Technical Tip Coccidiosis in Broilers

INTRODUCTION

Coccidiosis is a parasitic disease caused by protozoa of the Eimeria species. It is an economically significant disease of broilers and other major livestock species, estimated to cost the broiler industry alone €13.5 billion per year (Blake, 2020, FAO, 2022). Coccidiosis is ubiquitous and so is present in every poultry house, and all commercial poultry will at some stage contract coccidiosis.  

THE LIFECYCLE OF COCCIDIA

There are 7 common Eimeria species affecting broilers. Eimeria acervulina, maxima, tenella, praecox and mitis primarily affect broilers up to 6 weeks of age, and for older birds, brunetti and necatrix species are also important (Vetworks, 2021). These species-specific coccidia rely on the broiler as a host for sexual and asexual replication. They are transmitted from bird to bird by the faecal-oral route and each Eimeria species infects a particular portion of the digestive tract (Fig. 1).

Figure 1: Key Eimeria species and the locations of their activity in broilers (Pandey, 2018). 

The lifecycle of coccidia is between 4 and 7 days depending on the species. This is the time from when they are ingested to when oocysts are excreted in the litter. In order to become infective, the oocysts must firstly incubate and go through sporulation outside of the host. After sporulation, each oocyst has 8 infective sporozoites within its cell wall. Upon ingestion, the enzymes in the gizzard and proventriculus break down the cell walls around the sporozoites allowing for their invasion of intestinal epithelial cells where they undergo several cycles of asexual and sexual reproduction. Each reproduction cycle results in further intestinal damage (Fig. 2).  

Figure 2: Coccidiosis lifecycle in broilers.

CLINICAL SIGNS Coccidiosis is characterized by enteritis and intestinal cellular tissue damage, where the intestinal damage is due to coccidia replication in the epithelial lining of the gastrointestinal tract (Hendrix, 2021). Eimeria causes damage to the intestinal wall resulting in the inability to absorb nutrients effectively whilst compromising the barrier against bacterial pathogens. Damage to the intestinal barrier results in decreased performance, loss of appetite, poor uniformity, poor feathering, bloody diarrhoea, and increased mortality (Hendrix, 2021). Intestinal damage also enables Clostridium perfringens to proliferate causing necrotic enteritis. The most direct link is seen between C. perfringens and coccidiosis with E. maxima being the leading cause (Mathis, 2015). 

CONVENTIONAL CONTROL METHODS  Practical methods of control cannot prevent all cases of coccidiosis infection, and so rely on conventional pharmaceutical methods of control such as vaccination and anticoccidials.