The development of new disease challenge models is required where new pathogens (or pathogens of new significance), pathogen strains or serotypes emerge. It may also be necessary where pathogen strains have developed resistance to existing licensed products. In order to assess the efficacy of veterinary medicinal products (VMPs) against these pathogens, it is necessary to have validated disease models available for conducting tests under controlled conditions.
Validated experimental models of livestock and aquaculture infectious disease are important tools for the generation of efficacy data for candidate veterinary medicinal products (VMPs) or feed products / supplements. For efficacy data to be submitted for registration purposes in Europe, the US and Japan, pivotal efficacy studies must be conducted to the standards defined in the International Cooperation on Harmonisation of Technical Requirements for Registration of Veterinary Medicinal Products, Good Clinical Practice (VICH-GCP).
While it is widely acknowledged that the number and scale of in vivo assays should be reduced, replaced and or refined (the 3Rs) most of the dossiers submitted contain data from in vivo challenge model efficacy studies. It is unlikely that the requirements for animal challenge models will reduce markedly in the near future, since in most cases it is not possible to produce the same quality of efficacy data using in vitro assays. There is, however, a responsibility to ensure the in vivo models used to generate registration data are industry relevant, biologically valid and statistically robust.
Challenge model development can be complex and costly. It is generally undertaken by animal health companies, aquaculture companies, contract research organisations or academic groups. Prior to undertaking any development, it is necessary to have a clear idea of what will be required in the development, the resources required and the likely problems that may occur. In some cases, it is possible to review the scientific literature for similar models which can help to determine appropriate model design, however, such information is not always available and, even with well-established model designs, there can be considerable variability in clinical signs, pathology and other outputs for different isolates for the same pathogen species.
There are 5 main steps involved in the development of a new disease model:
1. Sourcing of pathogen isolates
2. Identification of challenge titre, volume and route
3. Identification of target species parameters (eg age range)
4. In vitro validation
5. In vivo validation
This article will review the critical success factors for each of these steps.
Sourcing of Pathogen Isolates
Once the need for a disease model for a particular pathogen has been identified, it should also be considered whether there is a particular serotype, subtype or toxin production profile of the pathogen that is of particular significance in the geographic region(s) where the VMP will be marketed. This will increase the likelihood that data generated from the use of the model will be acceptable to the regulatory authorities. With changing epidemiology of disease worldwide, this selection can prove challenging, but sufficient information is generally available within the literature and through contacts with industry to allow an informed choice to be made. With improvements in the speed and cost for DNA sequencing in recent years, the genotype of different isolates can now also be used to assist in determining the most appropriate strains to use, and identification of particular pathogenic gene sequences can aid in accurate selection of pathogen strains.
Generally, it is suggested that selection of isolates should be based on a number of factors, taking into account:
• Geographical location
• Where the isolate was sourced
• The year of isolation – (as often the regulators require the use of field isolates which are <5 years old for regulatory studies)
• The age of the animal – and, in fish, the development stage and production environment, from which it was sourced (selecting isolates from animals similar to the intended target for the model)
• The clinical history of the isolate (i.e. what clinical signs were observed in the host animal and what pathology was observed at postmortem, if applicable).
This can be particularly important where pathogens can cause a range of different clinical signs – offering a wide range of parameters where statistical significance can be evaluated.
Where possible, isolates should be selected from cases where a single isolate was recovered. This is important since with multifactorial infections it is difficult, if not impossible, to attribute clinical signs to a single pathogen. Secondly, opportunistic species may or may not be disease-causing
under certain conditions.
An additional factor to be considered for bacterial pathogens is antimicrobial sensitivity. For bacterial challenge models to be used in antimicrobial efficacy studies, the sensitivity of the pathogen against the common veterinary antimicrobial families in use should be established. For some studies, it may be appropriate to use a bacterial challenge isolate with high or low minimum inhibitory concentration (MIC). This is increasingly important as the incidence of resistant bacterial populations grows. The use of challenge isolates with higher MICs against a particular antimicrobial product provides the option of monitoring field effectiveness of the product against increasingly resistant pathogens.
