By Hybro veterinary department
The bacterium Escherichia Coli is present in the gut of every chicken but also in the gut of mammals including humans. The bacterium forms part of the normal intestinal flora that is necessary for the normal functioning of the digestive tract.
But the peaceful living together of chicken and E. coli can be disturbed and result in colibacillosis. Colibacillosis includes all different outcomes of an imbalance and infection with E. coli. In this article we will give an overview of the factors that can disturb the balance between poultry and Escherichia coli. Opportunist
Already in the beginning of the 20th century, bacterial culturing showed the presence of Escherichia coli in the digestive tract of healthy chicken. More recently DNA techniques revealed that E. coli is a minor habitant of the chicken gut. E. coli is one of the hundreds of bacterium species that are present in the gut. Estimations of the number of E. coli are 106 per gram feces.
E. coli is called an opportunistic bacterium that can cause disease at the moment the chicken has an impaired disease resistance. Apart from the host defence mechanisms, the virulence of the present E.coli bacterium and the number of present E.coli bacteria are factors that determine whether the balance between E.coli and the chicken becomes disturbed. When this happens, E.coli can be found on places in the body where it normally does not reside and where it can harm the chicken. Defence mechanisms of the host
In the defence against bacterial infections, the host has several mechanisms. The first way of defence is the skin and the mucosal barriers. At the moment these barriers are impaired or damaged, a bacterium can easily enter the body of the chicken. Damages of the skin are for example injuries through cannibalism, artificial insemination, bad litter, unhealed navels and others. Damage of mucous membranes of the respiratory and digestive tract can be induced by viruses, other bacterium species or toxic agents. Examples of viral infections of the respiratory tract are Infectious Bronchitis and Pneumovirus infections. Mycoplasma is a bacterium that damages the respiratory mucosa and presdisposes E.coli to cross the mucosa and enter the bloodstream. A whole list of circumstances can be mentioned that favour the entrance of E.coli into parts of the chicken body that can result in an unwanted E.coli infection. Infection pressure
High numbers of the bacterium E.coli can be found in feces. This means that dirty eggs carry a load of E.coli that can affect the hatchability and livability of the chicks hatched from the eggs. Also high numbers, up to numbers of 106 per gram, can be found in dust. In bad ventilated houses and in houses that have very dry litter, dust can be a problem. As the chickens inhale the dust, they are exposed to high numbers of the E.coli bacterium. During vaccinations with life respiratory vaccines or at the moment viral infections like Infectious Bronchitis are spreading within a flock, colibacillosis in the form of infections of the air sacs can be the result. Virulence factors
There are different methods developed to characterize E.coli isolates. Typing has been done by antibiotic resistance, toxigenicity, presence of adhesion factors like fimbriae and pilli, hemagglutination and others. Typing of DNA of the bacterium is done with different methods in epidemiology studies. From these studies it is concluded that only few genotypes of the bacterium can be found in the different forms of colibacillosis. These few genotypes can be found worldwide. Many different potential virulence factors have been investigated, but there is no single virulence factor that can be found in all pathogenic strains and is absent in non-pathogenic strains. Therefore it remains questionable whether the virulence of E.coli is an important determining factor in causing disease.
At the moment E.coli enters the chicken body and growths at unwanted places in the chicken, it can cause an infection. Unlike mammalian E.coli strains, the avian pathogenic strains produce no toxins or a low number of toxins. They are therefore relatively safe for humans. At the moment a local infection with E.coli occurs, exudates will be released from the local blood vessels around the spot of infection. The heterophils and macrophages, the phagocytic white blood cells of the host, will migrate to the spot and kill the bacterium. During the infection the exudate turns from colourless fluid to white, yellowish cheese-like material. Bacterium colonies, phagocytic cells and fibrin are present in this caseous exudate. As the bacterium are removed by the phagocytic cells during the inflammation process, which can take weeks to months, the fibrin will be organized and eventually turned into scar tissue. At the moment the bacterium is attacked by phagocytic host cells and falls apart, endotoxins are released. Endotoxins are the lipopolysaccharids that are a structural part of the cell wall of the bacterium. When many E.coli bacteria are present and killed by heterophils, the release of high numbers of endotoxins can cause septicaemia and as a result sudden death.
Local or systemic
Infections caused by E.coli can stay local or systemic. Local infections are limited to one or more organs of the chicken. A systemic infection occurs when the bacterium enters the bloodstream and spreads through the body via the bloodstream. Known examples of local infections include the navel infection (omphalitis), the oviduct infection (salpingitis), infection of the air sacs (airsacculitis) and infection of the skin (cellulitis).
Systemic infections occur when the bacterium spread through the body via the bloodstream. Bacteriaemia is the right word for this situation. Septicaemia describes the situation that bacteria are present in the bloodstream and causes toxic reactions in the host. After the bacterium has been cleared from the bloodstream, some E.coli can hide and colonize in small blood vessels where the bloodstream is low. Examples of such places are the joints and tendons. When the bacterium load is high a systemic infection can result in polyserositis, which includes inflammation of all serous membranes around internal organs like the heart, liver and intestines.
As mentioned above three factors increase the risk of colibacillosis: Infection pressure, impaired host defense and virulent E.coli strains. Prevention should therefore be focused on these three factors. First of all one should strive to minimize the factors that impair the primary and secondary defense mechanism of the host. Minimize the risk of other infections of viruses and bacterium species like Mycoplasma. Mycoplasma gallisepticum infections in broilers are a known predisposing factor for colibacillosis that is seen in the chronic respiratory disease syndrome. Reduce mechanical damage that can be caused by aggressive chicken, artificial insemination, sharp equipment, toxic agents like ammonia and other causes.
To minimize the risk of omphalitis it is important to optimize the hatching process to prevent unhealed navels. Too hot temperatures, but also too low temperatures during the hatching process can induce unhealed navels. Apart from damage of the primary barriers like skin and mucous membranes, can also the secondary defense, the immune system of the chicken be impaired by stress. Think about replacements, too high airflows, aggressive males, feeding during moment of laying etc.
Secondly, the bacterium load should be minimized by implementing good egg hygiene and hygiene in the hatchery in the poultry house. Reduce the amount of dust by sufficient ventilation or wettening of the litter. One can influence the virulence of the E.coli strains only limited. Regular use of antibiotics increases the risk of development of resistant strains.
Several vaccines have been developed, but the use of these vaccines have limited effect. In case hygiene and managerial circumstances are not optimal, some effect of vaccination is expected.