South African angora goats produce some of the finest mohair in the world (Goosen et al, 2010) and have therefore been exported widely including to Australia. Unfortunately, this selection for high quality fleeces has been linked to traits of reduced fitness, including a susceptibility to ventral oedema, known colloquially as 'water belly' or 'swelling disease'. (Thompson 1994, Byrne 1994). This condition was first reported in South African Angora in the 1970's (Snyman and Snyman 2006) and later in the Texan Angora (which was developed from the South African). The condition is rare in Australian Angoras (which descended from the original Turkish imports) (Thompson 1994). The oedema is often severe with massive subcutaneous swelling along the ventral abdomen and brisket, often extending up the neck to the submandibular region and down the legs (Thompson, 1994). Mortality may occur in severe cases (Snyman and Snyman 2006).
Ventral oedema may be linked to a congenital hypoadrenocorticism in South African Angora. Both conditions are considered inadvertent consequences of many years of selection for superior mohair production and fine fleece. These animals have a reduced adrenal function with decreased cortisol production and glucogenesis in response to stress (Engelbrecht and Swart 2000). Ventral oedema may therefore be a manifestation of stress in this breed.
On the 22nd of April 2013, 42 recently weaned October-November 2012 drop Angora goat kids were examined after most suddenly developed ventral oedema. Four days previously, the kids were shorn and treated with an anthelmintic.
The kids had been weaned three weeks previously, vaccinated with 5:1 clostridial vaccine, drenched and run on pastures dominated by weeping grass (Microlaena stipiodes), red grass (Bothriochloa macra), plantain (Plantago lanceolata), red root amaranth (Amaranthus retroflexus) and small patches of Paterson's curse (Echium plantagineum). Some clovers were seen but these were of a very low percentage of the pasture.
Approximately 60% of goats were affected with mild to marked ventral oedema. All were bright and alert and appeared to be untroubled by the oedema.
Three affected goats with marked, ventral pitting oedema were clinically examined and blood sampled. All has moist, pink mucous membranes with normal capillary refill and body temperatures of between 39.7 and 39.9 degrees C. One kid had oedema extending down the forelegs to the stifle and elbow joints. An arrhythmia was ausculated in this animal. The other two kids had normal heart and lung sounds.
Biochemistry: The blood clinical pathology results were unremarkable except for a marked hyperkalaemia in the 3 kids. The potassium ranged from 7.9 to 9.2 mmol/L (reference range; 3.5-6.7 mmol/L). Albumin and plasma levels were within normal range. Sodium was at the lower end of normal range. There was a mild decrease in vitamin B12 in two of the three goats.
Parasitology: Five fresh faecal samples were collected from the yards for a faecal egg count, larval differentiation and a fluke egg count. The worm egg count averaged 216 epg, with 100% Haemonchus contortus identified on larval differentiation. The fluke egg count was negative.
Ventral oedema or swelling disease in Angora goats is characterised by the sudden onset of severe oedema of the ventral neck, chest, abdomen, legs and sometimes the lower jaw (Bath and Vermeulen 1988; Snyman and Herselman, 2004). The swelling is characteristically cold, pitted, and painless and varies over different parts of the body (Synman & Synman, 2005). Affected goats are normally bright and alert, showing little to no discomfort, despite sometimes having quite marked oedema. This disease occurs in angoras with South African ancestry and is found in Angora populations in Australia, New Zealand and the United States (Thompson, 1994).
Swelling disease is most commonly seen immediately after shearing when a significant proportion of the flock may be affected to varying degrees (Thompson, 1994). However, ventral oedema is also associated with other high stress conditions including internal parasites, other diseases, inclement weather and management procedures such as weaning, castration, drenching or dipping (Bath and Vermeulen 1988; Snyman and Synman, 2005). In some cases, ventral oedema occurred without an obvious stressor (Thompson, 1994).
Oedema can occur in goats with hypoproteinaemia due to internal parasitism, Johne's disease, chronic liver disease and other diseases. However, hypoproteinaemia is not a feature of swelling disease. (Thompson, 1994). This disease should also be differentiated from the ventral oedema caused by cardiac insufficiency, including congestive heart failure due to congenital cardiac anomalies or subsequent to acute myocardial nutritional muscular dystrophy (Thompson, 1994).
It has been proposed that ventral oedema may be associated with congenital hypoadrenocorticism. The same reduced physiological response to ACTH by the adrenal cortex has also been reported to result in significant number of mortalities of young goats during cold, wet weather particularly after shearing (Storbeck et al, 2009).
In a normal response to stress, the anterior pituitary gland, under the influence of corticotrophin-releasing factor, releases ACTH, stimulating the adrenal cortex to secrete both glucocorticoids and mineralocorticoids. In goats with congenital hypoadrenocorticism, the synthesis of cortisol (a glucocorticoid) is reduced due to the bioactivity of two cytochrome P450 17 alpha- hydroxylase/17,20 lyase (CYP17) isoforms identified in the South African Angora). The secretion of glucocorticoids by the adrenal cortex is a central feature of the stress response in mammals and a functional hypothalamic-pituitary-adrenocortical axis is essential for glucose homeostasis (Storbeck et al, 2009). The reduced enzymatic activity of CYP17 isoforms in these animals does not allow them to maintain normal blood glucose levels under stressful conditions (Storbeck et al, 2009). Without adequate blood glucose levels, especially in cold conditions, the goats experience dangerous drops in body temperature. Without glucose as an energy source, the goats cannot thermoregulate effectively, resulting in significant mortalities.
Aldosterone, the main mineralocorticoid, increases sodium absorption by epithelial cells in the kidneys, salivary glands, sweat glands and the gastrointestinal tract. A reduction in the secretion of aldosterone results in marked alterations of serum levels of potassium, sodium and chloride. Potassium secretion by the kidneys is reduced resulting in a progressive increase in serum potassium (Bruyette, 2011). Hypernatraemia and hypochloraemia also result from renal tubular losses. Severe hyperkalaemia may result in bradycardia, an irregular heart rate (ventricular fibrillation) with alterations in ECG (Bruyette, 2011). In severe cases, it can cause asystole and ultimately death. The arrhythmia seen in the goat kid examined may have been due to hyperkalaemia.
In the absence of concurrent disease, most goats with ventral oedema recover spontaneously and treatment is considered unnecessary (Thompson, 1994). However, some goats have been treated with diuretics such as furosemide.