Effects of low-dose naltrexone on feed intake, growth, endocrine and immune parameters in the recently-weaned pig

Weaning is a stressful event for pigs and causes decreased feed intake, poor growth, and increased susceptibility to disease. Previous studies have shown that syndyphalin-33, a synthetic opioid, was effective in increasing feed intake, abrogating the changes in appetite regulating genes during weaning, and abrogating the effects of a salmonella challenge on immune cells in newly-weaned pigs. However, there are several concerns associated with the administration of an opioid in commercial swine operations. Low-dose naltrexone (an opioid antagonist) has been used to alleviate symptoms from fibromyalgia and Crohn’s disease in humans. As inflammation is a common factor in both auto-immune diseases in humans and weaning stress in pigs, a logical next-step would be to examine the effects of low-dose naltrexone on pigs at weaning. In this study, low-dose naltrexone administration was evaluated for its affects on feed intake, growth, endocrine, and immune parameters in newly weaned pigs. Four treatments of 0 mg/day (d), 1 mg/d, 5 mg/d, 10 mg/d naltrexone were administered orally daily beginning 2 wk prior to weaning to 48 commercial crossbred pigs. Each treatment group included 12 pigs. Body weights and blood samples were collected d 0 and at 1, 4, and 7 post-weaning. All animals treated with naltrexone had increased total gain as compared to the control animals (P < 0.05). A decrease in feed intake was seen in animals treated with 5 mg/d naltrexone as compared to the control animals (P < 0.05). Plasma cortisol concentrations were similar to previously published concentrations and increased 1 d post-weaning in the control animals (P < 0.01). Animals treated with 10 mg/d naltrexone had higher plasma concentrations of cortisol relative to all other treatments (P < 0.01). On 1 d post-weaning, animals treated with 1 mg/d and 5 mg/d naltrexone had lower plasma cortisol concentration than the controls (P < 0.01), and by d 4 post-weaning, all animals treated with naltrexone had lower cortisol concentrations relative to the control animals (P < 0.01). As a non-opioid, oral low-dose naltrexone may be a promising therapy to decrease the growth lag associated with weaning.