Fowl typhoid, the disease, immunity and control
Although vaccination against fowl typhoid is done widely in Kenya, some of the mechanisms pertaining to pathogenicity of Salmonella gallinavum isolates, their pathogenesis and the host's immune response to infection are not clearly understood. At times, the disease has been observed in flocks which had been vaccinated with the live vaccine. This necessitated the study of various aspects of the fowl typhoid disease with a view of discovering the cause of the vaccination breakdowns. These aspects included the study of the disease in general, the study of the immune response to S. gallinavum isolates and the study of the various ways of controlling the disease. A new vaccine was developed and the bird's immune response to it studied. This was compared with the immune response induced by the currently used vaccine strain, CN 180. Isolates from fowl typhoid outbreaks were characterised biochemically and serologically. Subsequently, their sensitivity to antibacterial agents was tested. This included antibiotics, disinfectants and phages. Virulence testing (LD^q - calculations) was also done. All this was carried out in search for markers for S. gallinavum bacteria and also to evaluate the possibility of occurrence of different strains. The pathogenicity of S. gallinavum isolates was studied in birds at various age groups Twenty (20) isolates at various doses ranging from 9 10 to 10 organisms per millilitre were used to challenge different groups of day-old-chicks intraperitoneally. Adult birds were challenged with a pool of 3 S. gallinarum isolates at concentrations ranging from 50 to 10^ organisms per millilitre through intraperitoneal and oral routes. The pathogenesis of S. gallinarum in the host was studied by dosing 2 groups of 45-day-old unvaccinated birds intraperitoneally and orally, separately, with S. gallinarum isolate L41 at the dosage of 9.6x10 organisms per millilitre and 3.2x10 organisms per millilitre, respectively. The birds from each group were then killed, two at a time, at various intervals and their heartblood, livers, spleens, caeca processed for bacterial isolations. To elaborate on the diagnosis of fowl typhoid disease, a study of the relationship between the immune status of an infected bird and any one of the three S. gallinarum somatic antigens was carried out. This was done by collecting sera from various birds that were suffering or had suffered from fowl typhoid outbreaks and monitoring the antibody titres using indirect haemagglutination test. The sera were screened with the various antigens, separately. To confirm the field findings, cockerels were experimentally vaccinated with CN 180 and their humoral and cellular immune responses were monitored by iridirect haemagglutination test and macrophage migration inhibition test, respectively. The specimens were screened wi£h the various antigens separately. From the LD5Q experiments, isolate L46, with an LD^q of 1 organism was chosen as the experimental'Vaccine strain. Preliminary studies showed that at a dose of 1,000 organisms per adult bird intramuscularly, L46 behaved as if it was non-pathogenic to the birds. This dosage was used henceforth to study the immune response to L46 as a model for S. gallinarum infections. A concurrent study of the currently-used vaccine strain, CN 180, was also carried out. The humoral and cell-mediated immune responses were assayed using indirect haemagglutination test (IHA) and macrophage migration inhibition test (MIT), respectively. Birds were vaccinated with the 2 vaccines, respectively, and bleeding for both sera (for IHA) and cells (for MIT) was done every week up to 7th week post-vaccination, then, after every alternate week 3 times, and later once every month for a total period of 37 weeks. For the control of the disease, the ability of the vaccines to protect pullets from the disease was studied. This was monitored by challenging the vaccinated birds with a virulent strain of S. gallinarum and calculating the liver and spleen hypertrophy indices and the liver clearance capacity of the respective birds. The calculations were made after sacrificing a few birds after one, two, three and six days post-challenge and recording their body weights, liver weights, and spleen weights. The antibiotic and disinfectant sensitivity tests were used to survey the various antibacterial agents that could be utilized to control fowl typhoid. The findings indicated that the isolates recovered and characterized by the various methods were different strains. The pathogenicity study done with day-old chicks indicated that various isolates had different pathogenicities. The adult birds infected orally did not show any signs of disease while those infected intraperitoneally died at a rate proportional to the dosage given. The post-mortem lesions of the dead birds were classically the same as those reported in literature on fowl typhoid disease. The currently-used vaccine strain, CN 180, was confirmed to be non-pathogenic to day-old chicks. The study on the pathogenesis of S. gallinavum in the host bird showed that the intraperitoneally-inoculated birds had organisms in their livers and spleens as early as 3 hours post-inoculation and these could be detected up to 7 days post-inoculation. The organisms were not detected in the caeca of these birds. The orally inoculated birds had organisms in their crops up to 6 hours post-inoculation but none was detected in the gizzard, duodenum and caecum throughout the experimental period. These birds had organisms in their livers and spleens 24 hours post-inoculation after which the numbers fell to below detectable levels. In studying the relationship between the immune status of an infected bird (IHA titre or MIT) and any one of the three S. gallinavum somatic antigens, it was observed that the 3 antigens gave rise to varying degrees of immunity as shown by IHA titres and degree of macrophage inhibition (MI), but there was no constant pattern relating higher IHA titres or higher degrees of MI to one particular antigen. Strain L46 was found to induce an immune response that was very similar to that of CN 180, although there were times when it showed superiority. Both gave good humoral and cellular responses. The liver and spleen hypertrophy indices and liver clearance rates compared well between L46 and CN 180 - vaccinated birds, which showed that both produced a good immunity which got rid of the S. gallinarum organisms as soon as they entered the body. The antibiotic sensitivity testing showed that antibiotics that had good effect in controlling the disease were furazolidone, neomycin, chloramphenicol, tetracycline, ampicillin gentamycin and nitrofurantoin. The effective disinfectants for controlling S. gallinarum contaminations were Bromosept, lysol and biodan. This study indicates that the various S. gallinarum isolates studied are of different strains. The difference is either in their biochemical processes as indicated by their reaction to the antibiotics and disinfectants used; or in their phage-receptors, as indicated in the phage typing experiment where they showed differing inhibition-zone diameters. The varying degrees of immunity to the various S. gallinarum somatic antigens as shown in the study on the relationship between the immune status of an infected bird and any one of the three S. gallinarum somatic antigens could either be due to a difference in the amounts of antigens exposed by the isolates surveyed, or due to a difference in individual chicken's immune response towards the various somatic antigens. This, together with the possible emergence of new virulent strains of S. gallinarum, may explain the occassional breakdown of vaccinations with the live-attenuated vaccine, CN 180. The virulence and pathogenicity tests indicated varying virulence as shown by the mortalities in the various groups tested. This indicates a strain difference although one should note, here, that any reduced virulence may be due to differences in the maintenance of different isolates on artificial media, resulting in varying degrees of attenuation. For self defence, the host initiates both humoral and cellular immunity which gets rid of the organisms. The new vaccine produced, L46, compared well with the currently-used vaccine strain, CN 180, in their induction of immune response and protection to disease; and in some cases, it has shown greater efficiency. To control the disease, treatment using the various antibiotics given above is possible, but it should be noted that most of the survivors tend to remain carriers of the disease organisms. Disinfection is also possible. For prophylaxis, vaccination has proved more convenient and this study has shown that both L46 and CN 180 can be used as the vaccine strains. They give good immune responses.