There are probably several reasons that these researchers chose footpad injection. 1) It is a standard procedure, although it is discouraged because the inflammation interferes with feeding and mobility of the animal. This becomes less of a concern since the animals in this experiment were sacrificed 48 hours post-injection. 2) The pathway of lymphatic drainage of the footpad is well known. 3) Footpad injections elicit stronger immune responses in some strains of rodents than injections at other sites.

Strongly agree with Brian L. Hoffman Brian L. Hoffman . In fact, it greatly depends on personal experience and reserch design. We prefer intraabdominal injection while studying abdominal sepsis, hepatic dysfunction, microbiota, gut permeability, etc. However, I know collegues who use feet injections as tradition without rational explanation. Some people use tail vien, too. Personaly, feel no strong scientific background for the method.