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Thrombocytopenia with tick infestation

Unusual presentation:

Sudden death of 19 heifers.

Significant abdominal and endocardial haemorrhage

Above: Significant abdominal and endocardial haemorrhage, with no clots and blood the consistency of weak red cordial.

Time and location: Northern Territory, October 2023​.

Case definition: Heifers appear weak, become recumbent and die within hours of routine speying and handling in yards.

Disease mapping: 19 of 1600 heifers died over 2 days after routine speying or processing. The heifers were 10-18 months of age (150-270 kg) and had a body condition score about 3/5. They had been brought to the station over the previous 4-8 weeks. They were speyed by the Willis technique, backlined, vaccinated, (some animals) dehorned and given pain relief. They were noted to have high burdens of immature ticks. There was no unusual or adverse behaviour or events on handling. They were fed good quality hay whilst in the yards.

Gross findings: Examination of two heifers at post mortem found significant abdominal haemorrhage with no clots present and blood with the consistency of weak red cordial, endocardial haemorrhages in both animals and generalised haemorrhages in one heifer. The surgical sites were normal. Sampling included a full set of fresh and fixed tissue and bloods from two animals and a blood smear from a third animal.

The field-based differential diagnoses were babesia/anaplasmosis (the heifers were from a tick region but were now further north with much higher tick burdens, and may not have had sufficient immunity), or a clotting defect to be determined.

Laboratory findings: Haematology confirmed severe thrombocytopaenia (low platelets). The blood smear (Slide 1) showed signs of a severe normocytic, normochromic anaemia with evidence of regeneration and no evidence of haemoparasites. BVDV infection was not implicated as leukocyte levels were not low. No significant infectious disease was indicated on histopathology. The profound thrombocytopaenia is the likely cause of the bleeding disorder.

Animal / management / environment risk factors: The heavy burden of blood sucking ticks in this case could result in platelets being consumed at sites of tick attachment and normal attempts at regeneration being unable to maintain adequate numbers of circulating platelets.

Recommendations: When a tick fever organism was initially suspected the manager was advised to ensure that a high quality parasite treatment be administered to cattle either prior to loading at the breeding blocks further south or upon unloading at this block. This recommendation remains appropriate with the final diagnosis.

Below: Slide 1 shows severe anaemia with lack of platelets (from this case). Slide 2 is a moderate anaemia with adequate numbers of platelets (for comparison).

Slide 1 - severe anaemia with lack of platelets
Slide 2 - moderate anaemia with adequate numbers of platelets (for comparison).