Density-dependent decline of host abundance resulting from a new infectious disease

Although many new diseases have emerged within the past 2 decades [Cohen, M . L. (1998) Brit. Med. Bull. 54, 523-532], attributing low numbers of anima l hosts to the existence of even a new pathogen is problematic. This is bec ause very rarely does one have data on host abundance before and after the epizootic as well as detailed descriptions of pathogen prevalence [Dobson, A. P. & Hudson. P. J. (1985) in Ecology of Infectious Diseases in Natural P opulations, eds. Grenfell, B. T. & Dobson, A. P. (Cambridge Univ. Press, Ca mbridge, U.K.), pp. 52-89]. Month by month we tracked the spread of the epi zootic of an apparently novel strain of a widespread poultry pathogen, Myco plasma gallisepticum, through a previously unknown host, the house finch, w hose abundance has been monitored over past decades. Here we are able to de monstrate a causal relationship between high disease prevalence and declini ng house finch abundance throughout the eastern half of North America becau se the epizootic reached different parts of the house finch range at differ ent times. Three years after the epizootic arrived, house finch abundance s tabilized at similar levels, although house finch abundance had been high a nd stable in some areas but low and rapidly increasing in others. This resu lt, not previously documented in wild populations, is as expected from theo ry if transmission of the disease was density dependent.