Advanced biomarkers and its usage in Arabian camel medicine – a review

ABSTRACT

Biomarkers are defined as organic or biological indicators of processes, events, or conditions occurring within the body. Over recent years, significant progress has been made in the detection, measurement and application of biomarkers in both companion and farm animal medicine. This review article summarizes the commonly used biomarkers in dromedary camels that include the cardiac, bone, inflammation, transport, stress and pulmonary biomarkers. The review also focuses on the use of these compounds in camel medicine either in diseased or physiological states; and finally summarizes and reviews, to the possible extent, examples of the types of biomarkers used in dromedary camels. Understanding the use of these biomarkers in camels has expanded significantly over recent years, and, with the insights provided by ongoing research, it is likely that these compounds will be increasingly used in the future in the diagnosis and prognosis of camel diseases.

KEYWORDS:

• Animals
• biomarkers
• diseases
• health
• pathophysiology

Introduction

A biomarker is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues (Hirsch and Watkins 2020). Evaluation of biomarkers using non-invasive samples is strongly recommended (Hunter 1986; Getahun et al. 2022). These biomarkers can indicate normal biological processes (such as growth and ageing), pathophysiological processes that occur with disease (e.g. cardiac damage and heart failure), or pharmacologic responses to a therapeutic intervention (Jesty 2012). The recent advancement of post-genomic technologies has resulted in rapid growth and progress in human biomarker research. In addition to the applications outlined above, such research has also focussed on the development of personalized treatments to reduce levels of attrition in clinical trials (Myers et al. 2017).

In veterinary medicine, a similar interest in biomarkers is now emerging, where there is enormous potential for their development and application. Research is not only relevant to the health and welfare of companion and food-production animals, but also to broader themes such as global food security (Mobasheri and Cassidy 2010). During the past decade, our research group evaluated various biomarkers in animals with physiological and diseased conditions and in those in response to drug administration. Of the physiological conditions are racing and transported animals, during the transition or periparturient period, and in response to bleeding (Tharwat et al. 2013a; Tharwat and Al-Sobayil 2014b, 2014d; Tharwat et al. 2014a; Tharwat 2020a; Tharwat and Al-Sobayil 2022a, 2022b). Of the pathological conditions are in animals with nutritional muscular dystrophy and endocardial vegetation (Tharwat et al. 2013d, 2014b). Of the pharmacological trials are the effects of the administration of the cardiac glycoside digoxin (Tharwat and Al-Sobayil 2014b).

A critical mass of knowledge on the use of these biomarkers in camel medicine has accumulated over the last decade. So there is now sufficient understanding of the pathophysiology of the response to support the use of these compounds as diagnostic and prognostic tools in clinical settings (Tharwat 2020b, 2020c; Tharwat and Al-Sobayil 2020). Unlike human diseases biomarker identification and use in livestock disease diagnosis for various diseases despite its huge potential in democratizing disease diagnosis is lagging behind thus it is important to document what is available and recommend some research directions to discover more biomarkers. This review article is written to focus on the six types of commonly used biomarkers in camel medicine and their potential use in clinical settings. It includes cardiac, bone, inflammation, transport, stress and pulmonary biomarkers. The review also focuses on the usage and application of these biomarkers in camel medicine either in diseased or physiological states. Considering the growing industrialization of camel products, such information about the use of biomarkers in camel medicine is therefore very important.

Cardiac biomarkers

In humans, cardiac biomarkers assist in the early discovery, diagnosis and outcome of cardiac diseases (Ginsburg and Haga 2006). It can be helpful in the management of cardiac and non-cardiac diseases (Jesty 2012). Among these markers, cardiac troponin I (cTnI), is a very sensitive and specific marker for cardiac injury in human beings (Ladenson 2007; Reagan et al. 2013) and in camels (Tharwat 2012; Tharwat et al. 2013a, 2013b, 2013c, 2013d; Tharwat and Al-Sobayil 2014c; Tharwat and Al-Sobayil 2015; Tharwat 2020b). The cTnI increases after acute cardiac degeneration because of seepage from the affected cardiac cells (O'Brien et al. 2006). In the veterinary field, it was similarly reported that cTnI is also a sensitive and specific marker in animals with cardiac diseases (Wells and Sleeper 2008).

The creatine kinase myocardial band (CK-MB) is another cardiac biomarker that has been reported to increase with exercise (Rahnama et al. 2011). With chest pain in humans, the level of CK-MB reaches its peak at 10–24 h subsequent to the initial injury and declines to the normal range within 72–96 h (Volz et al. 2012). Chronic occlusion of the coronary artery significantly increases the serum levels of CK-MB (Sharkey et al. 1991). However, a rise in CK-MB is not always indicative of myocardial damage; it has been elevated in patients with acute skeletal muscle trauma, dermatomyositis, polymyositis, muscular dystrophy and renal failure (Erlacher et al. 2001). Therefore, cTnI is nowadays the superior biochemical parameter in humans for assessing myocardial damage, with absolute specificity and sensitivity than CK-MB (Collinson et al. 2012). In camel medicine, the CK-MB is also used to assess myocardical injury in some affections (Tharwat et al. 2013b; El-Deeb and Elmoslemany 2015).

Usage of cardiac biomarkers in camel medicine

In humans, nonprimary cardiac diseases can induce myocyte damage leading to increased serum troponin concentrations (Mahajan et al. 2006). In addition, studies on the prognostic significance of cTnI concentrations in patients with non-primary cardiac disorders have found that cTnI can predict disease outcomes. Recently, in cattle with haemolytic anemia, long-term follow-up of serum cTnI concentrations was valuable in assessing the relationship between anaemia and myocyte damage (Fartashvand et al. 2013). Anaemia followed by hypoxia and increased oxygen consumption by the myocardium during a prolonged period of tachycardia will possibly cause myocardial injury and subsequently increased serum concentration of cTnI in animals with a parasitic infestation (Fartashvand et al. 2013).

In a study published in camels with tick infestation, 14 out of 15 recoverd camels had a serum concentration of cTnI lower than 1.0 ng/ml; the remaining camel had a higher cTnI concentration (1.65 ng/ml) (Tharwat and Al-Sobayil 2014c). In the later study, 8 camels were died because of severe anaemia and markedly reduced partial pressure of oxygen and oxygen saturation; died camels had a serum concentration above 1.22 ng/ml, with a maximum value of 5.22 ng/ml. It was therefore assumed that the increased serum concentration of cTnI above 1.0 ng/ml at initial examination was a bad prognostic indicator in the camels with tick infestation. In camels infected with Trypanosoma evansi (T. evansi), the values of cTnI and CK-MB were significantly higher in diseased camels compared to healthy ones. Successfully treated camels (43 out of 74; 58.1%) had lower levels of cTnI and CK-MB compared to camels with treatment failure. The cTnI values showed high degree of accuracy, sensitivity and specificity in predicting treatment outcome (success vs. failure) (El-Deeb and Elmoslemany 2015).

In another report carried out on 33 long-standing recumbent camels 11 out of the 33 camels (33.3%) were cured after treatment, 18 did not and the remaining 4 had died by day 20 after treatment (Tharwat 2012). Marked elevations of cTnI in the downer camels were considered a strong indication of myocardial damage and were used to predict treatment outcomes and mortality. In the same study (Tharwat 2012), the serum concentration of cTnI in the 11 cured camels was 0.05 ± 0.02 ng/ml. However, in the remaining 22 camels that did not recover, the serum concentration of cTnI was 0.53 ± 0.64 ng/ml. In the later study, compared to controls (median 0.02 ng/mL; range, 0.00–0.08 ng/mL), the serum cTnI concentrations (median 0.10 ng/mL; range, 0.01–2.20 ng/mL) differed significantly (P = 0.019) (Tharwat 2012). A recent study in dairy cows with downer cow syndrome concluded that cTnI concentrations could help to rapidly identify cows that have poor chances of recovery and would benefit from a more aggressive treatment or euthanasia (Labonte et al. 2018).

Transportation is a well-known stressor that has adverse effects on livestock production and health including muscular damages, generating concerns of an economic as well as a welfare-related nature. In 25 camels transported for a 500 km round trip, the mean cTnI concentration was 0.03 ± 0.02 ng/mL (median 0.02 ng/mL) and all camels had resting cTnI concentrations <0.08 ng/mL. Two hours after unloading from the transportation, cTnI concentration increased in all 25 camels and was significantly higher (P < 0.001) than values before and 24 h after transportation (Tharwat et al. 2013c). Following a 5 km race in 23 camels, 91.3% of the camels had increases in serum cTnI concentrations (Tharwat et al. 2013b). Twenty-four hours post-race, the cTnI concentrations had returned very nearly to their pre-race values. Post-exercise cTnI release and clearance were also reported in normal Standardbred racehorses. All horses experienced an increase in cTnI post-exercise, with the peak occurring 2–6 h post-exercise (Rossi et al. 2019).

After electroejaculation (EEJ) in 20 male camels, the mean serum concentration of cTnI increases significantly in camels following EEJ (Tharwat et al. 2014c). However, at 24 h post-EEJ, the serum concentration of cTnI did not differ significantly compared to baseline values (Tharwat et al. 2014c). In another study, the serum concentration of cTnI has been increased significantly in male camels with erectile dysfunction compared to healthy controls (Derar et al. 2017). The rise of cTnI in males with erectile dysfunction is probably indicative of myositis damage which supports the concept that failure to erect the penis or maintain an erection is primarily related to the inability to maintain a closed blood circuit at the penile tissue (Barassi et al. 2015). The cardiac injury had also been reported in camels with halothane and isoflurane general anaesthesia (Tharwat et al. 2013b). In the later study, in isoflurane group, serum cTnI concentrations did not rise above 0.04 ng/mL. On the other hand, in halothane group, serum cTnI concentrations increased markedly after 40 and 80 min of recovery to be 0.20 and 0.47 ng/mL, respectively. Serum cTnI concentrations remained significantly elevated at 24 and 48 h after recovery. Comparing halothane group to isoflurane group, mean serum concentration of halothane cTnI was significantly higher at 40 and 80 min of recovery and at 24 and 48 h after recovery. The later study had proved that halothane has marked effect on cardiomyocytes in healthy camels compared to isoflurane; therefore, the use of halothane should be restricted in camels with suspected cardiac diseases (Tharwat et al. 2013b). The cause of the cardiac cell compromise during halothane anaesthesia was likely due to extreme changes in heart rate and blood pressure, and the increased arterial concentration of PCO2. Based on the results of this study, it was concluded that isoflurane is superior to halothane as an inhalation anaesthetic in camels, especially in those with suspected cardiac problems.

Bone biomarkers

Bone is a dynamic tissue, characterized by continuously renewed processes of bone removal parallel to bone formation and replacement, which occur in the so-called basic multicellular units. The main cells in the basic multicellular units are osteoblasts, deputed to bone formation, and osteoclasts, to bone resorption (Al-Sobayil and Tharwat, 2021). Markers of bone metabolism are biochemical by-products that provide insight into the activity of bone cells. These biochemical markers are produced from the bone remodelling process including bone formation biomarkers and bone resorption biomarkers (Allen 2003). These markers are widely used in human clinical practice, mainly for non-invasive monitoring of bone metabolism and response to therapy of certain musculoskeletal and bone disorders (Sabour et al. 2014). In animals, bone biomarkers are mostly used in preclinical and clinical studies as a rapid and sensitive method for the assessment of bone response to medical treatment and surgical interventions and for the detection of musculoskeletal injuries (Frisbie et al. 2010; Tharwat and Al-Sobayil 2018a, 2018b; Tharwat and Al-Sobayi 2020; Al-Sobayil and Tharwat 2021).

The common biomarkers of bone formation include osteocalcin (OC), bone-specific alkaline phosphatase (b-ALP) and amino and carboxy propeptides of collagen type I (Al-Sobayil 1997). The non-collagenous protein OC, a product of the osteoblasts, is regarded as a sensitive indicator of bone formation (Pullig et al. 2000). The b-ALP, a glycoprotein found on the surface of osteoblasts, has also been shown to be a sensitive and reliable indicator of bone metabolism. Although b-ALP and OC are considered bone formation biomarkers, their correlation in camel blood was reported to be weak (Al-Sobayil 2010). The lack of a strong correlation between b-ALP and OC has been attributed to the fact that each of them reflects different stages of osteoblast function (Delmas et al. 1990). Moreover, b-ALP represents an early osteoblast biomarker because it presents in pre-osteoblasts and osteoblasts, whereas OC is considered a later biomarker of osteoblast differentiation and bone mineralization (Naylor and Eastell 1999). The most common biomarkers of bone resorption include pyridinoline cross-links (PYD), deoxypyridinoline (DPD) enzyme tartrate-resistant acid phosphatase and amino and carboxy telopeptides of collagen type I (Al-Sobayil 2010; Tharwat and Al-Sobayil 2021).

Usage of bone biomarkers in camel medicine

Similar to published results in mares (Filipovic et al. 2010), the serum concentrations of the bone formation biomarkers OC and b-ALP in female camels did not change significantly during the periparturient period (3wk before to 3wk after parturition). In contrast, the serum concentrations of the bone resorption biomarker PYD decreased significantly at parturition compared to 3wk before parturition and then increased significantly at 3wk after parturition. The decreased estrogen in camels at 3 wk after parturition could contribute to an elevated bone resorption rate. The decreased estrogen levels post-partum may enhance osteoclast activity that, in turn, would increase bone resorption (Tharwat and Al-Sobayil 2015). Compared to a value of 23.80 ± 11.29 ng/mL 3 weeks before parturition, the serum concentrations of OC measured 23.79 ± 8.30 ng/mL within 12 h of parturition and 18.76 ± 3.58 ng/mL at 3 weeks after parturition with no significant difference at the 3 time points (P > 0.05). Similar, compared to a value of 10.25 ± 1.94 U/L 3 weeks before parturion, the serum concentration of b-ALP measured 13.7 ± 3.73 U/L 12 h after parturion and 14.79 ± 4.94 U/L 3 weeks after parturion with no significant difference at the 3 time points (P > 0.05). On the other side, compared to a value of 10.65 ± 3.14 nmol/L at 3 weeks before parturion, the serum concentration of PYD decreased significantly 12 h after parturion to a value of 5.3 ± 1.22 nmol/L (P < 0.0001). At 3 weeks after parturition, the PYD serum concentration measured 6.75 ± 2.77 nmol/L, which differed also significantly from values at 3 weeks before parturion and at 12 h of parturition (P < 0.05) (Tharwat and Al-Sobayil 2015).

It was reported also that the serum concentration of OC increased significantly immediately after EEJ compared to baseline values (Tharwat and Al-Sobayil 2018a). In EEJ camels, the serum concentrations of OC increased significantly to a value of 21.4 ng/mL immediately after EEJ compared to baseline values of 13.5 ng/mL) (P = 0.04). However, the serum concentration of b-ALP (15.9 ng/mL after EEJ versus 14.2 ng/mL before EEJ) and PYD (6.3 ng/mL after EEJ versus 6.9 ng/mL before EEJ) did not differ significantly (P > 0.05) (Tharwat and Al-Sobayil 2018a). Although OC and b-ALP are considered bone formation biomarkers, their correlation in the serum of camels was reported to be weak (Al-Sobayil 2010). The lack of a strong correlation between the two biomarkers has been attributed to the fact that each of them reflects different stages of osteoblast function (Delmas et al. 1990). The non-significant changes in the serum concentration of the bone resorption biomarker PYD after EEJ may indicate that increased physical activity may have the potential to decrease collagen resorption in male camels. It has been shown also that increased levels of testosterone significantly reduce bone loss (Wiren et al. 2012), decrease collagen and glycosaminoglycan loss in the articular tissues (Ganesan et al. 2008) and increase the repair strength of the ligaments and tendons (Tipton et al. 1975).

It was found also that the serum concentration of OC and b-ALP increased but not significantly after a 5 km race. On the contrary, the serum concentration of the bone resorption biomarker PYD increased significantly after racing (Tharwat and Al-Sobayil 2018b). The non-significant elevations in the bone formation biomarker OC are consistent with the findings of another exercise study involving highly conditioned Arabian horses (Porr et al. 1988). Rudberg et al. (2000) reported short-lasting increases in b-ALP after 4.7 Km of jogging, a distance nearly similar to ours. The significant increases in PYD post-race in camels disagree with those of the study in horses assigned to 48-week race training (Caron et al. 2002), where no significant changes in serum PYD concentrations were detected post-race.

In camels with experimentally induced synovitis through injection of amphotericin-B into the carpal joint, it was found that a significant increase was seen in the levels of b-ALP at week 4 in the treatment group compared to the non-injected group. The concentration of OC did not change in both groups during the time of the study. The levels of PYD significantly increased in the treatment group at weeks 3 and 4 in the treatment group compared to the control group. A significant elevation of DPD was seen at week 4 in the treatment group compared to the control group (Al-Sobayil and Tharwat 2021). A significant increase was seen in the levels of b-ALP at week 4 in treatment group compared to control group (P < 0.05). However, the concentration of OC did not change in both groups during the time of study. However, the levels of PYD increased significantly in treatment group at weeks 3 and 4 in the treatment group compared to the control group. The significant elevation of DPD was seen at week 4 in treatment group compared to the control (P < 0.05) (Al-Sobayil and Tharwat 2021).

Inflammation biomarkers

Biomarkers of infection and inflammation or acute-phase proteins (APPs) are a class of proteins whose blood concentrations increase (positive APPs) or decrease (negative APPs) in response to infection, inflammation or trauma (Murata et al. 2004; Petersen et al. 2004; Ceron et al. 2005). This response is called the acute-phase reaction or acute-phase response (APR). The APR is a rapid, nonspecific, systemic response occurring secondary to many types of tissue injury and might be a physiological protective mechanism during inflammatory events (Yazwinski et al. 2013).

The origin of APR can be attributed to infection, inflammation, surgical trauma, or other causes, and the purpose of the response is to restore homeostasis and remove its disturbance (Ceron et al. 2005). The APR is induced by the pro-inflammatory cytokines IL-1, TNF-α and especially IL-6. These cytokines activate receptors on various target cells and promote hormonal and metabolic changes leading to local and systemic effects, including APP synthesis in the liver (Tizard 2009). In response to injury, local inflammatory cells (neutrophil granulocytes and macrophages) secrete a number of cytokines into the bloodstream. The liver responds by producing a large number of APPs (Petersen et al. 2004). The negative APPs include albumin, the most abundant constitutive plasma protein, and transferrin. The positive APPs include Haptoglobin (Hp), C-reactive protein, serum amyloid A (SAA), ceruloplasmin, fibrinogen and alpha 1-acid glycoprotein (Eckersall and Bell 2010).

In domestic animals, a critical mass of knowledge on the use of APPs as biomarkers of inflammatory conditions has accumulated over recent years, so there is now sufficient understanding of the pathophysiology of the response to support the use of these compounds as diagnostic tools in clinical settings (Eckersall and Bell 2010). In ruminants, APPs have been proposed as sensitive and rapid indicators of inflammatory disturbances (Schneider et al. 2013). Although not studied to the same extent, the APR in camels appears similar to that in cattle and chronic infections continue to stimulate APPs production (Tharwat and Al-Sobayil 2018a, 2018b; Tharwat 2020c). The changes in APPs due to various inflammatory and non-inflammatory conditions have been studied intensively in many animal species (Murata et al. 2004; Murata 2007). The APPs have received attention as biomarkers for APR due to their low physiological levels, fast incline, marked rise in concentration during APR that eases detection and a fast decline after cessation of a stimulus (Ceron et al. 2005).

Usage of inflammation biomarkers in camel medicine

In female dromedary camels, APR occurred at parturition which was manifested by significant increases in Hp and SAA as compared to values before or after parturition (Tharwat and Al-Sobayil 2015). Compared to a value of 0.58 ± 0.22 mg/dL at 3 weeks before parturition, the serum concentration of Hp increased significantly 12 h after parturition to 14.25 ± 4.31 mg/dL (P < 0.0001). At 3 weeks after parturition, the Hp serum concentration measured 2.60 ± 0.84 mg/dL, which differed significantly from values at 3 weeks before and at 12 h after parturition (P < 0.0001). Compared to a value of 0.73 ± 0.27 ng/mL at 3 weeks before parturition, the serum concentration of SAA increased significantly 12 h after parturition to a value of 81.00 ± 22.83 ng/mL (P < 0.0001). At 3 weeks after parturition, the SAA serum concentration measured 4.59 ± 1.42 ng/mL, a significant decrease from values at 3 weeks before and at 12 h after parturition (P < 0.0001). These elevations detected at parturition were not attributed to infectious causes, as the WBCs did not change significantly at that time, thus confirming the absence of infectious agents. These elevations could be due to cortisol and hormone release and to stress resulting in numerous changes (Huzzey et al. 2011).

APR manifested by significant increases in SAA has also been reported in camels subjected to EEJ (Tharwat and Al-Sobayil 2018a). It was not attributed to infectious causes, as the WBCs did not change significantly, thus confirming the absence of infectious agents. Therefore, these elevations, together with the cortisol increases, could be due to physical ‘stress’ resulting in numerous changes (Bauer et al. 2012). As a result of stress, APR manifested by significant increases in Hp and SAA was also detected in camels after the race (Tharwat and Al-Sobayil 2018b). These elevations could be due to physical ‘stress’ resulting in the numerous changes that occur during racing (Bauer et al. 2012). The mechanism behind the stress-induced SAA after the race is not known, but a hypothesis based on a neuroendocrine-immune network concept has recently been reported (Murata 2007). This indicates that the non-inflammatory and psychophysical stressors activate the combined action of the sympathoadrenal axis and the hypothalamic – pituitary – adrenal axis. This would affect both the immunity-related cells and the release of glucocorticoids, and would ultimately lead to the production and release of APPs (Murata 2007).

In a study with 74 camels with urinary tract infections, the acute phase proteins Hp (2.45 g/L versus 0.31 g/L in controls; P = 0.0002), SAA (15.7 µg/mL versus 9.5 µg/mL in controls; P < 0.0001), and fibrinogen (Fb) (4.28 g/L versus 3.27 g/L in controls; P < 0.0001) were significantly higher in diseased camels compared to healthy animals (El-Deeb and Buczinski 2015). In addition, in camels with paratuberculosis, the serum concentrations of the acute phase proteins HP, SAA and Fb were significantly higher in diseased camels compared to control camels (El-Deeb et al. 2014). In a study investigating the APPs in pneumonic camel calves, there was a significant increase in the levels of Fb, SAA and Hp compared to healthy camel calves (El-Deeb 2015). The APPs Hp, SAA, ceruloplasmin and Fb are significantly higher in camels with T. evansi compared to the controls (El-Bahr and El-Deeb 2016).

Transport biomarkers

Transportation is a well-known stressor that has adverse effects on livestock production and health (Ishizaki and Kariya 2010), generating concerns about economics and welfare. Unfortunately, the handling and loading of animals represent the most stressful aspect, as compared to the journey itself (Minka et al. 2009). Long-distance road transportation is commonly performed in draft and racing camels for industrial and racing purposes (Tharwat et al. 2013c). During environmental stress, marked changes in the levels of reactive oxygen species (ROS) scavengers occurred in the serum of camels (Kataria et al. 2010). As a very stressful factor, road transport of camels to the slaughterhouse is able to increase in free radical generation (Nazifi et al. 2009) in camels. Furthermore, the reaction to transport stress may have an impact on the quality of slaughtered animal meat and depends on the duration and intensity of this stressor (Kaoutar et al. 2016).

Usage of transport biomarkers in camel medicine

In a study investigating transport distance on stress biomarkers, El Khasmi et al. (2015) concluded that road transport is very stressful for the camel, and the effects of this stress on the relevant indicators increase much with distance which was categorized as short (72–80 km), medium (160–170 km) and long (350–360 km) distance. In the later study, the serum cortisol increased dramatically with a distance where it measured 88.32 ± 19.4 ng/mL, 152.4 ± 25.18, and 231.7 ± 23.75 ng/mL at the short, medium and long distance, respectively. The serum concentration of stress biomarker malondialdehyde (MDA) was estimated to be 1.58 ± 0.38 ng/mL, 3.88 ± 0.2, and 6.44 ± 0.52 nmol/mL at the short, medium and long-distance, respectively. Parallel, the serum concentration of the stress biomarker catalase (CAT) was found to be 60.08 ± 3.18 ng/mL, 79.13 ± 3.84, and 93.95 ± 3.62 KU/L at the short, medium and long distances, respectively (El Khasmi et al. 2015).

In a 300 km, 5-hour journey in the truck by road conducted during hot summer, Nazifi et al. (2009) found that the mean concentration of MDA (1.87 ± 0.26 nmol/mL) and the stress biomarker glutathione peroxidase (GSH-Px) activity (297.86 ± 25.68 U/g Hb) in basal pre-transport conditions show significant increase 24 h after arrival. However, the mean concentration of the stress biomarker superoxide dismutase (SOD) activity (1742.5 ± 74.36 U/g Hb) in basal pre-transport conditions had no significant change during and after transportation. It was concluded from this study that transport stress causes an oxidative challenge in dromedary camels and represents novel biomarkers for stress-associated disease susceptibility and welfare assessment (Nazifi et al. 2009).

Stress biomarkers

Oxidative stress (OS) is an imbalance between radical-generating and radical-scavenging activity processes necessary to detoxify these toxic molecules resulting in damage to all components of the cell, including proteins, lipids, and DNA (Tharwat and El-Deeb 2021; Almundarij and Tharwat 2023). OS can occur because of either heightened reactive oxygen species (ROS) generation, impaired antioxidant system, or a combination of both. In the presence of OS, ROS modify and denature functional and structural molecules leading to tissue injury and dysfunction (Vaziri 2008). Thus, OS can cause disruptions in normal mechanisms of cellular ability to detoxify the reactive intermediates or repair the resulting damage (Lands et al. 2000). A complex association exists between OS and inflammation. OS is believed to be an integrated part and major component in the pathogenesis of skin diseases expressed as erythema, oedema, wrinkling, hypersensitivity, keratinization abnormalities and cancer (Bickers and Athar 2006; Portugal et al. 2007).

Usage of stress biomarkers in camel medicine

In camels with paratuberculosis, the activities of SOD and CAT and reduced glutathione (RGSH) levels are reduced significantly in infected camels compared to the control. On the contrary, lipid peroxidation is increased significantly as reflected by higher MDA value in the serum of infected camels compared to the control. In addition, the values of all pro-inflammatory cytokines, IL-1α, IL-1β, IL-6, IL-10, TNF-α and IFN-γ were also increased significantly in paratuberculosis infected camels compared to control (El-Deeb et al. 2014).

In camels with trypanosomiasis, the activities of SOD, CAT and RGSH levels are reduced significantly in camels with trypanosomiasis compared with the control whereas, lipid peroxidation is increased significantly as reflected in higher MDA values in the serum of camels with trypanosomiasis compared to healthy controls. In addition, the values of all pro-inflammatory cytokines namely, interleukins-1α, 1β, 6, 10, tumour necrosis factor-α and interferon-gamma are significantly higher in T. evansi camels compared with the control (El-Bahr and El-Deeb 2016).

In camels with sarcoptic mange, concentrations of MDA did not differ in mild but elevated in moderate and severe cases compared to the values in the control group. In comparison with the healthy camels, the values of SOD and CAT are significantly higher in mild and significantly lower in moderate and severe cases. The same trend is noticed for GSH-Px concentration, where it is higher in mild and lower in moderate and severe cases compared to healthy animal values (Saleh et al. 2011).

Pulmonary biomarkers and its usage in camel medicine

In a study conducted on camels that suffered from bilateral nasal discharge, inappetence, dyspnea, cough and harsh lung sounds, there was a significant increase in the levels of MDA in diseased camels when compared to its levels in the healthy ones. In addition, there has been a significant decrease in the levels of total antioxidant capacity (TAC) and CAT compared to the healthy group (Kamr et al. 2020). The decreased antioxidants were attributed to the cell protection consumption of those enzymes by preventing the initiation of peroxidization that is capable of leading to serious cell damage (Kamr et al. 2020).

In another study conducted on female camels manifested by moist painful harsh cough, rhinitis, congested mucous membranes and serous or mucoid nasal discharges, the mean values of serum concentrations of MDA, nitric oxide (NO) and plasma hydrogen peroxide (H₂O₂) were significantly increased in the diseased group compared to the controls. On the other hand, serum enzymatic activities of CAT, Glutathione reductase (GR) and GSH-Px showed a significant decrease in the diseased group. Compared to controls, diseased female camels had significantly higher serum concentrations of pro-inflammatory cytokines (IL-1α, IL-1β, IL-6 and TNF-α), while the serum concentrations of anti-inflammatory cytokine (IL-10) were significantly decreased (Allam et al. 2017). In another study investigating the oxidative stress markers in pneumonic camel calves, there was a significant increase in the levels of MDA and a significant decrease in the levels of GSH, SOD and CAT in the pneumonic camel calves versus healthy controls (El-Deeb 2015).

Recommendation and future directions

It is believed that providing insights by ongoing research on the biomarkers of heart, bone, inflammation, transport, stress and lungs in camels, will be increasingly used in the future in the diagnosis and prognosis of camel disorders. In addition, it will give practical information on their actual use in camels and the interpretation of the results. It is essential in the future to focus on biomarkers for various camel diseases using noninvasive samples such as urine, saliva, milk and breath for ease of diagnosis and also for prognosis. A comparison between the biomarker-based diagnosis with conventional techniques for the diagnosis of camel diseases in terms of sensitivity, specificity, ease of use and cost is necessary. It will be also of great help to improve the use of these biomarkers for early disease diagnosis. However, the high price of the biomarkers at the present time limits the biomarker-based diagnosis in camels and therefore their use is limited in practice.

Acknowledgement

The author thanks Qassim University for technical support.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

The author extends his appreciation to the Deputyship for Research & Innovation, Ministry of Education, Saudi Arabia for funding this research work through the project number (QU-IF-2-1-3-25365).