Factors affecting the prevalence of endoparasites in pre-weaning calves in a warm humid climate of Mexico

Mexico ranks eighth in cattle production, with a territory of 196.4 million hectares, approximately 56 % of which is dedicated to livestock production (Parra-Bracamonte et al., 2020). The country has 1.1 million registered farms (SIAP, 2020) with the largest concentration of cattle fattening farms located in the southeast (Callejas-Juárez, 2024).

Although Mexico has a strong livestock vocation, several challenges hinder the profitable production of beef, including reproductive, nutritional, social, and health-related issues. Among these, parasitism is particularly significant in warm climates, where its main impact is on young animals. These animals are more susceptible to parasitic infections than older animals, which develop immunity after continuous or seasonal exposure (Palkumbura et al., 2024). The primary parasitic infections in ruminants include gastrointestinal nematodes (GIN), lungworms, and trematodes, such as liver and rumen flukes (Claerebout & Geldhof, 2020). Parasitic nematodes, in particular, affect livestock development predominantly in tropical and subtropical regions. The prevalence of GIN is influenced by the interplay between adaptability and environmental factors, including pasture quality and quantity, temperature, humidity, and the grazing behavior of the host (Datta et al., 2024). Parasitic infections result in both direct and indirect negative impacts, such as reduced production and reproduction, high treatment costs, and, in severe cases, animal mortality (Strydom et al., 2023). In female livestock rearing, GIN can impair the development and functionality of the reproductive system, delaying mating to 15 – 18 months of age. This delay affects 60 % – 70 % of females suitable for reproduction (Loyacano et al., 2002). Economically, GIN causes substantial losses in livestock farming world-wide. In Europe, studies estimate annual losses of €941 million in dairy cattle and €423 million in beef cattle, with 81 % of these costs attributed to production losses and 19 % to treatment costs (Ali et al., 2019). Similar studies conducted in Mexico estimate annual losses of approximately US $256 million due to GIN (Rodríguez-Vivas et al., 2017).

The primary method of controlling GIN has been the use of broad-spectrum anthelmintics, which were initially highly effective and had wide safety margins. Producers have relied heavily on these treatments to manage GIN (Rose-Vineer et al., 2020). However, excessive and inappropriate use has led to the development of anthelmintic resistance across major antiparasitic families (Libreros-Osorio et al., 2023). This issue is compounded by the fact that producers often apply anthelmintics without identifying the specific genera or species parasitizing the cattle. Prevalence studies are therefore essential to identify the primary internal parasites in a given region and guide management practices to reduce exposure to these parasites. The objective of this study was to investigate factors influencing the prevalence of internal parasites in pre-weaning calves in a warm, humid climate in Mexico.

The study protocol No 24027-C-67 was approved by the General Directorate of Research and Postgraduate Studies of the Autonomous University of Chapingo. Following the Mexican official standard NOM-051-ZOO-1995 for humane treatment in the mobilization of animals.

Gastrointestinal parasites are a major cause of reduced weight gain in cattle and negatively impact overall health (Rose-Vineer, 2020). This condition is one of the most significant diseases globally because of the economic losses it causes. A key challenge in parasite control is the accurate identification of the etiological agent to implement specific control measures (Vande et al., 2018), as some anthelmintics target specific parasites. For example, oxy closanide is effective against paramphistomids (Sanabria et al., 2014). This study focuses on the prevalence of the main nematodes and trematodes in a warm, humid climate, where environmental conditions promote the proliferation of these parasites in cattle, buffaloes (Nurhidayah et al., 2020) and equids (Nielsen et al., 2007).

GIN species are predominantly found in mixed infections, as observed in this study. While the highest prevalence of nematodes was associated with the family Trichostrongylidae (Haemonchus spp., Cooperia spp., and Oesophagostomum spp.) and Strongyloides spp., eggs of Trichuris spp., Toxocara spp., and Mammomonogamus spp. were detected in smaller proportions. Additionally, Monezia spp. eggs, Dictyocaulus spp. larvae, and coccidia oocysts were also identified.

This study compared FEC dynamics over time, considering the age and sex of calves, as genetic factors in host resistance to gastrointestinal parasites significantly influence parasite prevalence (Berton et al., 2019). In line with previous findings, males were observed to be more susceptible than females (Barger, 1993), a trend that was evident only in the FEC of dewormed calves. However, no statistical differences were observed between sexes for the FEC of Trichostrongylidae or trematodes, consistent with another study reporting no sex-based differences (Ngetich et al., 2019). The effect of age was confirmed, as observed in several studies indicating that calves' immunity increases with age (Thamsborg et al., 2017) As the animals grew, a marked reduction in the FEC of Strongyloides spp. was evident, and Trichostrongylidae counts decreased in both dewormed and non-dewormed calves. This is concerning because it suggests that the anthelmintics were ineffective. The resistance of calves to parasites increased with age, with both dewormed and non-dewormed groups showing similar patterns, indicating the development of anthelmintic resistance, which is widely reported globally (Baiak et al., 2018). Nearly all sampled animals tested positive for internal parasites, especially nematodes, reinforcing the idea that young animals grazing on pastures previously used by adult cattle are more likely to contract GIN (Romero-Hurtado et al., 2022). The high variability in EPG among calves of different ages suggests immunological development because higher FEC was observed early in the study, with a significant reduction in EPG by 8 months of age in both the de-wormed and control groups, aligning with findings reported in the literature (Greer & Hamie, 2016).

Climate is one of the main factors influencing the presence of parasites, as pre-parasitic periods depend on humidity and temperature. Some studies suggest that the highest number of infected animals occurs during summer and autumn due to rainfall, while infections gradually decrease in winter due to low temperatures, and parasitosis cases rise again in spring (Heckler & Borges, 2016). However, in the study region, a consistently high prevalence of trematodes and nematodes is observed year-round (Muñiz-Lagunes et al., 2015; Hernández-Hernández et al., 2023) attributed to ideal temperature and humidity conditions. As a result, the prevalence percentages for both nematodes and trematodes were notably high on the studied farms.

Various studies have identified Cooperia spp. as one of the most persistent parasites in fecal matter (Robi et al., 2023), posing a significant challenge in grazing livestock due to its high prevalence (80 %) and widespread anthelmintic resistance in tropical and temperate regions (Felippelli et al., 2014; Suarez et al., 2018; García-Ruíz et al., 2019). Research highlights the pathological effects of Cooperia spp. infection on the small intestine mucosa, which negatively impacts animal health and production, leading to economic losses (Stromberg et al., 2012; Höglund et al., 2018; Neves, Das et al., 2020). Despite the damage caused by Cooperia spp., infection in calves induces an immune response to regulate the infection, although acquiring immunity takes time (Matamoros-Mercado et al., 2021). By contrast, Haemonchus spp. and Trichostrongylus spp. were present at relatively lower levels, reflecting the complex interactions between parasite species and their environmental conditions.

The low prevalence of other parasite species observed in this study aligns with similar findings (Romero-Hurtado et al., 2022), where a prevalence of 1.7 % for Dictyocaulus viviparus was reported in cattle of various ages, including animals under 1 year old. The low prevalence of trematodes observed was consistent with previous reports from the region (Hernández-Hernández et al., 2023). For example, a study conducted in southeastern Mexico found a mean trematode egg count of 11.1 ± 14.0 EPG. However, another study reported high prevalences of F. hepatica (62.4 %) in cattle, particularly in animals raised on pasture (Jara-Campos et al., 2018).

The results of this study demonstrated a significant reduction in nematode FECs with increasing age; however, no differences in egg counts were observed between dewormed and non-de-wormed animals after the second treatment application. The prevalence of internal parasites fluctuated throughout the study, with coccidia being the most prevalent, detected in 54 % of animals during the final sampling in December. The primary genera of parasites were identified in the suckling calves, with Cooperia spp. being the predominant species among the larvae, accounting for approximately 80 % of the total.

With the results of this study, it is possible to reduce the use of anthelmintics by applying them only during the first three months of age of the calves to reduce the high initial counts of nematode eggs, since later the calves develop natural resistance, and the high prevalence of trematodes requires the use of specific products for their control which impacts livestock production in the tropical region.