1 Introduction

Sheep (Ovis aries) are ruminant species and their gastrointestinal tract (GIT) has many similarities to species within this classification. The large intestine is the last part of the GIT and is itself commonly divided into the following components for research and didactic purposes: caecum, colon, rectum and anal canal (Constantinescu 2018; König and Liebich 2020; WAVA 2017). The colon is the longest part of the large intestine and comprises of three sections: the ascending, transverse and descending colons. The ascending colon starts with an S-shaped segment called the proximal ansa, then narrows and continues ventrally to form a double spiral, which terminates in a distal loop connecting to the transverse colon. The middle part of the ascending colon, because of its spiral shape, is called the ansa spiralis, but it is sometimes referred to as the spiral colon, spiral ansa, or ansa spiralis coli (Gentile et al. 2019; Smith 1955a, 1959). The ansa spiralis is composed of both centripetal and centrifugal loops. Initially, the gut segment rolls up in a centripetal direction without deviation until it reaches the centre; then it moves from the centre to the periphery as centrifugal loops (Figure 1). This shape is considered normal or regular and is very similar to that observed in other ruminant animal species during dissection, such as those undertaken in slaughterhouses, and during necropsies or surgeries. However, cases with irregular ansa spiralis have been observed. In sheep and goats, there are usually three centripetal and three centrifugal gyri. The last centrifugal gyrus in sheep and goats differs from that generally observed in oxen. In sheep and goats, the ansa spiralis is more closely related to the jejunum in the small intestine than to the rest of the colon and has a pearl-string appearance due to the segmented contents (Dyce et al. 2017; Habel 1975; König and Liebich 2020; Nickel et al. 1979). An anatomical variation, variant, or variability consists of difference(s) between the anatomical structures of different animals of the same species. These variations may be considered normal if they are consistently found in a number of animals within the species and if they are primarily symptomless; thus, they are deemed anatomical variations rather than abnormalities.

Understanding variant anatomy versus abnormalities is essential for anatomists and clinicians working in fields involving diagnostic techniques and therapeutic interventions (Kachlík et al. 2020). Variations in shape, composition, topography, size, number, and frequency of anatomical structures or organs have been well documented in animal anatomy through various studies (Kiriki et al. 2011; Mik et al. 2024; Mochizuki and Makita 1996, 1998; Mohamed 2020a, 2020b).

The discovery of these changes, or variations, is of great importance not only from a theoretical perspective but also from a practical standpoint during various clinical, imaging, or surgical examinations, medical interventions, veterinary forensics, and evolutionary studies. It is essential to understand and recognise such physiological variations so that they can be distinguished from pathological conditions, especially since sheep can serve as comparative anatomical models for human medicine (Banstola and Reynolds 2022; DiVincenti Jr. et al. 2014; Sartoretto et al. 2016; Tryfon and Anastasia 2023).

The large intestines are described in detail in many anatomical textbooks (Dyce et al. 2017; Habel 1975; König and Liebich 2020; Nickel et al. 1979). A number of studies have also described the large intestines, and especially the ansa spiralis of the ascending colon in ruminant species including sheep (De Santis Prada et al. 1971; Smith 1955a, 1955b), goats (Borelli and Filho 1965; Nanda and Patel 1962; Paiva and Borelli 1965; Smith 1959), cattle (Braun et al. 2024; Gentile et al. 2019; Meadows and Smith 1956), and wild animal species such as the red brocket deer (Borrelli et al. 1971), pampas deer (Pérez et al. 2008), and Axis deer (Pérez et al. 2015). To evaluate the general discoid shape of the ansa spiralis, it is essential to consider the exact number of centripetal and centrifugal gyri, their arrangement, and the mesenteric folds that support and anchor these intestinal segments to other parts of the gastrointestinal tract. Various studies have also sought to explore variations in the ansa spiralis both between and within species. For example, in Gaddi sheep, the shape of the entire ansa spiralis was not completely discoid but rather somewhat hemispherical (Rajput 2006). The shape and regularity of the ansa spiralis in 213 cattle showed only 60.6% of them had a normal spiral colon (Wölfl 2011). In 24.9% of these cases, there was a slight deviation (grade 1), 9.8% had a moderate isolation from the mesenteric disc (grade 2), and finally, 4.7% were not embedded into the jejunal mesentery at all.

In both anatomical textbooks and published scientific literature, references to irregularities in the ansa spiralis in sheep are scarce. Despite the lack of research to date, this information is important since variation in the ansa spiralis gyri direction in sheep may affect functions related to the fermentation process, water absorption, and transit time of the digesta through the ansa spiralis gyri. The main objective of this cadaver-based study was to document a detailed morphological description of the ansa spiralis in sheep. This study contributes anatomically valuable and enriching data that can also serve as a basis for comparative studies.

2 Materials and Methods

The study was conducted in 2021–2022 in a slaughterhouse based in Tirana, Albania. No additional ethical permission was required as slaughterhouse material for non-research purposes was used, with permission from the slaughterhouse.

During this study, 24 visits were made to the slaughterhouse, and 555 sets of large intestines from sheep were randomly taken from slaughtered sheep aged 6 months and older. No records were available regarding exact age, sex, breed, or geographical region of origin. The study focused exclusively on the large intestine, specifically evaluating the frequency and types of variations in the ansa spiralis.

The intestinal tracts were always presented on the table for observation with the left side facing towards the observer. The same observer (SD) carried out all of the evaluations. Every sample underwent an evaluation to assess the normality of the centripetal and centrifugal coils; any samples with deviations from normality were recorded in detail. The observations made for each set of sheep intestines began at the caecum, followed by the proximal ansa and the ansa spiralis (including both centripetal and centrifugal coils; Figure 1), and the shape of each section up to the transverse colon was recorded. All samples with irregular shapes were carefully assessed and their differences were recorded, including the shape type (Figure 2), the starting location of the variation in relation to the centripetal or centrifugal coils and to the cecum, the type and number of spiralizations after the starting point of irregularity, and the position of the difference within the overall discoid structure of the ansa spiralis. The positions of the irregularities according to the caecum position and the central flexure were classified as either 1, 2, 3, or 4 (Figure 3).

3 Results

All of the irregular cases of the ansa spiralis, dependent on their shape and the starting location of the irregularity, were classified into 12 different types as demonstrated in Figure 2. The quantitative results of the observations, detailing the irregular ansa spiralis shapes in the ascending colon of slaughtered sheep, are presented in Table 1. From the 555 large intestine sets (from 24 slaughterhouse visits), 52 cases (9.37%) of irregular ansa spiralis anatomical shapes were recorded. Across the 24 visits, with randomised collection, the findings from each visit varied from zero cases of irregular ansa spiralis (e.g., 22 sets of large intestines evaluated and zero irregularities of the ansa spiralis of the ascending colon found) through to a maximum of 33.33% (nine large intestines evaluated, containing three ansa spiralis displaying irregularities).

Within the 52 non-typical ansa spiralis, almost 48% were classified as types A and B (Table 1). Types I, G, F, and J combined represented about 35%, and the remaining 17% consisted of the other types. The types classified as C, K, and L appeared only once each within the 555 samples (Table 1).

Irregularities were observed in both the centripetal and centrifugal gyri in 41 cases. An additional nine cases showed irregularities exclusively in the centrifugal gyri (eight were type B and one was type F), while only two cases had irregularities confined to the centripetal gyri. The irregularity began after the first normal centripetal loop in types B, C, D, G, I, J, and K, whereas in types A, E, F, H, and L, it started either at the beginning or in the middle of the first centripetal loop (Figure 2). Although some types appeared quite similar, for example, types A, E, I, and H, there were still differences in the location of the starting points of the irregularity and the number of centripetal ansa spiralizations.

The starting points of the irregularities were distributed as follows: 18 cases were located under the caecum base, 16 were near the starting point of the proximal loop, seven were at the beginning of the centripetal loop, and the remaining 11 were located ventrocaudally to the central flexure (Figure 3). During the observation process, irregularities in the sheep ansa spiralis were easily identified in types B, A, I, E, and J. However, understanding the course of the centripetal loop was particularly challenging in types D, H, G, K, and L. In the 10 cases of type B, the “S” shape deviation in the last two centrifugal loops varied in size, and in one case, the “S” deviation doubled. Observation of the large intestines also revealed that all of the intestines that were classified as normal had the spiral loop situated in a single plane. However, some cases deviated from this and exhibited disco-conusoid or hemispherical shapes.

During the assessment process of the sheep's large intestines, the presence of different fold attachments not previously described in Nomina Anatomica Veterinaria (WAVA 2017) was observed in three cases. As a novel finding in sheep, these folds have therefore been described. One fold fixed the caecum to the proximal ansa of the ascending colon, and another joined the terminal part of the proximal ansa to the last centrifugal gyrus of the ansa spiralis (Figure 4).

4 Discussion

The ansa spiralis is the middle part of the ascending colon and is comprised of centripetal, central, and centrifugal gyri. Initially, the gut segment coils in the centripetal direction without deviation until reaching the centre, after which it extends from the centre to the periphery as centrifugal gyri. All these parts resemble a disc in a single sagittal plane (Figure 1), and this shape is considered normal or regular (Dyce et al. 2017; Habel 1975; König and Liebich 2020; Nickel et al. 1979).

Analysing and recording the 52 cases of irregular variations of ansa spiralis from 555 sets of large intestines in slaughtered sheep showed that the phenomenon of anatomical variation exists and that the prevalence of these variations was 9.37%. A more detailed analysis of these variations showed that the forms were very diverse. Therefore, they were further classified into 12 different types based mainly on the shape, anatomical starting point of the variation, numbers, and location (e.g., type B and J) of these variations, and the number of spiralisations (e.g., type A and E) after the starting point of irregularity (Figure 2). The prevalence rate of ansa spiralis irregularity at 9.37% and the 12 different variation types found in the sheep within this study were different compared to previous studies. Smith (Smith 1955a) found that in 1061 sheep of different breeds and ages, 219 (or 20.64%) showed some irregularity in the spiral colon. These were grouped into just two categories: (a) an S-shaped insertion of varying size in an otherwise normal coil and (b) a completely different departure from the spiral arrangement. The latter two types described by Smith (Smith 1955a) correspond to the types B and L in the present study. In another study, a detailed analysis of irregular ansa spiralis was conducted in sheep (Smith 1955b). The study noted that approximately 25% of the irregularities exhibited U-, S-, or Z-shaped deviations occurring exclusively in the last coil, while the remaining cases displayed a variety of forms. Many of these forms occurred multiple times and could be classified into apparent sequences. Interestingly, the author (Smith 1955b) did not describe any changes in shape relating to the centripetal coils or to the anatomical starting points of these irregularities, something that the present study described in detail.

In the present study, within the 52 irregular ansa spiralis, the predominant types were types A and B, which appeared in 48% of cases. Additionally, the location of the irregular shape in both the centripetal and centrifugal gyri of the ansa spiralis was recorded in 41 cases (78.8%). Another novel feature of this study was related to the position of the starting point of the irregularity in relation to the caecum base and the central flexure of the ansa spiralis. The most frequently observed starting points of irregularity were points 1 and 3 (Figure 3), with 65.38% of cases demonstrating this pattern. By developing this classification system and describing the frequencies, anatomists, veterinary imaging specialists, surgeons, and researchers can discover the type and topography of ansa spiralis anatomical variations in sheep.

The spiral loop of the ascending colon in sheep is an elliptical to circular coil shape on a single plane and is called the ansa spiralis of the ascending colon (Figure 1). In this study, the ansa spiralis was typically situated in a single plane; however, some cases deviated and exhibited a disco-conusoidal or hemispherical shape, consistent with previous descriptions in Gaddi sheep (Habel 1975; Rajput 2006).

The large intestine is an important organ in the nutrition of ruminants (Hoover 1978) and the colon is the site of most of the water absorption in the large intestine (Reece and Rowe 2017). The caecum and the proximal ansa are the most important regions for fermentation, whereas the ansa spiralis and descending colons are involved in the absorption of water, ammonia, and volatile fatty acids (Dixon and Nolan 1982). The absorptive capacity of the large intestine may be greater per unit of volume than the rumen due to its long, narrow shape (Hecker 1971). Previous research experimentally recorded the retention time of digesta in the large intestine ranging from 10.2 to 26.5 h (Coombe and Kay 1965). This duration was influenced by the amount of food consumed, with higher intakes associated with shorter retention times. Additionally, the ansa spiralis in sheep is involved in the formation of pellets, with a colonic retention time averaging 20 h (Hecker and Grovum 1975). Since 9.37% of the ansa spiralis in slaughtered sheep had irregular shapes in the present study, we hypothesise that different shapes and lengths of the ansa spiralis may primarily affect the absorption of water, ammonia and volatile fatty acids.

According to a previously published study on the spiral loop (Fioramonti and Hubert 1980), short spike burst potentials lasting about 4 s occurred approximately 15 times per minute during nearly 95% of the recording time. This motor activity was disrupted in the first coil by strong spike bursts (8.5/h) propagated (about 3 cm/s) from the proximal colon, followed by quiescence for 10–30 s. Only 40% of these bursts reached the last centripetal coil. It is still unknown what happens with these short and strong spike burst potentials in cases of irregular colons in sheep and other ruminants, but this would be an interesting area for future research.

The centripetal gyri go from the periphery of the disc and coil gradually after 3–3.5 coils, terminating at the middle of the disc in a very sharp curve called the ansa spiralis. At this point, the direction reverses, and the centrifugal gyri run rigorously along to the periphery, running near the ventral surface of the caecum. From there, they extend caudoventrally and craniodorsally, terminating just cranial to the starting point of the first centripetal coil (Constantinescu 2018; König and Liebich 2020; Nickel et al. 1979). While the shape of spiral coils in the sheep observed was mostly standard as described previously, here, deviations from the normal shape were observed. According to Smith (Smith 1955b), about 25% of the irregularities in the sheep ansa spiralis were found in the last centrifugal coil. This corresponds to types B and F in our study, with a similar frequency finding of 26.92%. Studies of the large intestine in sheep (Smith 1958) and goats (Paiva and Borelli 1963) did not demonstrate differences in the degree of irregularity of the ansa spiralis when comparing foetuses, young, and adult animals. More examples of the irregularity of the ansa spiralis in different ruminant species are provided in Table 2.

A study of 735 large intestines in cattle (Meadows and Smith 1956) showed that only 24 samples (3.26%) had an irregular ansa spiralis shape. These values are much lower compared to the data from the present study and a previously published study (Smith 1955a) in sheep. A study on the analysis of the arrangement of the ansa spiralis of 100 Corriedale sheep colons (De Santis Prada et al. 1971) showed that 7 sets of large intestines (7.0% ± 2.55%) had an irregular ansa spiralis shape. Whereas a study investigating the large intestines of 400 merino sheep recorded that 65 (16.25% ± 1.84%) had an irregular ansa spiralis shape (Borelli and De Santis Prada 1966). A limitation of the present study was that the sexes were not recorded; like many other previous works, therefore comparisons between sexes could not be made. Interestingly, only one other previous publication specifically looked at this in 100 male sheep they had previously published on (males compared to whole cohort) and they showed no significant sex-specific differences or age-related differences in foetal (n = 50) compared to adult samples (Smith 1957).

In goats, the results varied too, both within the species and compared to other species and between goat studies. In one study, 6 out of 50 goats (12%) had an irregular ansa spiralis shape; the author hypothesized that this variation might be due to the mixed origin of the present domesticated breed (Smith 1959). In a larger study investigating 1136 goats, 215 (18.9%) had an irregular ansa spiralis shape (Nanda and Patel 1962). In another study in 946 goats (Borelli and Filho 1965), 70 (7.39% ± 0.85%) had irregularities in the coiling of the ansa spiralis, with no significant difference between sexes. In work undertaken on 77 goat foetuses (Borelli and D'Errico 1965), it was discovered that 9 specimens (11.68%) had an irregular ansa spiralis, but the age had no influence on the prevalence of the irregular shape of the ansa spiralis.

Cattle have also shown irregularities; for example, in a study including 1113 slaughtered calves, 472 (42.4%) exhibited irregularities (Gentile et al. 2019). These included conical-shaped looseness of the spiral colon (34.2% ± 2.8%), partial dystopia (6.1% ± 1.4%), and complete ectopy of the spiral colon, which was observed in 23 calves (2.1% ± 0.8%). In another retrospective study of 58 cattle treated in a veterinary clinic (Braun et al. 2024), torsion of the spiral colon was diagnosed in 13 cases (22.4%).

Other ruminants have also exhibited ansa spiralis variations. In four red brocket deer (Mazama Americana), two of them (50%) had ansa spiralis irregularities (Borrelli et al. 1971), but notably the numbers were smaller than comparable studies. Luginbuhl (Luginbuhl 1983), in their review of the gastrointestinal tract in ruminants, concluded that the proportion of irregular shapes of the ansa spiralis was greatest in sheep (ranging from 7% to 24%), followed by goats (7%–12%), and least common in oxen (3%). This data is now incorrect as later papers show completely different quantitative outcomes to those available when the review was published in 1983 (Luginbuhl 1983). These variations have been attributed to factors such as congenital and genetic predispositions. However, no relationship has been identified between the frequency of the irregular shapes of the ansa spiralis in sheep and their sex or age.

A final novel finding in this study was the folds that adhere the proximal ansa to the caecum and the centrifugal gyrus of the ansa spiralis, which were observed in three sets of large intestines in this study. These folds have only been described previously in wild ruminants such as pampas deer (Pérez et al. 2008), Axis deer (Pérez et al. 2015), and Brown brocket deer (Pérez and Vazquez 2012). In these studies, the folds of the proximal ansa were described as S-shaped, adhering to the cranial part of the caecum and then turning to the medial side, where they attached to the left sheet of the mesentery. Additionally, another fold, which joined the terminal part of the proximal loop with the last centrifugal loops of the spiral colon, was observed.

In conclusion, this study assessed the presence and frequency of the irregularities in the ansa spiralis of the ascending colon in sheep. Furthermore, 12 different irregular configurations of ansa spiralis in sheep were classified and described. The data provided on the morphology of the sheep's large intestine will serve as a foundation for further studies to explore additional details related to anatomical variation, such as the folds, fixation, and vascularisation of the ansa spiralis. Further research is also necessary to investigate the relationship between the frequency of the ansa spiralis shape deviations in sheep and factors such as breed, diet, and geographical region.

Understanding the ansa spiralis variations and the associated anatomical structures in different ruminant species is crucial for veterinary practice. It is essential for veterinary imaging specialists, surgeons, and forensics specialists to understand and recognise these variations to distinguish them from pathological conditions.

Conflicts of Interest

The authors declare no conflicts of interest.