A controlled-release capsule was developed by CSIRO Pastoral Research Laboratory, Armidale, in the early 1980's to provide a continuous dose of nutritional supplement for sheep. The device has been further adapted for sheep and cattle, including for the controlled sustained release of trace minerals, anthelmintics and a bloat prophylactic.
For sheep, controlled-release capsules containing anthelmintics became available in 1990. They are capable of dispensing a regulated dose of anthelmintic for a period of 100 days. The first capsules contained albendazole. Subsequent capsules were loaded with ivermectin, then ivermectin and albendazole sequentially and more recently abamectin and albendazole in combination.
The first deaths were noticed about three days after capsule administration. Because capsules were being administered to heavily pregnant ewes, many of these early deaths resembled pregnancy toxaemia. Owners reported that ewes were standing away from the mob, some apparently blind and unaware of their surroundings, for about twenty-four hours prior to being found dead.
Deaths ceased on about the seventh day post-administration. Sheep dying later suffered the effects of starvation and water deprivation, or septicaemia. A small number of these longer-standing cases had signs typical of choke, with ptyalism, frequent swallowing and wads of masticated green grass in the mouth.
Palpation of the neck of affected sheep identified a firm, capsule-size mass in the caudal pharyngeal area, or a more extensive firm linear swelling involving cervical muscles and fascia extending between the caudal pharynx and thoracic inlet.
Reported losses varied from five of 2500 ewes treated, three of 800 treated, seven of 120 ewes, to one case involving more than 100 of 1200 ewes treated. Deaths were reported in both Merino and cross-bred mobs. There were no apparent differences in body size, body condition score, age or litter size, between affected sheep and unaffected sheep in any mob examined. No sub-clinical or chronic effects were noted in sheep in affected mobs; all affected sheep died within the time-frame described.
Post mortem examination invariably identified the anthelmintic capsule lying sagittally either immediately caudal to the pharynx, or as far distally as the mid-cervical area. In all cases, the capsule was located outside the oesophagus, with the exit point in the caudal pharynx. The integrity of the oesophagus otherwise remained unaffected. A variable amount of smelly ingesta surrounded the capsule. In cases dying soon after administration, cellulitis was localised especially where the capsule was located near the pharynx and was extensive with a yellow-green appearance within the fascial planes of the neck especially in cases which survived longer (figure 1). Most cadavers had evidence of dehydration, and frequently also changes consistent with pregnancy toxaemia.
Controlled-release anthelmintic capsules are favoured by some sheep breeders for worm control at lambing. On the southern tablelands of New South Wales, they are commonly administered about two weeks prior to lambing, to coincide with moving pregnant ewes onto saved pastures in lambing paddocks. In the absence of worm resistance to the chemical within, the capsule pays out to protect the ewe during the period of peri-parturient relaxation of immunity. Improved growth rates of lambs, increased wool production of ewes (Barger et al 1993), and reduced prevalence of scouring and dag formation (Larsen et al 1994) have been reported following capsule use. Retail prices (2012) for capsules range from about $2.70 each for the albendazole adult capsule, to about $3.80 for the albendazole-abamectin combination capsule.
Seasonal conditions and results of parasite monitoring encouraged increased usage of controlled-release anthelmintic capsules in ewes lambing in 2012. In that year, rural merchants estimated that more than 100,000 capsules were administered to sheep on the southern tablelands of New South Wales. A small number of first-time users of the capsules reported deaths following their administration. Both owner-operators and livestock contractors were involved with mobs which experienced losses.
Where post mortem examinations were conducted, capsules had exited the caudal pharynx. Some capsules had then moved to the mid-cervical area. This suggests two application problems may be involved. The first involves a capsule which is not delivered into the oesophagus, but is ejected from the probang in the caudal pharynx in such a position that the sheep is unable to expel it. Pressure from the capsule subsequently weakens the mucosa, allowing the capsule to penetrate the wall of the pharynx.
Other cases appear to be the result of misdirected forceful application of the capsule. During application, the capsule exits the caudal pharynx, then is forced a variable distance sometimes as far as a mid-cervical intermuscular location before ejection from the probang.
Controlled-release anthelmintic capsules are integral to worm control programs on some properties and a valuable aid to worm control in difficult seasons on others. Few problems occur where they are administered by experienced operators who follow recommended practice and who recognise that the process is more time-consuming than routine oral drenching. Maladministration can result in significant losses.