Changes in the sciatic nerves produce the curled-toe paralysis in growing chicks. There is a marked enlargement of sciatic and brachial nerve sheaths with sciatic nerves reaching a diameter four to six times normal size. Histologic examinations of affected nerves show definite degenerative changes in myelin sheaths, which when severe may pinch the nerve, producing a permanent stimulus that causes the curled-toe paralysis (Scott et al., 1982). When the curled-toe deformity is long standing, irreparable damage has occurred in the sciatic nerve and administration of riboflavin no longer cures the condition. Retarded growth, splay and hock-resting postures and leg paralysis, rather than curled-toe paralysis, have been reported in some studies as the predominant signs of riboflavin deficiency in chicks (Ruiz and Harms, 1988; Chung and Baker, 1990).
Turkey poults and pheasants exhibit clinical signs similar to those of the chick, whereas ducks and geese are more likely to have a bowing of the legs in conjunction with perosis (NRC, 1994). In the poult, a dermatitis appears in about eight days; the vent becomes encrusted, inflamed and excoriated; growth is retarded or completely stopped by about the seventeenth day; and deaths begin to occur about the twenty-first day. However, when poults were fed a corn-soy diet analyzed to contain 2.7 mg per kg (1.2 mg per lb) of naturally occurring riboflavin, the only signs exhibited by poults were a paralysis of one or both legs, poor feathering, poor growth and, finally, mortality (Ruiz and Harms, 1989). In the duckling, diarrhea and cessation of growth are generally associated with riboflavin deficiency. Other signs of riboflavin deficiency are retardation of growth (Illus. 3 and 4), diarrhea after eight to 10 days, and high mortality after about three weeks. When chicks are fed a diet deficient in riboflavin, their appetite is fairly good but they grow very slowly and become weak and emaciated. There is no apparent impairment of feather growth; on the contrary, main wing feathers often appear to be disproportionally long. Increased hematocrit, increased mean corpuscular volume, decreased mean hemoglobin concentration and a marked heterophil leucocytosis appeared in the chick prior to neurological manifestations (NRC, 1994).
