A simulation model to investigate interactions between first season grazing calves and Ostertagia ostertagi

Abstract

A dynamic, deterministic model was developed to investigate the consequences of parasitism with Ostertagia ostertagi, the most prevalent and economically important gastrointestinal parasite of cattle in temperate regions. Interactions between host and parasite were considered to predict the level of parasitism and performance of an infected calf. Key model inputs included calf intrinsic growth rate, feed quality and mode and level of infection. The effects of these varied inputs were simulated on a daily basis for key parasitological (worm burden, total egg output and faecal egg count) and performance outputs (feed intake and bodyweight) over a 6 month grazing period. Data from published literature were used to parameterise the model and its sensitivity was tested for uncertain parameters by a Latin hypercube sensitivity design. For the latter each parameter tested was subject to a 20% coefficient of variation. The model parasitological outputs were most sensitive to the immune rate parameters that affected overall worm burdens. The model predicted the expected larger worm burdens along with disproportionately greater body weight losses with increasing daily infection levels. The model was validated against published literature using graphical and statistical comparisons. Its predictions were quantitatively consistent with the parasitological outputs of published experiments in which calves were subjected to different infection levels. The consequences of model weaknesses are discussed and point towards model improvements. Future work should focus on developing a stochastic model to account for calf variation in performance and immune response; this will ultimately be used to test the effectiveness of different parasite control strategies in naturally infected calf populations.

Keywords: Calves; Gastrointestinal parasites; Immunity; Modelling; Ostertagia ostertagi; Parasite-induced anorexia.

Fig. 1

A schematic description of the parasite-host interactions. The rectangular boxes and solid lines indicate the flow of ingested feed resources; the oval boxes indicate the host-parasite interactions and the hexagonal boxes represent the key measurabe stages of the parasite life-cycle. Host immune response is assumed to lead to parasite-induced anorexia (broken line).

Fig. 2

Predicted worm burdens (a), sampled daily faecal egg counts (FEC) (b) and daily faecal egg outputs (c) produced over time in calves administered one of 3 different infection doses of Ostertagia ostertagi L₃ larvae: 3500, 7000 and 14,000 L₃/day over a 200 day period. The FEC were subject to a random sampling error owing to external factors.

Fig. 3

The predicted daily feed intake (a) and total relative bodyweight losses (in comparison to uninfected controls) (b) over time in calves administered 3 different infection levels of Ostertagia ostertagi L₃ larvae: 3500, 7000 and 14,000 L₃/day.

Fig. 4

The sensitivity ratio of each of the 5 outputs considered (value and time of peak worm burden, peak faecal egg count, peak of reduction in feed intake and final bodyweight) in relation to each of the model parameters considered (1–12) when a calf was infected with 3500 L₃/d. The parameters were firstly the immune parameters (1–9): the combined effect of establishment and mortality on adult worm burdens (maximum, minimum and rate): EM_max (1), EM_min (2), k_EM (3); the effect of mortality of adult worms (maximum, minimum and rate): μ_max (4), μ_min (5), k_μ (6); the fecundity (eggs) of female adult worms (maximum, minimum and rate): F_max (7), F_min (8), k_F (9). The performance parameters (9–12) considered were; the rate of reduction in feed intake dependent on rate of immune acquisition: C₁ (10); the rate of protein loss caused by adult worms rWM (11) and by larvae rLB (12). The sensitivity analysis was conducted by the Latin hypercube sampling technique.

Fig. 5

A comparison of the observations (●) by Michel (1970) to simulated predictions (○) for worm burdens produced by Ostertagia ostertagi infections of (a) 2 00 L₃/d; (b) 340 L₃/d; (c) 570 L₃/d; (d) 950 L₃/d; (e) 1600 L₃/d. Each measurement was taken from 5 calves for each point.

Fig. 6

A comparison of experimental observations (●) by Michel and Sinclair (1969) to simulated predictions (○) for (a) worm burdens and (b) total eggs counts produced by an infection level of 1500 L₃/d. Each point is based on measurements from one calf, with the exception of day 63 which is based on measurements from 2 calves.

Fig. 7

A comparison of experimental observations (●) by Satrija and Nansen (1993) to simulated predictions (○) for faecal egg outputs per gram of fresh faeces resulting from a weekly infection of 1250 larvae. Each measurement was taken for 6 calves.

Fig. 8

A comparison of experimental observations (●) by Wiggin and Gibbs (1989) to simulated predictions (○) for faecal egg outputs per gram of fresh faeces produced by a weekly infection of 30,000 larvae. Each measurement was taken for 12 calves.