Twelve-week-old indigenous chickens, either immune-suppressed using dexamethasone (IS) or non-immunesuppressed (NIS), were challenged with a low virulent strain, Pasteurella multocida strain NCTC 10322 T , and developed clinical signs and pathological lesions typical of chronic fowl cholera. NIS birds demonstrated much more severe signs of fowl cholera than IS birds. With few exceptions, signs recorded in IS and NIS birds were of the same types, but significantly milder in the IS birds, indicating that immune suppression does not change the course of infection but rather the severity of signs in fowl cholera. P. multocida signals by fluorescent in situ hybridization (FISH) were observed between 1 h and 14 days in the lungs, trachea, air sacs, liver, spleen, bursa of Fabricius and caecal tonsils, while signals from other organs mostly were observed after 24 h. More organs had FISH signals in NIS birds than in IS birds and at higher frequency per organ. Many organs were positive by FISH even 14 days post infection, and it is suggested that these organs may be likely places for long-term carriage of P. multocida following infection. The present study has demonstrated the spread of P. multocida in different tissues in chickens and distribution of lesions associated with chronic fowl cholera, and pointed to a decrease of pathology in IS birds. Since dexamethasone mostly affects heterophils, the study suggests that these cells play a role in the development of lesions associated with chronic fowl cholera in chickens.
IntroductionPasteurella multocida subspecies multocida of capsular type A is the cause of different diseases in animals, including fowl cholera in chickens (Christensen & Bisgaard, 2000). This is a severe, systemic condition with sudden onset of clinical signs and high mortality (Glisson et al., 2008). However, in certain production systems, such as the common free-range scavenging poultry production systems in developing countries, the bacterium may be endemic present (Muhaiwa et al., 2001). Under such circumstances, outbreaks of acute fulminating disease are rare, and infected birds show more chronic clinical signs (Mbuthia et al., 2008).During fowl cholera infections, P. multocida is believed to enter primarily through the lungs (Christensen & Bisgaard, 2000), even though it has also been shown to invade the intestinal wall in an in vivo chicken loop model (Christensen et al., 2002). The bacterium probably adheres to the epithelial lining and subsequently associates with resident macrophages (Matsumoto et al., 1991), and, through a yet to be understood mechanism, it translocates to the bloodstream and from there to internal organs. Polymorphonuclear neutrophils (PMNs) (heterophils) are considered the first line of defence against the bacterium, but the role of this cell type seems dual, since selective removal of these have been shown to diminish pathological changes (Bojesen et al., 2004). Bacteria have been reported to be detectable in the blood as soon as 6 to 12 h post infection (p.i.) (Rhodes & ...