Trichostrongylid parasite populations in pregnant or lactating and unmated Florida Native and Dorset/ Rambouillet ewes

This study was undertaken to identify factors which might contribute to differences in the periparturient rise (PPR) in fecal egg counts observed in two sheep breeds. The population dynamics of naturally acquired trichostrongyle infections were studied in unbred and pregnant or lactating Dorset/Rambouillet and Florida Native ewes. The sheep grazed infective pastures in the fall and were housed from December until the end of the experiment in June. Fecal examination showed consistently higher egg counts in Dorset/Rambouillet sheep than in Florida Natives throughout the experiment. Lambing Dorset/Rambouillet ewes showed a significant periparturient rise (PPR) while the fecal egg counts of periparturient Florida Natives did not differ from controls of that breed. The total worm burdens and parasite species composition were determined in pregnant or lactating and unbred animals of each breed at about 65 and 125 days post-breeding and at approximately 6 weeks post-lambing. Dorset/Rambouillet ewes had significantly higher trichostrongyle burdens than Florida Natives throughout the experiment. At the point 6 weeks post-lambing, unbred Dorset/Rambouillet and Florida Native ewes had significantly lower worm burdens than lactating animals. When the developmental stages of the parasite were examined, pregnant animals of the two breeds were found to have fewer arrested larvae than unbred animals. These results suggest that although a distinct periparturient egg rise does not occur in Florida Native sheep, they may still be subject to some of the immunosuppressive changes associated with lactation.