LONG-TERM STUDY OF TOXOPLASMA-GONDII INFECTION IN A SWEDISH SHEEP FLOCK

The infection rate of Toxoplasma gondii was studied during 6 years in a sheep flock in central Sweden. The flock consisted of 165-249 breedi ng ewes of which 20-35% were lambs less than 1 year old. Most ewes wer e slaughtered when 5 years old. The sheep were kept indoors from end o f September to early May. Lambing took place in March and April. Indiv idual serum samples were collected twice a year, once just before turn ing the sheep out to pasture in the spring, and again after housing in the autumn. Sera were analysed by ELISA for antibodies to T. gondii. The seroprevalence varied between 10% and 45% during the 6 years of ob servation. Seroconversion was detected predominantly at the autumn sam pling, indicating that in most cases infection was acquired at pasture . Subclinical effects of T. gondii infection on lamb weight, litter si ze, total litter weight and ewe weight were also studied. Lambs born t o chronically infected ewes were lighter al birth than those of uninfe cted ewes, but this disparity was no longer evident at weaning.