**1. Introduction**

Avian coccidiosis is by far the most costly parasitic disease in poultry [1], and it may represent a threat to guarantee the supply for sufficient, safe, and nutritious food. According to some projections, the global population in 2050 will be 10 billion which will increase the demand for food production by 70% and therefore achieving global food security is a staggering challenge [2].

Coccidiosis is an infectious disease caused by protozoa, genus *Eimeria*. The parasite is host-specific and has a direct life cycle [3]. Birds become infected by ingestion of sporulated oocysts omnipresent in poultry houses. Once ingested by the chicken, the parasite invades and multiplies in the gastrointestinal tract, destroying epithelial cells [4]. The severity of infection will depend upon the number of infective oocysts ingested as well as the pathogenicity of the wild strains. Intensive methods of production of poultry favor the reproduction of *Eimeria.* Therefore, coccidiosis is a continuing problem requiring constant attention and, in the case of broilers, a need for continuous coccidiosis control tools [5].

Even today, coccidiosis is still ubiquitous, and it is generally accepted that, under the current production systems, coccidiosis control remains necessary [4, 6]. Coccidiosis is also one of the main triggers for other gastrointestinal disorders including necrotic enteritis, dysbacteriosis, *Salmonella,* among others [7–9].

Birds suffering with clinical coccidiosis will show typical signs such as diarrhea, bloody droppings, increased mortality, decreased feed intake, and impaired performance. Inadequate coccidiosis control may also result in impaired growth and an increased feed conversion ratio, even in the absence of obvious clinical signs (referred to as subclinical coccidiosis).

In a recent study, the global prevalence of clinical coccidiosis was estimated at 5% and subclinical coccidiosis at 20% of global poultry production [10]. This supports that, under current production systems, coccidiosis is still a major health and welfare issue, which needs to be controlled.

Synthetic anticoccidials were the first to be introduced in the market. The first paper on prophylactic use of anticoccidials was published in 1948 by Leland Grumbles and describes the continuous use of Sulfaquinoxaline for the control of coccidiosis in poultry [11]. After their introduction, synthetics were found to be very efficacious and were very popular. Up until 1971, they were the only available option for coccidiosis control as ionophores were only introduced in the 1970s.

The introduction of the first ionophore coccidiostat (monensin) in the 1970s has proven to be critical for the development of modern poultry production [12]. The use of ionophores has significantly helped in the development of poultry production and has improved the health and welfare of broilers (Report from the Commission to the Council and the European Parliament on the use of coccidiostats and histomonostats as feed additives, 2008).

As expected, suboptimal control of coccidiosis will result in the increased use of antimicrobials, some of which are medically important for human medicine.
