On Aug. 25, 1872, a disease that spread through Detroit’s horse population temporarily halted streetcar runs and forced express companies to deliver and collect in handcarts.
Sources:
Mich-Again’s Day.
Equine influenza wikipedia article.
Adoniram B. Judson, “History and Course of the Epizoötic among Horses upon the North American Continent in 1872-73.” Public health papers and reports 1 (1873): 88-109 online agrees but indicates the equine influenza probably reached Detroit in mid-October.