CASE NOTES

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Fatal Haemonchosis in Beef Heifers on the North Coast of NSW

Matthew Ball, Senior Technical Services Manager, Virbac Australia

Posted Flock & Herd March 2023

Introduction

Outbreaks of severe clinical haemonchosis are less expected in cattle than sheep but are reported each year during the warmer and wetter months in the higher rainfall areas of coastal New South Wales, Queensland, Northern Territory and in the Kimberley region of Western Australia. Haemonchosis can also cause unexpected problems for cattle in more temperate zones, especially after unseasonal spring or summer rainfall (Beveridge and Emery, 2014). Clinical cases of cattle haemonchosis may become more common due to the widespread drench resistance to single active anthelmintics (Ball and Gibbison, 2021). This case study outlines the clinical and laboratory findings from a heifer with severe haemonchosis on a farm that had only been using single active anthelmintics.

History

In May 2022 a beef farmer at Meerschaum Vale (North Coast NSW) sought advice on choosing a liver fluke drench. The farmer was concerned that liver fluke was the cause of two recent deaths and poor growth in a group of 15 mixed-breed 10-month-old beef heifers. Since March 2022 the region had experienced severe flooding and the farmer believed that the ongoing wet conditions were predisposing his cattle to liver fluke. The cattle had last been treated for parasites four months earlier at weaning using an abamectin/triclabendazole pour on. The farmer sought advice on whether he should use the same product or rotate to something else. He was also wondering if there were other treatments that could be considered for a heifer that was now in sternal recumbency (Figure 1).

Figure 1: 10-month-old beef heifer unable to stand and in poor body condition

FARM AND CLINICAL EXAMINATION

Cattle were on naturalised subtropical pastures. Due to recent floods, water was still pooling in low lying areas. The two heifers that had died previously were no longer available for examination. Twelve heifers were inspected in the paddock. They were small for their age and in low body condition. One was observed to have submandibular oedema. The heifer in sternal recumbency and unable to stand was examined and found to have:

• Sunken eyeballs and pale-dry mucous membranes (Figures 2 and 3)
• Mild ventral neck oedema
• Poor body condition
• Distended abdomen
• Dark firm faeces (Figure 4)
• Temperature 36.1 degrees Celsius
• Heart rate 80 beats per minute
• Inability to stay standing when lifted to feet

Blood and faecal samples were collected, and a decision was made for captive bolt euthanasia and necropsy due to poor prognosis.

Figures 2 and 3: Pale mucous membranes in a ten-month-old beef heifer

Figure 4: Dark firm manure from the recumbent ten-month-old beef heifer

Necropsy findings

Gross findings of necropsy examination included minimal fat deposits, pale lungs, pale liver and clear ascitic fluid (Figure 5). Otherwise, no significant gross abnormalities were identified. There was no evidence of liver fluke, either adults in bile ducts or migration tracts of immature fluke.

Figure 5: Gross necropsy left side recumbent in a 10-month-old beef heifer

The gastrointestinal tract was opened and examined. The only significant finding was a thick mat of Haemonchus in the abomasum (Figure 6).

Figure 6: Haemonchus nematodes in the abomasum of a 10-month-old beef heifer

Laboratory Findings

Samples were not collected for histopathology. Worm Egg Count (WEC) testing, biochemistry and haematology was undertaken by QML Pathology and funded by Virbac Australia. The McMaster Egg Count was 3650 strongyle eggs per gram (epg). The larval differentiation (Figure 7) was:

• Haemonchus spp 57%
• Cooperia spp 15%
• Trichostrongylus spp 14%
• Oesophagostomum spp 11%
• Ostertagia spp 3%

When the WEC results were entered into a worm calculator (Virbac, 2014) Ostertagia and Trichostrongylus were estimated to dominate the worm burden (Figure 8).

Figure 7: Larval differentiation results

Figure 8: Estimated proportion of adult worm species living inside heifer

Haematology (Figure 9) identified severe anaemia with a haematocrit (packed cell volume) of only 10% (normal 25-45%) and a red cell count of 2.1 (normal 5-7). There was a mild mature neutrophilia that was attributed to a likely stress response or inflammation. Pathologist interpretation commented that the anaemia was microcytic, marginally hypochromic and either non-regenerative or pre-regenerative. This finding is consistent with chronic blood loss.

Figure 9: Haematology results for a 10-month-old recumbent beef heifer

Biochemistry (Figure 10) demonstrated severe hypoproteinaemia of 33g/L (normal 58-77) with both globulin and albumin low. Both CK and AST were high, which was attributed to the extended period of recumbency. Fibrinogen 4.44g/L (2.00-6.00), serum glutamate dehydrogenase 21u/L (14-141) and serum B-hydroxybutyrate 0.5mmol/L (0.2-1.0) were all normal.

Figure Ten: Biochemistry Results for 10month old- recumbent beef heifer

Treatment

All heifers were treated with a combination moxidectin/levamisole drench (Cydectin Platinum, Virbac Australia) and the owner was advised to commence supplementary feeding. As of six weeks after treatment there had been no further deaths or illness.

Discussion

The clinical history, worm egg count and laboratory findings are consistent with haemonchosis as the primary problem in the recumbent heifer and likely the other affected heifers. This case study highlights the severe anaemia and hypoproteinaemia that haemonchosis can cause in post-weaned calves. In this heifer, the severity of the anaemia prevented any regenerative response before recumbency and likely death. While cattle develop an age-related immunity to Haemonchus, yearling-aged cattle are still susceptible, and a drench program needs to continue until cattle are at least 20 months of age.

The total worm egg count was very high and well above the threshold for needing drenching (Love, 2007). While 57% of the egg burden was Haemonchus it is important to recognise that this only represents 5% of the worm burden because of the high fecundity of Haemonchus spp. The calf was also infested with other worm species that will have contributed to ill thrift. For example, while Ostertagia was only 3% of the larval count this still represented 110 epg of Ostertagia eggs and 26% of the worm burden.

The recent wet weather, the necessity to group cattle at an increased stocking rate on higher ground and nutritional stress presumably contributed to this disease outbreak. It is also possible that the single active worm drench given at weaning was ineffective. Drench resistance to single active macrocyclic lactone drenches is very common (Ball and Gibbison, 2021). Even if the efficacy of the abamectin-containing weaning drench had been reasonable (70-85%) it would only have removed worms present at the time of treatment and would not have prevented rapid reinfestation. On high-risk properties a short acting drench at weaning may be inadequate to protect young cattle from developing severe disease a few months after treatment. An alternative program in these situations can be the use of 10% moxidectin injection, ideally primed same day with a different drench active such as oral oxfendazole. This program can protect at-risk animals for 120 days and significantly reduce paddock contamination with worm larvae (Virbac, 2022).

It is critical that drench selection in young cattle considers the use of actives that target the likely parasite risk and can be expected to have greater than 95% efficacy. This drench selection guidance will protect the health and productivity of growing cattle. In this case study the producer assumed that liver fluke was the cause of the problem when the necropsy results (admittedly on a single animal) suggest that the property may not have liver fluke. Diagnostic testing such as liver fluke ELISA can be used in homebred animals to check this.

In this case the moxidectin/levamisole combination drench prevented any further disease or deaths. While a post-drench check was not able to be carried out, multiple studies have demonstrated that a moxidectin/levamisole combination can be expected to have 99-100% efficacy against Haemonchus (Ball and Gibbison, 2021). Combination drench treatment will also be highly effective against other worm species, such as Ostertagia. While a single active levamisole drench may have been effective against the Haemonchus burden in these heifers it may not have been the most effective choice against the Ostertagia burdens they also carried. The use of ready-to-use combination or concurrent drench actives is now considered essential for cattle drench programs to ensure high efficacy against all key worm species, manage drench resistance and protect productivity. Principles of integrated parasite management, including grazing management, nutrition, and worm egg count monitoring, should also be implemented to reduce reliance on chemicals.

References

1. Ball M, Gibbison W (2021) Resistance pattern to avermectins and milbemycins in current strains of Australian cattle nematodes, Australian Cattle Veterinarians Journal, September 2021
2. Beveridge and Emery (2014) Australasian Parasites Inside and Out, The Australian Society of Parasitology E-Book
3. Love (2007) Worm test for livestock and guide to worm egg counts, Primefact 480 retrieved online 15 October 2022 www.dpi.nsw.gov.au
4. Virbac (2014) Cattle Calculator with worms per animal, available on request
5. Virbac (2022) Season long protection with Cydectin Long Acting for Cattle, available on request