LAUSR

Hydrops fetalis, or fetal ascites, is the accumulation of fluid within the peritoneum of the fetus prior to birth and when present in full-term calves can result in dystocia (Pandit and Singh 1990; Katiyar et al. 2016). The incidence of dystocia due to fetal ascites is likely to be low. In three retrospective reviews of dystocia in cattle, only 1/156, 2/236, and 1/560 dystocia cases were attributed to fetal ascites (Purohit and Mehta 2006; Sarkar et al. 2014; Stanciu et al. 2015, respectively). The ascites observed in hydrops fetalis can be due to either an overproduction of peritoneal fluid, or a lack of sufficient drainage of this fluid, and may occur as a result of a range of congenital abnormalities (Ravikumar et al. 2013). One such abnormality is the formation of congenital mesotheliomas in the fetal abdomen (Katiyar et al. 2016). Worldwide, reports of congenital mesotheliomas as a cause of bovine dystocia are very rare. A literature search conducted using the search terms “cattle AND mesothelioma AND dystocia” in Web of Science on 6 May 2021, retrieved only three peer-reviewed papers (Drieux et al. 1949; Charan et al. 1974; Tammen et al. 1994). A further two cases were described in Baskerville (1967). Mesotheliomas are tumours of the mesothelium and associated connective tissue of mesodermal origin, which form the lining of the thorax (pleura), abdomen (peritoneum) and viscera (Peli et al. 2018). In cattle, mesotheliomas most often occur in the peritoneal cavity, though can be seen elsewhere, including in the pericardium (Morita et al. 2019). Mesotheliomas may occur in animals of any age but in cattle, they are most commonly seen as congenital neoplasms in fetal or young calves (Parkinson et al. 2019). The ascites caused by these lesions, which is typically marked in young animals, is a common clinical sign in cattle with this disease and tends to be caused by a serosanguinous fluid produced by the neoplastic mesothelial cells (Milne et al. 2001; Morita et al. 2019). In humans, mesotheliomas are most commonly associated with the inhalation of asbestos particles (Selikoff et al. 1965), while in animals, the pathogenesis and aetiology of these tumours are not well understood, with inconsistencies in the current literature (Peli et al. 2018). This description of a case of bovine dystocia secondary to congenital mesothelioma in a calf is, to the authors’ knowledge, the first reported example in New Zealand. In August of 2020, a mature age, mixed breed dairy cow in the Eltham district of New Zealand, was examined because of calving difficulty. On arrival, it was reported that attempts had been made by farm personnel to assist the calving, with no progress noted for the last 20–30 minutes. The cow was bright and alert, with no signs of significant distress. The calf was partially exteriorised, with the forelimbs and head protruding from the vulva in a normal anterior presentation. On vaginal examination, it was found that the calf was alive and appeared to be breathing, and on palpation was noted to have a distended abdomen, suggestive of fetal ascites. The calf was euthanised via injection of an unspecified quantity of 500 g/L pentobarbitone sodium (Pentobarb 500; Provet, Auckland, NZ) directly into the tongue. Following euthanasia, the head of the calf was removed, and its abdomen was incised within the uterus with a finger knife. A piece of polythene pipe was then inserted through the vagina into the uterus to assist in removing the fluid and to allow sufficient space for the calf to be removed. The pipe was then passed into the abdomen of the calf via the oesophagus to further aid in the removal of the fluid. Once sufficient fluid had been removed from the abdomen of the calf and uterus of the cow, calving chains were placed on the forelimbs of the calf, and it was removed using controlled traction via a calving pulley, assisted by the uterine contractions of the cow. After removal from the dam’s uterus, an incision was made into the abdomen of the calf, causing exteriorisation of the remaining fluid along with multiple freefloating masses within the abdomen. Masses were variably soft to very firm, pink to white in colour, irregular ovoid to multinodular in shape and ranged in size from approximately 5–70-mm diameter (Figure 1). Upon further inspection of the calf’s abdominal cavity, numerous (>100) similar masses were present both free within the abdomen as well as adhered to and diffusely scattered across the abdominal viscera and peritoneum. A full necropsy was not performed, but the calf’s intestinal contents were noted to be more firm and more green than would be expected in a fetus. No further gross abnormalities were seen on the remainder of the limited post-mortem examination. For histopathologic examination, various tissues including samples of the free and peritoneal masses