Dairy profitability depends on the quantity and quality of the produced milk. Bovine mastitis is the infection of udder tissues of cows that reduces both, and therefore it causes considerable economic damage to milk producers. Nowadays, the most widely adopted method to detect mastitis is by determining the somatic cell count per milliliter of milk. However, it requires qualified personnel and sometimes the results take a long time to be available, hampering an effective solution. The electrical conductivity of the milk could also be used, but if the test is done manually by an operator neither is effective, since affects the normal operation of the parlour. In this work we propose a mastitis detection system based on the measuring of the electrical conductivity of the milk of each quarter during the milking. A new milking claw is designed to include the conductivity traducers inside it, which are connected to the rest of the measuring unit. As a result, the only necessary modification to the milking machine is to replace the original milking claw with the new one. The system also includes a central unit to process conductivity samples sent by each measuring unit to determine if a cow has mastitis or not. A prototype is successfully tested in field, obtaining a precision of 65% and a recall of 64% for infected cows, approaching to the state of the art. Nevertheless, our approach is, to the best of our knowledge, the first proposal that allows a cost-effective solution since it can be integrated to existing milking machines and capable of issuing early warnings.