Immunity in equine cyathostome infections

Defining the characteristics of immunity and immune responses to equine cya thostome infections is clearly important to advancing our understanding of the development of these nematodes within the host, the clinical conditions attributed to them, and in developing more rational and novel strategies f or their control. Nonetheless, little is currently known on this topic. Cur rent data based on field observations, worm burdens and fecal egg counts su ggest that horses acquire a resistance to cyathostome infection with age. T his response is slow to develop and incomplete in that most horses regardle ss of age harbor significant populations of these nematodes. More convincin g evidence has been obtained from experimental infections which indicate th at mature horses previously exposed to infection are resistant to re-infect ion and this resistance is directed at all stages of the parasite life cycl e. Further, some immunity against the developing stages within the mucosa a ppears to require less exposure and occurs in younger animals. Some non-spe cific events which induce expulsion of all species of lumenal dwelling nema todes also appear to take place post-infection with L3. Antibodies have bee n detected in Limited studies against somatic extracts of adult worms. Not surprisingly, titers of these antibodies do not correlate resistance to re- infection. Serendipitous observations have, however, associated a greater e xpression of the gene for IL-4 with the spontaneous expulsion of lumenal pa rasites. The development of a usable model is required to further advance o ur knowledge in this area. (C) 1999 Elsevier Science B.V. All rights reserv ed.