tetano
Editor, Senior Moderator
Published Date: 2013-05-14 13:51:25
Subject: PRO/AH/EDR> Avian influenza (58): China, zoonotic LPAI H7N9, animal, prevention
Archive Number: 20130514.1712913
AVIAN INFLUENZA (58): CHINA, ZOONOTIC LPAI H7N9, ANIMAL, PREVENTION POLICY
**************************************************************************
A ProMED-mail post
http://www.promedmail.org
ProMED-mail is a program of the
International Society for Infectious Diseases
http://www.isid.org
In this update:
[1] China: control and prevention, poultry, policy
[2] China: control and prevention, poultry, questions
[3] Southeast Asia: live bird markets and avian influenza
******
[1] China: control and prevention, poultry, policy
Date: Sun 12 May 2013
Source: Southern Metropolis Daily (China) [in Chinese, machine trans., summ., edited]
http://epaper.oeeee.com/A/html/2013-05/12/content_1855701.htm
On 10 May 2013, the Ministry of Agriculture (MOA), the Ministry of Public Security, Ministries of Health, of Transport, Commerce, Food, and other ministries jointly issued a notice which has been published on the website of the MOA, intended to minimize the risk of H7N9 virus spread within and in particular between provinces.
In spite of expected financial losses to the poultry industry in the short term, the movement of live birds originating from provinces where the virus was detected in humans and/or animal/environment has been temporarily suspended, except for breeding stock (such as, fertilized eggs for hatcheries and day-old chicks) provided they have undergone prescribed tests and certification procedures.
Transport of breeding stock will be allowed only between farms, not through live bird markets [see also items 2 and 3 below].
The authorities will encourage the consumption of frozen poultry meat, slaughtered in centralised, organised, industrial establishments ("compared to pig slaughter systems") rather than from live birds/poultry markets, which are clearly more hazardous. The need to increase consumer awareness to the safer consumption of frozen poultry products has been emphasized. This has been particularly underlined in relation to Guangdong.
The authorities will encourage the slaughter of live bird/poultry close to the poultry production sites [farms]. Frozen poultry meat can then be transported further with significantly reduced H7N9 risk.
Support to the industry, in order to minimize the economical burden due to the described policy requirements, will be safeguarded.
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
******
[2] China: control and prevention, poultry, questions
Date: Mon 13 May 2013
From: Nick Honhold <nick.honhold@gmail.com> [edited]
H7N9 questions
--------------
I've been watching the comments on ProMED-mail for some time about the lack of congruence between finding H7N9 in a variety of avian species at markets but rarely at farms. It seems to me that there is perhaps a lack of awareness (if only because it isn't mentioned) of the complexity of poultry marketing and trading systems, which contains many layers between a farm and a market. And that "live bird markets" themselves vary a lot from large wholesale markets and small weekly mixed markets at which some stalls sell birds. It seems to me that the virus could arise and circulate in the marketing system without needing to be found in commercial farms as such.
[The following questions represent an attempt to raise these issues.]
1. Many of the human cases of H7N9 have had contact with poultry markets, but not all. Do we have any handle of whether the proportion of human cases with these contacts is different from the proportion of unaffected people who have similar contacts? Many people are likely to have contact with poultry markets over a given period of time, so it is important to know whether the proportion in the cases is significantly higher than the general population. Hard to do but should be possible. This might help understand how important the bird markets are. Not everyone will report or remember such contacts even if they are relatively recent, but even so the proportions should be different if these markets are important in infecting people with H7N9.
2. Nonetheless, results indicate that it is quite possible to find the virus in various poultry species in live bird markets associated with human cases. No other relatively common source has been identified. That suggests that these markets must be somehow involved.
3. Do we understand these live bird markets? Are they dedicated poultry markets or are they general markets within which poultry are sold? Are they permanent markets working every day or markets held periodically? If the latter, are they part of a circuit around which a given set of poultry traders move from day to day? If permanent markets, are they cleared of animals at any point? If periodic markets, do traders move birds from market to market, maintaining a rolling population topped up every day but never fully depleted? Do the traders keep a stock of birds at their home base(s)?
4. Are there movements of birds, equipment, and people between markets? This could spread and maintain the disease between sites and reintroduce it if a site is cleared.
5. To date, finding the virus on poultry farms has been difficult. The only infected bird production unit so far seems to be a pigeon producer. But so far I haven't seen details of what types of poultry farms have been sampled. Are they large scale, small scale, backyard? Are they the same as the ones supplying the markets? I would assume so because presumably some level of trace back is involved, but I haven't seen that stated clearly.
6. Do we understand the poultry trading system in China? Whilst I cannot speak specifically from experience about live bird marketing in China, I suspect it will be more similar to what I have seen in other countries than it is different. Live bird marketing is by no means always (or even often) a simple linear system of birds moving from a producer to a market to a consumer. There may be collection points where birds come together from several sources and are then distributed to several markets. There can be several levels of intermediaries with local buyers consolidating birds to send on to more regional traders who may well have their own locations, which are not markets per se but are never empty of birds even if the markets may be. Some traders may specialise in one species but others may keep several at the same location, as can also happen in markets. Markets themselves, as well as selling to individual consumers, may also sell to traders who move birds to another location. Price differentials are often a driving force for this sort of trade. I suspect that the Chinese authorities have information on the overall system of marketing birds but I haven't yet seen it referred to. Understanding the full live poultry marketing system will be essential if this is an essential element in the maintenance of infection in poultry and spread to humans.
Some work has been published on poultry market chains in south China, for example at http://www.china-fetpv.org/cn/uploads/news/201208/1343963413948266.pdf and http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049712.
These suggest quite large movements of birds within China. This might indicate complex multi-level marketing systems. This, taken with and depending on the answers to some of the points above, may suggest that poultry farms may not have been necessary for the recombination of influenza A viruses or for the maintenance of a new recombination such as the current H7N9. Both could occur within a marketing system, which most people only come into contact with at markets where live poultry are sold but which may be only the visible tip of an iceberg.
7. On a separate issue, it is interesting to see the finding that this H7N9 shares the 6 "internal" genes of an H9N2 LPAI virus. If I remember correctly, the 1997 H5N1 outbreak in humans in Hong Kong involved a version of that virus that also shared its 6 "internal" genes with an H9N2 LPAI virus. I don't know how similar the "internal" genes of the 2 H9N2 viruses are but H9N2 has been circulating widely in poultry for many years in Asia and other parts of the world. Could that suggest a link between the pathogenicity of the (now extinct) 1997 H5N1 virus and the current H7N9 virus? If so, there was a hint that previous exposure of poultry to H9N2 was protective against H5N1 infection in that situation though an induced cell mediated immunity. Might that have (long term) implications for human immunisation strategies? I had suggested before that it should be possible to know the full genetic make-up of influenza A viruses and that this might help in understanding pathogenicity in humans as opposed to pathogenicity in poultry (which is clearly linked to a particular point in the 5 and 7 HA gene types). Full sequence genotyping is relatively easy and is likely to provide more information than the H and N type. Describing them simply by H and N type certainly doesn't give the whole picture.
--
Nick Honhold
BVSc, MSc, PhD, DipECVPH, MRCVS
Edinburgh
UK
<nick.honhold@gmail.com>
[Dr Honhold's timely questions deserve serious consideration.
We have approached a Chinese professional, familiar with the local situation. Interestingly, one of his observations was that live bird markets (LBM) are prevalent and particularly active in southern provinces, while in northern provinces chilled poultry meat is acceptable and familiar while LBMs are not as popular as in the south. This may explain also the concern, expressed in item 1 above, about the situation in (the southern) province Guangdong.
The following new paper, addressing LBMs in Viet Nam (though in relation to H5N1), may provide some additional insights. - Mod.AS]
******
[3] Southeast Asia: live bird markets and avian influenza
Date: Tue 14 May 2013 (accessed)
Source: Proceedings of the National Academy of Sciences, USA (PNAS), Direct Submission, Early Edition
http://www.pnas.org/content/early/2013/05/01/1220815110.full.pdf
[Ref: Fournie G, Guitian J, Desvaux S, Cuong VC, et al: Interventions for avian influenza A (H5N1) risk management in live bird market networks. Proc Natl Acad Sci USA. 2013. Epub ahead of print]
----------------------------------------------------------------------
Abstract
--------
Highly pathogenic avian influenza virus subtype H5N1 is endemic in Asia, with live bird trade as a major disease transmission pathway. A cross-sectional survey was undertaken in northern Viet Nam to investigate the structure of the live bird market (LBM) contact network and the implications for virus spread. Based on the movements of traders between LBMs, weighted and directed networks were constructed and used for social network analysis and individual-based modeling. Most LBMs were connected to one another, suggesting that the LBM network may support large scale disease spread. Because of cross-border trade, it also may promote transboundary virus circulation. However, opportunities for disease control do exist. The implementation of thorough, daily disinfection of the market environment as well as of traders' vehicles and equipment in only a small number of hubs can disconnect the network dramatically, preventing disease spread. These targeted interventions would be an effective alternative to the current policy of a complete ban of LBMs in some areas. Some LBMs that have been banned still are very active, and they likely have a substantial impact on disease dynamics, exhibiting the highest levels of susceptibility and infectiousness. The number of trader visits to markets, information that can be collected quickly and easily, may be used to identify LBMs suitable for implementing interventions. This would not require prior knowledge of the force of infection, for which laboratory-confirmed surveillance would be necessary. These findings are of particular relevance for policy development in resource-scarce settings.
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
[The following passage, extracted from the discussion section, may add some perspective:
Discussion
----------
"In our simulations, market disinfection had only a limited impact on the maximum epidemic size because of the high level of clustering in the province of Bac Giang. Although the 3 hubs mediate most of the traders' movements, substantial epidemics involving traders who do not visit these hubs still may occur.
The practical applications of mitigation strategies based on empirical networks need further investigation. To increase the uptake of such studies by policy makers, field trials might be conducted to demonstrate the efficacy and assess the feasibility of selected strategies (SI Text). Indeed, the behavioral changes required may make such interventions unfeasible. Disinfection of traders' vehicles and equipment may be particularly challenging. Additionally, some markets have particular physical characteristics that make environmental elimination difficult, such as nonsealed, earthen floors that would first require reinforcement. To ensure a high level of compliance and to minimize the negative impact on trading activities, the design of such interventions must involve all stakeholders.
Both the Hanoi and Bac Giang networks were only samples of wider networks, as only a fraction of the nodes and links were captured through the survey. Moreover, the markets included in the survey were not selected randomly. The results of the network analysis should be interpreted somewhat cautiously. Indeed, the sampling design may affect the structure of the observed networks and thus influence network parameter distributions (44). Such bias may have been introduced into the Hanoi network, in which the hub was the mediator in most contacts among other markets. The high impact of its removal on the network connectivity resulted from the low clustering: most of its neighbors were connected only to this hub and not to one another. In most markets of this network, traders were not interviewed. Although it is possible that additional market contacts might have been identified through further interviews, 22 of the network markets were visited and were observed to be small, with only one to six traders. Therefore, it is a realistic assumption that these markets were supplied by only one market. The centrality of the hub also is consistent with its role as a poultry supplier, being a wholesale market. Without doubt, it is the biggest market in northern Viet Nam in terms of the number of traders and volume of sales. Contrary to all other investigated markets, in which all or almost all traders operating in them were included in the survey, only a fraction of traders in this hub were interviewed. Therefore, it is possible that only a fraction of its contact markets were identified.
In contrast, half the markets classified as nodes with medium connectivity in the Bac Giang network were not included in the survey, and the most "important" hub was not surveyed. This suggests that the observed Bac Giang network indeed reflects some characteristics of the true network. A higher proportion of traders were thought to have been interviewed in the Bac Giang network than in the Hanoi network, and most markets in which poultry was sold regularly likely were included in the network. Markets from the provincial capital city and from all surrounding districts were visited, and because of the much lower human population density, the total number of markets and traders in Bac Giang likely is less than in Hanoi. Trader movements were driven by the opening schedules of periodic markets, so most traders were highly mobile. Although not all periodic markets were visited, traders likely were interviewed in other markets with alternative opening days.
Network analysis carried out in other livestock production systems has confirmed livestock markets as the main hubs for livestock movements (25, 26) and their contamination as a prerequisite for large epidemics (45). However, some farms also might act as bridges connecting markets. In Viet Nam, a nonquantified proportion of live poultry transactions are mediated outside markets at informal locations. These informal markets will modify the structure of the trader movement network.
In conclusion, although the northern Vietnamese LBM network may create conditions for maintaining HPAIV H5N1 and its spread across large areas, opportunities for targeted surveillance and control do exist. These strategies might be implemented effectively in a small number of hubs. Their identification might be based on egocentric measures without prior knowledge of the force of infection. These findings are particularly relevant for resource- poor settings where LBM systems are well developed."
The full, richly referenced paper is available online at the source URL above. - Mod.AS]
See Also
Avian influenza, human (77): China H7N9 update 20130512.1708336
Avian influenza (54): China, zoonotic LPAI H7N9, animal, update 20130509.1701937
Avian influenza (53): China (GD), zoonotic LPAI H7N9, animal, OIE 20130506.1695677
Avian influenza (52): China (GD), zoonotic LPAI H7N9, animal 20130505.1693137
Avian influenza (50): China, zoonotic LPAI H7N9, animal 20130504.1690089
Avian influenza (49): China zoonotic LPAI H7N9, animal: 20130502.1685345
Avian influenza (48): China, zoonotic LPAI H7N9, animal, OIE 20130430.1683206
Avian influenza (47): China zoonotic LPAI H7N9, animal 20130429.1679734
Avian influenza (45): China, zoonotic LPAI H7N9, animal, update, corr. 20130426.1674926
Avian influenza (45): China zoonotic LPAI H7N9, animal, update 20130426.1674487
Avian influenza (44): (China) zoonotic LPAI H7N9, animal, OIE, RFI 20130424.1670149
Avian influenza (43): (China) zoonotic LPAI H7N9, animal update 20130422.1664423
Avian influenza (41): (China) zoonotic LPAI H7N9, animal, RFI 20130420.1659952
Avian influenza (40): (China) zoonotic LPAI H7N9, source sought 20130418.1655718
Avian influenza, human (51): H7N9 update 20130418.1655610
Avian influenza (38): China, zoonotic LPAI H7N9, wild pigeon, OIE 20130417.1652959
Avian influenza (37): China, zoonotic LPAI H7N9, animals, RFI 20130414.1645731
Avian influenza (35): China, LPAI H7N9, update 20130412.1641185
Avian influenza (34): China (ZH), LPAI H7N9, duck, RFI 20130411.1639522
Avian influenza (33): China (JS, ZH) LPAI H7N9, live markets, RFI 20130410.1636843
Avian influenza (30): China, H7N9, multiple market closures 20130407.1629270
Avian influenza (29): China H7N9, FAO, biosecurity 20130407.1628375
Avian influenza (28): China (SH) H7N9, OIE, update 20130405.1624901
Avian influenza, human (16): China (SH, AH) H7N9 WHO 20130401.1614707
Avian influenza, human (14): China (Shanghai, Anhui) H7N9, fatal 20130331.1612370
.................................................arn/mj/lm
http://www.promedmail.org/direct.php?id=20130514.1712913
Subject: PRO/AH/EDR> Avian influenza (58): China, zoonotic LPAI H7N9, animal, prevention
Archive Number: 20130514.1712913
AVIAN INFLUENZA (58): CHINA, ZOONOTIC LPAI H7N9, ANIMAL, PREVENTION POLICY
**************************************************************************
A ProMED-mail post
http://www.promedmail.org
ProMED-mail is a program of the
International Society for Infectious Diseases
http://www.isid.org
In this update:
[1] China: control and prevention, poultry, policy
[2] China: control and prevention, poultry, questions
[3] Southeast Asia: live bird markets and avian influenza
******
[1] China: control and prevention, poultry, policy
Date: Sun 12 May 2013
Source: Southern Metropolis Daily (China) [in Chinese, machine trans., summ., edited]
http://epaper.oeeee.com/A/html/2013-05/12/content_1855701.htm
On 10 May 2013, the Ministry of Agriculture (MOA), the Ministry of Public Security, Ministries of Health, of Transport, Commerce, Food, and other ministries jointly issued a notice which has been published on the website of the MOA, intended to minimize the risk of H7N9 virus spread within and in particular between provinces.
In spite of expected financial losses to the poultry industry in the short term, the movement of live birds originating from provinces where the virus was detected in humans and/or animal/environment has been temporarily suspended, except for breeding stock (such as, fertilized eggs for hatcheries and day-old chicks) provided they have undergone prescribed tests and certification procedures.
Transport of breeding stock will be allowed only between farms, not through live bird markets [see also items 2 and 3 below].
The authorities will encourage the consumption of frozen poultry meat, slaughtered in centralised, organised, industrial establishments ("compared to pig slaughter systems") rather than from live birds/poultry markets, which are clearly more hazardous. The need to increase consumer awareness to the safer consumption of frozen poultry products has been emphasized. This has been particularly underlined in relation to Guangdong.
The authorities will encourage the slaughter of live bird/poultry close to the poultry production sites [farms]. Frozen poultry meat can then be transported further with significantly reduced H7N9 risk.
Support to the industry, in order to minimize the economical burden due to the described policy requirements, will be safeguarded.
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
******
[2] China: control and prevention, poultry, questions
Date: Mon 13 May 2013
From: Nick Honhold <nick.honhold@gmail.com> [edited]
H7N9 questions
--------------
I've been watching the comments on ProMED-mail for some time about the lack of congruence between finding H7N9 in a variety of avian species at markets but rarely at farms. It seems to me that there is perhaps a lack of awareness (if only because it isn't mentioned) of the complexity of poultry marketing and trading systems, which contains many layers between a farm and a market. And that "live bird markets" themselves vary a lot from large wholesale markets and small weekly mixed markets at which some stalls sell birds. It seems to me that the virus could arise and circulate in the marketing system without needing to be found in commercial farms as such.
[The following questions represent an attempt to raise these issues.]
1. Many of the human cases of H7N9 have had contact with poultry markets, but not all. Do we have any handle of whether the proportion of human cases with these contacts is different from the proportion of unaffected people who have similar contacts? Many people are likely to have contact with poultry markets over a given period of time, so it is important to know whether the proportion in the cases is significantly higher than the general population. Hard to do but should be possible. This might help understand how important the bird markets are. Not everyone will report or remember such contacts even if they are relatively recent, but even so the proportions should be different if these markets are important in infecting people with H7N9.
2. Nonetheless, results indicate that it is quite possible to find the virus in various poultry species in live bird markets associated with human cases. No other relatively common source has been identified. That suggests that these markets must be somehow involved.
3. Do we understand these live bird markets? Are they dedicated poultry markets or are they general markets within which poultry are sold? Are they permanent markets working every day or markets held periodically? If the latter, are they part of a circuit around which a given set of poultry traders move from day to day? If permanent markets, are they cleared of animals at any point? If periodic markets, do traders move birds from market to market, maintaining a rolling population topped up every day but never fully depleted? Do the traders keep a stock of birds at their home base(s)?
4. Are there movements of birds, equipment, and people between markets? This could spread and maintain the disease between sites and reintroduce it if a site is cleared.
5. To date, finding the virus on poultry farms has been difficult. The only infected bird production unit so far seems to be a pigeon producer. But so far I haven't seen details of what types of poultry farms have been sampled. Are they large scale, small scale, backyard? Are they the same as the ones supplying the markets? I would assume so because presumably some level of trace back is involved, but I haven't seen that stated clearly.
6. Do we understand the poultry trading system in China? Whilst I cannot speak specifically from experience about live bird marketing in China, I suspect it will be more similar to what I have seen in other countries than it is different. Live bird marketing is by no means always (or even often) a simple linear system of birds moving from a producer to a market to a consumer. There may be collection points where birds come together from several sources and are then distributed to several markets. There can be several levels of intermediaries with local buyers consolidating birds to send on to more regional traders who may well have their own locations, which are not markets per se but are never empty of birds even if the markets may be. Some traders may specialise in one species but others may keep several at the same location, as can also happen in markets. Markets themselves, as well as selling to individual consumers, may also sell to traders who move birds to another location. Price differentials are often a driving force for this sort of trade. I suspect that the Chinese authorities have information on the overall system of marketing birds but I haven't yet seen it referred to. Understanding the full live poultry marketing system will be essential if this is an essential element in the maintenance of infection in poultry and spread to humans.
Some work has been published on poultry market chains in south China, for example at http://www.china-fetpv.org/cn/uploads/news/201208/1343963413948266.pdf and http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049712.
These suggest quite large movements of birds within China. This might indicate complex multi-level marketing systems. This, taken with and depending on the answers to some of the points above, may suggest that poultry farms may not have been necessary for the recombination of influenza A viruses or for the maintenance of a new recombination such as the current H7N9. Both could occur within a marketing system, which most people only come into contact with at markets where live poultry are sold but which may be only the visible tip of an iceberg.
7. On a separate issue, it is interesting to see the finding that this H7N9 shares the 6 "internal" genes of an H9N2 LPAI virus. If I remember correctly, the 1997 H5N1 outbreak in humans in Hong Kong involved a version of that virus that also shared its 6 "internal" genes with an H9N2 LPAI virus. I don't know how similar the "internal" genes of the 2 H9N2 viruses are but H9N2 has been circulating widely in poultry for many years in Asia and other parts of the world. Could that suggest a link between the pathogenicity of the (now extinct) 1997 H5N1 virus and the current H7N9 virus? If so, there was a hint that previous exposure of poultry to H9N2 was protective against H5N1 infection in that situation though an induced cell mediated immunity. Might that have (long term) implications for human immunisation strategies? I had suggested before that it should be possible to know the full genetic make-up of influenza A viruses and that this might help in understanding pathogenicity in humans as opposed to pathogenicity in poultry (which is clearly linked to a particular point in the 5 and 7 HA gene types). Full sequence genotyping is relatively easy and is likely to provide more information than the H and N type. Describing them simply by H and N type certainly doesn't give the whole picture.
--
Nick Honhold
BVSc, MSc, PhD, DipECVPH, MRCVS
Edinburgh
UK
<nick.honhold@gmail.com>
[Dr Honhold's timely questions deserve serious consideration.
We have approached a Chinese professional, familiar with the local situation. Interestingly, one of his observations was that live bird markets (LBM) are prevalent and particularly active in southern provinces, while in northern provinces chilled poultry meat is acceptable and familiar while LBMs are not as popular as in the south. This may explain also the concern, expressed in item 1 above, about the situation in (the southern) province Guangdong.
The following new paper, addressing LBMs in Viet Nam (though in relation to H5N1), may provide some additional insights. - Mod.AS]
******
[3] Southeast Asia: live bird markets and avian influenza
Date: Tue 14 May 2013 (accessed)
Source: Proceedings of the National Academy of Sciences, USA (PNAS), Direct Submission, Early Edition
http://www.pnas.org/content/early/2013/05/01/1220815110.full.pdf
[Ref: Fournie G, Guitian J, Desvaux S, Cuong VC, et al: Interventions for avian influenza A (H5N1) risk management in live bird market networks. Proc Natl Acad Sci USA. 2013. Epub ahead of print]
----------------------------------------------------------------------
Abstract
--------
Highly pathogenic avian influenza virus subtype H5N1 is endemic in Asia, with live bird trade as a major disease transmission pathway. A cross-sectional survey was undertaken in northern Viet Nam to investigate the structure of the live bird market (LBM) contact network and the implications for virus spread. Based on the movements of traders between LBMs, weighted and directed networks were constructed and used for social network analysis and individual-based modeling. Most LBMs were connected to one another, suggesting that the LBM network may support large scale disease spread. Because of cross-border trade, it also may promote transboundary virus circulation. However, opportunities for disease control do exist. The implementation of thorough, daily disinfection of the market environment as well as of traders' vehicles and equipment in only a small number of hubs can disconnect the network dramatically, preventing disease spread. These targeted interventions would be an effective alternative to the current policy of a complete ban of LBMs in some areas. Some LBMs that have been banned still are very active, and they likely have a substantial impact on disease dynamics, exhibiting the highest levels of susceptibility and infectiousness. The number of trader visits to markets, information that can be collected quickly and easily, may be used to identify LBMs suitable for implementing interventions. This would not require prior knowledge of the force of infection, for which laboratory-confirmed surveillance would be necessary. These findings are of particular relevance for policy development in resource-scarce settings.
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
[The following passage, extracted from the discussion section, may add some perspective:
Discussion
----------
"In our simulations, market disinfection had only a limited impact on the maximum epidemic size because of the high level of clustering in the province of Bac Giang. Although the 3 hubs mediate most of the traders' movements, substantial epidemics involving traders who do not visit these hubs still may occur.
The practical applications of mitigation strategies based on empirical networks need further investigation. To increase the uptake of such studies by policy makers, field trials might be conducted to demonstrate the efficacy and assess the feasibility of selected strategies (SI Text). Indeed, the behavioral changes required may make such interventions unfeasible. Disinfection of traders' vehicles and equipment may be particularly challenging. Additionally, some markets have particular physical characteristics that make environmental elimination difficult, such as nonsealed, earthen floors that would first require reinforcement. To ensure a high level of compliance and to minimize the negative impact on trading activities, the design of such interventions must involve all stakeholders.
Both the Hanoi and Bac Giang networks were only samples of wider networks, as only a fraction of the nodes and links were captured through the survey. Moreover, the markets included in the survey were not selected randomly. The results of the network analysis should be interpreted somewhat cautiously. Indeed, the sampling design may affect the structure of the observed networks and thus influence network parameter distributions (44). Such bias may have been introduced into the Hanoi network, in which the hub was the mediator in most contacts among other markets. The high impact of its removal on the network connectivity resulted from the low clustering: most of its neighbors were connected only to this hub and not to one another. In most markets of this network, traders were not interviewed. Although it is possible that additional market contacts might have been identified through further interviews, 22 of the network markets were visited and were observed to be small, with only one to six traders. Therefore, it is a realistic assumption that these markets were supplied by only one market. The centrality of the hub also is consistent with its role as a poultry supplier, being a wholesale market. Without doubt, it is the biggest market in northern Viet Nam in terms of the number of traders and volume of sales. Contrary to all other investigated markets, in which all or almost all traders operating in them were included in the survey, only a fraction of traders in this hub were interviewed. Therefore, it is possible that only a fraction of its contact markets were identified.
In contrast, half the markets classified as nodes with medium connectivity in the Bac Giang network were not included in the survey, and the most "important" hub was not surveyed. This suggests that the observed Bac Giang network indeed reflects some characteristics of the true network. A higher proportion of traders were thought to have been interviewed in the Bac Giang network than in the Hanoi network, and most markets in which poultry was sold regularly likely were included in the network. Markets from the provincial capital city and from all surrounding districts were visited, and because of the much lower human population density, the total number of markets and traders in Bac Giang likely is less than in Hanoi. Trader movements were driven by the opening schedules of periodic markets, so most traders were highly mobile. Although not all periodic markets were visited, traders likely were interviewed in other markets with alternative opening days.
Network analysis carried out in other livestock production systems has confirmed livestock markets as the main hubs for livestock movements (25, 26) and their contamination as a prerequisite for large epidemics (45). However, some farms also might act as bridges connecting markets. In Viet Nam, a nonquantified proportion of live poultry transactions are mediated outside markets at informal locations. These informal markets will modify the structure of the trader movement network.
In conclusion, although the northern Vietnamese LBM network may create conditions for maintaining HPAIV H5N1 and its spread across large areas, opportunities for targeted surveillance and control do exist. These strategies might be implemented effectively in a small number of hubs. Their identification might be based on egocentric measures without prior knowledge of the force of infection. These findings are particularly relevant for resource- poor settings where LBM systems are well developed."
The full, richly referenced paper is available online at the source URL above. - Mod.AS]
See Also
Avian influenza, human (77): China H7N9 update 20130512.1708336
Avian influenza (54): China, zoonotic LPAI H7N9, animal, update 20130509.1701937
Avian influenza (53): China (GD), zoonotic LPAI H7N9, animal, OIE 20130506.1695677
Avian influenza (52): China (GD), zoonotic LPAI H7N9, animal 20130505.1693137
Avian influenza (50): China, zoonotic LPAI H7N9, animal 20130504.1690089
Avian influenza (49): China zoonotic LPAI H7N9, animal: 20130502.1685345
Avian influenza (48): China, zoonotic LPAI H7N9, animal, OIE 20130430.1683206
Avian influenza (47): China zoonotic LPAI H7N9, animal 20130429.1679734
Avian influenza (45): China, zoonotic LPAI H7N9, animal, update, corr. 20130426.1674926
Avian influenza (45): China zoonotic LPAI H7N9, animal, update 20130426.1674487
Avian influenza (44): (China) zoonotic LPAI H7N9, animal, OIE, RFI 20130424.1670149
Avian influenza (43): (China) zoonotic LPAI H7N9, animal update 20130422.1664423
Avian influenza (41): (China) zoonotic LPAI H7N9, animal, RFI 20130420.1659952
Avian influenza (40): (China) zoonotic LPAI H7N9, source sought 20130418.1655718
Avian influenza, human (51): H7N9 update 20130418.1655610
Avian influenza (38): China, zoonotic LPAI H7N9, wild pigeon, OIE 20130417.1652959
Avian influenza (37): China, zoonotic LPAI H7N9, animals, RFI 20130414.1645731
Avian influenza (35): China, LPAI H7N9, update 20130412.1641185
Avian influenza (34): China (ZH), LPAI H7N9, duck, RFI 20130411.1639522
Avian influenza (33): China (JS, ZH) LPAI H7N9, live markets, RFI 20130410.1636843
Avian influenza (30): China, H7N9, multiple market closures 20130407.1629270
Avian influenza (29): China H7N9, FAO, biosecurity 20130407.1628375
Avian influenza (28): China (SH) H7N9, OIE, update 20130405.1624901
Avian influenza, human (16): China (SH, AH) H7N9 WHO 20130401.1614707
Avian influenza, human (14): China (Shanghai, Anhui) H7N9, fatal 20130331.1612370
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http://www.promedmail.org/direct.php?id=20130514.1712913