So this requirement for bile salts of a specific kind may contribute to host specificity. However, such highly specific factors have only ever been demonstrated in the case of the eggs of the plant parasitic nematode Globodera rostochiensis , which is induced to hatch under the influence of root exudates from the host plants. Despite intensive efforts, the 'hatching factor' has never been isolated in a pure state and appears to be an unstable lactone, given the name eclepic acid. In general it is the overall pattern of stimuli that the parasite responds to, rather than a single specific stimulus. The actual attachment of parasites to the gut wall is not well understood.
Parasites usually occupy well-defined regions of the gut.
In species that have a wide host spectrum, the position of the parasite in the host gut can vary with the host species. In general, during infection, parasites usually attach fairly far back down the gut and then migrate forward to their preferred site of parasitism - presumably following gradients in the intestine. But many tapeworms, for example, regularly undergo diurnal migrations up and down the intestine. In order to grow parasites need a supply of nutrients. The synthetic capacity of helminth parasites is limited when compared to their vertebrate hosts.