The MHC class II region and resistance to an intestinal parasite in sheep

SUMMARY: RFLP analysis was used to investigate the effect of genetic variation in the Major Histocompatibility Complex (MHC) class II region on resistance of sheep to the intestinal parasite Trichostrongylus colubriformis, using faecal egg count (fec) as the measure of resistance. RFLP analysis of DNA from 335 sheep with the restriction enzymes TaqI, PvuII and HindIII, and DRB, DQA and DQB human class II MHC cDNA clones, revealed 238 bands, of which 233 were polymorphic. Sixteen bands were associated with a significant effect on fec, when analysed using a mixed model, best linear unbiased prediction statistical methods. However, when p values were corrected for the number of bands tested the associations were no longer significant. While no individual band had a significant effect on fec, Fisher's X(2) test of the distribution of p-values showed that RFLP bands, when considered together, do account for a significant proportion of variation in fec. One hundred and thirteen offspring from 11 halb-sib families were classified according to which MHC haplotype they had inherited from their sire. While there was overall no convincing statistical evidence of an effect of MHC haplotype on resistance, results in one family showed evidence for a repeatable MHC effect. ZUSAMMENFASSUNG: Die MHC-Klasse II Region und Resistenz gegenüber Darmparasiten beim Schaf RFLP Analyse betrifft die Auswirkung genetischer Unterschiede im Haupthistokompatibilit?ts-Komplex (MHC) Klasse II Region auf Resistenz von Schafen gegen den Darmparasiten Trichostrongylus colubriformis, wobei die f?kale Eizahl (fec) als Ma? der Resistenz verwendet worden ist. Die RFLP-Analyse der DNA von 335 Schafen mit Restriktionsenzymen TaqI, PvuII und HndIII und DRB, DQA und DQB menschlicher Klasse II MHC cDNA Klone ergab 238 Bande, wovon 233 polymorph waren. 16 Bande waren mit einer signifikanten Wirkung auf fec verbunden, wobei zur Analyse das gemischte Modell der BLUP verwendet worden ist. Allerdings, nach Korrektur der p-Werte für die Zahl der geprüften Bande zeigt sich die Assoziation nicht l?nger als signifikant. W?hrend kein individuelles Band eine signifikante Wirkung auf FEC zeigte, ergab Fisher's test hinsichtlich Verteilung der p-Werte Signifikanz bei Gesamtbetrachtung. 113 Nachkommen von 11 Halbgeschwister-Familien wurden nach dem MHC Haplotyp des Vatertieres klassifiziert. Es ergab sich kein insgesamt überzeugender statistischer Hinweis auf Wirkung der MHC Haplotypen, doch in einer Familie Hinweis auf wiederholbare MHC Effekte.     

High perinatal mortality associated with triple anthelmintic resistance in a German sheep flock

High perinatal mortality, low milk yields and occasional ewe deaths were investigated in a Dorper sheep flock in Southern Germany. Parasitic gastroenteritis due to Trichostrongylus spp. associated with severe weight loss despite regular anthelmintic treatments of the flock was identified as the underlying cause. A faecal egg count reduction (FECR) test revealed zero reduction after treatment with ivermectin or albendazole, respectively, and a FECR of 57.9% following treatment with levamisole. These results indicate a lack of, or considerably reduced efficacy of substances from all three classical groups of anthelmintics and demonstrate that triple anthelmintic resistance is also present in Germany. The introduction of resistant worm populations with imported livestock, excessive use of anthelmintic drugs and under-dosing of goats have possibly led to the problem in the flock described. Veterinary advice on anthelmintic treatments and responsible parasite control programmes are therefore crucial in small ruminant flocks.     

Benzimidazole resistance in Trichostrongylus axei in sheep: long-term monitoring of affected sheep and genotypic evaluation of the parasite

This is the first report of benzimidazole (BZ) resistance in the nematode Trichostrongylus axei in sheep. Trichostrongylus axei infects several species of herbivores including sheep, cattle and horses, and the emergence of anthelmintic resistance could lead to significant problems in its control. Benzimidazole resistance in two sheep flocks in central France was detected by post-treatment worm counts. The sequencing of a central region of the isotype 1 beta-tubulin gene from adult T. axei recovered post-mortem revealed only one, non-synonymous single nucleotide polymorphism at position 200 (Phe200Tyr), which had already been reported for other nematodes. Seven years after BZ treatment ceased, T. axei helminths present were still resistant to BZ suggesting these parasites do not revert to susceptibility to this anthelmintic, even when the selection pressure had been removed for many years. The findings also highlight major changes in the make-up of the nematode burden in sheep flocks that accompanies the emergence of BZ resistance.     

Survey of anthelmintic resistance in Western Australian sheep flocks 2. Relationship with sheep management and parasite control practices

Owners of 116 farms, whose flocks had been tested for anthelmintic resistance, were interviewed to determine their use of various sheep management and parasite control practices and their knowledge and adoption of recommended procedures for the prevention and control of resistance. Farmers knowledge of current recommendations related mainly to changing drenches and drench groups. Other aspects of the recommended program including reduction of drenching frequency and the use of alternative management strategies were not considered as important by farmers. For most questions a high proportion of farmers (greater than 20%) had no opinion. Associations between various strategies for nematode control and resistance of Trichostrongylus and Teladorsagia to thiabendazole and levamisole were examined. These relationships differed between anthelmintics and nematode genera. A number of factors were related to resistance of one or both nematode genera to one or both anthelmintic groups. These factors included flock size, percentage of ewes in the flock, cattle number, main sheep production activity, grazing strategy, frequency of drenching, changes in the frequency of drenching, number of summer drenches and the method of estimating dose rates. It was concluded that the methods employed to control anthelmintic resistance may vary with the nematode, its resistance status and the anthelmintic to which it is exposed. Modifications to the previously recommended program have been proposed which incorporate selection of the anthelmintic to be used following a test for anthelmintic resistance.     

Genome-wide scan for selective sweeps identifies novel loci associated with resistance to mastitis in German Holstein cattle

Domestication and selection significantly changed phenotypic and behavioural traits in modern domestic animals. In this study, to identify the genomic regions associated with mastitis, genomic data of German Holstein dairy cattle were analysed. The samples were genotyped using the Bovine 50 K SNP chip. For each defined healthy and sick group, 133 samples from 13,276 genotyped dairy cows were selected based on mastitis random residual effects. Grouping was done to infer selection signatures based on XP-EHH statistic. The results revealed that for the top 0.01 percentile of the obtained XP-EHH values, five genomic regions on chromosomes 8, 11, 12, 14 and 26 of the control group, and four regions on chromosomes 3, 4 (two regions) and 22 of the case group, have been under selection. Also, consideration of the top 0.1 percentile of the XP-EHH values, clarified 21 and 15 selective sweeps in the control and case group, respectively. This study identified some genomic regions containing potential candidate genes associated with resistance and susceptibility to mastitis, immune system and inflammation, milk traits, udder morphology and different types of cancers. In addition, these regions overlap with some quantitative trait loci linked to clinical mastitis, immunoglobulin levels, somatic cell score, udder traits, milk fat and protein, milk yield, milking speed and veterinary treatments. It is noteworthy that we found two regions in the healthy group (on chromosomes 12 and 14) with strong signals, which were not described previously. It is likely that future research could link these identified genomic regions to mastitis. The results of the current study contribute to the identification of causal mutations, genomic regions and genes affecting mastitis incidence in dairy cows.     

Scrapie resistance alleles are not associated with lower prolificity in Rasa Aragonesa sheep

Scrapie is a prion disease characterised by the accumulation of the pathological associated form of cellular prion protein (PrP(SC)) in the central nervous system. Susceptibility to scrapie is associated with polymorphism in the ovine prion protein (PrP) gene. The European Union has implemented scrapie control programs, relying on selective breeding for scrapie resistance; the use of ARR-carrier and the exclusion of VRQ-carrier were recommended. In this study, 4323 individuals from Rasa Aragonesa Sheep breed were genotyped for the PrP gene and the individual estimated breeding values (EBV) for prolificity were calculated. Most represented PrP alleles do not work against prolificity. Only a significant association between VRQ/VRQ genotype and a lower EBV was observed (p = 0.027, eta2 = 0.002). Therefore, avoiding reproduction of VRQ/VRQ individuals would not cause negative effect regarding prolificity.     

A systematic review and meta-analysis of factors associated with anthelmintic resistance in sheep

Background

Anthelmintic drugs have been widely used in sheep as a cost-effective means for gastro-intestinal nematode (GIN) control. However, growing anthelmintic resistance (AHR) has created a compelling need to identify evidence-based management recommendations that reduce the risk of further development and impact of AHR.

Objective

To identify, critically assess, and synthesize available data from primary research on factors associated with AHR in sheep.

Methods

Publications reporting original observational or experimental research on selected factors associated with AHR in sheep GINs and published after 1974, were identified through two processes. Three electronic databases (PubMed, Agricola, CAB) and Web of Science (a collection of databases) were searched for potentially relevant publications. Additional publications were identified through consultation with experts, manual search of references of included publications and conference proceedings, and information solicited from small ruminant practitioner list-serves. Two independent investigators screened abstracts for relevance. Relevant publications were assessed for risk of systematic bias. Where sufficient data were available, random-effects Meta-Analyses (MAs) were performed to estimate the pooled Odds Ratio (OR) and 95% Confidence Intervals (CIs) of AHR for factors reported in ≥2 publications.

Results

Of the 1712 abstracts screened for eligibility, 131 were deemed relevant for full publication review. Thirty publications describing 25 individual studies (15 observational studies, 7 challenge trials, and 3 controlled trials) were included in the qualitative synthesis and assessed for systematic bias. Unclear (i.e. not reported, or unable to assess) or high risk of selection bias and confounding bias was found in 93% (14/15) and 60% (9/15) of the observational studies, respectively, while unclear risk of selection bias was identified in all of the trials. Ten independent studies were included in the quantitative synthesis, and MAs were performed for five factors. Only high frequency of treatment was a significant risk factor (OR = 4.39; 95% CI = 1.59, 12.14), while the remaining 4 variables were marginally significant: mixed-species grazing (OR = 1.63; 95% CI = 0.66, 4.07); flock size (OR = 1.02; 95% CI = 0.97, 1.07); use of long-acting drug formulations (OR = 2.85; 95% CI = 0.79, 10.24); and drench-and-shift pasture management (OR = 4.08; 95% CI = 0.75, 22.16).

Conclusions

While there is abundant literature on the topic of AHR in sheep GINs, few studies have explicitly investigated the association between putative risk or protective factors and AHR. Consequently, several of the current recommendations on parasite management are not evidence-based. Moreover, many of the studies included in this review had a high or unclear risk of systematic bias, highlighting the need to improve study design and/or reporting of future research carried out in this field.     

Dermatitis in a dog associated with an unidentified Toxoplasma gondii-like parasite

Protozoal dermatitis was diagnosed in a 6-year-old female Great Dane dog from Rio de Janeiro, Brazil. The dog died because of a chronic illness with an Ehrlichia-like organism. Numerous apicomplexan parasites were identified histologically in the section of dermal lesions. The protozoan reacted with Toxoplasma gondii polyclonal rabbit serum but not with Neospora caninum or Sarcocystis neurona antibodies. Ultrastructurally, the protozoa was not T. gondii because it had schizont-like structures with merozoites arranged around a prominent residual body, and the merozoites had several rhoptries with electron-dense contents; rhoptries in T. gondii tachyzoites are electron-lucent and a residual body is not found in groups of tachyzoites. This is the first report of unidentified T. gondii-like protozoa in the skin of a dog.     

Farm management practices associated with macrocyclic lactone resistance on sheep farms in New Zealand

AIM: To identify farm practices associated with the presence of resistance to a macrocyclic lactone (ML) anthelmintic on sheep farms in New Zealand. METHODS: A cross-sectional study was conducted to test for associations between the presence of resistance to an ML anthelmintic (ivermectin) and management practices on sheep farms in New Zealand. Selection of farms was both random (n=80) and purposive (n=32; being farms with a history of suspected ML resistance). Resistance was inferred from faecal nematode egg count (FEC) reduction (FECR) tests (FECRTs) when there was <95% reduction in FEC 7-10 days after treatment with a half dose of ivermectin (0.1 mg/kg). A logistic regression model was built to identify farm-level factors that were associated with the presence or absence of ML resistance. RESULTS: Of the 112 flock managers that were approached for interview, 103 (92%) returned useable questionnaires. The odds of ML resistance were increased: on farms that had used long-acting ML products in ewes as a pre-lambing treatment for > or =3 of the previous 5 years (odds ratio (OR) = 7.2; 95% confidence interval (CI) = 1.7-30.3); on farms where <70% of the total stock units mid-winter were from sheep (OR=6.5; 95% CI=1.6-25.6); on farms which over the year purchased >10% of the number of sheep present mid-winter (OR=7.1; 95% CI=1.5-34.7); and on farms where the average wool diameter of the main flock was <37 (OR=4.1; 95% CI=1.1-14.7) microns. The model provided a good fit to the data (pseudo R2=0.64; Hosmer-Lemeshow statistic = 0.38). CONCLUSIONS: Explanatory factors identified as associated with the presence of ML (ivermectin) resistance on farms included the use of long-acting anthelmintic formulations in ewes pre-lambing, sources of refugia of unselected parasites on the farm, breed of sheep and their requirements for anthelmintic treatments, and the importing of resistant parasites with purchased stock. The study provides support for controls that aim to provide refugia of susceptible worms and that minimise the risk of introduction of resistance through effective quarantine drenching.     

Farm management practices associated with macrocyclic lactone resistance on sheep farms in New Zealand

AIM: To identify farm practices associated with the presence of resistance to a macrocyclic lactone (ML) anthelmintic on sheep farms in New Zealand.

METHODS: A cross-sectional study was conducted to test for associations between the presence of resistance to an ML anthelmintic (ivermectin) and management practices on sheep farms in New Zealand. Selection of farms was both random (n=80) and purposive (n=32; being farms with a history of suspected ML resistance). Resistance was inferred from faecal nematode egg count (FEC) reduction (FECR) tests (FECRTs) when there was <95% reduction in FEC 7–10 days after treatment with a half dose of ivermectin (0.1 mg/kg). A logistic regression model was built to identify farm-level factors that were associated with the presence or absence of ML resistance.

RESULTS: Of the 112 flock managers that were approached for interview, 103 (92%) returned useable questionnaires. The odds of ML resistance were increased: on farms that had used long-acting ML products in ewes as a pre-lambing treatment for ≥3 of the previous 5 years (odds ratio (OR) = 7.2; 95% confidence interval (CI) = 1.7–30.3); on farms where <70% of the total stock units mid-winter were from sheep (OR=6.5; 95% CI=1.6–25.6); on farms which over the year purchased ≥10% of the number of sheep present mid-winter (OR=7.1; 95% CI=1.5–34.7); and on farms where the average wool diameter of the main flock was <37 (OR=4.1; 95% CI=1.1–14.7) microns. The model provided a good fit to the data (pseudo R²=0.64; Hosmer-Lemeshow statistic = 0.38).

CONCLUSIONS: Explanatory factors identified as associated with the presence of ML (ivermectin) resistance on farms included the use of long-acting anthelmintic formulations in ewes pre-lambing, sources of refugia of unselected parasites on the farm, breed of sheep and their requirements for anthelmintic treatments, and the importing of resistant parasites with purchased stock. The study provides support for controls that aim to provide refugia of susceptible worms and that minimise the risk of introduction of resistance through effective quarantine-drenching.     

Horner's syndrome associated with parotid duct obstruction in a sheep

A 9-year old, Rasa Aragonesa ewe was presented with a left-sided, facial, soft fluctuant swelling. The postmortem examination showed grass awns filling the entire length of the parotid gland duct. The presence of parotid duct obstruction with Horner's syndrome, previously unreported in sheep, is discussed.     

Broomfield Corriedales: a strain of sheep selectively bred for resistance to footrot

Broomfield Corriedales originate from a 15-year programme of selective breeding from sheep that evaded footrot when deliberately subjected to field challenge. To determine whether this policy may have resulted in improved resistance to footrot, trials were set up in which representative animals from the Broomfield blood-line were challenged with footrot by either experimental or natural infection, together with Corriedale sheep of similar age and sex but from flocks in which the disease is not known to occur. In another approach, the occurrence of footrot was compared in the offspring of Broomfield and of non-Broomfield sires mated with randomly selected ewes in a commercial Corriedale flock in which the condition was prevalent. Finally, the effects of natural challenge in Broomfield lambs were compared with those in a selection line of Corriedale lambs of which both parents had a history of chronic footrot disease. Footrot affected significantly fewer sheep of the Broomfield blood-line and to a markedly lesser degree than those chosen for comparison under identical challenge conditions. Where present, foot infections in Broomfield animals also tended to be less invasive and less persistent. Reduced footrot prevalence was also demonstrated in three birth cohorts of first-cross offspring of Broomfield sires relative to the offspring of other sires, though differences were not significant at all inspections. These findings, considered as a supplement to the historical development of the Broomfield flock, offer encouraging prospects for breeding programmes to enhance the resistance of sheep to footrot disease.     

Functional investigation of a QTL affecting resistance to Haemonchus contortus in sheep

This study reports a functional characterization of a limited segment (QTL) of sheep chromosome 12 associated with resistance to the abomasal nematode Haemonchus contortus. The first objective was to validate the identified QTL through the comparison of genetically susceptible (N) and resistant (R) sheep produced from Martinik × Romane back-cross sheep. The R and N genotype groups were then experimentally infected with 10 000 H. contortus larvae and measured for FEC (every three days from 18 to 30 days post-challenge), haematocrit, worm burden and fertility. Significant differences in FEC and haematocrit drop were found between R and N sheep. In addition, the female worms recovered from R sheep were less fecund. The second step of the characterization was to investigate functional mechanisms associated with the QTL, thanks to a gene expression analysis performed on the abomasal mucosa and the abomasal lymph node. The gene expression level of a candidate gene lying within the QTL region (PAPP-A2) was measured. In addition, putative interactions between the chromosome segment under study and the top ten differentially expressed genes between resistant MBB and susceptible RMN sheep highlighted in a previous microarray experiment were investigated. We found an induction of Th-2 related cytokine genes expression in the abomasal mucosa of R sheep. Down-regulation of the PAPP-A2 gene expression was observed between naïve and challenged sheep although no differential expression was recorded between challenged R and N sheep. The genotyping of this limited region should contribute to the ability to predict the intrinsic resistance level of sheep.