El Niño and La Niña differentially drive transmission dynamics of the small ruminant parasite Haemonchus contortus across agroecological zones in Southern Africa

Abstract

The El Niño La Niña Southern Oscillation (ENSO) exerts a significant influence on agroecological environments and plays a crucial role in influencing crop productivity, yet its impact on animal diseases has been largely overlooked, despite the evident impact of climate on disease transmission. This study aimed to evaluate the impact of ENSO on the transmission potential (Q₀) of Haemonchus contortus (H. contortus) across agroecological zones (AEZs) in southern Africa across the region's typical rainy season (November-April). Key results indicate that stronger El Niño events can significantly decrease H. contortus transmission potential in subtropical AEZs. Tropical AEZs exhibit lower sensitivity to El Niño generally, but stronger events can lead to significant transmission potential reductions in certain zones. Conversely, La Niña increases transmission potential in tropical AEZs by small-to-medium effect magnitudes, depending on AEZ. Importantly, the analysis found significant increases in Q₀ across direct transitions from El Niño to La Niña, with statistically significant medium effects observed in all tropical AEZs, except for the cool-subhumid AEZ. In subtropical AEZs, notable increases in Q₀ were found in the warm-humid and warm-subhumid zones, also according to medium effect sizes. The study highlights the necessity for disseminating proactive and adaptive animal management practices for small ruminants among smallholder farmers, both during ENSO events and continuing into the post-event transition period, particularly through capacity building, nutritional supplementation and targeted selective treatment. The findings underscore the importance of considering small ruminants in ENSO preparedness and response plans, allowing farmers to optimise the resilience-enhancing roles of small ruminants to mitigate the impacts of climate shocks and food insecurity.

Fig. 1

Agroecological zones (AEZs) of southern Africa used to segment the analysis. Source: Author using ArcGIS Pro, data from.

Fig. 2

Effect sizes and statistical significance of ENSO phase transitions on Q0 during the typical southern Africa rainy season (November-April) across tropical and subtropical agroecological zones (AEZs). Red and blue tiles indicate positive and negative effect sizes respectively. Asterisks denote significance levels: * = p ≤ 0.05, ** = p ≤ 0.01, *** = p ≤ 0.001. Cohen’s d effect size classifications: negligible: d < 0.2, small: 0.2 ≤ d < 0.5, medium: 0.5 ≤ d < 0.8, large: d ≥ 0.8.

Fig. 3

Violin plots of rainy season Q₀ values across the seven different tropical and subtropical AEZs of southern Africa (A-N). Note the y-axis scale differs across plots due to the scale of differences in transmission potential. Red markers indicate the mean value, while blue indicates the median. Lines show the interquartile range.

Fig. 3

Violin plots of rainy season Q₀ values across the seven different tropical and subtropical AEZs of southern Africa (A-N). Note the y-axis scale differs across plots due to the scale of differences in transmission potential. Red markers indicate the mean value, while blue indicates the median. Lines show the interquartile range.

Fig. 4

Area of effect map showing areas of southern Africa where a statistically significant difference in ENSO phase means was detected. For effect direction, refer to text and section “Specific ENSO events”.

Fig. 5

Effect sizes and statistical significance of differences of mean Q₀ during major El Niño events compared to neutral phase values during the typical southern Africa rainy season (November-April) across tropical and subtropical agroecological zones (AEZs). Red and blue tiles indicate positive and negative effect sizes respectively. Asterisks denote significance levels: * = p ≤ 0.05, ** = p ≤ 0.01, *** = p ≤ 0.001. Cohen’s d effect size classifications: negligible: d < 0.2, small: 0.2 ≤ d < 0.5, medium: 0.5 ≤ d < 0.8, large: d ≥ 0.8. Note difference of legend scale compared to Fig. 2, reflective of higher detected effect sizes in this analysis.

Fig. 6

Effect sizes and statistical significance of differences in mean Q₀ during major La Niña events compared to neutral phase values during the typical southern Africa rainy season (November-April) across tropical and subtropical agroecological zones (AEZs). Red and blue tiles indicate positive and negative effect sizes respectively. Asterisks denote significance levels: * = p ≤ 0.05, ** = p ≤ 0.01, *** = p ≤ 0.001. Cohen’s d effect size classifications: negligible: d < 0.2, small: 0.2 ≤ d < 0.5, medium: 0.5 ≤ d < 0.8, large: d ≥ 0.8. Note difference of legend scale compared to Fig. 2, reflective of higher detected effect sizes in this analysis.

Fig. 7

Choropleth map of rainy season Q₀ anomalies relative to neutral conditions across the three strongest El Niño and La Niña events between 1987 and 2016 across southern Africa. Countries included in the analysis include Angola, Namibia, Botswana, Zambia, Zimbabwe, Eswatini, Mozambique, Malawi, South Africa and Lesotho. A southern portion of the Democratic Republic of the Congo is also visible, in addition to a southern portion of Tanzania.