Rib anomalies in cats are rare, with few reports in veterinary literature. In particular, bifid ribs—a congenital anomaly where a rib splits into two distinct branches—are uncommon in animals. This anomaly, more frequently reported in human medicine (3,8), features a rib bifurcation typically at the sternal end, resulting in two branches joined by a bifid section of costal cartilage (3,8). Bifid ribs can present as isolated or complex with other congenital anomalies and are often discovered incidentally by radiographic examination. While typically asymptomatic, certain cases may involve thoracic wall deformation or compression of adjacent structures, potentially leading to respiratory or neurological symptoms under specific conditions, such as trauma or congenital abnormalities (1,3,5,8). Despite their rarity and usually being clinically insignificant, understanding bifid ribs is crucial for precise diagnosis and effective management in veterinary practice.

This report aims to describe the radiographic findings and clinical significance of a bifid rib in cats, which are seldom documented in veterinary literature. Additionally, it provides a detailed description of these radiographic findings and discusses their clinical relevance in veterinary patients, particularly in cats.

A 7-year-old, 4.6 kg spayed female Korean short hair cat was evaluated at a local animal medical center for a health screening. According to the owners, the cat experienced only occasional vomiting and displayed no other notable symptoms. There was no history of significant illness, surgery, trauma, thoracic disease, or other serious conditions. Both physical and laboratory examinations were unremarkable. Radiographs revealed a bifurcation of the proximal part of the right 9th rib extending towards the caudo-ventral direction of the thoracic cavity, while the vertebrae showed no abnormalities (Fig. 1). No other organs had abnormal findings.

Figure 1. Radiographic images showing the bifurcation of the proximal part of the right 9th rib in a cat. (A) Right lateral view showing the caudo-ventral orientation of the bifid rib (white arrow). (B) Ventrodorsal (VD) thoracic view highlighting the bifid rib (white arrow) extending into right lateral of the thoracic cavity. Note the absence of abnormalities in adjacent thoracic structures.

Bifid ribs account for approximately 1.2% to 28% of rib abnormalities in humans (1,4). Although bifid ribs are more commonly observed in humans, this case demonstrates that such anomalies can also occur in cats, underscoring the need for thorough diagnostic imaging in veterinary settings. Moreover, there is a slightly higher prevalence in males than in females (9), and the condition more frequently affects the right-side ribs than the left (4,7). This study shows a bifid rib on the right side, akin to prior human studies, occurring in a female cat.

The exact etiology of bifid ribs is uncertain; however, it is thought to arise from disruptions in the segmentation process of developing somites during embryogenesis (1,5). Some cases of bifid ribs are associated with genetic syndromes such as Gorlin-Goltz syndrome, also known as nevoid basal cell carcinoma syndrome (1,4,9). Consequently, bifid ribs are sometimes linked with other skeletal or systemic abnormalities, such as vertebral defects, cardiac anomalies, and renal malformations, with varying clinical significance (9). In this case, although no genetic tests or similar diagnostic analyses were performed, no specific clinical findings suggestive of a genetic disorder were identified.

On radiographs, bifid ribs typically occur at the sternal end and can present as either a “fork“ or “hole“ type, depending on the bifurcation nature (1). The fork type is characterized by a long and slender fissure, whereas the hole type exhibits a more rounded bifid space (6). Moreover, the fork-type bifurcation appears to develop through a division occurring in the anterior portion of the rib. In contrast, the hole-type bifurcation is formed by the upper branch extending from the lower branch at the bifurcation site (1). Additionally, a rarer condition known as ’bifid intrathoracic ribs’ extends into the thoracic cavity, which is identified using radiographs and computed tomography (CT) (1,3). CT scans play a crucial role in visualizing the bifurcation point of bifid ribs and the space between the upper and lower divisions (6). This detailed visualization is essential for assessing the relationships between the rib segments and surrounding structures such as muscles, nerves, and blood vessels (6). This particular case featured bifurcation of bifid ribs extending inward into the thoracic cavity on radiograph. If 3D imaging had been acquired via CT scan, a diagnosis of ’intrathoracic bifid ribs’ might have been possible. Unfortunately, a CT scan was not performed in this case. Further studies, including advanced imaging techniques like CT, could provide deeper insights into the prevalence and clinical impact of bifid ribs in animals.

In humans, bifid ribs are typically asymptomatic and often discovered incidentally during thoracic radiographic examinations. They rarely coincide with neuroskeletal anomalies, respiratory, and neurological difficulties (1,3,5,8). Although generally benign, bifid ribs can complicate surgical procedures and diagnostic evaluations due to possible misinterpretations such as costal fractures and rib tumors (2,4,5). These differential diagnoses are particularly relevant when the bifurcation extends into the thoracic cavity. Additionally, pulmonary or esophageal foreign bodies should be distinguished depending on the location of the bifid rib. The clinical significance of rib anomalies in veterinary patients is complex, as these conditions are rare and prone to misdiagnosis, which makes accurate diagnosis challenging.

This case report describes a bifid rib in the right 9th rib of a cat, emphasizing the critical need for precise diagnostic imaging to identify and manage incidental findings with potential clinical significance. It also underscores the importance of recognizing rare rib anomalies and considering their implications during surgeries or trauma management. Verifying whether the bifurcation extends into the thoracic cavity is crucial, as it may require special consideration during surgical procedures or in trauma cases. Further research could explore the genetic and developmental factors associated with rib anomalies in veterinary patients.

This study was supported by a grant from the Cooperative Research Program for Agriculture Science and Technology Development (project no. RS-2023-00231792), RDA, Republic of Korea.

The authors have no conflicting interests.