Introduction
A recent European survey of turkey veterinarians and production professionals identified necrotic enteritis and coccidiosis as leading health concerns in commercial turkey production.
Caused by protozoan parasites of the genus Eimeria, coccidiosis compromises intestinal integrity, leading to impaired nutrient absorption, reduced growth performance, increased feed conversion ratios, and, in severe cases, mortality. By affecting intestinal integrity, coccidiosis predisposes birds to secondary infections such as necrotic enteritis (NE).
To date, seven Eimeria species have been described in turkeys. Among these, Eimeria meleagrimitis, Eimeria adenoeides, Eimeria gallopavonis, and Eimeria meleagridis are considered pathogenic, whereas Eimeria dispersa, Eimeria innocua and Eimeria subrotunda are generally regarded as low- or non-pathogenic (Chapman, 2008; Vrba and Pakandl, 2014). E. meleagrimitis, E. dispersa, E. innocua, and E. subrotunda have been reported to replicate primarily in the duodenum and/or mid-intestinal regions. Lesions in the lower intestine have also been described and may be associated with infections caused by E. adenoeides and E. meleagridis, which can result in caecal cores, or by E. gallopavonis, which is linked to caseous lesions in the lower jejunum, ileum and caecal neck (El-Sherry et al., 2018; Gadde et al., 2019). With the exception of E. subrotunda, the morphology and pathogenicity of turkey Eimeria species have been confirmed using pure Eimeria lines.
The availability of molecular diagnostic techniques provides possibility for more accurate identification of Eimeria species than conventional microscopy, as overlap in oocyst size among species increases the risk of misclassification. With the exception of E. subrotunda, species-specific primers have been described for all recognised turkey Eimeria species. Application of these molecular approaches has revealed a close genetic relationship between E. adenoeides and E. meleagridis. As a result, these species have been classified by some authors as a single species (Vrba and Pakandl, 2014), whereas others continue to recognise them as two distinct species (El-Sherry et al., 2015; El-Sherry et al., 2018).
Prevalence and identification of Eimeria species in European turkey flocks
Despite the high prevalence of coccidiosis in turkey flocks, data on the prevalence of the different Eimeria species in turkey flocks are limited. Therefore, a survey was conducted on prevalence and identification of coccidiosis in turkeys, using microscopy and PCR techniques. In total, 387 samples collected between 2019 and 2025 from 127 different commercial turkey farms located in 7 different EU countries and the UK were investigated. The age of the turkeys at sampling ranged between 1 and 15 weeks, with a median of 6 weeks. Samples were collected using a standardised protocol with the request to collect intestinal and caecal droppings from well distributed places, according to a sampling scheme, in the turkey houses. It should be noted that samples were collected both from farms experiencing gastrointestinal health issues and from farms without reported problems for monitoring purposes. In addition, for the majority of samples obtained from birds younger than 8 weeks of age, coccidiostats were present in the feed at the time of sampling.
First, samples were microscopically examined for identification of Eimeria species and determination of oocyst per gram (OPG) in the same laboratory. In 118 of the 387 samples no oocysts could be detected. Of the 269 positive samples:
- OPG ranged between 200 and 1000 in 25%
- OPG ranged between 1000 and 10,000 in 27%
- OPG ranged between 10,000 and 100,000 in 33%
- OPG was higher than 100,000 in 15% of samples
Microscopic examination differentiated between the following groups:
- E. meleagrimitis / E. adenoeides (identified in 95% of positive samples)
- E. dispersa (identified in 10% of positive samples)
- E. meleagridis / E. gallopavonis (identified in 39% of positive samples)
- E. innocua (not identified in any positive samples)
Further identification of Eimeria species was performed by qPCR on 47 samples from 31 different farms using the primers described by Vrba et al. (2010). Overall, qPCR results were consistent with microscopic identification but allowed for individual differentiation. The prevalence of E. meleagrimitis, E. meleagridis, E. adenoeides, E. gallopavonis, E. dispersa and E. innocua as determined by qPCR was 72%, 33%, 17%, 9%, 26% and 0%, respectively.
E. meleagrimitis was the most prevalent species and was detected throughout all ages. With increasing age of the turkeys, co-infections with multiple Eimeria species or infections with other species alone were more frequently observed. In Europe, control of coccidiosis is achieved through the inclusion of coccidiostats in feed, the majority of which are ionophores. In other markets outside Europe, coccidiosis vaccines are also available, with vaccination representing a potential alternative control strategy.
Necrotic enteritis
As Eimeria meleagrimitis primarily replicates in the duodenum, infection with this species may compromise intestinal integrity and thereby predispose turkeys to the development of NE. Outbreaks of NE have emerged as an increasing health concern in commercial turkey production. These outbreaks typically occur in turkeys between 4 and 8 weeks of age, are often observed during the spring period, and are characterised by a sudden increase in mortality. The most prevalent Clostridium perfringens toxinotype associated with NE outbreaks in turkeys is toxinotype A. Additional toxins detected in C. perfringens strains isolated from clinical outbreaks in turkeys include beta2 toxin and, less frequently, netB (Giovanardi et al., 2016; Lyhs et al., 2013; Vereecken et al., 2022).
In susceptibility testing against the most frequently used ionophores in turkeys (monensin and lasalocid) low minimum inhibitory concentration (MIC) values have been observed, indicating the importance of other predisposing factors for the development of clinical NE (Vereecken et al., 2022).
In a recent study (Cordioli et al., 2025), NE was experimentally induced using established predisposing factors commonly applied in chicken models, including a sudden switch to a high-protein diet and infection with Eimeria meleagrimitis. Turkeys were subsequently challenged with C. perfringens strains differing in their genetic virulence profiles, isolated from chickens and turkeys affected by NE. Infection with a netB-positive C. perfringens strain originating from a chicken NE outbreak resulted in high mortality (80%) as well as pronounced macroscopic and microscopic intestinal lesions in turkeys, whereas no mortality was observed in birds infected with a netB-negative strain isolated from a turkey NE outbreak. These findings demonstrate that successful reproduction of NE in turkeys is strain-dependent and questions the toxins involved in the pathology.
In conclusion, Eimeria meleagrimitis is currently the most prevalent Eimeria species detected in commercial turkey flocks. Nevertheless, the importance of Eimeria meleagrimitis in the pathogenesis of NE, additional factors, including strain-specific virulence factors, are critical contributors to disease development.
References are available on request.
