CASE NOTES
An Outbreak of Haemonchosis in Ewes on Pastures Previously Grazed by Cattle
Bruce Watt, Central Tablelands Local Land Services, Bathurst, Garry Telford, 'Glendower', Vittoria and Brown
Besier, Brown Besier Parasitology, Albany WA
Posted Flock and Herd June 2024
Introduction
Where there is little cropping, sheep-cattle pasture interchanges are potentially the best option for creating safe paddocks for sheep. Where Haemonchus in sheep is not an issue, there are few problems with such pasture changes if grazing intervals are sufficiently long. Trichostrongylus axei is a risk. However, counts of Trichostrongylus species (including T. axei) are low in cattle.
History
In late April-early May 2022 an outbreak of haemonchosis occurred in a mob of 808 merino ewes despite them being drenched on 22 March 2022 with 10 mls of derquantel plus abamectin (Startect, Zoetis containing derquantel 10 mg/ml plus abamectin 1 mg/ml) onto pastures that had almost entirely run cattle since 2 August 2021, eight months previously.
Property and livestock management
One of the authors (GT) had managed this 1420 ha mixed grazing property on the Central Tablelands of NSW property since 1984 and during this time had grazed cattle on sheep pastures and sheep on cattle pastures over the summer and early autumn, in part for internal parasite control. The property had improved pastures, mainly cocksfoot (Dactylus glomerata) and phalaris (Phalaris aquatica), on granite-derived soils.
The property had recently run 2740 merino ewes joined to either Merino or white Suffolk rams. The same paddocks were used for lambing each year. In early December ewes and weaned lambs were drenched onto pastures previously grazed by cattle. Ewes were joined on 5 March for five weeks, scanned in June and treated with an effective drench and vaccinated with a clostridial and caseous lymphadenitis vaccine pre-lambing before being moved onto these lambing paddocks. Lambing usually commenced on 14 August and the lambs were marked in late September-early October. At marking, lambs were ear tagged, vaccinated with 6 in 1 plus selenium, treated on the breech for fly prevention (most recently using dicyclanil) and the ewe portion were vaccinated against ovine Johne's disease (Gudair, Pfizer). The lambs had not been mulesed for the previous five years. A worm egg count (WEC) was conducted prior to lamb marking and, in some years, ewes were drenched at lamb marking due to a high Haemonchus WEC.
After marking, the ewes and lambs were amalgamated into bigger mobs and rotationally grazed (in a four-paddock 1-2-week duration rotation) and then moved onto pastures grazed by cattle after weaning. Cattle were introduced onto pastures previously rotationally grazed by sheep on about 5 November, the usual joining time for the cattle.
The property carried 450 Angus cows joined on 5 November for six weeks (heifers) to eight weeks (cows). Calves were weaned from January to April depending on the season and steers sold at approximately 500 kg and 18 months of age. In July 2021 the cows were injected with ivermectin and chlorsulon (Vetmec F, containing 10 mg/ml ivermectin and 100 mg/ml of chlorsulon) and the calves were treated with an injectable macrocyclic lactone at weaning.
History of the affected mob
The ewes in the affected mob of 808 mixed-age Merinos were drenched with 10 ml of derquantel plus abamectin (Startect) on 22 March 2022 onto a paddock that had run either cows and calves or dry cattle since 11 August 2021 (earlier than usual as there was an abundance of feed and a reduced stocking rate, post the drought). On 13 April 2022 the ewes were moved to another paddock that had run either cows and calves or dry cows since 2 August 2021 (apart from wether lambs that were drenched onto the paddock with derquantel plus abamectin and grazed there with for six days, 24-30 April). On 21 April, as the ewes were moved to another paddock, four ewes fell to the back of the mob. These ewes were noted to be weak and anaemic, and one had marked submandibular oedema. The four ewes all survived after being drenched with a combination of abamectin, closantel, albendazole and levamisole.
On 26 April, faecal samples from this mob were submitted to the laboratory revealing an average WEC of 2736 epg, composing 98% Haemonchus. Four more ewes were found dead in the paddock. On 6 May 2022 the ewes were mustered for drenching with monepantel plus abamectin (‘Zolvix Plus'). Again, four ewes were noticed to be anaemic with submandibular oedema. Two of these ewes died but no deaths occurred subsequently.
Laboratory findings
| Mob | N | Drenched Startect | Lab | WEC | Haem % | Trich % | Oster % | Paddock history |
| Autumn wethers | 270 | 22 Mar | 06274 | 368 | 59 | 38 | 3 | crop |
| Wool ewes | 1193 | 18 Mar | 06271 | 156 | Spelled since Oct, then C&C | |||
| Yellow ewes | 808 | 22 Mar | 06285 | 2736 | 98 | 2 | Weaners until Nov then cattle | |
| Ewes to XBs | 789 | 18 Mar | 06266 | 432 | 54 | 14 | 32 | Spelled since Oct, then C&C |
Table 1. WEC results from four mobs, samples collected 26 April 2022
Discussion
We suggest three possibilities as to why the ewes developed clinical haemonchosis in just over three weeks since they were drenched onto the cattle paddock. The first is that the ewes carried a significant burden prior to drenching and that the drench was ineffective. The second is that infective L3 larvae derived from sheep previously grazing the pasture survived for eight months. The third is that the ewes acquired the infection following pasture contamination with H. contortus by cattle.
Unless a significant number of Haemonchus survived the derquantel plus abamectin treatment (considered most unlikely as this combination had been used sparingly with no indication of resistance), we presume that the ewes picked up their burdens in just over three weeks following the 22 March 2022 drench into the paddock grazed by cattle. The paddock they were moved to on 13 April 2022 would not have been involved, as they were there for only a week, and signs appeared a few days later.
It is unlikely that the Haemonchus infections originated from previous pasture contamination by sheep. While some long-term survival may have been possible, significant numbers of Haemonchus L3s would not be expected to survive for eight months, especially over summer, as sheep last grazed this paddock in August 2021.
It is therefore most likely that the young cattle cycled enough H. contortus to contaminate the pasture and infect the ewes. Relatively large numbers of Haemonchus L3s would need to have developed for the ewes to pick up lethal burdens in a short time, as with a 17-day pre-patent period, the adult Haemonchus (capable of blood sucking) present on 22 April 2022 would have been acquired over about 10 days. This timeline suggests that the cattle cycled significant populations of H. contortus.
In our experience young cattle often have high burdens of Haemonchus. We also understand that cattle older than about 15-18 months are relatively refractory to infection with H. contortus and even to H. placei. Although the Haemonchus were not identified to species in this investigation, it was assumed that they were H. contortus. Although young sheep can be infected with H. placei, outbreaks of haemonchosis in sheep due to this species have not been reported and are especially unlikely in mature ewes.