Surra is one of the most important hemoprotozoan diseases of livestock with varying prevalence between countries and regions. The study involved cattle, buffaloes and equines of different agro-climatic Zones of Himachal Pradesh. Overall sero-prevalence of T. evansi shown by indirect ELISA in equine was 1.82%. Among equines, prevalence between mules and horses was not found to differ significantly in the present study. Similar finding has been reported in previous study (Yadav et al., 2019). However, there are reports of significantly higher prevalence of trypanosomosis in mules as compared to horses (Kumar et al., 2013; Prashar et al. 2018). In cattle and buffaloes, the prevalence of Surra was observed to be 22.52 and 23.57% respectively, with no significant difference, although the prevalence was significantly lower in equines. Similarly, higher sero-prevalence of Surra was reported in cattle 42.7 and buffaloes 48.0 %, in comparison to horses (1.7 %) in Indonesia (Payne et al., 1980). Abera (2016) also reported significantly higher sero-prevalence of surra in cattle (37.3%) in contrast to equines (10.7%) in Northern Ethiopia. Cattle and buffaloes are usually kept together in a tie-stall housing system and due to the interrupted feeding habit of tabanid flies, usually, the whole of the herd gets affected which can be related to the higher prevalence of the disease. On the other hand, the equine population is scattered and mostly employed for transportation, resulting in less contact with other affected animals. In addition, equines are kept loose, and their defensive behavior deters tabanids from biting (Muzari et al., 2010). Furthermore, T. evansi infection in equines is thought to be very fatal, with death of most of the infected animals in early stages (Desquesnes et al., 2013a), potentially resulting in the reduction in number of sick animals and the creation of an equine population with minimal signs of infection.
Prevalence of surra in equines from Zones 1 (1.43 %), 2+3 (2.86 %), and 4 (1.24%) showed no significant differences. Similarly, no significant variation was observed in prevalence of surra in equines between the Western and Central Plain Zone of Punjab (Sumbria et al., 2014). In cattle and buffaloes, no significant difference was found in prevalence between Zone 2+3 and Zone 4; however prevalence was found to be significantly lower in cattle from Zone 1 as compared to Zone 2+3 and Zone 4. Lower prevalence in Zone 1 can be attributed to its temperate and dry conditions which are unfavorable for proliferation of different vectors responsible for transmission of surra. A study on effect of climate variables on vector and prevalence of bovine trypanosomosis revealed higher prevalence of flies of genus Tabanus and Stomoxys in lowland in comparison to midland and highland regions of Ethiopia (Zekarias et al., 2017). The finding of this study was consistent with previous study showing higher seroprevalence of bovine trypanosomosis in Zones with higher temperatures (43.02% in the undulating Zone) compared to those with lower temperatures (27.36% in the Sub-mountain Zone) in Punjab (Singla et al., 2013).
In the present study, prevalence of T. evansi infection was not found be affected by sex of animals which is in agreement with previous studies (Tafese et al., 2012; Singla et al., 2013; Singh et al., 2016; Parashar et al., 2018). The possibility of both the sexes being equally exposed to the bites of disease-carrying vectors may explain the similar prevalence between them. In contrary, several authors have reported higher prevalence of surra infection in female than male animals and was linked to numerous stress factors such as pregnancy and lactation in female animals, rendering them more susceptible to infection (Agrawal et al., 2013; Sumbria et al. 2017; Sharma et al. 2019; Dodiya et al. 2020).
Similar to sex, age of the animals was not found to have any significant effect on prevalence of surra in equines, cattle and buffaloes suggesting that all age groups of animals are equally exposed to and affected by T. evansi. This corroborates previous observations, which demonstrated similar prevalence of surra in all age group animals (Tehseen et al., 2017; Batu et al., 2017; Alanazi et al., 2018 and Singh et al., 2019). Previous researchers have revealed varied results regarding the effect of age on the incidence of surra, which contradicts the current data. Prevalence was reported to be higher in older as compared to younger animals (Payne et al., 1991; Singla et al., 2013; Prashar et al., 2016; Gangwar et al., 2019; Sharma et al., 2019; Dodiya et al., 2020). In contrast, young equines were found to be at significantly higher risk of infection as compared to the adults (Sumbria et al., 2017). In bovines, increasing sero-positivity with age could be attributable to antibody persistence due to the chronic nature of disease, rather than an age-specific component.
In T. evansi infected equines, the BUN value was observed to be significantly higher when compared with the non-infected group. This finding is in agreement with earlier reports (Singh et al., 2011, Chavda et al., 2016, Yadav et al., 2016). BUN levels rise as a result of parasite damage during pathogenesis, which causes mononuclear infiltration of the renal glomeruli, interstitial nephritis, tubular degeneration, and tissue deterioration in the visceral organs (Hilali et al., 2006; Bal et al., 2014). The mean levels of both AST and ALT/GGT enzymes were found to be significantly higher in T. evansi infected group of animals. These findings in the present study are in consonance with those reported in previous studies (Sivajothi et al., 2015; Amin et al., 2020). The increase in AST levels might be attributed to tissue damage induced by trypanosomes, as well as the destruction of the parasite by the host immune system, resulting in the release of trypanosomal AST (Takeet and Fagbemi, 2009; Pandya et al., 2018a). This rise in ALT levels could be due to hepatic damage caused by the parasite (Akinseye et al., 2020). The glucose levels in infected animals were considerably lower than those in the non-infected group. Significant reduction in glucose levels in infected as compared to the control group is also reported by several workers (Cadioli et al., 2006), Singh et al. (2011), Sivajothi et al. (2015), and Pandya et al. (2018b). This phenomenon of hypoglycemia can be attributed to direct utilization of glucose by the trypanosomes. In addition, fever and hepatocellular damage associated with trypanosomes infection which causes an increase in metabolic rate and therefore greater utilization of glucose by the host (Von Brand, 1973; Stephen, 1986; Opperdoes et al., 1987). Contrary, Aquino and coworkers (2002) observed no significant reduction in glucose levels of infected animals. The polypeptide pattern of WCL antigen prepared was identical to earlier reports (Yadav et al. 2013). Major polypeptides found were in molecular weight range of 66.2-38 kDa in addition to some high and low molecular weight polypeptides. These observations are similar to those reported in previous study in which a comparable polypeptide pattern of T. evansi was observed with WCL antigen of buffalo, horse, and cow isolates (Laha and Sasmal, 2008). Similarly, major polypeptides of T. evansi of equine isolate were observed in the range of 62–66 kDa, 52–55 kDa and 41–43 kDa (Yadav et al., 2013). Immunoblot analysis of T. evansi positive field serum of equines revealed major immunogenic polypeptides present in molecular weight range of 62-66 kDa and some minor polypeptide band in the range of 72- 25 kDa. The polypeptide clusters identified in our study were similar to those observed by previous researchers (Aquino et al., 2010; Yadav et al., 2013). In bovines, immunodominant polypeptides bands ranging from 85 to 32 kDa were detected. However, the main poypeptide bands seen in all of the bovine serum samples were between 62-66, 52-55 kDa and 48-46 kDa range. Immunodominant proteins of molecular weight range of 178–24 kDa were observed with antigen prepared from the T. evansi isolate of horse, cattle and buffalo (Laha and Sasmal, 2008). The 48-46 and 38 kDa bands were recognized in both cattle and buffalo which are similar to those reported by Aquino et al. (2010) mainly in later stages of infection suggesting the chronic nature of the disease in bovines. The presence of these polypeptides in the chronic stages of infection might be due to the release of internal antigens following the destruction of parasites by VSG-specific antibodies (Aquino et al., 2010). The 62-66 kDa polypeptide cluster was found to be recognized by the immune serum of all the infected animals including cattle, buffalo, and equines. Immunodominant bands in a similar molecular weight range have been reported previously in different studies (Giardina et al., 2003; Laha and Sasmal, 2008; Aquino et al., 2010; Yadav et al., 2013).