Abstract
Background:
Ostertagia ostertagi is considered a great epidemic gastroenteric roundworm affecting cattle in temperate regions and places a central limitation on productivity. This fact is supported by a large number of reports.

Aim:
This study was carried out to detect specific antibodies against O. ostertagi in cow’s milk, hence, investigating the epidemiology of this parasite and its spread in the cows of Mosul city.

Methods:
Milk samples were obtained from 200 cows each separately. The animals were suffering from emaciation, weight loss, and watery green diarrhea. For ten minutes, milk was centrifuged at 1500 rpm, and the sediment was discarded while the supernatant was stored at -20 °C until analysis. Indirect enzyme-linked immunosorbent assay (ELISA) was employed to detect antibodies for O. ostertagi.

Results:
Among 200 animals, 115 animals were infected with O. ostertagi resulting in a total percentage of infection of 57.5% The infection increases significantly with the age of animals, with the highest percentage of infection recorded in animals up to 6 years 82.5% (P < 0.001). The relationship between the infection with O. ostertagi showed a significant seropositivity rate of infection in native animals 69.9% (P < 0.001) compared with imported ones (37.7%). The outdoor animals recorded a high prevalence of infection with O. ostertagi (70.2%), while the lowest prevalence of infection was shown in indoor animals. Some animals were treated with anthelmintic drugs, while others were not treated, so the animals that were not treated with any anthelmintic drugs showed a high prevalence of infection (64.9%).

Conclusion:
The infection by O. ostertagi is affected by different parameters, including the age of the animal, source and breeding management; moreover, in Mosul city the infected cows showed more seropositivity to parasites than the unaffected cows.

Key words: Cattle; ELISA; Mosul; Ostertagia ostertagi.