A.C.G. Heath

The epidemiology of nematode infections of sheep

Abstract Nematode parasites have been a major factor limiting sheep production in New Zealand for more than 100 years. Twenty-nine species of nematodes were unintentionally introduced with sheep into New Zealand, but it is principally species of Haemonchus, Ostertagia, Trichostrongylus, Nematodirus and Cooperia that are associated with production losses and clinical disease. The seasonal dynamics of nematode infection are the consequence of complex inter-relationships between the sheep, their husbandry and the prevailing climate. The patterns of pasture contamination by nematode eggs and then larvae, and the subsequent levels of infection in ewes and lambs are broadly similar throughout New Zealand. Numbers of infective larvae on pasture build up over summer to a peak in autumn/early winter with, in some years, a spring peak derived from the parturient rise in faecal nematode egg counts (FEC), expressed in eggs per gram of faeces (epg), in lactating ewes. The immune capability of lambs is initially low but increases with the magnitude and duration of exposure to infection. Once significant immunity has developed (usually by 10–12 months of age), sheep are capable of markedly restricting parasite infection, except during times of disease, malnutrition or stress. For the effective control of nematode parasites, farmers have come to rely almost exclusively on broad-spectrum anthelmintics. However, issues relating to resistance, residues and eco-toxicity increasingly threaten the sustainability of chemotherapy. In order to maintain present levels of parasite control and productivity in the long term, farmers need to integrate management practices aimed at minimising animal exposure to parasites with reduced reliance on anthelmintics.

Anthelmintic resistance in New Zealand

Abstract Anthelmintic resistance was first confirmed in New Zealand in 1979 and since then has become common-place; more than 50 % of sheep farms now have detectable levels of resistance to one or more chemical classes of anthelmintic. Farmer drenching practices have changed little over the last 15–20 years and are clearly exerting a significant level of selection for resistance. In the absence of new chemical classes of anthelmintics, current parasite control practices will be unsustainable in the long-term. Once substantial resistance has developed, significant reversion to susceptibility is unlikely and re-introduction of failed drugs is likely to result in the rapid re-emergence of control problems. The number of anthelmintic treatments applied is not necessarily a reliable indicator of selection pressure and should not be the only factor considered in strategies for minimising the development of resistance. The relative potential of the different anthelmintics now available, particularly the long- acting products, to select for resistance varies with the way they are used and with other epidemiological and management factors; generalisations about their respective roles in the development of resistance are often unreliable. In many cases, literal extrapolation of recommendations for the management of resistance from Australia to New Zealand is unsupportable, given the differences in climate, parasite ecology and farming practices between the 2 countries. In the absence of a refuge for susceptible genotypes, as occurs when anthelmintic treatments are used as a means of generating low-contamination ‘safe’ pasture for young stock, the rapid development of resistance is likely. Anthelmintic treatments applied to animals with a high level of immunity, or which become immune while the anthelmintic is active, are likely to select for resistance faster than treatments applied to non-immune stock.

Preliminary investigations into the aetiology and treatment of cockle, a sheep pelt defect

A defect of sheep pelts known as cockle, detectable after depilation, but usually first noted only in the pickled pelt or tanned stage of processing, was studied to establish causal factor(s) and effective treatments. In addition, data on the histology and seasonal prevalence of the disease were obtained. Samples collected soon after slaughter from pelts identified at the pickled pelt stage as having cockle, had a superficial dermatitis with infiltration of eosinophils. This may represent an immediate hypersensitivity reaction of the sheep to lice. Treatments of sheep with either insecticides, disinfectants or shearing showed that where biting lice (Bovicola ovis) were removed, cockle lesions had either disappeared or regressed on pickled pelts. In Trial 1 diazinon reduced cockle prevalence and severity substantially; cypermethrin had a less pronounced effect. In Trial 2 diazinon, cypermethrin, Hibitane and Savlon were equally effective in reducing biting louse numbers as shown by counts of lice at 35 and 63 days post-treatment. Reduction of cockle on pelts from sheep slaughtered at 39 days post-treatment was achieved best by both diazinon and shearing. Examination of other pelts at 67 days post-treatment showed diazinon and Hibitane to be equally effective in reducing cockle. Furthermore, shearing in the absence of insecticides reduced the severity and extent of lesions on cockled pelts. The diazinon excipient and zinc sulphate were consistently poor at removing lice and reducing cockle prevalence and severity. The results have important implications for the leather industry in that shearing and good dipping practice with appropriate chemicals at the right time can lead to improved pelt quality. However, an incentive scheme for farmers, and a means of identifying individual pelts to the farms or origin, are both necessary before a marked improvement is likely to occur.

Evidence for the role of the sheep biting-louse Bovicola ovis in producing cockle, a sheep pelt defect.

The prevalence and severity of cockle, a sheep pelt defect characterised by raised lumps, was assessed on lambs that were either louse-infested (Bovicola ovis) or louse-free at birth. Assessments were made on pickled pelts, 10 months after the lambs had entered the trial. Lambs kept free of lice did not develop cockle, whereas the defect was common in lambs (about 90%) that were infested with lice. There was a direct correlation between louse scores and cockle, high louse scores being associated with a more severe degree of cockle. The results give support to the strong causal relationship between B. ovis and cockle, but questions such as the minimum number of lice needed to cause cockle and the time required for cockle to develop still need to be answered.

The population biology of the Australian sheep blowfly, Lucilia cuprina, in New Zealand

Abstract The population biology of Lucilia cuprina in the lower North Island of New Zealand was investigated using trap data and estimates of gene flow from genetic data. The results from the trap survey provided evidence that L. cuprina may be restricted to sheep farms and, within these, is predominantly found in the presence of sheep. Gene flow (Nm) estimates are moderate with no isolation by distance pattern evident. This is an indication that one of the major contributors to fly migration between regions is the movement of infested sheep rather than movement of the flies themselves, demonstrating that L. cuprina has a low tendency for dispersal when favourable habitat conditions exist. These results suggest that localised control measures such as large scale trapping and genetic control techniques may have potential for controlling L. cuprina numbers while reducing reliance on insecticide use.

Future technologies for control of nematodes of sheep.

Abstract New approaches to nematode control in sheep are urgently needed as anthelmintic drench resistance becomes ever more widespread among worm populations. Here we briefly describe and assess a number of new technologies which will become increasingly important in anti-nematode control programmes in the future. These include vaccines, immunomodulants, strategic grazing practices, the use of biological agents to destroy nematode larvae, biological anthelmintics and targeted silencing of genes regulating nematode development.

The development of cockle, a sheep pelt defect, in relation to size of infestation and time of exposure to Bovicola ovis, the sheep-biting louse

Groups of ten louse-naive lambs were infested with one, ten or 100 female Bovicola ovis and killed 84 days later when an examination of their pelts was made to detect cockle. In a second experiment groups of ten lambs were infested with ten or 100 female B. ovis and groups of lambs were killed every fortnight up to 84 days post-infestation. The pelts were examined in order to detect the earliest time at which cockle could be detected following a louse infestation. Cockle is a nodular condition of the skin arising in response to infestation with B. ovis and is possibly a hypersensitivity on the part of some sheep to antigens of louse origin. In the first experiment cockle did not develop in lambs that had remained louse free or which had been initially infested with one louse. However, five of ten lambs that had been infested with ten lice and all lambs that had been infested with 100 lice developed cockle. In general cockle severity was positively related to the size of the terminal louse population. Group mean louse counts only slightly exceeded the initial infestation in the lambs infested with either ten or 100 lice, and were less than the initial infestation in lambs given only one louse. In the second experiment cockle was first seen 54 days post-infestation, but only in sheep infested initially with 100 lice.

Insecticide resistance status of Lucilia cuprina in New Zealand using biochemical and toxicological techniques.

The resistance status to diazinon based organophosphorus insecticides has been examined in the sheep blowfly, Lucilia cuprina, a recent arrival to New Zealand. Samples, representative of the distribution of L. cuprina in New Zealand, were scored for the presence or absence of a microsomal esterase (E3) associated with resistance. Iso-female lines were generated from four sites to examine variation in resistance levels using toxicological data. Results from both techniques show resistance at a high frequency with some variation between sites.

Distribution, seasonality and relative abundance of Stomoxys calcitrans (stablefly) (Diptera: Muscidae) in New Zealand

Abstract AIMS: To determine the current distribution, seasonality and relative abundance of Stomoxys calcitrans in New Zealand in order to provide information that could be used to assess risks of transmission of equine infectious anaemia (EIA). METHODS: Adhesive yellow traps were distributed to schools throughout New Zealand and used to detect the presence of S. calcitrans between November 1999 and April 2000 at sites considered likely to be a focus for S. calcitrans breeding and activity. In addition, researchers undertook monthly trapping at six other sites between August 1999 and June 2000 to measure the duration and seasonal periodicity of S. calcitrans flight activity. Veterinary practices and farmers were also surveyed to provide anecdotal evidence of the presence or absence of S. calcitrans, particularly in areas where no flies were trapped by schools. RESULTS: Stomoxys calcitrans was found to occur in both North and South Islands, but principally in locations where dairy farming occurred. The fly was active during most months of the year, except perhaps during July and August, and was particularly active in warmer North Island districts such as the Waikato. Peak activity was recorded from January to May. The fly was more abundant in Northland, Auckland, Waikato, the Marlborough Sounds and Nelson than in other districts, as determined by the number of occasions flies were caught relative to the number of traps set. There are only a few areas, such as around Taupo, the Otago Lakes, Central Otago and the Mackenzie Basin, where S. calcitrans was not trapped or, on the basis of anecdotal evidence, thought not to occur. CONCLUSIONS: Given that other potential vectors of EIA are absent from New Zealand, Taupo, the Otago Lakes, Central Otago and the Mackenzie Basin districts could be considered areas of low risk for EIA transmission to horses. In addition, teachers and school children were capable, in most instances, of supporting a nationwide survey providing the methods were simple and the aims few.

A New Zealand record for the cat fur-mite, Lynxacarus(Felistrophorus) radovskyi Tenorio (Acarina: Astigmata: Listrophoridae)

Abstract Extract The cat fur-mite, Lynxacarus(Felistrophorus) radovskyi is a relatively recent discovery (Tenorio 1974) and to date only rarely noted. Excluding its first discovery in Hawaii (Tenorio 1974), the mite has been recorded only seven times previously, i.e. from Puerto Rico (Fox 1977), Australia (Bowman & Domrow 1978), Fiji (Munro & Munro 1979); Florida (Greve & Gerrish 1981), Brazil (Faccini & Coutinho 1987); Florida keys (Foley 1991) andTexas (Craig et al. 1993).The mite can cause intense itching, particularly on the posterior half of the cat, and gastrointestinal symptoms as well as alopecia and gingivitis have been recorded, most from an epizootic in >300 cases (Foley 1991).