Hendriks J, van Vliet G
Tijdschr Diergeneeskd. 1980 Sep 15;105(18):764-70.
Although the lungworm vaccine was found to be useful under field conditions, some questions remain, particularly epidemiological ones. Experiments with housed and grazing calves were carried out to elucidate a number of these problems. From these experiments in which vaccinated and unvaccinated calves were challenged, it was apparent: 1. that the immunity developed by vaccinated calves could resist a heavy challenge without clinical symptoms; 2. that vaccinated calves exposed to a heavy challenge could produce small numbers of lungworm larvae. It was shown that, despite the hard winter of 1978/1979, infectious lungworm larvae were present on the pasture in the spring of 1979.
