Fever detection in cattle is today a time-consuming manual procedure. Isolating infected animals to avoid the spread of the diseases is therefore not straightforward with the consequence of massive dose of antibiotics delivered to the whole cattle. This work proposes a subdermal implantable flexible battery-less wireless sensor for un-cooperative monitoring of core temperature in pigs. The sensor consists in a shaped loop antenna sewed into a textile scaffold, as anti-migration support, for a natural integration with the animal fatty tissue. By resorting to the UHF-RFID communication protocol, the device can be interrogated from a distance of up to 1.7 m that is compatible with the automatic temperature reading when the animal approaches the cattle feeder. © 2020 URSI.