Attempts were made to induce increased resistance to caecal worm infections in chickens and turkeys by giving 5-week-old birds embryonated Heterakis eggs. To test for such resistance, the birds were challenged 28 days later by feeding comparable numbers of embryonated eggs. To study the possible influence of any response on the transmission of blackhead, some of the birds received Heterakis eggs from worms grown in birds that had blackhead.

In blackhead-free chickens there was only a slight reduction in the number of worms recoveied from the initial infection as a result of the superimposed second infection. However, there were 37% fewer worms from the second feeding of eggs than were present in the control birds that had had no previous Heterakis infection. In blackhead-free turkeys, the immunized birds had fewer worms from both doses than did the controls. These reductions averaged about 40% in both instances.