The North West of NSW was severely affected by drought from 2017 until early 2020. Most livestock enterprises in the Walgett and Narrabri districts were offloading stock and full hand feeding by early 2018, and this continued with little respite until February 2020.
The 2020 drought-breaking rains were notable in having excellent follow-up rains for the following months. The pasture and climatic conditions produced by this break resulted in a rapid increase in livestock health problems. Several of the multi-case health conditions had presentations outside the author’s 30 years’ experience in the region.
As well as providing assistance to livestock managers, investigation of these livestock disease reports was prioritised to exclude the possibility of serious notifiable diseases, including bluetongue (the rains produced high midge numbers), anthrax (while it’s been decades since the last known case in the region, soil movement during the drought and short pasture increased the risk), and footrot (due to major re-stocking of sheep). These conditions were excluded in all relevant investigations.
Conditions seen with unique presentations included the following.
This was the most significant condition with the drought breaking. Nine properties reported cases, the first a week after the break, with the last case seven weeks later. There is a high likelihood of other cases.
Normally in these districts nitrate-nitrite poisoning is limited to hungry stock placed into yards containing nitrate-accumulating species growing on rains after a dry period. The plant most associated with poisoning is marshmallow (Malva parviflora). Losses can be large, often only halted after all the toxic plants have been consumed. Restocking after a drought into yards with thick Malva parviflora is the typical scenario.
The 2020 cases were distinguished by the initial cases occuring in yarded sheep and cattle released onto pasture. Also all cases were or likely to have been due to pigweed (Portulaca oleracea) despite other nitrate accumulators being in the unimproved pasture mix.
It’s assumed that the poisoning on pasture resulted from the drought, namely that three years in a fallow state was sufficient to increase soil nitrogen to similar levels found in yards with nitrogen from faeces. As pasture nitrogen declined with initial plant growth, cases were confined to animals transported and placed in yards or holding yards.
A major diagnostic learning was that nitrate/nitrite test strips could be successfully used at least 48 hours post-mortem on eye fluids.
In one case, 153 ewes died including 80 from one mob of 240 ewes. This followed yarding for 24 hours for crutching.
While the pasture contained pigweed (Portulaca oleracea), yellow vine (Tribulus micrococcus) and button grass (Dactyloctenium radulans), all known accumulators, the ewes had preferentially grazed the Portulaca. As many of the ewes had bloody discharge from the nares, anthrax was excluded by the use of the carcass side anthrax ICT kits.
In a second case, four of 165 weaner steers died after being released from drought confinement feeding onto a nearby small paddock for three hours. The steers were normal on returning to the yards. The cattle were not checked until two days later when the dead steers were found. Decomposition was consistent with death on the evening of the pasture grazing.
Carcass side testing 48 hours post-mortem excluded anthrax and confirmed nitrate/nitrite poisoning.
Most of the pasture consisted of summer grasses but inspection of the paddock found a tennis court-sized area of Portulaca that had been preferentially heavily grazed by the mob.
Orf virus/contagious pustular dermatitis
This normally presents in these districts as lesions on the mouth, muzzle and face of weaner sheep (hence scabby mouth) that are both nutritionally stressed and exposed to pasture (e.g. edible burr species) that causes mouth abrasions. Lesions on the lower leg are also occasionally seen in lambs grazing wet long green grass pasture. From the third to fifth month after the break, several cases of orf infection with very different presentations were investigated.
Most unusual of these presentations was in two flocks where adult ewes had severe lesions on the lower leg, with some that progressed to foot abscesses. The foot abscesses were caused by severe orf lesions on the coronet band, providing an entry point for foot abscess-causing bacteria. These cases also occurred without the usual long wet pasture conditions.
In the worst case, 550 three-year-old ewes presented 30 days after transport from Western Australia (the other flock was home-bred ewes). Fifty per cent had orf lesions of the lower leg and in five per cent of ewes this had progressed to foot abscesses. The odd ewe had mild mouth lesions. Local origin Border Leister rams running with the ewes were lesion free, as were ewes in the same Western Australian consignment that went to other local properties. Abscessed ewes responded well to a single treatment of oxytetracycline 20mg/kg, though many abscesses may have already begun to resolve.
Two flocks experienced very severe outbreaks of orf in weaner sheep that were neither nutritionally stressed or grazing abrasion-causing pasture. Both flocks had severe scabby mouth as well as a few lower leg lesions. In both, recovery was rapid—perhaps due to their excellent nutritional status.
Contagious ophthalmia of sheep in these districts is largely limited to sheep under 12 months old exposed to eye irritation from either long grass or dust.
Yet two to 13 weeks after the break, cases were reported in adult ewes from 10 flocks. In all flocks, the majority of adult sheep were affected. In some flocks, there were concerns that this might impact on the joining occurring at the time of the outbreak, but subsequent reported scanning and marking percentages were in line with district averages.
Four flocks were post-trucking but in six flocks there was no predisposing eye irritation (grazing short dust-free pasture). However, the district experienced very high bush fly numbers (Musca vetustissima) over this period (so severe that it also caused joining concerns and in one case, fly worry caused condition loss in a small weaner mob). In two cases the outbreak moved through the flock after neighbours introduced affected sheep to paddocks adjacent to the affected mobs.
Bovine ephemeral fever
Epidemics occur in these districts every five to 10 years when high late summer/autumn rains coincide with low herd immunity. Almost all herds are affected, there are high mosquito numbers and infection moves through the districts’ herds over a three to four-week period. In all these epidemics over the last 30 years, stringing drools from the nose and mouth has been the key clinical sign to differentiate from other causes of lameness and recumbency.
In contrast, the 2020 epidemic was prolonged over two months. It was also extended in individual herds, with one herd reporting six weeks from the first to final case. Mosquito numbers were relatively low but midges were high. Most herds had a low prevalence and mild disease but in a small number death was quick and followed nervous signs. Most notably, stringing drools were largely absent.
Dr SEAN SLATTERY
Dr Shaun Slattery is a LLS district veterinarian based at Narrabri since 1991. This industry-funded role has a strong focus on livestock disease surveillance as well as regulatory functions. In the 1990s, he worked on the cotton pesticide beef residue campaigns and at the same time turned an interest in internal parasites of sheep into membership of the ANZCVS in Sheep Medicine.
Following working on poultry EAD responses in the 1990s, Dr Slattery developed an interest in emergency management, spending three months with the 2001 UK Foot and Mouth Disease campaign, several stints as LCC investigations manager during the 2007-08 EI response and three months at the National Control Centre for the NZ M.bovis response in 2018.
With the RLPB to LHPA restructure, Dr Slattery was appointed as senior district veterinarian and widened his fieldwork area to include the Walgett district. With the Local Lands Service restructure in 2014, Dr Slattery left fieldwork for the manager biosecurity and emergency services position for the North West LLS.
In 2017 Dr Slattery returned to a field technical veterinary role as a district veterinarian. Dr Slattery is a past president of the District Veterinarian’s Association.