Immunity to Dictyocaulus viviparus in Calves: II. Vaccination with Normal Larvae or Larvae Attenuated by Triethylene Melamine (TEM) or X-irradiation and Exposure to Experimental Challenge

Abstract

Experiment I. Calves vaccinated with 2 doses of 1,000 D. viviparus larvae attenuated with triethylene melamine (TEM), together with calves similarly dosed, hut with normal larvae, and untreated controls, were challenged with 10,000 normal larvae. The TEM vaccine was fully attenuated and non-pathogenic, and protected calves against the challenge which produced severe symptoms in, and killed, 2 of the 4 controls. Mild clinical signs occurred in the vaccinates after challenge with increased respiratory rate and low grade faecal larval output (6°/0of controls) but weight gain was maintained. The calves receiving normal larvae showed moderate clinical signs with high respiratory rate, weight loss and faecal larval excretion. The challenge did not exacerbate the symptoms and appeared to have no effect on the course of the initial infection which was eliminated with a return to normal weight gain. The initial weight loss was, however, not regained. Experiment II. Calves vaccinated at intervals of 2 or 4 weeks with 2 doses of 1,000 D. viviparus larvae attenuated with TEM were compared with calves vaccinated with X-irradiated or normal larvae with a 4-week interval between doses and, together with untreated controls, exposed to a challenge of 7,500 larvae. The groups vaccinated with TEM-attenuated larvae showed no untoward clinical effects during vaccination but a slight reduction in weight gain was apparent and a few faecal samples were positive for larvae. The irradiated larval vaccine was not associated with reduced weight gain and only one faecal sample was positive for larvae. Following challenge, mild clinical symptoms were present in all the vaccinates with low levels of faecal larval output and moderately elevated respiratory rates. Weight gain continued steadily in contrast to controls which suffered severely from the challenge with one fatality, substantial weight loss and high respiratory rate and faecal larval excretion. One death occurred in the TEM vaccinated group receiving vaccine at a 2-week interval. This calf unlike any of its fellows in the group, behaved like the controls and apparently failed completely to respond to vaccination. The group receiving normal larvae suffered moderately from the initial infections but did not appear to be affected in any way by the challenge infection.