A field study on the occurrence of gastrointestinal nematodes in sheep over the wet and dry seasons in two West Indian Islands.

Gastrointestinal nematode (GIN) infestations are a major constraint to sheep production in the West Indies (WI). Intensive and semi-intensive management systems are most commonly employed. These islands display tropical weather patterns with wet and dry seasons. Semi-intensive farming combined with increased rainfall during the wet season has been reported to be most favourable for development and survival of GIN. This study was conducted to determine whether there was a relationship between GIN burdens in sheep with seasonality and management practices of farmers in Trinidad and Tobago (T&T). Farms were visited on a monthly basis from January to December 2017. A maximum of ten sheep, three to nine months of age, were selected from each farm. A total of 3,053 faecal samples were collected and analysed using the Modified McMaster technique. Environmental data on daily precipitation and temperature were collected from the Trinidad and Tobago Meteorological Office during the period of sampling. A mixed effects negative binomial regression model was constructed to analyse the relationship between gastrointestinal nematode faecal egg counts (GINFEC) with season, management system and use of dewormers as fixed effects and farm as a random effect. Average diurnal temperatures of T&T fluctuated between 23.2°C to 32.6°C and 23.9°C to 32.3°C in the dry and wet seasons, respectively. Average daily precipitation ranged between 1.6-1.8 mm and 6.1-8.5 mm during the dry and wet seasons, respectively. A decreased risk of 0.34 (95% CI 0.25-0.47, p < .0001) for GINFEC was observed in dewormed animals versus not dewormed. Semi-intensive management systems experienced an increased risk of GINFEC by 1.39 (95% CI 1.05-1.84, p = .021) compared with intensive and the risk of GINFEC was reduced by 0.40 (95% CI 0.33-0.49, p < .0001) in the wet season versus the dry. A higher prevalence of GIN was observed during the drier months of the year. Climatic conditions observed in T&T remained favourable for GIN survival, development and distribution year-round. Proper management is therefore required for reducing the occurrence of GIN in sheep of T&T throughout the year. This is the first reported study in the WI on the influence of seasonality and management on GIN infestations in sheep during the dry and wet seasons. Further investigation is needed to elucidate why GIN burdens appear to be higher in the dry season than the wet season. This study can be used as a baseline for public education in T&T as well as other developing countries.