Modelling bodyweight to avoid anthelmintic underdosing of goats in resources-limited settings

Tropical Animal Health and Production (2023) 55:72 https://doi.org/10.1007/s11250-023-03479-6 REGULAR ARTICLES Modelling bodyweight to avoid anthelmintic underdosing of goats in resources‑limited settings M. J. Evans1 · C. L. Kaona2,3 · A. K. Barros4 · J. L. Burdon Bailey3 · P. Chikungwa5 · L. M. Costa‑Junior4 · L. Gamble3 · A. M. Hopker1 · R. F. Kelly1 · F. Lohr3 · R. Silva4 · N. D. Sargison1 Received: 1 August 2022 / Accepted: 23 January 2023 © The Author(s) 2023 Abstract Improved management of livestock in resource-limited settings can provide a means towards improved human nutrition and livelihoods. However, gastrointestinal nematodes (GIN) are a significant production-limiting factor. Anthelmintics play a role in GIN management; however, few anthelmintic classes are available in many low-middle-income countries. Utilising a limited range of classes may increase selection for anthelmintic resistance; therefore, strategies to reduce other selective pressures are of heightened importance. Avoiding anthelmintic underdosing is one such strategy, but it can be challenging without access to accurate bodyweight measurement. Many previous studies have used thoracic girth as a practical proxy for bodyweight in goats; however, they have rarely considered the potential impact of natural variation on therapeutic doses. Here, the relationship between bodyweight and thoracic girth was modelled using data from 820 goats from three Malawian biomes in two seasons, with the specific aim of avoiding underestimation of bodyweight. The internally cross-validated linear regression (∛Weight ~ 0.053 + 0.040*Girth, R2 = 0.92, rounded up to the nearest 5 kg) was validated against data from an additional 352 Malawian goats (1.4% of goats allocated an underdose and 10.2% allocated a dose > 200% of bodyweight). The equation was further externally validated using an historical dataset of 150 goats from Assam, India (2.7% of goats were allocated to an underdose and 24.8% allocated to a > 200% of bodyweight). These results suggest that a more globally generalisable approach may be feasible, provided the accuracy of the estimate is considered alongside the therapeutic index of the pharmaceutical. Keywords Goat · Bodyweight · Anthelmintic · Sustainable development Introduction Small ruminant production has been identified as a potentially sustainable, climate-resilient route towards improving livelihoods in many resources-limited settings (Sargison M.J. Evans and C.L. Kaona are joint first authors. * M. J. Evans 1 Royal (Dick) School of Veterinary Studies, University of Edinburgh, Roslin, UK 2 Lilongwe University of Agriculture and Natural Resources, Lilongwe, Malawi 3 Worldwide Veterinary Services, Cranborne, UK 4 Universidade Federal Do Maranhão, São Luís, Brazil 5 Department of Animal Health and Livestock Development, Lilongwe, Malawi et al. 2020). Malawi is one such example, remaining one of the world’s economically poorest nations, with approximately 50% of the population living in poverty despite recent economic growth (World Bank 2020). Approximately 80% of the population of Malawi are employed in agriculture, and this sector has been identified as particularly vulnerable to climatic shocks (World Bank 2020; Ignaciuk et al. 2021). Small ruminants (principally goats) are the most numerous mammalian livestock species in Malawi (Food and Agriculture Organization 2005). Whilst there is little research quantifying the economic impact of endemic livestock diseases in lower- and middle-income countries (LMICs) (Perry and Grace 2009), haemonchosis is a well-recognised problem across sub-Saharan Africa (Emery et al. 2016); therefore, sustainable control of this gastrointestinal nematode (GIN) infection is vital, if small ruminant production is to function as a route towards poverty alleviation (Bessell et al. 2018). 13 Vol.:(0123456789) 72 Page 2 of 8 Sargison et al. (2021) demonstrated that FAMACHA scoring and faecal egg counting can be an effective way to engage Malawian smallholders with the concept of animal health management; however, that study also identified a potential lack of efficacy with albendazole treatments. In order to improve animal health and production, and to maintain farmer engagement, it is essential that the efficacy of advocated treatments is maximised. At the same time, for interventions to be sustainable longer term, it is key that the development and spread of anthelmintic resistance are minimised. This is particularly important given that benzimidazoles and ivermectin are the only anthelmintics commonly available in Malawi (author observation). Utilising a limited range of anthelmintic classes may increase selection for anthelmintic resistance, as well as reducing mitigation strategies once resistance establishes. There are very few studies investigating the presence and scale of anthelmintic resistance in LMICs and efficacy may currently be high (Seyoum et al. 2017); however, genetic analysis of Haemonchus spp. in Pakistan revealed evidence of multiple rapid emergences of benzimidazole resistance (Ali et al. 2019) similar to the progression previously observed in intensive production systems situated in high-income countries (Kotze et al. 2020). Affordable strategies aimed at reducing other selective pressures for anthelmintic resistance may therefore be very valuable in resource-limited settings. One such strategy is to avoid the administration of subtherapeutic doses of anthelmintics (Smith et al. 1999). However, it is unusual for Malawian farmers to have access to weigh scales; therefore, it is necessary to develop proxy measurements that can be used to estimate goats’ bodyweight accurately. The relationships between goat bodyweight and morphometrics (particularly thoracic girth at the level of the heart) have been extensively studied with 42 worldwide examples found during a literature search for this study (Supplementary Tab.S1). However, there have been no studies utilising goats in Malawi, and Chinchilla-Vargas et al. (2018) demonstrated variation in the relationship between chest girth and bodyweight between local breed goats in neighbouring east African countries. Furthermore, the majority of studies have focussed on defining the relationships as a selective index; only four studies discuss the variation around the line of best fit (Mayaka et al. 1996; Mahieu et al. 2011; Eyduran et al. 2017; Hopker et al. 2019); and only one of these discusses the impact that this variation would have on dose rates if the relationship was used to inform dose calculations (Hopker et al. 2019). Machila et al. (2008) investigated the impact of weight estimation on dose rates in Zebu cattle; however, they optimised their weigh tape to estimate ± 20% of true bodyweight, as they were considering medicines with narrow therapeutic indices. In contrast, ivermectin and benzimidazoles have much wider therapeutic indices (Abdou and Sharkawy 2004; Foreyt 1988; Mohsen et (...truncated)