A survey of the economic impact of subclinical Eimeria infections in broiler chickens in Norway

Figures & data

Figure 1.  Distribution of oocysts by length categories in Study 1 (2000/01) and Study 2 (2003/04). The length of the pointer indicates mean relative frequency of respective oocyst category.

Table 1.  Studies 1 and 2: basic data and significance of the differences between studies in univariate analysis

Sample characteristics (mean)	Study 1 (2000/01) (n=58)	Study 2 (2003/04) (n=86)	Significance of difference (P value)
Flock size (range)	10,682 (3,500 to 25,612)	12,205 (2,800 to 24,000)	0.049
Weight day 0 (g) (range)	40.6 (35.0 to 46.0)	41.8 (35.0 to 47.0)	0.071
Slaughter age (days) (range)	31.6 (29.0 to 35.0)	31.00 (28.0 to 35.0)	0.003
Carcass weight^a (g) (range)	945 (823 to 1120)	1051 (840 to 1480)	<0.0001
Daily carcass weight gain^b (g) (range)	29.9 (26.4 to 33.9)	33.9 (26.3 to 42.3)	<0.0001
FCR^c (range)	2.50 (2.12 to 2.83)	2.26 (1.94 to 2.66)	<0.0001
EPI^d (range)	12.04 (9.56 to 14.43)	15.08 (9.97 to 18.42)	<0.0001
Liver condemnations^e (%) (range)	0.11 (0.00 to 0.65)	0.07 (0.00 to 0.94)	0.023
Mortality (%) (range)	2.5 (1.0 to 5.4)	2.5 (0.5 to 16.5)	0.181
Prevalence of coccidial infection (confidence interval)	36.2 (25.1 to 49.1)	70.9 (60.6 to 79.5)	<0.0001
Infection intensity^f at slaughter (range)	25,779 (600 to 1,561,000)	14,928 (100 to 1,485,000)	0.389
Infection intensity index during rearing^g (range)	31,879 (200 to 529,420)
Relative frequency of length categories^h at slaughter (range)
AM	(0.0 to 74.0)	65.3 (0.0 to 100)	<0.0001
NTP	(0.0 to 85.0)	29.7 (0.0 to 72.0)	0.002
BM	27.1 (0.0 to 100)	2.9 (0.0 to 53.1)	0.013
Relative frequency of flocks with OPG > 50 000 at slaughter	42.1	34.4	0.546
Number of risk flocks^i at slaughter (%; confidence interval)	5 (8.6; 3.7 to 18.4)	2 (2.3; 0.6 to 8.1)	0.118
Number of risk flocks^j during rearing (%; confidence interval)	13 (22.4; 13.6 to 34,7)
Frequency of Eimeria species in coccidia positive flocks (%)
E. acervulina		100.0
E. maxima		27.5
E. praecox		9.8
E. tenella		76.5
Number of Eimeria species involved (%)
One Eimeria species		17.6
Two Eimeria species		54.9
Three Eimeria species		23.5
Four Eimeria species		39.2
Season of sample collection (%) (confidence interval)			<0.0001
Summer (April to September)	36.3 (25.1 to 49.1)	90.7 (83.7 to 95.2)
Winter (October to March)	63.8 (50.9 to 74.9)	9.3 (4.5 to 17.3)
^aCarcass weight = live weight × slaughter yield (constant set to 0.62% for birds slaughtered at average slaughter age).
^bDaily carcass weight gain = carcass weight/slaughter age.
^cFCR = feed consumed per flock (kg)/total carcass weight of approved chickens per flock (kg).
^dEPI = mean carcass weight gain per day per bird (g)/feed conversion rate (FCR).
^eCorrelated to occurrence of netrotic enteritis (Løvland & Kaldhusdal, 1999).
^fExpressed as OPG of sample.
^gExpressed as the average of the OPG levels found in three samples collected during rearing.
^hAM = small oocysts (i.e presumptive Eimeria species are E. acervulina or E. mitis); NTP = medium-sized oocysts (i.e. presumptive Eimeria species are E. necatrix, E. tenella or E. praecox); BM = large sized oocysts (i.e. presumptive Eimeria species are E. brunetti or E. maxima).
^iRisk flock criteria = > 50 000 OPG (slaughter) with >50% oocysts of NTP and/or BM group.
^jRisk flock criteria = > 50 000 OPG (average of three samples collected during rearing) with >50% oocysts of NTP and/or BM group.

Figure 2.  Association between the EPI and coccidial infection levels in Study 1 (2000/01) and Study 2 (2003/04), respectively. The infection level categories were 0 = below detection limit; 1 = 100 to 999; 2 = 1000 to 9999; 3 = 10 000 to 49 999; and 4 = > 50 000. The box indicates the 25th, 50th and 75th percentiles, and the whiskers the 2.5th and 97.5th percentiles.

Table 2.  Studies 1 and 2: test statistics for univariate analysis with EPI as the test outcome

	Study 1 (2000/01) (n=58)^a			Study 2 (2003/04) (n=86)^a
Study variable	Regression coefficient (standard error)	Mean EPI (confidence interval)	P value	Regression coefficient (standard error)	Mean EPI (confidence interval)	P value
No coccidial infection		12.38 (12.04 to 12.72)	0.002		15.19 (14.48 to 15.89)	0.658
Coccidial infection		11.44 (10.94 to 11.93)			15.03 (14.75 to 15.32)
Infection level^b at slaughter	−0.35 (0.23)	11.44	0.154	−0.09 (0.14)	15.03	0.534
Average infection level during rearing^c	−0.41 (0.22)	11.44	0.075
Infection levels at slaughter >50 000		10.97 (10.33 to 11.61)	0.121		14.95 (14.48 to 15.41)	0.657
Infection levels at slaughter <50 000		11.79 (10.94 to 12.63)			15.08 (14.71 to 15.45)
Defined risk flock^d based on one sample
Not risk flock		12.13 (11.82 to 12.42)	0.049^e		15.08 (14.79 to 15.36)
Risk flock		11.09 (10.32 to 11.87)			15.23 (13.39 to 17.08)
Defined risk flock based on average of three samples
Not risk flock		12.32 (12.02 to 12.62)	0.0003
Risk flock		11.08 (10.52 to 11.64)
Number of Eimeria species detected^f						0.164
0 (25 flocks)					15.18 (14.65 to 15.72)
1 (9 flocks)					15.56 (14.67 to 16.46)
2 (28 flocks)					15.19 (14.68 to 15.69)
3 (12 flocks)					14.49 (13.72 to 15.27)
4 (2 flocks)					15.54 (13.64 to 17.43)
Pathogenicity categories^g						0.613
0 (25 flocks)					15.19 (14.48 to 15.89)
1 (9 flocks)					15.56 (14.56 to 16.57)
2 (3 flocks)					15.42 (13.15 to 17.68)
3 (27 flocks)					15.03 (14.55 to 15.52)
4 (12 flocks)					14.84 (14.32 to 15.36)
Statistical significances of differences in mean production indexes of nominal study variables, and statistical significances of associations between continuous study variables and the production index were tested.
^aDue to variable sample qualities, some positive samples did not enable measurement of 50 oocysts and/or PCR identification: n(morphometry, Study 1) = 18, n(morphometry, Study 2) = 52 and n (PCR) = 51.
^bExpressed as number of OPG of sample.
^cAverage OPG of the two litter samples and one faecal sample during rearing.
^dRisk flocks; OPG > 50 000, and >50% of the oocysts measured belonged to NTP and/or BM.
^eFive risk flocks in Study 1. Only two risk flocks in Study 2 (i.e. no reliable statistical analysis).
^f0 = coccidia-negative flocks; 1 = single Eimeria species infection; 2 = two concurrent Eimeria spp.; 3 = three concurrent Eimeria spp.; 4 = four concurrent Eimeria spp.
^g0 = coccidia-negative flocks; 1 = E. acervulina; 2 = concurrent E. acervulina and E. maxima or E. acervulina, E. maxima and E. praecox; 3 = concurrent E. acervulina and E. tenella or E. acervulina, E. tenella and E. praecox; 4 = concurrent E. acervulina, E. maxima and E. tenella or E. acervulina, E. maxima, E. tenella and E. praecox.

Table 3.  Study 1: test statistics for multivariate analysis of the impact of coccidial infection, based on a single faecal sample collected at slaughter, on the EPI

	Coefficient	95% confidence interval	P value
Ordinal infection levels^a
Level 1 versus 0	−0.16	−1.49 to 1.16	0.807
Level 2 versus 0	−0.31	−1.11 to 0.49	0.441
Level 3 versus 0	−1.11	−2.23 to 0.02	0.054
Level 4 versus 0	−1.11	−1.88 to −0.34	0.006
Region
Region 2 versus region 1	−0.88	−1.44 to −0.31	0.003
Region 3 versus region 1	−0.19	−1.05 to 0.67	0.660
Slaughter age	−0.27	−0.48 to −0.07	0.010
Constant	21.47
P(F) ≤ 0.0001 Adjusted R ^2=0.36. Residual standard deviation = 0.91.
^aInfection level categories: 0 = below detection limit (n=39); 1 = 100 to 999 (n=2); 2 = 1000 to 9999 (n=6); 3 = 10 000 to 49 999 (n=3); 4 >50 000 (n=8).

Table 4.  Study 1: test statistics for multivariate analysis of the impact of coccidial infection, based on the average of three samples taken during rearing, on the EPI

	Coefficient	95% confidence interval	P value
Ordinal infection levels^a
1 versus 0	−0.28	−1.28 to 0.72	0.574
Level 2 versus 0	−0.19	−1.57 to 1.19	0.788
Level 3 versus 0	−0.53	−1.93 to 0.87	0.452
Level 4 versus 0	−0.95	−1.59 to −0.32	0.004
Region
Region 2 versus region 1	−0.93	−1.53 to −0.32	0.003
Region 3 versus region 1	−0.44	−1.31 to 0.43	0.315
Slaughter age	−0.27	−0.48 to −0.06	0.013
Constant	21.42
P(F) ≤ 0.0001. Adjusted R ^2=0.35. Residual standard deviation = 0.91.
^aInfection level categories: 0 = below detection limit (n=37); 1 = 100 to 999 (n=4); 2 = 1000 to 9999 (n=2); 3 = 10 000 to 49 999 (n=2); 4 >50 000 (n=13).

Table 5.  Study 1: test statistics for multivariate analysis of the impact of being classified as a risk flock, based on three samples collected during rearing, on the EPI

	Coefficient	95% confidence interval	P value
Risk flock^a versus non-risk flock^b	−0.89	−1.49 to −0.29	0.004
Region
Region 2 versus region 1	−0.91	−1.46 to −0.36	0.002
Region 3 versus region 1	−0.46	−1.25 to 0.34	0.255
Slaughter age	−0.28	−0.48 to −0.07	0.008
Constant	21.61
P(F) ≤ 0.0001. Adjusted R ^2=0.37. Residual standard deviation = 0.90.
^a n(risk flock) = 13.
^b n(non-risk flock) = 45.

Løvland , A. and Kaldhusdal , M. 1999 . Liver lesions seen at slaughter as an indicator of necrotic enteritis in broiler flocks . FEMS Immunology & Medical Microbiology , 24 : 345 – 351 .

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