Skin immune responses in cattle after primary and secondary infections with Hypoderma lineatum (Diptera: Oestridae) larvae.

Cellular immune responses were examined in the skin of cattle after primary and secondary experimental infections with Hypoderma lineatum larvae. Skin biopsies were taken at 0, 6, 12, 48 and 96 h post-infection (h.p.i.). In primary infected animals the penetration of Hypoderma larvae was characterized by moderate inflammatory responses. The pattern of cellular changes in previously infected animals suggested an allergic or rheumatic process, probably as a consequence of the development of a type III hypersensitivity reaction against the second or successive reinfections. Perivascular infiltration with CD3(+) T lymphocytes was marked in infected groups, but especially in previously infected animals, with a significant increase with respect to uninfected controls at 48 h.p.i. B cells remained close to control values during primary infection, while they increased significantly 12 h.p.i. in reinfected animals. IgG(+) plasma cells were also very abundant during secondary infections, with significant differences from primary infected and uninfected animals from 6 h.p.i. onwards. These results suggest that secondary infections allow cellular responses which may be effective in killing some of the entering larvae, resulting in a degree of resistance.