Monocephalic thoracopagus tetrabrachius in twin lambs

Congenital defects result from errors during fetal development, and can be caused by genetic defects, infectious agents, or environmental factors (1). Conjoined or fused twins arise from incomplete division of a single embryo during the primitive streak stage. The components of the fetuses are, therefore, monozygotic (2) (Table 1). The defects present in conjoined twins may range from minor duplications to near complete separation and can be broadly classified in terms of the general location of the failure of complete separation. For example, fusion located in the cranial region is termed dipygus; however, there are more specific terms such as, thoracopagus, craniopagus, or abdominopagus (1). This case report describes monocephalic twin fetuses with failure of separation at the level of the thorax or thoracopagus.

Table 1.

Glossary of terms

-pagus	A combining form or suffix that means conjoined twins where the first element of the word denotes the parts that are fused. See examples that follow.
Abdominopagus	Conjoined twins in which the abdomens are fused.
Asymmetrical conjoined twins	Conjoined twins in which one individual is nearly normal and the other is small, incomplete, and dependent upon the nearly normal individual. Also referred to as unequal conjoined twins.
Congenital	A condition that is present at and existing from birth.
Conjoined twins	Monozygotic twins whose bodies are joined to a varying extent.
Craniopagus	Conjoined twins in which the skulls are fused.
Chromosome	The linear thread of DNA contained within the nucleus of eukaryotic cells. During cell division the chromosomal material is compactly coiled to permit its movement within the cell with minimal entanglement. Photographs of mitotic chromosomes can be arranged in homologous pairs for chromosomal analysis, also known as karotyping.
Dipygus	Conjoined twins in which the head and thorax are fused and the pelvis and pelvic limbs duplicated.
Dizygotic twins	Two offspring produced in the same pregnancy from separate ova fertilized at the same time. Compare with monozygotic twins.
Monozygotic twins	Two offspring developed from one fertilized ovum.
Siamese twins	A term, now out of use, to refer to a pair of conjoined twins.
Symmetrical conjoined twins	Conjoined twins in which both individual are approximately the same size and nearly normal except for the areas of fusion. Also referred to as equal conjoined twins.
Teratology	The branch of biology (embryology and pathology) concerned with abnormal development and congenital malformations.
Tetrabrachius	An individual, typically a fetus, with 4 thoracic limbs.
Tetrascelus	An individual, typically a fetus, with 4 pelvic limbs.
Thoracopagus	Conjoined twins in which there is fusion of the thoracic region.
Twins	Two offspring born at the same time from the same pregnancy.

Cross-bred, fetal, twin lambs, both male, from northeastern New South Wales, Australia, were submitted for necropsy to the Regional Veterinary Laboratory following an unsuccessful assisted delivery by the owner. The ewe had died during parturition and the owner had experienced difficulty in removing the fetuses from the ewe. The lambs were markedly autolyzed and there was a traumatic opening of the abdominal cavity extending from the prepuce to the midline of both bodies.

The combined weight of the lambs was 5.1 kg. Grossly, there was a single head, an apparent duplication of the thorax, and 8 limbs — 4 brachial (tetrabrachius) and 4 pelvic (tetrascelus) (Figure 1). The crown-to-rump lengths of the 2 fused bodies were 36 cm and 35 cm, respectively; however, curvature rendered accurate measurement difficult (Figure 2). There was bifurcation of the cervical spine at the level of C4 that extended caudally to include 2 distinct tails. There was marked scoliosis of the entire vertebral columns. Rostral to C4, there was a single, normal appearing, cervical spinal cord. The thoraces were fused (thoracopagus), but there were 2 sternums and 2 complete sets of ribs encasing a single heart and lower respiratory system. The heart was 6 cm from base to apex and 5 cm in greatest diameter with an irregular and small atrioventricular valve. The abdomen contained a single stomach, but complete duplication of the spleen, colon, both kidneys, and both testes. Autolysis and trauma prevented complete examination and description of the intestines. Antibodies of Akabane Virus were not detected in pleural fluid obtained during necropsy, using an enzyme-linked immunosorbent assay (ELISA) developed by the Elizabeth Macarthur Agricultural Institute.

Figure 1.

External view of twin lambs showing a single head and duplication of the forelimbs, hind limbs, and tail.

Figure 2.

The twin lambs with the skin removed and bodies separated to identify a complete duplication of skeletal components below the cervical spine.

The veterinary literature contains few descriptions and studies examining teratological malformations. Within the realm of conjoined twins, such as in this case, semantic descriptions become convoluted and ungainly, and lack methodical nomenclature. This stems largely from comparisons between veterinary teratology, and human conditions such as conjoined symmetrical and asymmetrical twins (so-called Siamese twins). For obvious reasons, in the human condition, each portion of the fetus tends to be considered as an individual. This has led to the use of terms such as “fusion” that, while descriptively accurate, does not reflect the pathogenesis of these conditions.

In the scant veterinary literature, there is frequent use of the terms “fusion,” “duplication,” and “incomplete separation” — often within the same paper and describing the same animals. In the present communication, the authors have attempted to address the descriptive pathology with reference to the fundamental pathogenesis of these conditions, which is, distinctly, failure of separation.

While fetuses presented with failure of separation (conjoined twins) are not commonly reported in the veterinary literature, they are described most fully in cattle (2), and have been described in pigs (2), sheep (3,4), goats (5), and a dog (6). In sheep, the failure of separation most commonly involves the cranial region, whereas in cattle, the defect is most typically caudal (4). In sheep, the condition typically occurs in males, and is frequently associated with atresia ani (4). In the case described here, the failure of separation involved most structures cranial to C4, leading to the appearance of duplication of both the axial and non-axial portions of the skeleton and associated organs. Atresia ani was not present. The lack of cephalic duplication was unusual. This particular combination has only been reported in a single dog (6).

The incidence of congenital defects in extensive production animals varies greatly, and may arise sporadically (7), or in clusters, as occurred during the autumn 1960 lambing season in western Australia (3,4). In sheep, it has been reported that 1.5% to 2.1% of lambs that die within 7 d of lambing have a notable congenital defect (8). Despite investigations, the underlying cause of congenital abnormalities often remains elusive (3–7). In the current case, Akabane virus was ruled out as a potential infectious cause and because this case was an isolated case on this property, other infectious or toxic causes were considered unlikely.

Teratology is an understudied field of veterinary science, possibly due to the paucity of reports and the relative lack of uniformity in classification. However, congenital defects occur in production animals and constitute a loss of productivity. While chromosomal and genetic testing are not routinely performed in such cases, the facilities do exist in many larger laboratories to investigate teratological malformations should clinicians note an increased incidence of congenital defects in the absence of obvious infectious or toxic etiologies.