Identification of Candidate Components of a Novel Bordetella Avium Vaccine to Prevent Turkey Coryza

The bacterium Bordetella avium causes upper respiratory tract disease in avian species writes Dr. Andrew Preston of the University of Bristol in a paper presented by the Canadian Poultry Industry Council.

23 May 2008

7 minute read

The Challenge

Commercially raised turkeys are particularly susceptible to B. avium infection, leading to turkey coryza, a disease that is often characterized by mouth breathing, oculonasal discharge and collapse of the trachea (windpipe).

Although turkey coryza is rarely fatal, it is associated with poor weight gain and thus many birds have stunted growth. In addition, it is highly contagious and infection spreads quickly through flocks. The damage caused to the airway of turkeys by B. avium increases the susceptibility of these birds to other respiratory infections. The increased incidence of concurrent infections during outbreaks of turkey coryza contributes to the poor performance of B. avium-infected flocks and is an important consequence of turkey coryza.

Antibiotic treatment of turkey coryza has variable success, it is likely that antibiotics in fact treat secondary infections rather than the B. avium infection. There is only one B. avium vaccine licensed for use in Canada, Art-Vax that comprises an attenuated B. avium strain. Although some studies suggested that vaccination with Art-Vax reduced the risk of B. avium infection, for example, a larger number found that the vaccine only moderately decreased the severity of disease or delayed its onset and did not protect young poults.

Since B. avium survives for many months on litter waste and in drinking water systems it is very difficult to eliminate the bacteria from the turkey’s environment. Thus, extensive clean up measures are required to remove the bacterium from contaminated premises, including complete removal of all litter and disinfection of all surfaces, feeders and drinking water systems. It is very easy to spread B. avium from an infected flock to a clean one. Prevention of this requires strict biosecurity measures. In summary, turkey coryza presents a significant problem to the turkey industry, causing poor flock performance and increasing the incidence of other infections. Currently available vaccines to prevent infection or antibiotic treatment of cases do not appear to work well and disinfection of premises following an outbreak is a costly and disruptive process.

A major barrier to the development of improved therapies against B. avium infections is that very little is known about this bacterium or how it infects birds to cause disease. However, a major breakthrough in understanding the biology of B. avium is on the horizon, as the genome sequence of a representative strain of B. avium has been generated and analysed. This has revealed the entire genetic make-up of the bacterium.

Within the genome sequence information are all of the bacterial factors involved in the infection of turkeys and causation of disease. Thus, there is the potential to identify and characterize the B. avium components that are important to the disease process. The availability of genome sequence information will hugely accelerate the progress of research by replacing conventional methods for gene identification that are both slow and costly. Thus there is now the potential to dramatically increase our knowledge of B. avium, the ultimate use of which will be to design new and improved therapies.

The Research

This project conducted a detailed analysis of the genome sequence information as a first step towards better understanding B. avium. Part of this analyses identified that B. avium contains a number of genes that are predicted to enable the bacterium to express fimbriae on its cell surface. Fimbriae are protein appendages that radiate away from the bacterial cell surface and as such reach out in the bacterium’s environment. Fimbriae expression is common among many different bacteria. In pathogenic bacteria they have important functions in attaching the bacteria to host tissues, enabling the bacteria to gain a foothold in a host and are thus very important to establishing infections. Preston and his research team chose to conduct a detailed investigation of B. avium fimbriae, with the aim of characterising their role in the infection of turkeys and in inducing immune responses in infected birds. Bacterial components that induce immune responses are potential candidates for inclusion in vaccines.

B. avium contains genes that suggest it might produce several different types of fimbriae, that might have different roles in infection. Before being able to study the functions of B. avium fimbriae, it is important to confirm that it actually produces these structures, and that each of the different fimbrial genes is functional.

Firstly, the research team investigated if the genes were read (transcribed) by the bacterial RNA synthesising apparatus and the conditions under which this happened. They verified that each fimbrial gene is read, suggesting that the entire fimbrial gene repertoire of B. avium is functional. Furthermore, they revealed that this process is regulated such that it is responsive to the temperature at which the bacteria are growing and is greatest at host body temperature, suggesting that fimbriae might be involved in the survival of the bacteria within hosts.

The team attempted to correlate the reading of the fimbrial genes with the production of fimbriae on the cell surface. A form of microscopy, transmission electron microscopy (TEM), was used to look for characteristic appendages. Initial attempts were unsuccessful. To introduce greater sensitivity to the procedure, they generated antibodies that recognised particular fimbrial proteins. The use of antibodies in TEM allows ‘tagging’ of these particular proteins and introduces specificity in to the process. This refinement allowed the successful identification of fimbriae on the surface of B. avium, and they confirmed the temperature dependence of the presence of fimbriae.

Having characterised the production of fimbriae by B. avium and established the particular conditions under which this happens, the research team has begun to investigate their role in the interaction between bacteria and turkeys. To do this, they have developed a model that utilises respiratory tract tissue from commercially raised turkeys. This tissue can be kept alive in the laboratory for several days after it has been dissected from the birds. Tissue pieces can be infected with B. avium in order to study the interactions between tissue and bacteria that the researchers propose are pivotal in the establishment of infection in the airways of turkeys by B. avium. In this way, they can test the involvement of fimbriae in the attachment of the bacteria to this tissue and this work is about to start.

The Bottom Line

In summary, this work has characterised the production of fimbriae by B. avium at the genetic and functional level, the first definitive study of this in this bacterium. The researchers plan to characterise the role of fimbriae in the adherence of B. avium to turkey respiratory tract tissue and ultimately in the establishment of disease in live birds.

These studies are an important first step to better characterising the mechanisms by which B. avium causes disease and in turn, are important first steps towards developing new therapeutic strategies against this bacterium.