Association between microsatellite markers of sheep chromosome 5 and faecal egg counts

Small ruminant resistance against gastrointestinal nematodes: a case of Haemonchus contortus

Gastrointestinal nematode (GIN) infections are a common constraint to small ruminant industry throughout the world, and among those, haemonchosis has its own significance. Control of GIN primarily relies on the use of anthelmintics, but this approach has become less reliable due to the development of resistance in GINs against commonly used anthelmintics and an increased consumer demand for environmentally friendly animal products. These issues have stimulated investigations to find alternative sustainable control strategies, which are less reliant on anthelmintic input. One of such strategies is breeding of small ruminants for their resistance to the GINs. The susceptibility and resistance of animals to GIN infections varies within and between breeds. Various parasitological, biochemical and immunological parameters are employed to evaluate natural resistance status of animals both in natural pasture and artificial infections. The immune mechanisms responsible for resistance are not completely understood, but it has a significant effect in inherited resistance. Relatively resistant or tolerant animals show better local and generalised immune response as compared to susceptible. Immune response against GINs is influenced by many physiological factors. Determination of specific genes linked with host resistance will provide a valuable approach to find out the molecular mechanism of host resistance to GINs. Resistance has been reported to reduce pasture contamination, which in turn reduces re-infection and thus the requirement of the frequent anthelmintic treatments. The efficiency of control can be increased through objective and accurate identification of genetically tolerant individuals by natural and artificial infections with GINs. Complete resistance is the ultimate solution, but this has generally been ignored as a commercial reality. This paper reviews the published reports on natural resistance in small ruminants and discusses the prospects of developing small ruminants, which could be resistant to GINs.

Frequency of treatment and production performance using the FAMACHA method compared with preventive control in ewes

The rapid escalation of anthelmintic resistance in parasites suggests the adoption of different strategies for parasite control in small ruminants instead of relying only on the use of chemicals. The integrated system of parasite control (SICOPA) program uses the FAMACHA method, which is used to separately identify sheep that are resistant/resilient or susceptible to Haemonchus contortus, or together with other management practices, such as pasture rotation or the monitoring of drug efficacy. The objective of this work was to compare two methods of parasite control in ewes. A flock of ewes (n=90) of Ile de France and Texel breeds was monitored from August 2001 to July 2003 under suppressive treatment (45-day treatment interval) and from August 2003 to July 2006 under the partial selective regimen based on FAMACHA evaluations. Fecal samples were collected for fecal egg counts (EPG) and larval identification to monitor infection during the 5-year period. After the partial selective evaluations, an average of 19% of the animals was treated each year between 2003 and 2006. The reproductive indexes of parturition (89%), birth rate (119%), lamb weight (4.1 kg) and lamb mortality (5.2%) were similar under the partial selective regimen and the previous suppressive treatment period. H. contortus was the most prevalent species, and EPG values were not statistically different across periods or among treatment regimes. The results demonstrate that the FAMACHA method allowed a large number of naturally infected ewes to be raised with reduced anthelmintic treatment without a negative impact on their reproductive indexes. It is indicated, though, that this method be applied together with other available strategies. Culling was based on clinical signs and ranged from 5.5 to 20% per year. The results suggest that an important host-specific effect may have regulated the rate of infection, expressing the resistant/resilient characteristic within the flock with an important economical benefit.

Alternative approaches to control--quo vadit?

The increasing prevalence of anthelmintic resistance has provided a spur for research into 'alternative/novel' approaches to the control of helminthoses that are intended to reduce our reliance upon using chemoprophylaxis. The different approaches either target the parasite population in the host or on pasture, but the goal of all of them is to restrict host parasite contact to levels which minimise the impact of helminths on host welfare and/or performance. Infrapopulation regulation can be achieved through methods that enhance immunity such as optimised nutrition (immunonutrition), genetic selection and vaccination, or by an 'anthelmintic' route using bioactive forages, copper oxide wire particles, or use of targeted selective treatment strategies such as FAMACHA, which reduce the selection pressure for the development of resistance by maintaining a population in refugia. Suprapopulation control can be achieved through grazing management, or by using predacious fungi such as Duddingtonia flagrans. All of these approaches have been developed beyond the proof of concept stage and some are capable of being employed currently. However, some still require knowledge transfer, or commercialisation before they can be tested and widely applied in the field. All of the different approaches present unique challenges to the researchers engaged in developing them, and in comparison to simple prescriptive anthelmintic treatments, their use appears complex and requires some expertise on behalf of the advisor and/or end user. At present, most of our data are derived from trials using single approaches, but it is apparent that we need to move towards integrating some of these technologies which again represents a further challenge to the extension/advisory services. Progress in establishing different approaches requires not only the funding to support their scientific development but also to support the development of computer based models which can be used to highlight deficiencies in our understanding of the control mechanisms and to identify impediments to their introduction. It is inevitable that some of the approaches currently under investigation will fail to become widely applied for a variety of reasons that are not solely financial. These include issues concerned with practicability/applicability, affordability/appropriateness, availability/deliverability and above all, the failure to provide a consistent, reliable effect when used under commercial farming conditions.

Gastrointestinal nematode infection in sheep--a review of the alternatives to anthelmintics in parasite control

Efforts to curb production losses caused by nematode parasitism in sheep have led to the development of a number of control methods to complement or replace anthelmintics. The need for alternative control measures stems from the emergence of anthelmintic-resistant parasitic nematodes with reports of multi-class resistance to these drugs now emerging. A number of these control methods such as predacious microfungi, protein supplementation, plant extracts in feed and vaccines have demonstrated potential to control infection but require development and examination under natural conditions. Breeding for natural resistance to nematode infection has already shown success in controlling the disease under natural conditions. Selection for resistance is currently based on fecal egg count measurements but identification of genetic indicators of resistance will provide a more efficient method of selection. Current quantitative trait loci for nematode resistance include the MHC genes, interferon gamma gene, IgE gene and microsatellites on chromosome 1, 5 and 6. This paper reviews the current alternatives to anthelmintics to control infection, with an emphasis on breeding for host resistance and identification of genetic indicators of resistance.

Intron 1 of the interferon γ gene: Its role in nematode resistance in Suffolk and Texel sheep breeds

Genetic variation at intron 1 of the interferon gamma gene has recently been associated with variation in faecal egg count in Romney, Merino and Soay breeds of sheep. The Texel breed is more resistant to gastrointestinal nematode infection than the Suffolk breed, based on faecal egg count. Hence, the objective of this experiment was to characterise the polymorphisms of intron 1 of the interferon gammagene in Suffolk and Texel sheep, and to determine if the characterised haplotype variants were associated with faecal egg count variation in these breeds. Intron 1 haplotypes and faecal egg measurements were determined for Suffolks (n=113) and Texels (n=135). Four haplotypes were identified in Suffolks (A, B, C and D), two haplotypes as previously described, and two previously unknown haplotypes. However, there was no association between interferon gamma intron 1 haplotype and faecal egg count in Suffolks. In contrast, two previously identified haplotypes (A and B) were present in the Texel breed, and the B haplotype was associated with resistance to nematode infection (P=0.02). These results suggest that intron 1 of the interferon gamma gene has an important role in resistance to nematode infection in the Texel breed.