Avian Salmonellosis and Colibacillosis: Risk Factors, Antibiotic Resistance, Public Health Impact and Biological Control

INTRODUCTION

Poultry farming is one of the most important sources of animal protein and income in the world in general and particularly in Africa¹. It is a sub-sector that contributes considerably to the economy of several African countries and thus play key role in fighting against hunger and poverty. Despite of its importance, this sub-sector is facing major disease challenges that hinder the agricultural and socio-economic development of many countries. Among these, salmonellosis and colibacillosis are the main bacterial diseases caused by Salmonella spp. and Escherichia coli respectively and are considered as one of the main causes of morbidity and mortality either as a primary or secondary pathogen². In West Africa, salmonellosis alone causes significant economic losses with mortalities of up to 90%³. Antibiotic therapy with synthetic molecules is one of the ways to control these diseases. These molecules are used either for curative or preventive purposes, or as growth promoters in feed⁴. Their use in poultry farming has undoubtedly improved the productivity⁵. However, the frequent and uncontrolled use of these molecules has progressively contributed to the emergence of resistant bacteria, in this case Salmonella and Escherichia coli strains that are multi-resistant to different families of antibiotics⁶. According to Chang et al.⁷; Economou and Gousia⁸, the use of antibiotics in veterinary medicine and farm animals is of constant concern because of the possible transmission of resistant bacteria to human through food consumption and environment. New effective and accessible treatment methods must therefore be envisaged to reduce the speed at which this microbial resistance develops. Medicinal plants, through their pharmacological effects, are an option to be considered. Used for thousands of years, they represent a significant source of new drugs. The abundance of research work in this area confirms the renewed interest in their use in the treatment of animals⁹. In Benin, the poultry sub-sector is booming but bacterial resistance related to Salmonella and Escherichia coli is one of the major challenges for it. The objective of this article is to: (1) Synthesize recent information on the different serotypes isolated and their associated virulence genes, (2) Determine the bacterial resistance to antibiotics and and their impact on public health (3) Review some phytosanitary treatments performed against these bacteria in several regions of the world.

Avian salmonellosis: Avian salmonellosis is one of the most common bacterial infections of poultry. It is caused by the multiplication in the body of germs of the Salmonella genus, a facultative intracellular pathogenic bacterium causing local or systemic infections and belonging to the Enterobacteriaceae family¹⁰. The genus Salmonella currently includes 2659 serovars belonging to two species: Salmonella enterica which has 6 subspecies (enterica, salamae, arizonae, diarizonae, houtenae, indica) and Salmonella bongori. Of these many serovars, only 10% have been isolated from poultry and over 50% from humans². In poultry farming, there are two types of infection due to Salmonella Pullorum and Salmonella Gallinarum are the etiological agents of pullorosis and avian typhosis, respectively, which cause huge economic losses to the poultry industry. Besides the serovar Gallinarum/Pullorum recognized as specific to poultry, there are other serovars (Salmonella enteritidis, S. Typhimurium, S. Hadar, S. Heidelberg, S. Saintpaul and S. Infantis) less or non-specific, also responsible for the development of poultry infections and moreover, involved mainly in the public health problem via the consumption of food of animal origin¹¹. Two types of transmission are possible in poultry farms: vertical and horizontal transmission. Vertical transmission can be transovarial (direct contamination of the egg during its formation from the infected ovary or oviduct)¹². Horizontal transmission, on the other hand, occurs orally, through contact with virulent materials (feces), the environment as well as water and food. It can infect chicks, pullets as well as adult hens. Thus, several Salmonella serotypes have been isolated from poultry farms, their environments, poultry carcasses and also table eggs^13,14. Apart from the different Salmonella serotypes isolated till date, different virulence genes associated with them have also been determined by PCR technique from DNA and also plasmid analysis. Table 1 summarizes the recently identified Salmonella serovars and their associated virulence genes.