Colibacillosis is a common infection in broilers and broilers breeder’s flocks caused by avian pathogenic E.coli (APEC) and resulting in major economic losses.
Mortality, the cost of treatment and the decrease in feed conversion efficiency result in significant economic losses cost to the poultry industry. Clinical signs of colibacillosis can be observed in form of high mortalities, respiratory manifestations and poor growth rate.
The pathogenicity of E.coli is depending on the presence of one or more virulence factors including invasiveness factors invasins, heat labile and heat stable enterotoxins, verotoxins and colonization factors or adhesins. It is quite difficult to differentiate between pathogenic and nonpathogenic E.coli strains on the basis of antigenic structural and biochemical characteristics.
APEC infection can be controlled by antimicrobial agents based on antibiotic susceptibility testing which considered the proper technique to determine the antimicrobial of choice due to resistance issue resulting from misuse of antibiotics.
The decision to use antimicrobial therapy depends on the susceptibility of the microorganism and the pharmacokinetics of the antimicrobial achieves the desired therapeutic concentration at the site of infection and thus clinical efficacy.
Neonatal infection of chicks can occur horizontally, from the environment, or vertically, from the hen. A laying hen suffering from E. coli-induced oophoritis or salpingitis may infect the internal egg before shell formation. Faecal contamination of the eggshell is possible during the passage of the egg through the cloaca and after laying.
Before hatching, APEC cause yolk sac infections and embryo mortality. The chick can also be infected during or shortly after hatching. In these cases, retained infected yolk, omphalitis, septicaemia and mortality of the young chicks up to an age of three weeks is seen .
Broilers may be affected by necrotic dermatitis, also known as cellulitis, characterized by a chronic inflammation of the subcutis on abdomen and thighs . The affected animals usually do not show any clinical signs, and the lesions are sometimes only detected at the slaughter plant .
Airsacculitis is observed at all ages. The bird is infected by inhalation of dust contaminated with faecal material, which may contain 10 6 CFU of E. coli per gram . In animals with respiratory defenses compromised by other pathogens or by an unfavourable housing climate , inhalation of contaminated dust causes airsacculitis with sero-fibrinous exudates.
Septicaemia also affects chickens of all ages, and is mainly described in broilers. It is the most prevalent form of colibacillosis, characterised by polyserositis . It causes depression, fever and often high mortality. When E. coli reaches the vascular system, the internal organs and the heart are infected. The infection of the myocard causes heart failure . Septicaemia occasionally also leads to synovitis and osteomyelitis .
Virulence Factors of APEC
Virulence factors enable E. coli to colonise the mucosal epithelium, evoke an inflammatory reaction and eventually proceed to tissue invasion. The capacity of E. coli to produce virulence factors contributes to its pathogenicity. These virulence factors enable some members of the normal flora to elicit an infection by overcoming the host defense mechanisms.
Control and prevention of Collibacillosis
Approaches to prevent and control APEC infections in the poultry industry include improved hygienic methods, vaccination, use of competitive exclusion products, and the introduction of novel immunopotentiators, however, each of these practices have had limited success.
This has necessitated the use of antimicrobial chemotherapy to control outbreaks of colibacillosis, however, recent reports have described increased resistance to those antimicrobial agents commonly used for treatment .
Avian pathogenic E. coli are often highly resistant, especially to tetracycline, streptomycin and sulfonamides. High levels of resistance to ciprofloxacin have been also reported.
In medicine, frequency of strains resistant to ampicillin, trimethoprim–sulfamethoxazole, streptomycin, Ciproflaxin, chloramphenicol, tetracycline and cephalosporins (ceftriaxone or ceftazidime) is a cause of growing concern.
Commensal E. coli strains from poultry have similar patterns of resistance but at lower frequencies. Antibiotic administration is considered the most important factor promoting the emergence, selection and dissemination of antibiotic-resistant microorganisms in both veterinary and human medicine.