Helminths (parasitic worms) can exert protective effects on autoimmune diseases by modulating the type of immune response, and deliberate infection with helminths is being explored as a potential therapeutic strategy for autoimmunity. However, the use of live helminths as therapeutic agents for autoimmune disease has a number of drawbacks, and it would be preferable to identify and use the immunomodulatory components of the helminths. Previously it has been shown that the immunomodulatory activity of the liver fluke Fasciola hepatica resides in its excretory-secretory products (FhES), and further analysis of FhES has identified 3 major components: an 68 amino acid alpha helical cathelicidin-like peptide (FhHDM1), a cathepsin L-cysteine protease (FhCL1), and peroxiredoxin (FhPrx). We have found that FhHDM1 is the most effective at preventing development of the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis (MS). In the current study, we have investigated the therapeutic effects of FhHDM1 when it is given after the first attack of disease in a relapsing-remitting EAE model. FhHDM1 significantly (p<0.0001) reduced the overall severity of the disease and the number of relapses compared to mice treated with vehicle alone. The effects were long-lasting, with mice continuing to show benefits for up to 70 days following a single course of FhHDM1 treatment. Preliminary investigation of the mechanism of action of FhHDM1 suggests that it is not affecting the adaptive arm of the immune response, but is exerting significant effects by modulation of innate pro-inflammatory immune responses and by limiting egress of immune cells across the glia limitans. The data suggest that this parasite-derived peptide has potential as a novel treatment for patients with MS.