The emergence and spread of finch trichomonosis in the British Isles

Abstract

Finch trichomonosis, caused by the protozoal parasite Trichomonas gallinae, was first recognized as an emerging infectious disease of British passerines in 2005. The first year of seasonal epidemic mortality occurred in 2006 with significant declines of greenfinch Carduelis chloris and chaffinch Fringilla coelebs populations. Here, we demonstrate that large-scale mortality, principally of greenfinch, continued in subsequent years, 2007-2009, with a shifting geographical distribution across the British Isles over time. Consequent to the emergence of finch trichomonosis, the breeding greenfinch population in Great Britain has declined from ca 4.3 million to ca 2.8 million birds and the maximum mean number of greenfinches (a proxy for flock size) visiting gardens has declined by 50 per cent. The annual rate of decline of the breeding greenfinch population within England has exceeded 7 per cent since the initial epidemic. Although initially chaffinch populations were regionally diminished by the disease, this has not continued. Retrospective analyses of disease surveillance data showed a rapid, widespread emergence of finch trichomonosis across Great Britain in 2005 and we hypothesize that the disease emerged by T. gallinae jumping from columbiforms to passeriforms. Further investigation is required to determine the continuing impact of finch trichomonosis and to develop our understanding of how protozoal diseases jump host species.

Figure 1.

The number of dead greenfinches and/or chaffinches reported per finch trichomonosis incident.

Figure 2.

Distribution of finch trichomonosis incidents in Great Britain, 2005, obtained from opportunistic surveillance reports. The shading by county indicates relative frequency of finch trichomonosis (incidents per thousand households) during the period from 1 April to 30 September. The index case is shown with a black star. Incidents with onset in April and May (black squares), June and July (grey circles), August and September (white circles) are shown.

Figure 3.

Distribution of finch trichomonosis incidents in Great Britain, 2006–2009, obtained on opportunistic surveillance reports. The shading by county indicates relative frequency of finch trichomonosis (incidents per thousand households) during the period from 1 April to 30 September for each year, (a) 2006, (b) 2007, (c) 2008 and (d) 2009.

Figure 4.

Reporting rate for (a) greenfinch, (b) chaffinch and (c) dunnock in all GBW gardens for the years 1996–2005 (grey lines), 2006 (purple), 2007 (blue), 2008 (red) and 2009 (black).

Figure 5.

GBFS mean maximum count per garden for UK for (a) greenfinch, (b) chaffinch and (c) dunnock.

Figure 6.

Annual rate of population change as measured by the BBS by country. Boxes show mean and quartiles of annual changes and whiskers minimum and maximum annual change observed in the period 1994–2006; points, the population change recorded in 2006/2007 (purple), 2007/2008 (blue), 2008/2009 (red), 2009/2010 (black). Dotted line indicates no population change.

Figure 7.

Annual population size of chaffinch (dotted line), dunnock (dashed line) and greenfinch (solid line) in the United Kingdom 1994–2010. The population size in 2000 for each species was taken from Newson et al. [20] and scaled by the annual BBS index in each year [22].