Blood samples were collected from all selected commercial chickens while cloacal swabs were collected from 326 of the total sampled commercial chickens. Among the scavenging local chickens, the sero and cultural prevalences were 6.3% and 0% respectively whereas in commercial layers the sero and cultural prevalences were 26.9% and 11.3% respectively. Sero and cultural prevalences were found to be significantly higher (P<0.001) in commercial layers than in scavenging local chickens. Also, the risk of infection in scavenging local chickens kept in-contact with commercial chickens was found to be five times higher than the risk of infection in scavenging local chickens with no contact with commercial chickens (RR=5). The difference in infection rates between the scavenging local chickens and commercial layers was attributed to extensive and intensive type of management respectively, or due to resistance mechanisms among local chickens to fowl typhoid.

The genetic diversity of 63 S. gallinarum isolates from chickens in Tanzania was determined using plasmid profiling and ribotyping (Chapter 5 Publication 5.2). Plasmid profiling demonstrated two plasmids of different molecular weights (85kb and 2.5kb). Basing on plasmid carriage, three different plasmid profiles were observed. Fifty one isolates carried both 85kb and 2.5kb plasmids, five isolates carried only 85kb plasmid and seven isolates had no plasmids. Seven different ribotypes designated Ha through Hg were demonstrated indicating the chromosomal differences that exist in Tanzanian S. gallinarum isolates. Of the seven ribotypes, ribotype Ha was the commonest. Both typing methods, plasmid profiling and ribotyping were capable in tracing back the source of infection and in identifying one strain which had persisted for many years. However, ribotyping using HindIII restriction enzyme, was more discriminatory than plasmid profiling. Forty seven of the total isolates typed had similar results on plasmid profiling and ribotyping indicating that probably they belong to one clone. The experimental study on the pathogenicity of S. gallinarum infection in local and commercial broiler chickens is described in chapter 5 publication 5.3. This experiment was conducted for a period of 14 days on 19 local and 19 broiler chickens. Basing on clinical signs, mortalities, pathological features, serological response and antibody titres, both groups of chickens were found to be susceptible to S. gallinarum infection However, basing en severity of the disease and mortalities, local chickens were found to be more susceptible than commercial broilers. It was concluded that, under experimental conditions, local chickens are more susceptible to fowl typhoid than commercial chickens. This suggests that, the low infection rate observed in scavenging local chickens in the field studies (Publication 5.1) was likely due to extensive type of management and not resistance mechanisms to fowl typhoid Nonetheless, within local chickens, some were found to be less susceptible to S. gallinarum infection as they did not die to the end of the experiment.