Formic acid treatment drastically reduces sporadic Creutzfeldt–Jakob disease and variant Creutzfeldt–Jakob disease infectivity in histological samples as tested in a high-sensitivity mouse bioassay

Formic acid treatment is widely used in diagnostic neuropathology to reduce the infectivity of prion-containing tissues; however, quantitative in vivo evidence supporting its effectiveness under routine laboratory conditions remains limited. Here, we assessed the impact of formalin fixation and formic acid treatment on the infectivity of type 1 sporadic Creutzfeldt–Jakob disease (sCJD) and variant CJD (vCJD) prions using highly sensitive transgenic mouse models overexpressing human-PrP M129 (Hu-Tg340) or bovine PrP (Bo-Tg110). Brain tissues were processed under conditions closely resembling standard histopathological workflows and tested as untreated, formalin-fixed or formalin-plus-formic-acid–treated inocula. Untreated samples produced short incubation times and full attack rates, whereas formalin fixation caused only a modest prolongation of incubation times. In contrast, formic acid treatment markedly extended incubation times and reduced attack rates for sCJD. Based on incubation-time interpolation, the estimated infectivity reductions were on the order of 4.4 log₁₀ for vCJD and 5 log₁₀ for sCJD. These estimates indicate a major reduction in infectious titre under the conditions tested, although residual infectivity was still detected. The findings support formic acid treatment as an important risk-reduction step in routine neuropathology workflows for the two prion strains examined.