One thing about being the editors of The Vet Gazette is you never know what is going to come through the inbox. This wonderful and truly unique submission describing a boa constrictor dissection comes from Daniel Newham (with help from Leanne Smaller) from the Royal Veterinary College. Thank you for the wonderful submission Daniel!
Snakes have a very linear digestive system. Initial digestion by swallowing is conducted in two motions, one via muscles in the walls of the oesophagus which contracts to squeeze the food down. The second swallowing movement is specific to snakes, and involves wave movements of the backbone and body (Moon 2001).
In smaller snake species, gastro intestinal problems can happen when the snake ingests articles larger than the lumen. These can include accidental ingestion of substrate with food through inadequate husbandry.
Further diseases of the gastro intestinal tract include parasites, bacteria and viruses which are common plagues for snakes. Snakes kept in closed environments reinfection is common where many parasites can live an entire life cycle on the same host, laying their eggs in the host's intestines which are then passed out in their droppings only to then reinfect the host again. Good husbandry management is necessary to mitigate reinfection risk (Johnson 2012).
Display the gastrointestinal tract in situ
The materials needed in this dissection include: boa constrictor, a blunt metal probe, a pair of scissors, a scalpel, a pair of rat-toothed and blunt tweezers.
8mm and 18mm tubes for supporting structures.
Ventral parallel incisions were made just caudal to the head running the length of body finishing three scutes cranial to the vent. The incisions followed at the level of the ventral end of the ribs approximately two scales width dorsal to the scutes.
The tail vein was removed (coccygeal vein) along with the most superficial epaxial muscles (Mm. semispinalis-spinalis, longissimus dorsi and iliocostalis) exposing the internal visera and paired coelomic adipose stores.
A left sided section of skin was reflected just caudal to the head to reveal the upper oesophageal tract. The pleura surrounding the tract is closely associated with that surrounding the carotid arteries and trachea, the muscles ribs and pleura were removed to reveal the opening to the head gut, trachea and carotid arteries. A small section exposing the different layers of muscles ribs, oesophageal wall (with membrane containing sensory nerves) was exposed and a window of tissue was cut and reflected in the oesophageal wall to expose the lining of the oesophagus. A 18mm tube was inserted from this window to the mouth to support this area for plastination.
Clinical note: The wall of the upper oesophagus appears thin and easily damaged and retains its integrity through close association with the connective tissues of the surrounding muscles (and trachea), though apparently not damaged by ingestion of large prey it may prove an area prone to iatrogenic damage by the surgeon. The delicate nature of the upper oesophagus could also be due to degradation of specimen.
Caudal to this area (1/8 of the body length from the head) the oesophagus changes from bridging the entire width of the body to running to the left and dorsal of the trachea. Some connective tissue was reflected to expose the path of the oesophagus and around the thyroid to show it in situ lying ventrally across the trachea and oesophagus (1/4 from the head).
Clinical note: Caudal to the thyroid (3/8) the trachea and oesophagus are obscured by the fat pads of the cardiac region, the cranial 2/3 of these were removed to show the oesophagus running closely within connective tissue dorsal to the major cardiac vessels and trachea.
Bifurcation of trachea and association of oesophagus and left lung. Oesophagus runs along left side of liver with left lung. Level with the last third of the liver (1/2) is the division on oesophagus and stomach. The ventral fat pads moulded over the liver lungs and oesophagus, were removed. Caudal to the liver the fat pads also run centrally alongside the stomach and intestine, these were also removed. Removing the fat pads exposed protective pleural covering the abdominal organs and associated circulatory system. This connective membrane was carefully pared and removed exposing portal vein running within a secondary membrane (dorsal to the caudal liver). The membranes attaching stomach to spleen, pancreas and gall bladder were also removed.
Clinical note: there is a significant gap (approximately a third the length of the stomach) between the caudal end of the liver and the spleen and gall bladder. This area is filled with convulsions of vessels running to and from the organs, including the portal vein and distinctive (two vessels that run alongside each other in a separate pleura). The entire ventral surface of the spleen and part of the gall bladder and covered by a mass of vessels that would make surgical access difficult. Additionally, the spleen lies medial to the stomach and as with all these vessels is closely associated by connective tissue/ pleura.
The snake was gravid and the intestines had been displaced dorsally by the presence of multiple eggs. The mesentery of the small intestine closely is associated with the ovaries and more caudal with the oviduct and kidneys. The left ovary runs dorso-lateraly to the small intestine which runs more medially as it becomes the large intestine.
The ingesta in the boa’s large intestine though small meant the intestine ran medial to both pairs of kidneys and ovaries. The connective tissue was paired and the mesentery partially separated at its dorsal wall attachment to allow the oviducts and kidneys to be partially reflected. Parallel to the dorsal two thirds of the left kidney the snake had an area of intestine with a thicker more muscular appearance. This area appeared to contain ingesta. On palpation most of the area was soft with the most cranial part feeling firmer. A window was cut and reflected in this area revealing a mix of thick liquid and hair. Further investigation revealed a canine tooth of a small rodent.
Fat pads and connective tissue were removed down to the level of the vent, an 8mm tube was inserted into the vent to support the cloaca and demonstrated the merging of the intestine and oviducts just cranial to the vent.
If indigestion and loss of appetite is caused by impaction surgical intervention of impaction can be challenging. Rehydration and administration of lubricants and food may allow passage of some foreign bodies. However, in the case of complete obstruction, surgery to remove impactions and foreign bodies may be challenging for smaller patients. Small suture and careful tissue handling must be used to prevent stricture (Johnson 2012).
In many cases however, indigestion is caused by parasites (Veterinärmedizinische Universität, 2011). There are many parasites that can have adverse impacts on snakes, such as hookworms, roundworms and cryptosporidiosis to name a few. Given the brevity of this document, just a few will be mentioned. Infections of hookworms in the upper gastrointestinal tracts, can cause attachment wounds and can leave large mineral deposits, in turn causing impaction (Pets4homes 2016).
Regurgitation of partially digested food and little appetite can also indicate roundworms. Again, these worms leave mineral deposits in the reptiles gastrointestinal tracts which can be tender. If the wounds are left untreated, they can cause holes in the gut. Parasite burdens can be identified through analysis of droppings. Treatment is essential using prescribed products specific to the species (Johnson 2012).
Clinical signs of cryptosporidiosis in snakes may include anorexia, lethargy, intermittent or chronic regurgitation of undigested prey several days after feeding, chronic weight loss, and firm mid-body swelling caused by gastric hyperplasia (In Practice 2015).
We found the snake dissection challenging to compared to other available references due to the range of species differences. Particularly the spleen, stomach and gall bladder.
The dissection was a lengthy process and due to the tight association of the surrounding tissue to the organs made the organs appear to be very delicate. What was surprising was the thin tissue of the oesophagus but whether this was a post mortem artefact or would be delicate during an operation would remain to be seen.