Coccidiosis Overview

Coccidiosis is the most important protozoan disease affecting poultry industry worldwide having an annual loss of more than US $ 4 billion (Williams, 1999a; Shirley et al., 2004). Eighty % losses are due to mortality, reduced weight, and inefficient feed conversion as well as temporary loss of egg production in layers (Dalloul &Lillehoj, 2005). Good management practices can help in controlling the disease by reducing its spread, but still chemoprophylaxis and live-vaccines are the key tool for Coccidiosis control.Feed medication and water treatments result in tremendous costs responsible for this loss.

The etiologic agents of Coccidiosis are various Eimeria spp., which invade the lining of the intestine and are transmitted from bird to bird via the ingested sporulated oocysts found in the environment. The life cycle of Eimeria is complex, having both sexual and asexual stages. Eimeria infections are site specific by which each Eimeria spp. infects specific part of the intestine, and host specific, with varying pathogenicity in different poultry breeds (Jeffers et al., 1970; Levine, 1985; Lillehoj, 1988; Lillehoj et al., 1989). Seven species of Eimeria are considered crucial in pathogenesisnamely, Eimeria acervulina, E. maxima, E. tenella, E. necatrix, E. brunetti, E. mitis and E. praecox; however, Eimeria hagani and E. mivati validity is under review (Long, 1973b; Shirley et al., 1983; Barta et al., 1997; Tsuji et al., 1997; Vrba et al., 2011).

The pathogenicity of coccidia is dependent on the successful replication of parasites inside the intestine. Theoretically,a single oocyst of virulent Eimeria tenella gives 2,520,000 invasive parasites following the second merogony stage in its life cycle (Levine, 1982).

Coccidia invade the intestinal mucosa causing a certain degree of damage to the epithelial cells, inflammation and villous atrophy (Pout, 1967; Assiset al., 2010).The signs observed during coccidiosis infection depend on extent of the damage caused to the mucosal intestinal lining and inflammation,and may include whitish diarrhea in Eimeria acervulina or hemorrhagic fecal material in E. tenella, petechial hemorrhages and plenty of orange mucus in E. maxima, dehydration and weight loss (Conwayand Mckenzie, 2007). The abundant bleeding found in the ceca is a characteristic sign of E. tenella infection due to erosion and sloughing of the intestinal mucosa (Witlock et al.,1975).

In general, young chickens are more susceptible to coccidiosis and display the signs of the disease, while older chickens are more resistant(Lillehoj, 1998). Young animals that have recovered from coccidiosis are able to compensate for their lost growth; however, their overall growth might be severely compromised. The magnitude of clinical signs resulting from Eimeria infection is influenced to a great extent by the host genetic factors (Lillehoj et al., 1989; Lillehoj et al., 2004).

Infection with Eimeria induces protective immunity that last lifelong and is species-specific to that particular Eimeria species(Akhtar et al., 2005).Large numbers of oocysts are required to generate a protective immune response against Eimeria, but not in case of E. maxima that is highly immunogenic, in which lower numbers of oocysts are needed to give almost full immunity. Moreover, the early endogenous stages of the Eimeria life cycle are more immunogenic than the later stages (Roseet al., 1984).However, gamete antigens of E. maxima were confirmed to be immunogenic (Akhtaret al., 2005).

Since the discovery of the sulfanilamide by Levine in 1939, curing coccidiosis in chickens, many anticoccidial drugs were introduced to the poultry industry all over the world as feed additives (Allenet al., 1998; Peek and Landman,2011). Currently, polyether ionophorous antibiotics are mainly used.These anti-coccidials are effective for avian coccidiosis but, their continuous use in feed and their misuse have resulted in the emergence of drug-resistant strains (Ruff & Danforth, 1996). Furthermore, there are increased concerns of consumers about drug residues in the poultry products (McDougald & Seibert, 1998). This triggered the need for alternative control of avian coccidiosis. Interactions between poultry coccidiosis and Clostridium perfringens that causes necrotic enteritis (NE) occurs and results in increasing health risk to poultry (Al-Sheikhly & Al-Saieg, 1980). NE is controlled by feed Antibiotic Growth Promoters AGPs such as bacitracin, avoparcin, avilamycin, and some ionophores like Narasin and Monensin. (Elwinger et al., 1992; Vissiennonet et al., 2000; Martel et al., 2004). However,the role of AGPs in the emergence of antibiotic resistance in humans has been questioned and the European Union decided to ban AGPs. An important example of antibiotic resistance in humans is that observed in Campylobacter jejuni loss of susceptibility to Quinolones family (Ciprofloxacin) (Wieczorek et al., 2013).Hence, the potential for interactions between Coccidia and Clostridia in the chicken host is becoming greater nowadays,due to this ban of AGPs, and stressing the need for investing in research for alternative products (Van Immerseel et al., 2009).

Eimeria spp. induces immunity and vaccination programs, on a commercial scale, have proved to be of limited scope. Existing vaccines consist of live virulent or attenuated Eimeria strains with limited scope of protection against Eimeria due to antigenic variability among the different species(Peek and Landman,2011). Thus the most common disease control remains largely dependent on routine use of anticoccidial drugs (Allen & Fetterer, 2002; Williams,2002a). In the past years,several different live vaccines composed of either virulent or attenuated strains were commercially introduced to the market. Major disadvantages of live vaccines are their high cost due to their in vivo production of oocyts, and the addition of multiple Eimeria strains in the vaccine (Dalloul& Lillehoj, 2005; 2006). Live vaccines provide limited alternative to prophylactic medication, and there is a need for a recombinant vaccine composed of parasite antigens or antigen-encoding genes (proteins or DNA), that should result in Eimeria-specific immunity. Several difficulties are encountered in identifying the protective antigens or genes that are responsible for eliciting protective immune response, and the use of an efficient delivery method to induce protective immune response in birds.The poultry industry is obliged to rely on prophylactic chemotherapy for Eimeria control until more efficient vaccines become commercially available (Dalloul & Lillehoj, 2006).

The introduction of alternative prevention and treatment measures, such as herbal immunopotentiators that effectively improve productivity and boost non-specific immunity of birds, maybe of help to limit the use of chemotherapy. In the meantime, the unavailability of efficient recombinant vaccines, and the increasing drug-resistant strains of Eimeria, along with growing public worry over the use of chemicals and their residues in poultry products, provide enough justification for the search to find alternative control methods.