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Abstract. Sera from 53 sheep belonging to Castellano, Churro, Manchego, and Merino breeds were analyzed to test the diagnostic value of a 26-kD antigen from adult Haemonchus contortus at prepatency and early and late patency of experimental haemonchosis. Animals that received zero, 1, or 2 infections with the parasite were tested. In addition, sera from 20 experimentally infected and 10 noninfected Texel sheep were used to test the antigen. Sera from 37 infected animals at prepatency as well as at patency in primary and secondary infection were found positive with the 26-kD antigen. However, sera from 10 animals with the lowest worm burdens (second infection) did not recognize the antigen during early patency (day 28 postinfection). IgG1 was the only isotype implicated in antigen recognition because IgG2, IgA, and IgM, in the same sera, showed no reactivity with the peptide. Antigen specificity was confirmed because hyperimmune sera against infective larvae and adult stages of the most common gastrointestinal nematodes found in natural infections in sheep (Trichostrongylus colubriformis and Teladorsagia circumcincta) did not recognize this peptide. The antigen was recognized only by anti-adult H. contortus hyperimmune sera and appeared to be absent in the L3 parasite stage. In addition, the partial N-terminal amino acid sequence of the diagnostic peptide is reported.Sheep haemonchosis is a widespread parasitic disease in the temperate areas of the world. Diagnosis is based on coprological methods, which are time consuming and require specially trained personnel. A rapid and simple test with which a considerable number of samples could be processed at the same time would be advantageous. Circulating antigens could not be detected in infected animals, 22 and the assays developed to detect parasite antigens in the feces of infected animals, although occasionally able to detect surface antigens of the parasite, were not always related to the parasite burden. 10 In addition, methods used to detect specific DNA are restricted to patency and are expensive to use routinely. 6,23 The detection of specific serum antibodies against the parasite by ELISA has produced inconsistent results because of the complex nature of the antigen extract used 5 and limited value of the soluble extracts as an antigen source. 21 In addition, cross-antigenicity has been widely described among related and unrelated nematodes. 1,7,9,27,28 The use of less complex antigen mixtures, such as excretory/secretory products, has led to better results in ELISA tests, 24 but there is still some cross-reactivity between related nematodes, both in ELISA and in Western blotting. 25 From the Departamento de Patología Animal I, Facultad de Veterinaria, Universidad Complutense de Madrid, 28040 Madrid, Spain (Gó-mez-Muñoz, Domínguez, Fernández-Pérez, Méndez, de la Fuente, Alunda, Cuquerella), and the Servicio Territorial de Agricultura y Ganadería, Medina de Rioseco, 47800 Valladolid, Spain (Gómez-Iglesias).Received for publication July 31, 1998.A partially ch...

Resistance to primary and secondary infections with Haemonchus contortus was studied in 10‐month‐old Manchego and Merino sheep. No notable interbreed differences were observed after primary infections in the parameters determined (prepatency period, faecal egg output, abomasal worm burden). Previously infected sheep (200 L‐3/kg live weight (lw)) from both breeds showed notable protection after challenge (400 L‐3/kg lw), evidenced by lower eggs/g faeces (epg) values and worm burdens. A protective response in the Manchego breed was associated with arrested development of fourth stage larvae in the abomasal mucosa, whereas in the Merino breed a more rapid expulsion mechanism seems to be involved. Serum antibody levels (IgG, IgA) were infective dose‐dependent and protection from re‐infection was not clearly related to the parasite‐specific IgG response estimated by enzyme‐linked immunosorbent assay (ELISA) and Western blotting.