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Haemonchosis in lambs one month post Ivermectin drenching in low rainfall South Australia

Jeremy Rogers, Senior Veterinary Officer, PIRSA, Bremer Rd, Murray Bridge, South Australia

Posted Flock & Herd July 2017


Haemonchosis is rare in the low rainfall areas of SA but the unusually wet spring and summer conditions of the 2016-17 spring and summer brought unusual conditions with them including unusual parasite problems. 

The owner of a well-managed property near Lameroo (mean annual rainfall 350 mm) was caught by an outbreak of haemonchosis that killed lambs rapidly, without signs of scouring. These lambs died one month after being drenched with ivermectin suggesting that drench resistance contributed to the problem.

Phone advice from a range of lay sources was misleading and diagnoses ranged from ARGT to PEM, but none mentioned internal parasites. Sheep dying from a range of conditions can present similarly and mistaken assumptions can lead to poor outcomes.


In mid-December 2016 a producer initially reported 10 deaths in a mob of 5-6 month old Suffolk ewe lambs. No deaths were reported in two other mobs. Five days late the producer reported that a further 7 lambs had died including some from another mob. Prior to the deaths all lambs appeared to be healthy and had been drenched with ivermectin and vaccinated with 6:1 one month previously and were weaned two weeks previously. The lambs were either found dead or were observed weak without struggling or neurological signs prior to death. There was some dry annual ryegrass in the paddock the lambs had been weaned into but a week later the lambs were moved onto a paddock without ryegrass and were supplemented with barley hay. 


A ram lamb and a ewe lamb were necropsied on the 22/12/16 with a tentative diagnosis of Annual Ryegrass Toxicity. A property visit was conducted on the 23/12/2016 when the sheep were inspected and an additional lamb was necropsied. A thick mat of immature Haemonchus  worms was observed in the abomasum.

haemonchosis south australia worms open abomasum
Figure 1. A section of abomasum showing numerous worms


Very high faecal work egg counts (up 10,400epg) were noted in all samples but larval cultures were not done. Profound anaemia was noted in blood samples. 

Histopathology revealed diffuse, chronic, moderate, lymphoplasmacytic and eosinophilic abomasitis with mucous neck cell hyperplasia/metaplasia and strongyle nematodes. There was also diffuse, chronic, moderate, lymphoplasmacytic and eosinophilic enteritis with villous blunting and intraluminal strongyle nematodes. 

The histological changes within the abomasum and intestinal tract are compatible with gastrointestinal parasitism (verminosis). Mucus neck cell hyperplasia and metaplasia of the abomasal glands is most often seen in association with ostertagiasis; however, mixed strongyle infections are common. In this case, the clinical history and presence of severe anaemia with hypoproteinaemia is compatible with haemonchosis.

The faecal egg counts (individual animal and bulk sample) were very high. Faecal strongyle PCR could be considered, if further quantification of the relative percentages of different strongyle species is desired. Note that (suspect) Mycoplasma ovis (Eperythrozoon ovis) organisms were identified on the blood smear attached to the erythrocytes, although these bodies could be due to smear artefact. This haemoparasite can be associated with haemolytic anaemia +/- jaundice and ill thrift or occasional mortality in sheep. Infected sheep suffering from poor nutrition or worm infestation are more likely to show reduced growth or ill thrift when infected with M. ovis. A blood sample has been submitted for PCR testing to look for further diagnostic confirmation. There were no microscopic changes within the brain to implicate ARGT; however, note that histological lesions can be subtle and minimal in many cases.

These lambs were clearly dying from GI parasites- Haemonchus or Barber’s Pole worms. No testing has demonstrated ARGT in samples of pasture or lambs.


The Lameroo/ Pinaroo area is low rainfall (about 250-300mm annually) and so internal parasite problems are not usually seen, except occasionally in winter and spring. In 2016 there was a lot more rain and the rain continued throughout Nov & Dec, when rainfall is usually low. As a result an ideal environment has arisen for outbreaks of parasitic and fungal conditions. 

Haemonchus is almost unknown in this region, but it is one of the few internal parasites that can be clearly seen in abomasum, produces very high faecal worm egg counts and specific rapid faecal test (occult blood sensitivity) is available for diagnosis.

This producer usually drenches with moxidectin but was unable to obtain it and so used ivermectin There is known resistance of Haemonchus to this drench in other states. The producer used a triple combination when advised of the diagnosis and  after treatment did not lose more lambs. In total 20-30 lambs died in this event.

There have been other documented cases of haemonchosis in the SA Mallee area in 2016/17 which is very unusual in this area.


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