The present study was conducted to assess different candidates for biomarkers of helminth infection in Zebuine cattle naturally infected by the genera Haemonchus and Cooperia as well as correlations with both the fecal egg count (FEC) and physiological variables. Pepsinogen achieved the best results in the first step and its use in the field was validated in the second step.
The physiological, hematological and biochemical variables did not enable differentiating treated animals from untreated animals. Erythrocyte and hemoglobin values remained within the reference range and it was not possible to differentiate the groups based on CV and albumin. Although some animals sporadically exhibited values above the reference range, this must have been due to individual factors that interfere with these indices, such as nutritional factors and age (Gennari et al., 1995). These results are similar to data on experimental infection with H. placei or H. contortus in calves exhibiting discrete submandibular edema, but with no changes in hematological or biochemical variables, as changes were only found in the histological analysis of the abomasum (Fávero et al., 2016).
It was also not possible to differentiate the animals of the treated and control groups based on the phosphorus results, which limits its use in the field as a biomarker for helminth infection, despite changes in its plasma concentration in infections by Trichostrongylus colubriformis and Cooperia sp. (Louvandini et al., 2009; Dias e Silva et al, 2018; Dias e Silva, 2021). However, this biomarker cannot yet be discarded, as various factors can interfere with its metabolism, such as the quantity of parasites, the ingestion of phosphorus and the diagnostic method. Moreover, the most indicated method for the assessment of phosphorus kinetics is radioactive identification (Lovadini et al., 2009), which limits its use in the field.
Gastrin - another possible biomarker - also did not differentiate the CON A and TREAT A groups. This may have occurred due to the infection profile in the present study, as the animals that had a predominance of Haemonchus sp. and Cooperia sp. as well as low mean FEC. For a significant change in plasma gastrin, it is necessary for the parasites in the abomasum to cause sufficient harm to stimulate the release of gastrin by G cells (Lawton et al., 1996). Thus, gastrin is a biomarker that enables differentiating whether there is high or low infection by adult helminths in the abomasum (MCKELLAR et al., 1987; PITT et al., 1988). The level of infection and high participation of intestinal nematodes, such as Cooperia, may have resulted in low gastrin levels in the present study. The maximum peak of 92.5 pg/ml found in the CON A group is equal to results in animals without contact with the parasite in O. ostertagi inoculation studies, with values of 93 pg/ml to 100 pg/ml (Entrocasso et al., 1989; Berghen,1993), whereas values can be from 184 to 1000 pg/ml in animals with ostertagiasis (Entrocasso et al., 1989; Berghen, 1993; Forbes, 2009). Comparing gastrin to pepsinogen, one can infer that pepsinogen indicates the degree of harm, whereas gastrin may be related to the quantity of adult worms (Pitt et al., 1988).
A reduction in pepsinogen was found beginning on Day 21 after anthelminthic treatment in the first step of the study and this enzyme also enabled differentiating the treated and control groups on Day 28 in both steps. In studies involving experimental inoculation with O. ostertagi (COOP R. L., 1971; PITT et al., 1988; HILDERSON, et al., 1991) and the assessment of the increase in pepsinogen after infection, changes were identified at around 24 to 35 days after inoculation. The determination of serum/plasma pepsinogen and changes in the abomasum result from the increase in the permeability of the mucosa (FOX et al., 1991). Thus, our results help determine the post-treatment interval to assess whether pepsinogen would return to acceptable levels in animals experimentally infected with Haemonchus sp. and Cooperia sp.
Pepsinogen proved to be a possible biomarker for zebuine cattle raised in tropical regions in step 1 and these results were confirmed in step 2 on a ranch with a history of clinical helminth infection and a situation of the animals exhibiting weight loss. The low (0.9 Utyr/ml), medium (2.1 Utyr/ml) and high (5.1 Utyr/ml) reference values have been established (DORNY and VERCRUYSSE, 1998) for cattle infected by Ostertagia sp. When compared to the present results, the values of the TREAT A and CON A groups were low in the first step and all groups had medium reference values on D0 in the second step, which diminished to low reference values after treatment in the TREAT B and TREAT C groups, whereas medium values remained the CON BC group. As the animals in step 2 had clinical signs of helminth infection but were still not classified in the high stratum of pepsinogen, it is perhaps necessary to reassess the threshold values for stratification into low, medium and high for the conditions of the present study, which are representative of a large part of livestock farming in tropical regions, with zebuine cattle permanently exposed to mixed infections predominantly by the genera Haemonchus and Cooperia.
Other values that can be considered in the monitoring of the herd are those of CV and albumin, which were negatively correlated with pepsinogen. This is coherent with parasitism, as haemonchosis causes changes in the composition of the plasma, specifically a reduction in total serum protein, with evident hypoalbuminemia and the development of submandibular edema and ascites in severe cases (Holmes, 1985).
Therefore, CV and albumin can assist in the monitoring of GINs in cattle raised in tropical regions, whereas phosphorus and gastrin proved not to be good biomarkers. Pepsinogen was capable of identifying animals with clinical and subclinical helminth infection and can also be useful as a parameter for the evaluation of anthelminthic treatments.
Considering these important results for the monitoring of GINs in cattle raised in tropical regions, new research opportunities arise, as experimental infection was not used in the present study. Future studies should validate low, medium and high thresholds of pepsinogen for situations similar to this and gain a precise understanding of the correlation between pepsinogen and both CV and albumin.