Immune response following Ascaridia galli infection in free range laying hens

A recent study suggests that artificial Ascardia galli infection has no effect on performance and egg quality of free-range laying hens from the point of lay until 40 weeks of age (Sharma et al., 2017). However, the lower hepatic lipid reserve of infected hens observed in the same study indicated the potential for A. galli to affect production at a later stage. In this study, serum and yolk antibody levels of A. galli were measured to investigate the immune response to natural infection in hens with different infection levels.

Sixteen week old Lohmann brown laying hens (n=200) were divided into 4 treatments with 5 replicates of 10 hens per pen. Hens had access to the range between 9:00 am and 5:00 pm. Hens of group 1 served as negative controls (NC) and ranged on a decontaminated range, hens of group 2 (low) and 3 (medium) ranged on an area previously contaminated using hens that were artificially infected with 250 and 1000 A. galli eggs/hen respectively. Hens of group 4 (positive control-PC) were artificially inoculated with 1000 A. galli eggs/hen and ranged on an area that was previously contaminated using hens that were artificially infected with 1000 A. galli eggs/hen. Infection intensity of all treatment groups was measured by counting A. galli eggs in the hen excreta weekly, as well as by counting mature A. galli worms in the hens’ intestines after sacrificing them at 30 weeks of age. Serum and egg yolk were collected and analysed for anti-A. galli antibody (Ab) using ELISA method before range access at 16 weeks of age, and after range access at 20, 25 and 30 weeks of age. Data were analysed using JMP statistical software version 8 (SAS Institute Inc, Cary, NC). The experimental unit was the pen. Data were subjected to a twoway ANOVA with repeated measures, and difference between group means were assessed by Tukey's HSD test at a probability level of 0.05. Fixed effects used were treatment, age and their interation.

Hens of the medium infection group had higher worm counts in the intestine (43.9±3.96) and excreta egg counts (3437±459 eggs/g) compared to hens of the low infection group (23.8±3.96 and 1820±449 eggs/g) but similar worm numbers compared to hens of the PC (34.4±3.96 and 2918±474 eggs/g; P < 0.01). An interaction was detected between infection level and time for serum and yolk Ab (P < 0.01). While serum Ab levels were not different in hens between all groups before exposure (P > 0.05), they gradually increased overtime after exposing to the contaminated ranges. Only hens of the NC group maintained serum Ab constant all the time (P > 0.05). At 20 weeks, higher serum Ab was observed in hens of the medium (OD 1.07±0.03) and PC (OD 1.17±0.03) compared to the low (OD 0.38±0.03) (P < 0.01) group. At 25 weeks, serum Ab was similar in hens of low and medium infection groups but significantly lower compared to PC (P < 0.01). At 30 weeks, serum Ab levels were similar in the hens of low, medium and PC group (P < 0.01). Yolk Ab followed the same pattern except for medium infection group where yolk Ab remained same at 25 or 30 weeks of age (OD 0.72±0.04 and 0.74±0.04, respectively).

These findings suggest that hens naturally infected with A. galli produce both serum and yolk Ab at different levels depending on infection intensity and duration of exposure. Presence of A. galli eggs in excreta throughout the experiment suggests that increasing Ab levels in serum may not protect hens from continuing A. galli infection.

ACKNOWLEDGEMENTS: This study was funded by the Poultry CRC, Australia.