Formalizing host competence
Host competence, the capacity to acquire, sustain and transmit parasites or pathogens, is typically quantified for species, although it is also an individual property. We derive an individual-level competence index from infection biology and link it to reproduction numbers. Our index separates infectiousness and susceptibility (individual properties) from contact and abundance (environmental properties); combined, they yield host potential and capacity, analogues of vector potential and vectorial capacity. We simulated populations in four systems (Lyme disease, West Nile virus, chytridiomycosis, avian influenza), using experimental data where available and plausible values otherwise. Individual competence varied widely within and overlapped among species; species means approximated an established index for West Nile virus, to which the model was calibrated. Under the assigned variances, intraspecific variation was greatest in low-competence species, whose loads sit near the transmission threshold; there, load-contact covariation roughly halved or doubled mean potential. Where contact was measured (Lyme tick burdens), nearly all individuals of high-competence species replaced themselves across plausible tick survival, whereas replacement by most individuals of low-competence species depended on tick survival. In West Nile virus, the most infectious 20% of mourning doves supplied about half of transmission, so species averages could understate the role of some individuals.