Pet ownership, particularly of dogs and cats, is common throughout the world. Gastrointestinal disorders in companion animals, especially diarrhea, are a common reason for pet owners to seek for veterinary advice. There is a modest prevalence of Cryptosporidium sp. infections in companion animals. Although clinical disease is observed infrequently (e.g., Mtambo et al., 1991; Nash et al., 1993; Xiao and Herd, 1994a; Abe et al., 2002a; Ederli et al., 2005), severe clinical illness has been reported in dogs, cats, and horses, occasionally associated with concurrent infections such as canine distemper virus, isosporiasis, or feline leukemia virus (e.g., Snyder et al., 1978; Monticello et al., 1987; Goodwin and Barsanti, 1990; Miller et al., 2003; Aydin et al., 2004).