Role of lipids in the transmission of the infective stage (L3) of Strongylus vulgaris (Nematoda: Strongylida)

Infective larvae (L3) of Strongylus vulgaris have limited energy stores for host finding and for infection. For transmission to occur, the larvae must have sufficient energy to (a) migrate onto grass, where they are ingested by their equine host (host finding), and (b) penetrate into the host gut. This study is designed to test the hypothesis that L3 larvae of S. vulgaris partition their energy stores between locomotory activity (used in host finding) and infection activity (penetration). Chronic locomotory activity was stimulated by incubating S. vulgaris L3 larvae at a constant temperature (38 C). After 8 days of treatment, locomotory activity ceased (exhaustion). Exhausted L3 larvae had significantly decreased total lipid when compared to controls (P < 0.05), but there was no decrease in levels of protein or carbohydrate. Lipids of S. vulgaris L3 larvae are comprised of 9 fatty acids, some of which are depleted in exhausted worms (14:0, 14: 1, 16:0, 16:1, 18:1, 18:2), whereas others (18:0, 20:4, 24: 0) remain unchanged. These data suggest that specific fatty acids provide the energy source for locomotory activity in S. vulgaris. Exhausted L3 larvae were also less able to penetrate host cecal tissue in vitro penetration assays when compared to controls (P < 0.05), suggesting that the depletion of individual fatty acids during locomotory activity also reduced infectivity. These data do not support the hypothesis that S. vulgaris L3 larvae partition their energy stores between host-finding and infection activities. A comparison of lipid storage profiles in the L3 larvae of 4 nematode species with similar transmission strategies (S. vulgaris, Strongylus edentatus, Strongylus equinus, and Haemonchus contortus) revealed similarities in the fatty acid composition of these species. These data suggest a relationship between transmission patterns and energy storage strategies in the L3 larvae of nematode parasites of vertebrates.