Changes in food intake and circulating leptin due to gastrointestinal parasitism in lambs of two breeds

A reduction in food intake is a prominent feature of many infectious diseases. However, the underlying mechanisms of parasite-induced anorexia in sheep are poorly understood. Here, we tested the hypotheses (a) that the degree of parasite-induced anorexia in lambs is influenced by their growth potential and (b) that nematode infection results in elevated plasma leptin concentration in lambs. The hypotheses were tested with Suffolk x Greyface (S) and Scottish Black-face (B) lambs that are known to differ in their growth potential (S lambs are of greater growth potential than B lambs). During a primary parasite infection, 24 out of 48 lambs per breed were trickle-infected with 7,000 infective Teladorsagia circumcincta larvae per day, 3 d/wk, for a period of 12 wk (experiment I). The lambs were then dewormed, and after a 2-wk interval, half of the 24 lambs per breed that were previously infected were reinfected for another 12 wk with the same parasite and dose as used in the primary infection (experiment II). In both experiments, infected lambs were fed grass pellets for ad libitum intake, whereas noninfected lambs were fed grass pellets for either ad libitum or restricted intakes. The S lambs were more susceptible than B lambs to nematode infection, as judged from the differences in fecal egg counts (P = 0.007). Parasitized lambs of the more susceptible breed (S) showed anorexia [i.e., a decrease in intake of 13% compared with uninfected controls (P = 0.01)], whereas no significant reduction in food intake was observed in lambs of the more resistant breed (B). Reexposure to nematode infection of previously infected animals tended to result in renewed anorexia in S lambs but not in B lambs (P = 0.08) in a similar extent as during primary infection. Plasma leptin concentrations did not differ between ad libitum-fed infected and control lambs but were greater in infected than in noninfected lambs at a similar level of food intake during both the primary (P = 0.02) and the secondary parasitic infection (P = 0.004) in both breeds. The results show that leptin may be involved in the response of lambs to infection but that it is unlikely that leptin alone is responsible for the parasite-induced anorexia in lambs.