Wild bird migration across the Qinghai-Tibetan plateau: a transmission route for highly pathogenic H5N1

Abstract

Background: Qinghai Lake in central China has been at the center of debate on whether wild birds play a role in circulation of highly pathogenic avian influenza virus H5N1. In 2005, an unprecedented epizootic at Qinghai Lake killed more than 6000 migratory birds including over 3000 bar-headed geese (Anser indicus). H5N1 subsequently spread to Europe and Africa, and in following years has re-emerged in wild birds along the Central Asia flyway several times.

Methodology/principal findings: To better understand the potential involvement of wild birds in the spread of H5N1, we studied the movements of bar-headed geese marked with GPS satellite transmitters at Qinghai Lake in relation to virus outbreaks and disease risk factors. We discovered a previously undocumented migratory pathway between Qinghai Lake and the Lhasa Valley of Tibet where 93% of the 29 marked geese overwintered. From 2003-2009, sixteen outbreaks in poultry or wild birds were confirmed on the Qinghai-Tibet Plateau, and the majority were located within the migratory pathway of the geese. Spatial and temporal concordance between goose movements and three potential H5N1 virus sources (poultry farms, a captive bar-headed goose facility, and H5N1 outbreak locations) indicated ample opportunities existed for virus spillover and infection of migratory geese on the wintering grounds. Their potential as a vector of H5N1 was supported by rapid migration movements of some geese and genetic relatedness of H5N1 virus isolated from geese in Tibet and Qinghai Lake.

Conclusions/significance: This is the first study to compare phylogenetics of the virus with spatial ecology of its host, and the combined results suggest that wild birds play a role in the spread of H5N1 in this region. However, the strength of the evidence would be improved with additional sequences from both poultry and wild birds on the Qinghai-Tibet Plateau where H5N1 has a clear stronghold.

Figure 1. Migration routes, stopover locations, breeding, and wintering areas for bar-headed geese marked at Qinghai Lake.

Migration routes are shown for the 15 individuals (each in unique color) that completed at least one fall migration. White polygons represent stopover areas. Individual 82081 (red path, full path in inset) wintered in India. Individuals 74898 (orange) and 82084 (yellow) are highlighted in Figure 3.

Figure 2. Brownian bridge utilization distributions in relation to poultry density and HPAI H5N1 outbreaks.

Brownian bridge utilization distributions (A) describe fall (yellow; 27 Sep–9 Dec) and spring (orange; 6 Mar–22 May) goose migrations. Fixed kernel home ranges depict (B) population level breeding and post-breeding areas (C) and wintering areas, with only locations near outbreaks shown. Brown shading indicates poultry densities. H5N1 outbreak events in wild birds (white) and poultry (black) are indicated for 2003–2009. Two shading levels indicate isopleths containing 95% (red) and 99% (yellow-orange) of total locations.

Figure 3. Concurrent use of natural wetlands and agricultural fields by wintering bar-headed geese near Lhasa.

(A) Winter movements for goose 74898 (3 November 2007–2 April 2008; 1205 locations) in relation to a confirmed HPAI H5N1 outbreak in chickens on 21 January, 2008 (black circle). (B) Winter movements (9 November 2008–5 April 2009; 961 locations) for goose 82084 in relation to a captive bar-headed goose farm (red circle).

Figure 4. Bar-headed goose farming in Tibet.

(A) Captive bar-headed geese in Gonggar County, Tibet as shown in a December 2007 China Tibet Information Center article (Wu 2007). (B) Wild bar-headed geese (foreground) shown in close proximity to a captive bar-headed goose farm (blue building in background) in a January 2007 photo from an anonymous source. Approximately 250 bar-headed geese were counted in outdoor net pens attached to the building (out of view in photo B). Approximate location of this farm is shown in a red circle (Fig. 3b).

Figure 5. Classification and regression tree describing poultry (A) and wild bird (B) outbreaks on the Qinghai-Tibet Plateau.

Numbers of random points (numerator), outbreak locations (denominator), and percentage of total sample are reported at each terminal node.

Figure 6. Comparison by season of wild and domestic outbreaks on the Qinghai-Tibet Plateau.

(A) Observed versus expected values differed for both poultry and wild bird outbreaks when tested separately (Fisher Exact Test, poultry P = 0.079, wild P = 0.017). (B) Temporal distribution of wild bird outbreaks differed from domestic bird outbreaks (Fishers Exact Test, P = 0.008). Expected numbers were calculated under the assumption that outbreaks are proportional to the number of days within the seasonal period (Winter = 87 d, Spring Migration = 78 d, Breeding = 126 d, and Fall Migration = 74 d).

Figure 7. Phylogenetic relationships of HPAI H5N1inferred by neighbor-joining analysis based on 1550 bp fragment of the HA gene.

Viruses isolated from the bar-headed goose are highlighted (blue) and the monophyletic grouping of isolates from Tibet and Qinghai are indicated by a symbol (♦).