Ubiquitous parasites drive a 33% increase in methane yield from livestock

[Display omitted] •Parasitic worms are globally ubiquitous, affecting livestock health and productivity.•We measured methane emissions from parasitised and parasite-free lambs.•Parasitism in lambs can lead to a 33% increase in methane yield (g CH4/kg of feed).•First known study to empirically demonstrate disease-driven increases in methane yield.•Implications for calculating effects of animal health on greenhouse gas emissions. Of anthropogenic methane emissions, 40% can be attributed to agriculture, the majority of which are from enteric fermentation in livestock. With international commitments to tackle drivers of climate change, there is a need to lower global methane emissions from livestock production. Gastrointestinal helminths (parasitic worms) are globally ubiquitous and represent one of the most pervasive challenges to the health and productivity of grazing livestock. These parasites influence a number of factors affecting methane emissions including feed efficiency, nutrient use, and production traits. However, their effects on methane emissions are unknown. This is to our knowledge the first study that empirically demonstrates disease-driven increases in methane (CH4) yield in livestock (grams of CH4 per kg of dry matter intake). We do this by measuring methane emissions (in respiration chambers), dry matter intake, and production parameters for parasitised and parasite-free lambs. This study shows that parasite infections in lambs can lead to a 33% increase in methane yield (g CH4/kg DMI). This knowledge will facilitate more accurate calculations of the true environmental costs of parasitism in livestock, and reveals the potential benefits of mitigating emission through controlling parasite burdens.