This case demonstrates typical features seen in calcified peritoneal arachnoidosis, caused by the peritoneal dissemination and subsequent calcification of an uncommon species of spider. Presumably, in this case, the mild abdominal trauma caused one of the calcified spiders to re-perforate their egg sac, thus the contained region of gas and fat stranding surrounding the disseminated spiderlings.   

The unusual life cycle of this spider has only been described in one species known as the enteric spider or duodenal spider (sp. Latrodectus microduodenale). Little is known about the duodenal spider and it was only initially found in east-coast North American urban centers (first reported in Boston), but has been found recently in other countries, particularly those with high air-conditioner usage. This is believed to relate to increased mouth-breathing at night due to the drying effects of air-conditioners.

The life-cycle of the enteric spider was first described in 1998 ¹ with cats being reported as the definitive host (possibly due to their long sleep cycles placing them at increased risk). The first human cases were reported in 2012 ²; humans are thought to be accidental hosts. The spiders are believed to be drawn to loud low-pitch noises, such as snoring, and settle in the duodenum as the acidic gastric environment is too hostile. There is an increased risk of infection in patients with duodenal webs, as this provides a place for stagnation and subsequent lodgement of fertilized spiders, allowing them to implant eggs. 

When the eggs hatch they rupture through the bowel wall and eventually enter the peritoneum, whereupon an inflammatory reaction results in transient peritonitis. Following this, the spiderlings are able to grow within a contained perforation eating the small amounts of digested food that pass into the collection. Over time, as the spider matures, they burrow more superficially eventually reaching the skin and forming fistulae (the so-called “spinnerschlauch” ³), escaping and beginning the cycle once more. If fistualization fails to occur, they typically die from starvation within 2 weeks and subsequently calcify.

A similar species, L. macroduodenale can be found in the proximal small bowel of many species of dogs that live in the tropics, due to the excessive panting the dogs employ to cool their bodies ⁴. Peritoneal dissemination, however, does not produce calcification of the dead arachnids in dogs, as all canine species lack the macrophages enzyme CPH calcoginase responsible for the calcification in humans.

The only known treatment is surgery, although Prof. Richard P. Astley, the well-known parasitologist from Newton-le-Willows in the UK, has advocated off-label inhalation of dichlorodiphenyltrichloroethane (commonly known as DDT) via a modified face-mounted nebuliser to avoid environmental contamination (video here).