1. Modes of transmission and the origin of H5N1 viruses
Avian Influenza (A.I.) is transmitted by infected birds and their excrements. Also, AI is mechanically transmitted by surface means via contaminated food, water, feeds, soil, vehicles, humans, animals, flies, feathers etc. AI viruses can be spread by national or international trades of infected birds and contaminated products. Wild birds, especially migratory waterfowl, are a recognized source and reservoir for all subtypes of AI viruses. Some mammals such as dogs and cats are susceptible to the virus, but they are usually considered as the dead ends.
In 1996, H5N1 virus was first detected in Guangdong Province, China. In 1997 the virus became widespread in poultry markets in Hong Kong, and killed 6 of 18 infected persons. The virus was wiped out by culling all domestic poultry in Hong Kong. In 2002, a new H5N1 genotype appeared again in Hong Kong, and the variant strains spread across Southeast Asia and South Asia between 2002 and 2007. The viruses can be divided into several clades such as V1, V2, V3 and Indonesian clades. The strains of H5N1 virus appeared in Korea (2003) and Japan (2004) were closely related to Guangdong strain/174/04 which is distinct from the abov 4 clades.
In April 2005, a new variant H5N1 virus, which caused high mortality in both wild birds and poultry, was observed in Quinghai Lake, China. The virus was spread westward through migratory birds into Siberia, Kazakhstan and Turkey. This unprecedented mortality of wild birds associated with H5N1 viruses opened a new window for its movement within wild and domestic birds across Eurasia, the Near East and Africa. Virus strains are divided into 3 clades (EMA 1, 2, 3). The virus isolated in 2007 in Japan is closely related to one of those viruses of Quinghai origin (EMA clades).
2. AI situation in Europe
The European Union decided to make risk assessments of H5N1 virus entering via migratory birds into Europe, and active and passive surveillance for AI virus in wild birds started in July 2005. The conclusion of this study indicated a high risk of introducing the virus via migratory birds, and also a risk of the infection to become enzootic in Europe.
The EU encouraged each member country (a) to make an extensive survey of AI viruses in both wild and domestic birds, (b) to vaccinate zoo birds and poultry that cannot be kept in houses (c) to keep all domestic birds in closed housing in high risk areas or zones and (d) to vaccinate domestic birds that cannot be housed.
Between 2005 and 2006, H5N1 viruses were detected in wild birds in 25 countries. AI outbreaks in poultry farms were reported from 13 countries in Eastern Europe, and 4 countries in Western Europe (Sweden, Denmark, France and Germany). It is considered that migratory birds played a major role in spreading H5N1 viruses in Europe.
The results of the risk control measures in Western Europe can be summarized as follows :
(i) It was successful to protect the zoo birds by vaccination, but several birds died due to trauma of vaccination.
(ii) Surveillance of wild birds was useful in improving early warning systems for poultry producers, and was effective in reducing the exposure risks of poultry.
(iii) Mass culling of poultry and ornamental birds could be avoided.
In 2007, H5N1 virus surfaced again in Hungary, UK, Czech R., Germany and France. It seems that H5N1 viruses became enzootic in some countries in Eastern Europe including Russia.
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Highly pathogenic avian influenza caused by H5N1 strains has spread in Asian region in recent years. This disease has seriously affected poultry and other related industries. There are several constraints on the disease control in this region. However, activities have been enhanced to overcome these constraints under the cooperation with international organizations and donor countries. Development of harmonized control strategies is required taking account of active movements of people and commodities across the country borders in this region.

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Data files were collected from 134 herds which participated in a data-share program in Japan. Of 129 farrow-to-finish swine herds, 93 herds were selected for recording their growth performance over a period of three years from 2001 to 2003. A database was created by abstracting 12 three-month mortality records of each herd. Mortality risk (%) and mortality rate per 10,000 pig days (incidence density) were used for measuring mortality in fattening pig operations. The pig days were calculated as the total number of days for which pigs in a herd were fed during each three-month period. Mortality rate was defined as the number of dead pigs divided by the pig days during a three-month period×10,000, whereas mortality risk was defined as the number of dead pigs divided by the number of an initial pig inventory and the number of pigs moved in the herd during a three-month period. Pearson correlation analysis was used to obtain a relationship between the mortality risk and the mortality rate. The 12 three-month data were analyzed by using mixed effects models for repeated measures. Tukey-Kramer post-hoc multiple comparisons were used to compare the means of the year, season and herd size groups. The overall means of the mortality risk and the mortality rate were 2.77% and 4.98 deaths per 10,000 pig days, respectively. High correlations across 12 three-month periods were found between the mortality risk and the mortality rate (r>0.97, P<0.05). Year effect was not associated with both the mortality risk and rate. Pigs fed in the spring and summer seasons and those in small herds (<1,000 pigs) had higher mortality risks and higher mortality rates than those fed in the fall season and those in large herds (>2,800 pigs) (P<0.05).

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For the investigation of swine Toxoplasma infection in Vietnam, a total of 41 blood samples were collected from 16 animals at the abattoirs, 13 animals at the rural pig farms, and 12 animals at the Hue University farm in Thua Thien Hue Province (central Vietnam). They were analysed by the antibody method using of the quick inspection kit (TgICT). All samples from the Hue University Farm were negative for Toxoplasma, while 14 of the abattoirs, and 5 of pig farms were positive against the quick detection. As we detected cat (definitive host of Toxoplasma) at the rural farms, the infection source of Toxoplasma may be oral oocysts from foods polluted by cat feces. Pigs from positive regions tended to be shown a growth deficiency. This report suggested that educational efforts for local veterinarians must be conducted continuously to eradication of Toxoplasma infections in Vietnam.

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This paper briefly reviewed the origin of H5N1 virus and spreading of the virus from China to the other parts of Asia, the Near East, Europe and Africa.
The new measures implemented by the EU in 2006 and 2007 were also reviewed, and the effectiveness of the measures such as surveillance of infected wild birds, strategic vaccination of zoo birds and poultry kept out-door and DIVA tests prior to their release etc was confirmed.
In the USA, the surveillance of infected wild birds has been intensified since 2006 with emphasis on Alaska, but so far H5N1 virus of Asian origin has not been detected. Emphasis was also placed on preparedness for emergencies, and the detailed plans for preemptive culling and strategic vaccination have been prepared.
Both the EU and the USA are putting emphasis on risk assessments, surveillance of infected wild birds, early warning systems and strategic vaccination prior to the invasion of AI into poultry farms. They are confident that vaccination is safe and effective if vaccination is carried out under the general supervision of the governments (EU and AHIS) and vaccinated birds are strictly controlled and disposed. How to get the maximum benefit by vaccinating the minimum number of birds is the subject assigned to the epidemiologists concerned.

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[in Japanese]

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[in Japanese]

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