Research Note: The administration schedule of coccidia is a major determinant in broiler necrotic enteritis models

Abstract

A reliable and reproducible in vivo experimental model is an essential tool to study the pathogenesis of broiler necrotic enteritis and to evaluate control methods. Most current in vivo models use Eimeria as predisposing factor. Nevertheless, most models only result in a limited number of animals with intestinal necrosis. This research describes the necrotic enteritis incidence and severity using 2 previously described experimental models varying in the time point and frequency of Eimeria administration: single late and early repeated Eimeria administration models. In an in vivo model in which Clostridium perfringens is administered at 3 consecutive days between day 18 and 20 of age, birds belonging to the single late Eimeria administration regimen received a single administration of a tenfold dose of a live attenuated Eimeria vaccine on the second day of C. perfringens challenge. Broilers belonging to the early repeated administration regimen were inoculated with the same Eimeria vaccine 4 and 2 d before the start of the C. perfringens challenge. Early repeated coccidial administration resulted in a significant increase in average necrotic lesion score (value 3.26) as compared with a single late Eimeria administration regimen (value 1.2). In addition, the number of necrotic enteritis-positive animals was significantly higher in the group that received the early repeated coccidial administration. Single Eimeria administration during C. perfringens challenge resulted in a skewed distribution of lesion scoring with hardly any birds in the high score categories. A more centered distribution was obtained with the early repeated Eimeria administration regimen, having observations in every lesion score category. These findings allow better standardization of a subclinical necrotic enteritis model and reduction of the required numbers of experimental animals.

Keywords: coccidiosis; experimental model; necrotic enteritis.

Figure 1

Timeline of the necrotic enteritis in vivo experiment. The feeding regimen was soybean-based and replaced with fishmeal from day 17 onward for all models. Predisposing factors are indicated below. Oral administration of a tenfold dose of Paracox-5 at d 14 and 16 for group 1 (early repeated Eimeria administration, 4 and 2 d before Clostridium perfringens challenge) and day 19 for group 2 (single late Eimeria administration, during C. perfringens challenge). All broilers were challenged with C. perfringens CP56 (black bar), resulting in the induction of subclinical NE. Here for 1 mL overnight culture of the pathogenic C. perfringens strain CP56 was orally administered. Afterward, birds were euthanized.

Figure 2

Lesion scoring and distribution after single and repeated coccidial challenge in in vivo NE trials using 2 different coccidial administration models. (A) NE trials described in literature using the single late coccidial administration model (trials A and B by Mot et al. (2013), trial C by Van Waeyenberghe et al. (2016), and trial D by Da Costa et al. (2013)) and the early repeated coccidial administration model (trial E by Dierick et al. (2019) and trial F by Van Damme et al. (2020)). (B) NE lesion score obtained in present in vivo study. Birds were pretreated by administration of a tenfold dose of Paracox-5 on day 19 (single late coccidial challenge) or at day 14 and 16 (early repeated coccidial challenge). Feed and water was provided at libitum. From day 17 onward the feed was supplemented with 30% fishmeal. On day 18, 19, and 20, the birds were challenged by oral administration of 1 mL overnight culture of the pathogenic Clostridium perfringens strain CP56. Birds were euthanized and lesions were scored on day 21. In short, score 0: no gross lesions; score 2: focal necrosis and ulceration (1-5 foci); score 3: focal necrosis and ulceration (6-15 foci); score 4: focal necrosis and ulceration (16 or more foci); score 5: patches of necrosis 2 to 3 cm long and score 6: diffuse necrosis. The distribution of the lesion scores is shown in panel C. Black and open bars indicate the necrotic enteritis–negative and positive birds, respectively.