| dc.description.abstract | Infectious bursal disease (IBD), an economically important viral disease of chicken was
first encountered in Kenya in 1991. Despite vaccinations done to control the disease,
outbreaks occur in vaccinated birds. Factors that lead to vaccination failure have not been
investigated and documented. This study was designed to document outbreaks in
vaccinated birds, isolate the virus and study the pathogenicity in chicken embryos and
further investigate the risk factors at vaccine handling that lead to vaccine failure in
broilers in Kwale, Kenya. In addition, determine antibody titers in four; three vaccinated
and one not vaccinated, selected broiler flocks in Kwale.
Eight outbreaks in vaccinated and four in unvaccinated broiler flocks in Kwale were
investigated. The bursa of Fabricius, spleen, caecal tonsils, thymus and Harderian gland
were examined histologically and the lesions scored on a scale of 0 to 5. The virus was
isolated in specific pathogen free chicken embryos. Embryo mortality and pathological
lesions observed were recorded. One hundred and six structured questionnaires were
administered to broiler farmers, breeding farms and agrovet shops to determine types of
vaccines in use, their handling and administration. One flock in each of the four randomly
selected farms was monitored for presence of antibodies for six weeks; from week one to
week six.
The results showed that morbidity in the outbreaks ranged from 16% to 80%. Mortality in
the vaccinated flocks varied from 5% to 75% while in the unvaccinated flocks the range
was 13% to 56%. The clinical signs were sudden, severe and characterized by depression,
white watery diarrhea, and death in .all the outbreaks. The carcasses had haemorrhages on
the thigh and breast muscles, adipose tissue and on the serosal surface of the intestines.
The bursa of Fabricius was swollen, oedematous, haemorrhagic, atrophied or contained
cheesy exudates depending on the stage of the disease. Lymphoid depletion, infiltration
by inflammatory cells, cystic formation and fibrosis were seen in the bursae. Congestion
and cellular changes were observed in the spleen, thymus, caecal tonsils and Harderian
gland. Bursal lesion scores were high in the range of 4 and 5. The high mortality, severe
clinical signs and high bursal scores strongly suggest the virus as the highly pathogenic
type. Mortality rate in inoculated embryos was high; up to 100%, in the first passage and
lesions severe as observed with hypervirulent strains. Intermediate vaccines from six
different companies were used by broiler farmers. None of the farmers handled vaccines
correctly. The estimated relative risk (R') value for vaccine failure was highest for
storage of vaccines in the un-recommended temperatures at the farms (R'=8.7), use of
few drinkers to administer the vaccine (R'=5.3), presence of disinfectants in drinkers at
vaccination (R'=2.8), use of non-ffiD vaccines (R'=2.1), long duration taken to
administer vaccine (R'=1.8), use of inappropriate diluents (R'=1.6), and failure to follow
the recommended vaccination regime (R'=1.5). Storage in the un-recommended
temperature and use of few drinkers were statistically significant: p=0.0193 and p=0.001
from chi-square and p=0.0261 and p=0.0015 from Fisher's exact test respectively. The
antibody titers in all the four farms monitored were below protective levels (log26)
throughout the production period. Maternal antibody titers ranged from 1:2 to 1:16 and
the post-vaccination titers ranged from 1:2 to 1:32. In conclusion many factors seem to
lead to vaccine failure. | en |
