Africa, possibly via migratory routes. There has, however, been no observed spread to the Americas or to Australia.
Human cases appeared in Vietnam, Thailand, Cambodia in 2004-5. This group of virus is antigenically distinct from later clades, and is designated clade 1.
Indonesia started to have human cases in mid-2005, with rising numbers, increasing clusters, high case fatality, and instances of human-to-human transmission through to the present time. This group of virus is designated Clade 2 subclade 1. Although the virus causing the biggest cluster of human cases (Karo cluster) belongs to this group, antigenically it is closer to subclade 2 viruses.
The Qinghai-like sublineage (Clade 2, subclade 2) viruses have caused human cases in Azerbaijan, Turkey, Iraq, and Egypt. Samples from this group carry the E627K mutation on PB2 gene, which, from previous studies, is consistently found in human but not avian viruses. This mutation is also found in avian H9N2 viruses that have caused human infections as well as in the single fatal case of H7N7 infection in the Netherlands in 2003.
A third broad group (Clade 2, subclade 3) are viruses in China. While there had been multiple sublineages before, since mid 2005, this has been gradually replaced by a single dominant sublineage, the Fujian-like group. These now form 95% of positive samples from markets from various provinces. In addition, there had been 22 confirmed human cases in China, and those for whom sequences were available show that they were all infected by viruses of this sublineage. The sequences bear a high degree of homology to avian sequences, so it would appear that infections are mainly from avian sources. However, since China started mass vaccination of poultry, there had been fewer poultry outbreaks, and many of the human cases have happened in the absence of poultry deaths in the area, with a few from densely populated urban areas.
Vaccination has resulted in low rates of seroconversion. At the same time, since vaccines are primarily geared towards chickens, their efficacy in ducks is unknown. Ducks and geese continue to shed virus despite positive antibody tests, and, according to Webster, ducks could be the ‘Trojan horse’ in the fight for eradication of the virus. The virus undergoes antigenic drift in ducks, with different strains being isolated from one duck. There is also a tendency towards increasing environmental stability – viruses from 1997 were stable at 37oC for 1 day, now they can live up to 3 days.
On the question of what might be driving the antigenic diversity or selection of variants, Webster thinks that vaccines might be a reason, but transmission between species could be more important. Specifically, he wonders whether passage between different mammalian hosts, eg ‘some small mammal’ such as the civet cat, might be an important driver.
H5N1 has caused infections in cats and other felids, the most striking incident being the death of all the tigers in a zoo in 2004. Since then, it has been shown that it can transmit from cat to cat soon after it switched species. The virus has also been isolated from various small mammals, notably the stone marten in Germany and civet cat in Thailand. The latter may be significant as it was implicated as a possible intermediate host for SARS virus where it acquired the ability to transmit from human to human. It is also a known culinary delicacy in certain areas of Asia, especially in the Guangdong province of China, and so would frequently be caged in close proximity to poultry in markets. Other possible mammalian hosts would include cats, and, to a lesser degree, pigs.
In terms of epidemiology of human disease, 2006 saw a further increase in the number of human cases, with 111 cases so far (Dec 22) and 76 deaths (CFR 68.4%) as compared to 97 cases in 2005 with 42 deaths (CFR 43.3%) and overall 258 cases with 154 deaths (CFR 59.6%). The median age for disease was 20 years, case fatality was highest in the 11-19 age group at 73% (up to June 2006).
The issue of host susceptibility has been raised, partly to explain the apparent observation that those who are related by blood are more likely to become infected in a cluster than those who are not. This issue is unresolved but is IMO worth keeping in mind. For example, one thing that I’ve found intriguing has been the absence of human cases on the island of Bali in Indonesia, despite reports of avian outbreaks, cats dying, and pigs said to have tested positive. Since the Balinese are Buddhists and have traditionally been segregated from other Indonesian, eg Islamic, communities in marriage, could we speculate some genetic selection eg by the prevalence or absence of certain HLA types? However, be warned that this is entirely speculative on my part. ;-)
The evidence for tamiflu sensitivity is inconclusive, since there are a couple of anecdotal cases of resistance. De Jong in Vietnam and others are adamant that tamiflu does reduce the viral load dramatically. However, it would appear that the drug needs to be given very early in the course of the disease. Beyond a certain point, a reduction of viral titre is no longer associated with positive clinical benefit once ‘cytokine storm’ or ARDS has set in. Study in ferrets would suggest that the normal 5-day course for seasonal flu would be insufficient, and that one would need double the duration, and possibly double the dose for treatment of H5N1 infections.
H5N1 in 1997 was sensitive to amantanes. Subsequent samples were no longer sensitive. Webster has an interesting speculation about this. At the time, there were concerns and questions were raised as to whether the Chinese were using amantadine in their poultry feeds. “Word went out’” according to Webster, and recent samples are now again sensitive to amantadine.
Efforts to produce a vaccine have had mixed results. The normal subunit inactivated vaccines are not very immunogenic, and require high doses and/or the addition of an adjuvant, either alum which is not very effective, or one of the proprietary oil adjuvants that are not licensed in the US. The Chinese are having preliminary success with a whole virus alum-adjuvanted vaccine which appears to be antigenic at low dose. There is also a recombinant HA protein vaccine which appears promising in terms of antigen-sparing. However, the appearance of antigenically distinct sublineages is adding to the difficulties. Only yesterday, the WHO announced another seed vaccine strain from Anhui China, ie from the FJ-like sublineage. Current recommendation from WHO is to be cautious and not to commit to prepandemic vaccines, as there are still many unresolved issues.
There is some suggestion from recent data that vaccination with seasonal flu vaccine ie containing H1N1, may confer some degree of protection due to N1 cross-immunity. In addition, a recent review of N1 sequences in the database shows that the N1 from H5N1 is distantly related to the H1N1 from 1918 and its descendants, further adding some support to that idea. This is however speculative, and any protection is likely to be partial, but there may be enough to prevent death. |
