Comparison of outbred lines of chickens for resistance to experimental infection with coccidiosis (Eimeria tenella)

BREEDING AND GENETICS Comparison of Outbred Lines of Chickens for Resistance to Experimental Infection with Coccidiosis (Eimeria tenella) M.-H. PINARD-VAN DER LAAN,*,1 J.-L. MONVOISIN,* P. PERY,† N. HAMET,‡ and M. THOMAS‡ *Laboratoire de Ge´ne´tique Factorielle and †Laboratoire de Virologie et Immunologie Mole´culaires, Institut National de la Recherche Agronomique, 78352 Jouy-en-Josas Cedex, France, and ‡Unite´ de Recherche Virologie Immunologie et Parasitologie Aviaires et Cunicoles, Centre National d’Etudes Ve´te´rinaires et Alimentaires, Beaucemaine BP53, 22440 Ploufragan, France tenella (150,000 oocysts) and slaughtered 8 d postinoculation. Innate resistance was assessed individually by measures of lesion score, mortality, and body weight gain at slaughter, and plasma coloration 4 d postinoculation. Large differences in resistance to E. tenella were observed between lines. The Fayoumi line appeared clearly as the most resistant line, showing no mortality, less severe lesions than other lines, and a 30% reduction of growth as compared to control birds. The WLDW line was the most susceptible, with 27% mortality and a 85% reduction in growth. No major effect of MHC or dwarfism on resistance to E. tenella was found.

(Key words: coccidiosis, chicken, resistance, major histocompatibility complex, dwarf gene) 1998 Poultry Science 77:185–191

Coccidiosis is a worldwide problem in poultry production, especially in broiler lines. It is an infectious disease caused by at least seven protozoan Eimeria species that affect specific organs of the birds. For example, Eimeria tenella causes moderate to severe cecal lesions and sometimes death. Clinical coccidiosis has been treated mostly by drugs only. But other approaches need to be investigated because, despite medication, subclinical infections remain. Therefore, economic losses due to weight gain loss and decreased feed efficiency have to be added to the price of the medication. In France, about 90 million French Francs (1 French Franc = 0.19 US $) are spent yearly for treating more than one billion broilers (P. Rault, SYSAAF, INRA, Tours, France, personal communication, 1996). Total yearly losses due to coccidiosis were estimated to be over one billion dollars worldwide (Danforth and Augustine, 1990). In addition, this extensive use of drugs has led to the

development of resistance to anticoccidial products (Chapman, 1993). Approaches alternative to drugs include the induction of protective immunity (vaccination) and the use of naturally resistant birds (genetic resistance). Research has been carried out to find an effective but nonpathogenic vaccine (Lillehoj and Trout, 1993; Yvore´ et al., 1993b) with recent work on the development of recombinant vaccines (Bumstead et al., 1995). Composition of the diet may also significantly limit the development of coccidial lesions (Allen et al., 1996). In a broader sense, all environmental factors (e.g., temperature, housing management, and stress) and host immunity influence the outcome of the disease and may be integrated into an epidemiological approach (Henken et al., 1992; Graat, 1996). It is likely that no single approach will be sufficient, but a comprehensive combination of strategies may help to reduce significantly the prevalence of coccidiosis. Previous work demonstrated that increasing resistance to coccidiosis was possible through selection

Received for publication February 24, 1997. Accepted for publication September 22, 1997. 1To whom correspondence should be [email protected]

Abbreviation Key: C = control; RIR = Rhode Island Red; T = tested; WLB21 = a White Leghorn line homozygous for the MHC B21; WLDW = a White Leghorn line segregating for the sex-linked dwarfism gene DW, and for three MHC haplotypes B15, B19, and B21.

INTRODUCTION

addressed:

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ABSTRACT Coccidiosis causes dramatic economic losses in the poultry industry. Next to the extensive use of anticoccidial drugs, improving genetic resistance of birds to this parasitic disease represents an attractive alternative. An experiment was run in order to identify lines of chickens resistant and susceptible to coccidiosis as a tool to search for genetic markers of resistance. Five outbred lines were used: two Egyptian lines (Mandarah and Fayoumi), a Rhode Island Red line, and two White Leghorn lines (WLB21 and WLDW). The WLDW line segregated for three MHC haplotypes, B15, B19, and B21, and for the sex-linked dwarf gene, DW. Chicks were challenged at 4 wk of age with a high dose of Eimeria

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MATERIALS AND METHODS

Challenged Lines A total of 539 chicks (279 males and 260 females) from five unselected lines were used: a White Leghorn (WLB21) line, homozygous for the MHC B21 (n = 64), a White Leghorn (WLDW) line, segregating for the sex-linked dwarfism gene DW, and for three MHC haplotypes B15, B19, and B21 (n = 127), two Egyptian lines, Fayoumi (n = 111), and Mandarah (n = 125), and one Rhode Island Red (RIR) line (n = 112). Cocks were mated to two hens each, to produce about four male and four female chicks per hen. Eggs were hatched at the Laboratoire de Ge´ne´tique Factorielle and unvaccinated day-old chicks were sent to the Laboratoire de Parasitologie at Ploufragan, where chicks were housed in clean wire-floored cages and provided with ad libitum access to feed and water.

Coccidial Challenge Challenge. Chickens were weighed at 26 d and separated, within line, sex, and dam family into two groups of identical weight distribution: unchallenged control (C) and tested (T) birds. Control and tested birds were housed in separate cages by groups of four of similar body weight. Challenged (T) birds were inoculated per os at 28 d of age with a high dose of E. tenella (150,000 oocysts) from the PT5 strain maintained at the Laboratoire de Parasitologie of Ploufragan since 1965. All C and T birds

were slaughtered and weighed 8 d after inoculation at 36 d of age. Resistance Criteria. Body weight was measured at 26 (BW26) and at 36 (BW36) d of age. Between 26 and 36 d of age, relative weight gain was calculated individually as 100 (BW36 – BW26)/BW26 on all birds surviving the test, and feed conversion (weight gain:feed intake) was calculated per treatment group, line, and sex. At slaughter, cecal lesions scores were assessed according to Johnson and Reid (1970), from 0 (no lesion) to 4 (most severe lesions). Mortality due to coccidiosis was recorded. Plasma coloration, as a measure of blood carotenoid level, was analyzed at 4 d postinoculation by optical density at 480 nm (Yvore´ et al., 1993a). Oocyst production (105 per gram of feces) was assessed between 5 and 7 d postinoculation, per line and sex.

Serological MHC Typing The MHC genotype was determined in the WLDW line by direct hemagglutination, using alloantisera obtained from the line. Matings were made to yield segregation of the B15, B19, and B21 haplotypes in the WLDW progeny. Identification of the B-haplotypes in the WLDW lines was performed by serological typing with reference sera (Briles and Briles, 1982), and molecular characterization of these B-haplotypes for B-F, B-G, and B-L (classes I, IV, and II of the chicken MHC) was obtained in comparison with reference Danish population (M. H. Pinard-van der Laan and C. H. Jacobsen, 1995, unpublished data).

Sex-Linked Dwarfism Typing In the WLDW line, cocks heterozygous for DW were mated to dwarf hens to produce progeny segregating for DW. As challenged birds were weighed too young to discriminate between dwarf and normal chicks, a PCR test was performed on the growth hormone receptor to identify carriers from noncarriers. Allele-specific oligonucleotide analysis was performed as described by Duriez et al. (1993).

Statistical Analysis Comparison between lines and effect of sex were analyzed by two-way analysis of variance with interaction by using the General Linear Models procedure of SAS (1989). Main effects were line and sex. For weight gain and plasma coloration, in order to remove effects due to differences between lines, each individual observation was expressed as a difference from the mean value of the control group, within line and sex (and also within DW genotype in the WLDW line) as follows: adjusted WG: WG′ = (WG – WGc)/WGc, adjusted PC: PC′ = (PC – PCc)/PCc,

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(Johnson and Edgar, 1986). However, this method requires direct challenges and therefore cannot be a routine procedure for commercial lines, for economical and ethical reasons. On the contrary, genetic markers linked to the resistance could be readily applied. Until now, only the MHC, known to be associated with many immune response mechanisms and various diseases, has been looked into (Lillehoj et al., 1989). In the absence of other candidate genes, one may search for anonymous genetic markers of resistance. Briefly, a strategy to achieve this goal may be to identify one resistant line and one susceptible line and then, in an appropriate crossing, for example an F2, to search for genetic markers segregating with the resistance to coccidiosis. The main objective of this experiment was to compare several outbred lines of various genetic origins for innate resistance to an acute infection by Eimeria tenella. Various parameters of resistance were individually measured to identify relevant markers of resistance to infection. The lines tested here included a line of the Egyptian Fayoumi breed, which has a local reputation of hardiness and had experimentally showed resistance to coccidiosis in a preliminary test (Hamet and Me´rat, 1982). Another aim of this study was to evaluate the effect of the MHC and the sex-linked dwarf gene on innate resistance to E. tenella in one of the tested lines that segregated for these two genetic polymorphisms.

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TABLE 1. Numbers of birds and mean body weights at 26 and 36 d of age (BW26 and BW36, respectively) and relative weight gain between 26 and 36 d of age (WG1) per line, sex, and treatment (T = tested; C = control) Line Fayoumi Variable

WLB21

Mandarah

WLDW

(T)

(C)

(T)

(C)

(T)

(C)

(T)

(C)

(T)

(C)

28 27

28 28

28 28

28 28

16 16

16 16

32 31

31 31

36 27

36 28

249.8 211.7

249.8C 212.1C

215.0 191.4

215.0D 191.4D

226.4 210.2

225.3CD 209.6CD

348.8 328.2

349.7A 327.9A

281.0 276.1

281.0B 272.7B

343.6 281.1

378.6C 315.2C

286.9 247.1

362.4CD 311.4C

266.0 246.7

336.0D 308.8C

406.3 370.8

565.8A 502.2A

318.7 287.9

441.9B 403.0B

33.5 29.3

68.6A 62.5A

17.5 17.3

16.6 13.1

62.0B 53.3B

12.6 4.3

56.8C 47.4C

37.3 33.0

51.5CD 48.6BC

48.9D 47.3BC

A–DValues 1WG

within lines with no common superscript differ significantly (P < 0.005) between control groups. (%) = 100 (BW36 – BW26)/BW26.

where WG = weight gain; PC = plasma coloration, and WGc and PCc, are the mean values of WG and PC, respectively, in the control group. In the WLDW line, effects of MHC genotype and DW on individual data were estimated by analysis of variance, according to the following model: Yijklmn = m + sexi + DWij + MHCk + sirel + damlm + eijklmn where, sexi is the fixed effect of the sex i; DWij is the fixed effect of the DW genotype j within the sex i; MHCk is the fixed effect of the MHC genotype k; and sirel and damlm are the random effects of sire l and dam m within sire l, respectively. Interactions between main effects were tested but were not significant (P > 0.10) and therefore were removed from the model. In addition, effects on resistance criteria of the different MHC genotypes were analyzed by testing the contrasts between genotypes.

RESULTS

Body Weight and Weight Gain Numbers of animals, mean body weights, and relative weight gains by line, sex, and challenge group are given in Table 1. Lines were compared for body weights at 26 and 36 d, and for relative weight gain, by comparing unchallenged birds. At 26 and 36 d of age, the Egyptian Mandarah line was the heaviest, and the RIR and the WLB21 lines the lightest, with the Fayoumi and the WLDW lines having intermediate body weights. The RIR birds had the greatest weight gain, and the WLB21 males had the lowest weight gain. Weight gain was similar in the WLDW line and in the Fayoumi line, but it was lower than in the Mandarah line.

Resistance to Eimeria tenella Effect of line was highly significant (P < 0.001) on all individually analyzed traits. Sex and line by sex effects were generally not significant (P > 0.10). Within the WLDW line only, females showed a lower (P < 0.01) weight gain than males (–99 and –82 %, in females and males, respectively) (data not shown). Lines were ranked in Table 2 for the following resistance criteria: adjusted weight gain and plasma coloration, and feed conversion. For these three criteria, ranking of the lines was in agreement: the Fayoumi line was the most resistant line, whereas the WLDW was the most susceptible one. For example, body weight gain was severely reduced (–85%) in the tested WLDW chickens (–30% in the Fayoumi only), whereas feed conversion was dramatically increased in the WLDW (362%) as compared to the Fayoumi (27%). The RIR, WLB21, and Mandarah lines showed intermediate resistance for weight gain, plasma coloration, and feed conversion. Phenotypic variability of weight gain and plasma coloration per line and sex is given in Table 2. Whereas standard deviations of weight gain showed moderate variation between lines, phenotypic variation of plasma coloration increased with resistance, showing little variation in the susceptible WLDW line. Mortality due to E. tenella was nil in the Fayoumi line only, it was low in the RIR and Mandarah lines, and it was significantly higher in the two White Leghorn lines (38 and 27% in the WLB21 and WLDW lines, respectively). As expected for a high inoculation dose, oocyst production was high in all the lines and most animals showed lesion scores of 3 or 4. Nevertheless, Fayoumi birds had a significantly (P < 0.005) lower average lesion score than the other birds: most Fayoumi birds (84%) showed lesion scores of 3, whereas birds from the other lines had more frequently (79% on average) lesion scores of 4.

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Number Males Females BW26, g Males Females BW36, g Males Females WG,1 % Males Females

RIR

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TABLE 2. Comparison of five poultry lines for traits associated with resistance to Eimeria tenella. Least squares means (± SE) of adjusted weight gain (WG′), and plasma coloration (PC′), adjusted feed conversion (FC′), mortality (M), and lesion score (LS), phenotypic standard deviations (SD males/SD females) of WG′ and of PC′. Distribution of LS and oocyst production (OP) per chicken line Line Variable

Fayoumi

RIR

WG′,1

–29.9 ± 29.1/27.8 –27.2 ± 2.6A 25.6/24.8 +26.8 0A 3.13 ± 0.39A

–52.1 ± 20.5/17.6 –51.4 ± 2.6B 28.7/22.6 +65.7 3.57A 3.80 ± 0.40BC

1.82 83.64 14.54 38.85

WLB21 3.1B

0 19.64 80.36 30.80

–63.7 ± 24.1/18.8 –66.9 ± 3.8C 20.7/14.1 +96.2 37.50B 3.63 ± 0.49B 4.1BC

0 37.50 62.50 42.18

Mandarah

WLDW

–71.6 ± 20.1/23.1 –72.3 ± 2.5C 11.7/12.1 +162.4 7.94A 3.94 ± 0.25C

–84.6 ± 3.0D 18.0/27.8 –83.3 ± 2.5D 4.8/9.8 +361.5 26.98B 3.81 ± 0.39BC

2.9C

0 6.35 93.65 20.45

0 19.05 80.95 23.97

A–DValues 1WG′

within lines with no common superscript indicate significant differences (P < 0.005) between chicken lines. = (WG – WGc)/WGc, and PC′ = (PC – PCc)/PCc with WGc, and PCc, the mean values of WG and PC, respectively, in the control

group. 2FC′ = (FC – FCc)/FCc, with FCc the value of FC for the control group. 3105 per gram of feces.

Correlation Between Resistance Criteria to Eimeria tenella Phenotypic correlations between adjusted weight gain and plasma coloration were calculated per line and sex (Table 3). Correlations were mostly significant and positive. Significances and values varied between lines and sexes, the significant correlation values varying between 0.35 and 0.83. Because animals were mostly suffering from lesion score of either 3 or 4 only, average weight gains were compared between animals showing the two lesion grades in order to estimate the relationship between these two resistance criteria. Within all lines, except in the WLDW line, animals suffering from more severe lesions (4 vs 3) showed also significantly (P < 0.005 to 0.01) higher weight gain depression (Table 3). In these lines, the average difference for weight gain between Lesion score 3 and Lesion score 4 animals was 25%. In the WLDW line, average weight gain depression was very high (over –83%) in both Lesion 3 and Lesion 4 animals.

Effects of the MHC Genotype, and of the Dwarf Gene DW on Resistance to E. tenella in the WLDW Line All birds from the WLDW were genotyped for MHC. The three present haplotypes (B15, B19, and B21) segregated in the families, showing the six different homozygote and heterozygote genotypic combinations (about 20 birds per possible genotype). The overall effect of the MHC genotype was not significant (P > 0.10) on any of the resistance criteria in the WLDW line (data not shown). However, when testing contrasts between genotypes for each resistance criteria, one could notice a

slightly lower weight gain depression (P < 0.05) in birds with B15B21 genotype (–79% of WG′) than in the B15B19 birds (–107%). Also, B15B21 birds had less severe lesion scores (3.53) than B21B21 birds (3.92) (P < 0.05). In addition, B15B15 birds showed less severe lesion scores (3.30) than B21B21 birds (P < 0.01). A total of 124 birds from the WLDW families were genotyped for DW. There were 33 normal and 21 dwarf females, and 39 heterozygous and 31 dwarf males, spread within families in control and challenged groups. Effect of the dwarf gene DW within sex was not significant (P > 0.10) on any of the resistance criteria analyzed in the WLDW line (data not shown).

DISCUSSION Large genetic variation for resistance to E. tenella has been demonstrated here by comparing several outbred lines. The importance of the host genetics on the outcome of the disease had been already demonstrated by comparisons of inbred lines (e.g., Bumstead and Millard, 1987; Lillehoj et al., 1989) or more infrequently of outbred lines as done here (Albers and Verheijen, 1992). Johnson and Edgar (1986) were successful in divergently selecting for survival to acute infection with E. tenella. Significant genetic variability of resistance to this coccidiosis has been assessed by within population analysis (Mathis et al., 1984). When considering weight gain, plasma coloration, feed conversion, mortality, and lesion score, the Fayoumi line appeared in this work as the most resistant line to an acute primary infection with E. tenella. The Fayoumi line originates from Egypt and has been maintained at the Laboratoire de Ge´ne´tique Factorielle since 1978. A preliminary test in 1980 had indicated a higher degree of resistance of these birds to

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% SD, WG′ PC′,1 % SD PC′ FC′,2 % M, % LS LS, % 2 3 4 OP3

3.1A

189

RESISTANCE TO COCCIDIOSIS TABLE 3. Relations between resistance criteria. Phenotypic correlations (rWG′–PC′) between adjusted weight gain (WG′) and plasma coloration (PC′), and least squares mean values (± SE) for WG′ (LSM WG′) between animals with lesions (LS) 3 and 4 Line Variable rWG′–PC′1,2 WG′ LS 3 LS 4

Fayoumi 0.45**/0.59*** –25.98 ± 3.84A –57.22 ± 9.27B

RIR

WLB21

0.80***/0.35†

0.83***/0.55*

–34.67 ± 5.15A –56.38 ± 2.55B

–49.55 ± 5.44A –72.19 ± 4.21B

Mandarah 0.65***/0.63*** –47.10 ± 10.40x –73.24 ± 2.71y

WLDW 0.13/0.54** –83.01 ± 6.68 –85.77 ± 3.39

E. tenella (Hamet and Me´rat, 1982), and our results confirm the local reputation of resistance that these birds have in Egypt (Hossary and Galal, 1994). If the genetic resistance has been conserved for 15 yr, despite some increase in inbreeding, part of the genetic resistance might be fixed in the Fayoumi line. This conclusion would validate the strategy of producing an F2 with this line in order to identify markers of resistance. Interestingly, the Fayoumi line showed a higher resistance than a White Leghorn line to Marek’s disease (M. TixierBoichard, 1993, Laboratoire de Ge´ne´tique Factorielle, INRA, Jouy-en-Josas, France, personal communication), and to Rous Sarcoma virus (reviewed by Hossary and Galal, 1994), two viral diseases that involve different resistance mechanisms. This broad genetic resistance validates searching in this line for genes to introduce disease resistance. As found by Albers and Verheijen (1992) and Bumstead and Millard (1987), lines having a higher mortality rate did not always show the largest weight gain depression, as illustrated here with the WLB21 line compared to the Mandarah line. Albers and Verheijen (1992) made the assumption that some lines would be able to cope better than others with the effect of the disease and would survive. In most of our lines, birds having lesion scores of 4 had, on average, a higher weight gain depression than those presenting lesion scores of 3, indicating some correlation between these two parameters (Table 3). The exception to this assumption was the most susceptible WLDW line, in which the weight gain depression might have been too large to show enough variation between the two groups of lesion scores. Mathis et al. (1984) concluded from their genetic analysis that lesion score might be a suitable parameter, correlated with other infection parameters. Because of its quantitative nature, weight gain may be

considered as the parameter of choice in a genetic analysis, especially when the range of lesion scores is small. Whether the plasma coloration is a valid criterion to assess resistance to E. tenella may be questioned. It is easy to measure and it does not require that the birds be killed. The yellow coloration of the plasma is due to carotenoids pigments in the feed only. From a physiological point of view, this criterion might be more relevant in the case of an intestinal coccidiosis, such as with E. maxima, than for a cecal infection with E. tenella. Yvore´ et al. (1993a) found that plasma coloration was only a discriminating criterion for high inoculation doses of E. tenella, in contrast with E. maxima for which plasma decoloration was significant at very low doses. In the present experiment, a high level of infection was chosen and plasma coloration appeared to be a relevant criterion to compare the different lines, giving very similar results to those obtained with the weight gain (Table 2). However, within lines, correlation of plasma coloration with weight gain was significant in all the lines except in the WLDW males (Table 3). In this very susceptible line, plasma coloration showed little phenotypic variation (Table 2), which definitely reduces the usefulness of plasma coloration as a discriminating resistance criterion within a given susceptible line. Whereas related endocrinological changes and effects on production traits of DW are extensively documented, studies on the effects of this gene on immune responses and specific diseases, especially coccidiosis, are limited (Me´rat, 1990), with little or inconsistent effects on disease traits. For example, Meurier and Me´rat (1972) found a positive effect of DW on resistance to Marek’s disease, whereas no difference in resistance to Newcastle disease was reported by Stephens and Dreyfuss (1978). In the WLDW line used here, a negative effect of DW on

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A,BMeans within columns with no common superscript indicate significant differences within chicken lines (P < 0.005). x,yMeans within columns with no common superscript indicate significant differences within chicken lines (P < 0.01). 1Correlations in males/females within chicken lines. 2WG′ = (WG – WGc)/WGc, and PC′ = (PC – PCc)/PCc with WGc, and PCc, the mean values of WG and PC, respectively, in the control group. †P < 0.10. *P < 0.05. **P < 0.01. ***P < 0.001.

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controls of innate and acquired immunities may, in fact, differ. Varying effects of the MHC on the two types of resistance were reported (Clare et al., 1989, Ruff and Bacon, 1989). The current study showed great genetic variability for genetic resistance to a cecal coccidiosis, E. tenella. The Fayoumi line appeared as the most resistant line, in contrast to a susceptible White Leghorn line (WLDW), as the two lines differed highly significantly for the resistance criteria analyzed, especially body weight depression. Consequently, these two lines may be considered as candidates of choice to produce F2 crosses to search for genetic markers of resistance. Control birds from the two lines differed for body weight, but not for body weight gain, which means that variation of weight gain in a mixed F2 population may be directly related to resistance to infection and not to line origin. As expression of genetic variability may depend on the infection dose chosen, additional challenge tests need to be performed first in order to identify the most discriminant dose of E. tenella for the F2 experiment. Also, gain in improving genetic resistance to E. tenella will be of limited value if it implies a lower resistance to other Eimeria species. Previous studies using other chicken lines did not report a clear correlation between resistances to E. tenella and to E. acervulina (Albers and Verheijen, 1992) or to E. maxima (Bumstead and Millard, 1987). Therefore, resistance to other Eimeria species will have to be assessed in the Fayoumi line. Finally, such resistant and susceptible lines may be used as models to discover novel underlying immune mechanisms of resistance.

ACKNOWLEDGMENTS The authors thank the staff at the poultry experimental unit, La Minie`re, for the chicken line management, J.-L. Coville for his technical assistance for the dwarf gene typing and the CNEVA, Beaucemaine BP53, 22440 Ploufragan, France, for providing challenge facilities. This research was supported by an INRA program “Genes of Resistance to Diseases”.

REFERENCES Albers, G.A.A., and F. Verheijen, 1992. Genetic resistance to coccidiosis in broiler lines. Pages 753–756 in: Proceedings of the 19th World’s Poultry Congress. Vol. 1. Amsterdam, The Netherlands. Allen, P. C., H. D. Danforth, and O. A. Levander, 1996. Diets in n-3 fatty acids reduce cecal lesion scores in chickens infected with Eimeria tenella. Poultry Sci. 75:179–185. Briles, W. E., and R. W. Briles, 1982. Identification of haplotypes in the chicken major histocompatibility complex (B). Immunogenetics 15:449–459. Bumstead, N., and B. Millard, 1987. Genetics of resistance to coccidiosis: Response of inbred chicken lines to infection by Eimeria tenella and Eimeria maxima. Br. Poult. Sci. 28: 705–715.

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several in vivo immune responses had been shown earlier (Pinard and Monvoisin, 1994), but no effect of DW on resistance to E. tenella was found in the present work. Haas et al. (1975) found no significant effect of DW on resistance to Eimeria acervulina, Eimeria brunetti, or Eimeria necatrix, and Zulkifli et al. (1993) reported even a higher percentage of lesions to E. tenella in dwarf than in normal chickens. However, in the latter experiment, effects of infection were limited, lesion scores being low and both body weight and feed consumption were unaffected by the inoculation. Moreover, the last two studies did not analyze the effect of DW in segregating families, but compared dwarf and normal lines having a similar but not identical backgrounds. One might conclude that if DW had a major and direct unfavorable effect on the outcome of the disease, this effect would have appeared in all studies. And fortunately, considering the widespread utilization of DW in the poultry industry, this was not the case. No major effect of the MHC on resistance to E. tenella was observed here either. Only the B15B21 genotype seemed to be associated with both the reduced effect on growth and lower lesion scores. The number of animals per genotype, however, was limited and a specific trial on MHC should be performed to study this putative associated resistance of the B15B21 genotype in this line. Moreover, the most resistant genotype might vary according to the resistance criterion used, as suggested by the results. Ruff and Bacon (1989) showed different degrees of resistance of congenic lines differing for MHC haplotypes when considering either weight gain or lesion score. Even if correlated, resistance parameters would not be controlled by identical sets of genes. It was not possible to directly compare our results with those from other studies because most of these compared congenic lines that were homozygous for a given haplotype (Lillehoj et al., 1989; Nakai et al., 1993). The absence of major and consistent effects of the MHC on coccidiosis is reported in the literature. It shows that if the MHC has some effect on resistance to coccidiosis, other genes are also involved. Moreover, the MHC effect may vary depending on background genes, as illustrated by two types of studies: the work of Lillehoj et al. (1989), previously quoted, which compared congenic lines and lines having identical MHC haplotype but different genetic background, and the work of Johnson and Edgar (1986), which concluded that there was an interaction of MHC genes with other genes that explained the observed changes in MHC type frequencies as a response to selection for resistance to E. tenella. The magnitude of the MHC effect may also depend on the inoculation dose. The high dose used in our study may have limited the expression of the genetic variability for resistance criteria, as found for lesion score in the work of Clare et al. (1989), and it may have hidden the MHC effect, as shown by Uni et al. (1995). In a future trial, it would be interesting to study the effect of the MHC on acquired resistance to E. tenella. Genetic

RESISTANCE TO COCCIDIOSIS

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