CASE NOTES
Acute and subacute aspiration pneumonia in merino ewes post immersion cage dipping
Alicia Moses, Central West Local Land Services, Grenfell NSW, Anne Jordan and Emily Winkenwerder, Elizabeth
Macarthur Agricultural Institute, Menangle NSW
Posted Flock and Herd November 2024
Introduction
Immersion cage dipping is an efficient and effective lice-control technique considered to be a physically less strenuous alternative to plunge dipping. Mobile cage dipping is offered by livestock contractors in New South Wales.
Aspiration or inhalation pneumonia is an inflammatory lung disease initiated by the inhalation of foreign material. Death from aspiration pneumonia may be acute and sudden in the case of large volume or toxic chemical inhalation, or protracted, taking days to weeks and involving secondary bacterial infections.
This is a case report where 57 out of 4200 merino ewes died between 12 hours and one week following a cage dipping event with suspected aspiration pneumonia.
History
On 27 June 2024 a Central West Local Land Services District Veterinarian was called regarding the sudden death of 14 merino ewes from a mob of 600. They were from a flock of 4200 mixed age ewes in multiple mobs that were five weeks from lambing. All sheep had been cage dipped over the previous 2-3 days by a contractor.
The contractor's dipping protocol was either a single or double immersion for up to ten seconds. The dip solution was thiacloprid (480g/L; Piranha™, Elanco), 1% zinc sulphate and chlorhexidine (4.5g/L; Hibitane, Coopers), and the dip sump had been topped up immediately prior to this mob entering.
The mob of 600 ewes had been dipped the previous day in the early afternoon. They had been shorn 1-2 weeks prior and were put into a new paddock of short clover pasture following dip treatment. The last clostridial vaccination was six months previous and they were to be drenched the following day. There were no other recent husbandry or management activities and no reports of coughing or other symptoms whilst moving to the paddock following dipping.
On 28 June 2024 another ewe from the affected mob (Ewe 3) collapsed in severe respiratory distress with rapid progression to death whilst being yarded for drenching. Another 6-10 ewes from other mobs had died overnight - these ewes had been dipped 2-3 days prior. Deaths continued for 5-6 days with a total loss of 57 ewes.
Clinical findings
On 27 June 2024 two ewes (Ewe 1 and Ewe 2) were selected for post-mortem examination - they had been dead approximately 12-18 hours.
Ewe 1
An Anthrax ICT test was negative. She was in excellent body condition with a singleton pregnancy. There was a moderate quantity of straw-coloured fluid in the thoracic cavity, stable foam in the trachea and a small quantity of thick yellow mucus in the trachea lumen extending into the bronchi. The cranioventral lung lobes were symmetrically darker and heavier than the remaining lung parenchyma - approximately 15-20% of the tissue was affected. The kidneys were moderately dark with reduced distinction between cortex and medulla. The abomasal mucosa was diffusely dark red.
Ewe 2
This ewe was in good body condition with a twin pregnancy. The lungs were similar to Ewe 1. There were no other significant findings.
On 28 June 2024 one ewe was selected for post-mortem examination (Ewe 3).
Ewe 3
This ewe was necropsied within an hour of death. She was in good body condition with a singleton pregnancy. There was a moderate quantity of straw-coloured fluid in the thoracic cavity and symmetrical cranioventral lung consolidation involving approximately 45% of the left lung lobe and 30% of the right lung lobe and accessory lobe (Figure 1). The affected areas were dark red, firm, and sank in water. The remaining lung tissue was heavier than usual and oozed blood from the cut surface. The trachea had a moderate amount of blood-tinged foam extending into the bronchi. The rest of the examination was unremarkable.
Figure 1. Lateral view of Ewe 3 showing the dark red consolidated cranioventral lung
Laboratory results
Zinc, ammonia, calcium and nitrate levels in aqueous humour were within reference ranges. Bluetongue Virus PCR on lung was negative. Bacterial culture of the lung of Ewe 1 produced a sparse mixed, but not significant, growth. Ewe 3 had a pure growth of Carnobacterium maltaromaticum.
On histopathology of lung tissue from Ewe 1 there was a moderate, acute, necrosuppurative and fibrinous bronchopneumonia with interstitial, alveolar and perivascular oedema and congestion. There was rare foreign material within airways (Figure 2). Within the liver, there were occasional small foci of acute hepatocellular degeneration and necrosis and within the abomasum, a moderate, suppurative and lymphoplasmacytic abomasitis with congestion.
Figure 2. Ewe 1. Alveoli were multifocally filled with eosinophilic fluid (oedema), degenerate neutrophils and fibrin, with rare foreign material (asterisk). Blood vessels were congested (Lung, H&E, x10)
Ewe 3 had subacute changes within the lungs characterised by a fibrinosuppurative and necrotising bronchopneumonia with alveolitis, type II pneumocyte hyperplasia, alveolar histiocytosis, necrotising bronchial and bronchiolitis and multifocal bronchial and bronchiolar epithelial hyperplasia. In addition, vessels multifocally had a necrotising vasculitis and thrombi (Figure 3) and foreign material observed occasionally in the airways (Figure 4).
Figure 3. Ewe 3. Within bronchioles there was frequent loss of epithelium replaced by cellular and nuclear debris, oedema, necrotic debris and degenerate neutrophils, with some bronchioles lined by attenuated epithelial cells. Within alveoli and the interlobular septae there was abundant oedema and multifocal degenerate neutrophils. Blood vessels were multifocally occluded with thrombi (Lung, H&E, x4)
Figure 4. Ewe 3. Occasionally within bronchioles there were small fragments of brown-to-grey, refractile foreign material, with frequent degenerate neutrophils and necrotic cell debris and loss of bronchiolar epithelium (Lung, H&E, x20)
Results interpretation
Possible causes of the pneumonia in these ewes included inhaled irritants, viruses or bacterial infection. The bacteria Carnobacterium maltaromaticum isolated from Ewe 3 is found in the environment and is considered non-pathogenic in mammals, whilst no significant bacteria were isolated from Ewe 1 making primary bacterial causes of the pneumonia highly unlikely.
Given the history of a recent dipping event, the common pattern of symmetrical cranioventral lung consolidation typical of aspiration pneumonia, occasional foreign material within airways and exclusion of other causes of the bronchopneumonia, inhalation of dip during immersion was considered the most likely cause of the mortality outbreak.
Discussion
Cage dipping involves sheep entering a mobile hydraulic cage with a platform that lowers into the dip sump. Contractors report they can dip up to 5000 sheep a day using the machine. The LiceBoss website makes the following statement about immersion times:
"Operator procedures vary a little. Some will immerse the sheep such that their backs are covered by solution, then raise the cage, before again completely submerging the cage momentarily (i.e. they are dunked twice). Others will simply submerge the cage for about 10 seconds (to achieve the equivalent of two short dunkings) before raising the cage and releasing the sheep." (Levot 2013)
Risk factors for inhalation of dip would be similar to those of plunge dipping, which include physical exhaustion, heat stress, dehydration, not drafting sheep to size and any disease or an ailment (e.g. lameness) (Joshua 2011). Inappropriately long immersion times would be another significant risk factor.
In this case the ewes were reported to have been physically well with no lameness or other signs of disease and were dipped in cool to mild weather. Iatrogenic 'drowning' from submerging for longer than the reported ten seconds cannot be ruled out, however seems unlikely given the contractor's expertise.
A common factor is that the affected animals were late-term pregnant ewes. It is generally contraindicated to dip 'heavily pregnant' ewes, considered to be within the last four to six weeks of pregnancy, due to the potential effects of stress on the ewe and foetus. These ewes were at least five weeks off lambing, however, the physiological effects of pregnancy on the respiratory system at this stage could still be considered significant. Increased circulating progesterone during pregnancy increases resting minute ventilation, tidal volume and oxygen consumption (LoMauro & Aliverti 2015). The growing uterus also places physical pressure on the diaphragm, resulting in reduced thoracic volume and functional residual capacity, which in turn increases the risk of atelectasis and reduces oxygen reserve (LoMauro & Aliverti 2015; Hopkins & Sharma 2018). These physiological changes combined with stress could have impacted respiratory function of the ewes and increased the risk of inhalation. Other risk factors such as smothering and underlying disease cannot be completely ruled out.
An interesting feature of this case is the sudden death of ewes from one single mob within hours of the dipping event, and a delayed onset of mortalities in other mobs. Possible explanations may include more significant inhalation of fluid and irritants from a longer immersion time or toxicity from an increased concentration of inhaled chemicals from the topped-up dip sump.
Thiacloprid is a neonicotinoid insecticide used as a lousicide in backline and dipping formulations. It has a moderate toxicity profile in mammals with the liver being the target organ. Studies in rats have a reported LD50 as >2000mg/kg for dermal exposure, 225mg/kg for oral exposure and 1223mg/m3 for inhalation of vapour (Elanco 2022). The study of vapour rather than direct inhalation of fluid may affect the accuracy of this value and may underestimate the level of toxicity given the high absorptive ability of the lungs. However, it is highly unlikely that thiacloprid toxicity was responsible for the acute deaths given that the histopathologic liver changes were mild.
Chlorhexidine is a known respiratory irritant in both humans and animals. In humans, cases of acute respiratory distress syndrome (ARDS) have occurred with both the inhalation and intravenous administration of chlorhexidine. A study in rats demonstrated severe acute pulmonary inflammation in response to an instillation of 0.2% chlorhexidine into the lungs and less severe pulmonary changes with 0.02%. The study mentioned preliminary tests using 0.5% and 0.3% solutions caused severe dyspnoea and rapid death of rats following exposure (Xue et al 2011). Commercial chlorhexidine products recommend a rate of approximately 0.0009% chlorhexidine in dipping solutions as a bactericide.
Zinc sulphate is also reported to cause airway and gastrointestinal irritation on exposure (Cooper 2008). Toxicity by zinc absorption was ruled out in both Ewe 1 and Ewe 2.
No post-mortem examinations were done on ewes from other mobs that died days following dipping. It would have been interesting to determine whether secondary infections from aspiration became involved and contributed to disease in these ewes.
Conclusion
Cage immersion dipping, though potentially less strenuous on the sheep than plunge dipping, still involves a risk of inhalation of fluid. The importance of immersion times and other factors that increase the risk of inhalation should be highlighted to contractors and producers. The pregnancy status of sheep may increase the risk of adverse outcomes and increased care when dipping these animals should be exercised.
References
- Levot G (2013) Plunge and cage dipping LiceBoss liceboss.com.au Accessed August 2024
- Joshua E (2011) Sheep plunge dip management NSW DPI primefact 1141 first edition
- LoMauro A and Aliverti A (2015) Respiratory physiology of pregnancy: physiology masterclass Breathe 11(4):297-301
- Hopkins E and Sharma S (2018) Physiology, functional residual capacity researchgate.net
- Elanco (2022) Pirahna dip for sheep Safety Data Sheet
- Xue Y, Zhang S, Yang Y, Lu M, Wang Y, Zhang T, Tang M and Takeshita H (2011) Acute pulmonary toxic effects of chlorhexidine (CHX) following an intratracheal instillation in rats Human & experimental toxicology 30(11):1795-1803
- Cooper RG (2008) Zinc toxicology following particulate inhalation Indian journal of occupational and environmental medicine 12(1):10-13