Dynamics of the free-living stages of sheep intestinal parasites on pasture in the North Island of New Zealand. 1. Patterns of seasonal development.

AIM: To describe the seasonal pattern of development of third-stage infective larvae (L3) from eggs of Teladorsagia (=Ostertagia) circumcincta, Trichostrongylus colubriformis and Haemonchus contortus on pasture in the North Island of New Zealand. METHODS: Sheep faeces containing known numbers of eggs of all three nematode species were deposited on, or buried in, pasture plots at three sites, viz coastal Manawatu, Upper Hutt Valley, and East Cape hill country. Development was measured by recovering L3 from faeces, herbage and soil 28-31 days after deposition on 13-18 occasions, between January 2005 and July 2006. Analysis of the number of larvae recovered used a mixed model including number of eggs deposited, weight of faeces recovered (an assumed indicator of earthworm activity), site, contamination date, and position of deposited faeces, i.e. on the surface or buried. RESULTS: There was a significant effect of contamination date on development of all three species, with maximum numbers of L3 developing between late spring (November) and early autumn (March), and minimum numbers in June and July. There were large differences between species, with H. contortus exhibiting a long period (April to October) where development was close to zero, whereas T. circumcincta developed to some extent all year round. Development of T. colubriformis was intermediate between the other two species. Burying faeces containing nematode eggs increased the number of L3 recovered compared with surface deposition (p ≤ 0.001), although there were a small number of exceptions involving only T. colubriformis. The weight of faeces recovered at harvest, which was assumed to be an indication of earthworm activity, was correlated with the number of L3 recovered for all species (p<0.001). In a separate analysis, earthworms were assumed to have been active if <5 g faeces remained at harvest. Where this occurred, the number of L3 of T. colubriformis and T.circumcincta recovered was reduced by 56% and 58%, respectively (p<0.001). CONCLUSIONS: A marked seasonal pattern of development was observed for all three species, with the most larvae developing in spring-early autumn and the least in winter. This seasonal pattern was most pronounced in H. contortus and least obvious in T. circumcincta. Burying faeces containing eggs generally resulted in more L3 being recovered, whilst the apparent activity of earthworms resulted in fewer larvae being recovered.

Dynamics of the free-living stages of sheep intestinal parasites on pasture in the North Island of New Zealand. 2. Weather variables associated with development.

AIM: To identify weather variables associated with the development of eggs of Teladorsagia (=Ostertagia) circumcincta, Trichostrongylus colubriformis and Haemonchus contortus to third-stage infective larvae (L3) under a range of climatic conditions on pasture in the North Island of New Zealand. METHODS: Sheep faeces containing known numbers of eggs of all three nematode species were deposited on, or buried in, pasture plots at three sites, viz coastal Manawatu, Upper Hutt Valley, and East Cape hill country. Development was measured by recovering L3 from faeces, herbage and soil 28-31 days after deposition, on 13-18 occasions between January 2005 until July 2006. Weather data were recorded at each site, and the association between weather variables and number of L3 recovered was analysed using subsets regression to select best-fitting models from several candidate variables, after adjustment for efficiency of recovery of L3. A multiple linear regression model was then developed for each species, to select weather variables that had both significant and substantive effects on the number of L3 recovered. RESULTS: For all species, mean daily temperature was the best predictor of the number of L3 recovered (p=0.001). For T.circumcincta the final model included mean daily temperature and soil temperature (R²=51%), and for T. colubriformis the model only included mean daily temperature (R²=55%). For development of H. contortus, mean daily temperature was the most significant variable, but moisture in the form of rainfall entropy, i.e. the temporal distribution of rainfall, over the first 14 days was also significant in the final model (R²=34%). CONCLUSIONS: Temperature was the most important determinant of developmental success of free-living nematodes on pasture at the study sites, and probably also for other parts of New Zealand with similar climates. Moisture was not significant in the development of T. circumcincta or T.colubriformis, implying that under the generally moist temperate climate in New Zealand, moisture is seldom limiting for development of these species. Haemonchus contortus appeared to be sensitive to moisture availability and precipitation in the first 14 days after deposition of faeces. The results of this study will be further developed as part of species-specific climate-driven models of parasite development in New Zealand.

A stochastic model accommodating the FAMACHA© system for estimating worm burdens and associated risk factors in sheep naturally infected with Haemonchus contortus.

A previously developed multiple regression algorithm was used as the basis of a stochastic model to simulate worm burdens in sheep naturally infected with Haemonchus contortus over five consecutive Haemonchus seasons (November to January/February) on a farm in the summer rainfall region in South Africa, although only one season is discussed. The algorithm associates haemoglobin levels with worm counts in individual animals. Variables were represented by distributions based on FAMACHA(©) scores and body weights of sheep, and Monte Carlo sampling was used to simulate worm burdens. Under conditions of high disease risk, defined as the sampling event during the worm season with the lowest relative mean haemoglobin level for a class of sheep, the model provided a distribution function for mean class H. contortus burdens and the probability of these occurring. A mean H. contortus burden for ewes (n=130 per sample) of approximately 1000 (range 51-28,768) and 2933 (range 78-44,175) for rams (n=120 per sample) was predicted under these conditions. At the beginning of the worm season when the risk of disease was lowest (i.e. when both classes had their highest estimated mean haemoglobin levels), a mean worm burden of 525 (range 39-4910) for ewes and 651 (range 37-17,260) for rams was predicted. Model indications were that despite being selectively drenched according to FAMACHA(©) evaluation, 72% of the ewes would maintain their mean worm burden below an arbitrarily selected threshold of 1000 even when risk of disease was at its highest. In contrast, far fewer rams (27%) remained below this threshold, especially towards the end of the worm season. The model was most sensitive to changes in haemoglobin value, and thus by extrapolation, the haematocrit, which is used as the gold standard for validating the FAMACHA(©) system. The mean class haemoglobin level at which there was a 50% probability of worm burdens being ≤ 1000 worms was 7.05 g/dl in ewes and 7.92 g/dl in rams.

Application of ROC curve analysis to FAMACHA© evaluation of haemonchosis on two sheep farms in South Africa.

Test sensitivity and specificity for the FAMACHA(©) clinical test for anaemia due to haemonchosis have previously been shown to be adequate in differentiating between heavily/less heavily infected sheep, but these properties give no objective guidance for setting the optimum threshold at which anthelmintic treatment should occur. The aim of this work was to use Receiver Operating Characteristic (ROC) curves to evaluate the diagnostic accuracy of FAMACHA(©) testing by estimating the area under the ROC curve, and to use two-graph ROC curves to decrease subjectivity in selecting treatment thresholds on two farms with contrasting management. Test diagnostic accuracy, and thus discriminating power as determined by the area under the ROC curves, ranged from "moderate to good" on the first farm, and from "moderate to high" on the second farm for haematocrit (the Gold Standard for the test) cut-offs of ≤ 22% and ≤ 19% on both farms respectively. Accuracy of classification between haematocrit cut-offs was not significantly different within farms, but did differ significantly between farms, with test accuracy being highest on the second farm at both haematocrit cut-offs (p<0.05). The results also showed the suitability of the two-graph ROC curve approach for discriminating not only between different levels of accuracy of evaluators, but also to give an indication of the so-called ROC cut point (i.e. the desired threshold level) at which animals should be treated for a given level of risk of loss. The approach appears to have the potential not only to validate the diagnostic accuracy of the test across the complete testing range (i.e. all FAMACHA(©) categories from 1 to 5), but also to compensate for such inaccuracy by allowing objective adjustment of the threshold treatment level according to the output of the two-graph ROC method.

Blueprint for an automated specific decision support system for countering anthelmintic resistance in Haemonchus spp. at farm level.

This article is the first of a series aimed at developing specific decision support software for on-farm optimisation of sustainable integrated management of haemonchosis. It contains a concept framework for such a system for use by farmers and/or their advisors but, as reported in the series, only the first steps have been taken on the road to achieve this goal. Anthelmintic resistance has reached such levels of prevalence and intensity that recently it evoked the comment that for small ruminants the final phase of resistance was being entered, without effective chemotherapeutic agents on some farms with which to control worms at a level commensurate with profitable animal production. In addition, in the case of cattle, a recent survey in New Zealand showed 92% of worm populations to be resistant to at least one anthelmintic group. Ironically, new technology, such as the FAMACHA(©) system which was devised for sustainable management of haemonchosis, is at present being adopted relatively slowly by the majority of farmers and it is suggested that an important reason for this is the complexity of integration of new methods with epidemiological factors. The alternatives to the simple drenching programmes of the past are not only more difficult to manage, but are also more labour-intensive. The problem is further complicated by a progressive global shortage of persons with the necessary experience to train farmers in the new methods. The opinion is advanced that only computerised, automated decision support software can optimise the integration of the range of factors (such as rainfall, temperature, host age and reproductive status, pasture type, history of host and pasture infection, and anthelmintic formulation) for more sustainable worm management than is obtainable with present methods. Other than the conventional method (in which prospective analysis of laboratory and other data is mainly used to suggest when strategic prophylactic drenching of all animals for preventing excessive helminthosis should be conducted during the relevant worm season), the computer model being proposed is to be based on targeted selective treatment, supported by progressive periodic retrospective analysis of clinical data of a given worm season. It is emphasised that, in order not to repeat the mistakes of the past, such an automated support system should ideally be developed urgently in a attempt to engineer greater sustainability of any unrelated new anthelmintics which may reach the market.

Validation of the FAMACHA© eye colour chart using sensitivity/specificity analysis on two South African sheep farms.

A validation study of the FAMACHA(©) system for clinical evaluation of anaemia due to Haemonchus contortus was conducted on two commercial sheep farms in the summer rainfall region of South Africa. In this region, the Haemonchus season lasts from October to April. On Farm 1 the system was tested over a period of five successive years in consecutive sets of young stud Merino replacement rams and ewes examined at intervals of 3-5 weeks over each Haemonchus season, under routine farming conditions. When FAMACHA(©) scores of 3, 4, and 5 and haematocrit values of ≤ 22%, ≤ 19%, and ≤ 15% were separately considered to be anaemic, sensitivity on Farm 1 ranged from a maximum of 83% for a haematocrit cut-off of ≤ 15%, to 40% for a haematocrit cut-off of ≤ 22%. Sensitivity increased to 93% when FAMACHA(©) scores of 2, 3, 4, and 5 were considered anaemic at a cut-off value of ≤ 19%, but the positive predictive value decreased to 0.43, indicating that many non-anaemic animals would be treated. The analysis indicated a high level of classification bias on Farm 1, with the animals consistently being classified one FAMACHA(©) category lower (i.e. less anaemic) than reality. On Farm 2 the test was conducted over two successive years in yearling rams evaluated at weekly to fortnightly intervals during each worm season. Every ram judged to be in FAMACHA(©) category 4 or 5 was bled for haematocrit determination, and it was only dewormed with effective anthelmintics if the haematocrit was 15% or lower. When FAMACHA(©) scores of 3, 4, and 5 and haematocrit values of ≤ 22% and ≤ 19% were separately considered to be anaemic on Farm 2, sensitivity ranged from 64% for a haematocrit cut-off of ≤ 22%, to 80% for a cut-off of ≤ 19%. For identical haematocrit cut-off values and proportions of the sampled flock considered to be diseased as for Farm 1, sensitivity was always higher for Farm 2. On the other hand, further analysis of the data indicated that the magnitude of the error on Farm 1 was very consistent on average over the entire trial period. The results of this study indicate that (i) persons introduced to the system should not only be trained, but also be evaluated for accuracy of application; (ii) the sensitivity of the FAMACHA(©) diagnostic system should ideally be evaluated at shorter intervals to avoid production losses due to failure to detect anaemic animals which may be at risk of death; (iii) that calibration of the FAMACHA(©) scoring is essential per individual evaluator, and (iv) that animals should be examined at weekly intervals during periods of the highest worm challenge.