Nematodirus battus is a roundworm that affects grazing livestock. The lifecycle of this nematode is a direct parasitic cycle similar to those of other roundworms, albeit with differences in timings for egg development and hatching. In brief, infective larvae (L3) present on pasture, are ingested with herbage and develop to adulthood within two to three weeks. Adult worms mate and produce eggs which are then passed out in the hosts faeces. Unlike other roundworms the parasite develops to become L3 within the egg; the timing of development and hatching generally depends on climatic conditions (chilling followed by a mean day/night temperature of greater than 10°C for 10 consecutive days) and hence most larvae will overwinter within the egg and hatch in the following year.
Nematodirus species are highly seasonal. Developed eggs undergo a mass-hatching event, typically between April and June when environmental conditions fall within specific ranges. Once on pasture, infective larvae have a short lifespan due to their sensitivity to ultra violet radiation; the population of larvae on pasture can be reduced to 20% of that observed following the peak hatch within one month. Hatching of N. battus was initially reported once annually. Recent climatic changes and alteration of parasite behaviour have resulted in a second hatching event in autumn in some N. battus populations, thus providing the potential for a second generation per annum. Hatching of N. battus outwith the expected spring peak hatch will likely alter the epidemiology of this species which could impact upon the success of control strategies implimented by farmers. The apparent shift in epidemiology may benefit the parasite in a number of ways; allowing for infection of different hosts with different immune statuses which may avoid triggering the strong immune response associated with high parasite challenge in young stock.
What does it do?
N. battus infects young lambs, typically 6-8 weeks of age, causing acute yellow/green diarrhoea and subsequent dehydration. Symptoms result from inflammation of the small intestine, causing alteration of intestinal enzymes and the physical structure of the microvilli. Intensity of the infection varies between lambs; the majority will recover within a month, however mortality rates for N. battus infection can be as high as 10-30% of the lamb crop in which animals suffer intense scouring and succumb to dehydration quickly. Other clinical signs include sunken eyes and rough wool. Lambs which recover develop a strong, protective immune response which is maintained throughout life. The acquired immune response mounted by young lambs provides protection against re-infection, causing retardation of adult worms and significantly lower establishment rates following a subsequent challenge. Infection is typically mild in adult animals with low egg output and few clinical signs observed.
White drench resistance in Nematodirus battus
White drench resistance (1-BZ) has recently been identified in Nematodirus battus populations in the UK. Due to the acute nature of disease caused by this parasite, treatment failures could have a profound impact on production in young lambs.
A study is being conducted by Moredun Research Institute to provide a benchmark of the current scale of resistance in this species, identify potential risk factors for the development and spread of resistance and to develop a diagnostic test for resistance in this species. The output of this research could inform future management strategies and minimise the economic impact of resistance.
Prevalence of white drench resistance in Nematodirus battus
Analysis of Nematodirus populations from around 200 farms from across mainland UK indicated that the genes (genetic information) which codes for white drench resistance in this species are present throughout the country. Resistant genes were found in 1 in 4 of the populations tested however, the overall frequency (number of worms carrying the genes) appears to be relatively low. The wide distribution of resistant genes, even at low frequency, indicates that the potential for the development of clinical white drench resistance in Nematodirus is widespread in the UK. Focal regions of high prevalence were also identified and further research is being conducted to investigate the cause of this apparent localisation.
Diagnostic test development
Un-identified resistance could lead to treatment failures and potential production losses in future years. A novel, rapid test is currently in development to identify white drench resistance genes in Nematodirus populations. This test could be useful in the rapid assessment of treatment decisions and the evaluation of the need for alternative treatment strategies by determining whether eggs identified in post treatment samples are due to re-infection or as a result of anthelmintic resistance. Preliminary results suggest that the DNA-based test could identify resistant genes from pooled eggs extracted from a faecal sample.
Risk factor analysis
The mechanism of development of resistance in Nematodirus is not yet understood. The project aims to investigate potential risk factors associated with the development and spread of resistance in Nematodirus. A questionnaire has been launched to study farm management strategies in use throughout the UK. The development of anthelmintic resistance is believed to be due to a complex interaction of many factors, analysis of data collected by the questionnaire may allow for the identification of management practices or farm demographics which are more closely associated with resistance thus, informing future management advice.
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