Dynamics of infection in rats given low doses of homogonic and heterogonic Strongyloides ratti

SUMMARY The results of control infections from 39 experiments using the ‘exact dose’ technique are analysed and compared with earlier large dose experiments with homogonic S. ratti. A smaller error variance and less added variance between experiments accompany the new method, which therefore provides a more precise experimental system. Using the exact dose technique, it is established that the migratory bahaviour of small numbers of L3 of the homogonic strain in lactating animals is the same as that of large numbers. In previous work the rate of migration of the homogonic strain estimated from conventional large dose experiments (Wilson, Cameron & Scott, 1978b) differed from that deduced from the use of radio-isotope labelled L3 (Wilson, 1979). ‘Exact dose’ data support results with the latter technique. There are significant differences between strains. The heterogonic larvae take longer to migrate in lactating hosts and appear to distribute themselves differently between mother and offspring. The consequences of these findings for theories of pathfinding in hosts are discussed. Experiments are also reported which failed to detect any artifact which might have resulted from ether anaesthesia of rats at injection of larval worms. This observation has implications for other studies in progress in this laboratory.