General Overview Of Intestinal Nematodes
Nematodes are worms with bodies that are round in cross-section. They come in two broad categories: Intestinal nematodes and tissue nematodes. The distinction between these groups may seem arbitrary, because some intestinal nematodes migrate through tissue on their way to the gut, and some tissue nematodes spend part of their lives in the intestines! However, the difference between the groups will be clear if you focus on whether the adult form spends its time chiefly in the intestines or in other body tissues.
Intestinal nematode parasites are very common infections of man and his domestic animals. When the host becomes infected, often by oral ingestion of infective eggs, the parasite establishes in its intestinal niche. Parasite antigens are carried by intestinal dendritic cells, or drain freely in the lymph, to the local draining lymph node, the mesenteric lymph node. In the mesenteric lymph node antigen is presented to T cells in the context of MHC class II. The T cell becomes polarised towards one of several possible CD4+ T cell subsets, according to the cytokine environment at the time of antigen presentation. Elimination of intestinal nematode parasites is controlled by T helper 2 cells (Th2). Th2 cells secrete cytokines such as IL-4 and IL-13 which are capable of orchestrating the sets of effector mechanisms able to eliminate nematode infections. Typically these Th2-dependent effector responses are no good at controlling intracellular pathogens, which require a T helper 1 (Th1)-mediated response.
In order to eliminate the infection Th2 cells have to migrate from the mesenteric lymph node back to the site of infection where they control locally, through the secretion of type 2 cytokines, innate effector cells. Although numerous nematodes infect humans, six spend the majority of their lifecycle in the bowel lumen and are classified as intestinal nematodes: Ascaris lumbricoides; Trichuris trichiura (whipworm); Ancylostoma duodenale and Necator americanus (the two human hookworms); Enterobius vermicularis (pinworm); and Strongyloides stercoralis. Ascaris, Trichuris, and hookworms are commonly grouped together as the soil-transmitted helminths (STHs) due to a shared aspect of their life cycles.
Formalin and mercuric chloride-based low-viscosity polyvinyl alcohol (LV-PVA) are widely used by most diagnostic parasitology laboratories for preservation of helminth eggs and protozoan cysts and trophozoites in fecal specimens. Concerns about the toxicity of formalin and the difficulty of disposal of LV-PVA are powerful incentives to use alternate preservatives. Such alternatives have been marketed by several companies and are often presented as one-vial, non-mercuric chloride fixatives that aim at performing the same role as formalin and PVA combined.
