Thermotolerance of L3 Ostertagia (Teladorsagia) circumcincta and some of its enzymes

A mathematical model to predict the risk arising from the pasture infectivity of four nematode species in Australia

Gastrointestinal parasites cost the Australian sheep industry AU$436 million annually. Early warning of impending worm risk may reduce this cost by providing producers with sufficient time to implement control strategies. A biophysical model was developed to simulate the on-pasture lifecycle stages of the four predominant nematode species in Australia (Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus colubriformis and Trichostrongylus vitrinus). The influence of climatic variables (temperature and water availability) on the survival, development and migration of each lifecycle stage was incorporated and parameterised to available point estimates (H. contortus: R2 = 0.88, n = 1409; T. circumcincta: R2 = 0.56, n = 243; T. colubriformis: R2 = 0.61, n = 355; T. vitrinus: R2 = 0.66, n = 147). Constant fecundities (eggs/worm.day) provided the daily quantity of eggs deposited per sheep (H. contortus = 3275; T. circumcincta = 140; T. colubriformis = 300; T. vitrinus = 300). Farm management practices were considered via the specification of stocking rates (sheep/ha), and the administration of anthelmintic treatments (reducing egg deposition by a defined efficacy and duration for each nematode species). Pasture infectivity per nematode species was calculated as the quotient of larvae on herbage and herbage availability (t/ha). Risk was calculated as the product of pasture infectivity and the potential productive impact of each nematode species (H. contortus = 3.9%; T. circumcincta = 9.22%; T. colubriformis = 9.31%; T. vitrinus = 9.31%), and then summed across nematode species. This predictive model has been incorporated into the Sheep CRC’s ‘ASKBILL’ application (www.askbill.com.au, verified 13 April 2018), which uses 90-day weather forecast data (5-km grid resolution) provided by the Australian Bureau of Meteorology.

The initial kinetics of NH3/NH4(+) efflux from L3 Teladorsagia circumcincta

The initial rate of NH(3)/NH(4)(+) accumulation in a medium containing L(3) Teladorsagia circumcincta was 0.18-0.6 pmol h(-1) larva(-1), which increased linearly with larval density. However it appeared that the larva-generated external concentration of NH(3)/NH(4)(+) did not exceed about 130 μM. The rate of NH(3)/NH(4)(+) accumulation increased with temperature between 4 °C and 37 °C, declined with increasing pH or increasing external NH(3)/NH(4)(+) concentration and was not significantly affected by the concentration of the phosphate buffer or by exsheathing the larvae. We infer from these data that the efflux of NH(3)/NH(4)(+) is a diffusive process and that the secreted or excreted NH(3)/NH(4)(+) is generated enzymatically rather than dissociating from the surface of the nematode. The enzymatic source of the NH(3)/NH(4)(+) is yet to be identified. Since the concentration of NH(3)/NH(4)(+) in the rumen and abomasum is higher than 130 μM, it is unlikely that T. circumcincta contributes to it, but NH(3)/NH(4)(+) may be accumulated from the rumen fluid by the nematode.

The kinetic properties of the glutamate dehydrogenase of Teladorsagia circumcincta and their significance for the lifestyle of the parasite

Like other nematodes, both L(3) and adult Teladosagia circumcincta secrete or excrete NH(3)/NH(4)(+), but the reactions involved in the production are unclear. Glutamate dehydrogenase is a significant source NH(3)/NH(4)(+) in some species, but previous reports indicate that the enzyme is absent from L(3)Haemonchus contortus. We show that glutamate dehydrogenase was active in both L(3) and adult T. circumcincta. The apparent K(m)s of the L(3) enzyme differed from those of the adult enzyme, the most significant of these being the increase in the K(m) for NH(4)(+) from 18mM in L(3) to 49mM in adults. The apparent V(max) of the oxidative deamination reaction was greater than that of the reductive reaction in L(3), but this was reversed in adults. The activity of the oxidative reaction of the L(3) enzyme was not affected by adenine nucleotides, but that of the reductive reaction was stimulated significantly by either ADP or ATP. The L(3) enzyme was more active with NAD(+) than it was with NADP(+), although the activities supported by NADH and NADPH were similar at saturating concentrations. While the activity of the oxidative reaction was sufficient to account for the NH(3)/NH(4)(+) efflux we have previously reported, the reductive amination reaction was likely to be more active.