Platelets in horses

Laboratory diagnostics

Platelet counts are usually included in every complete blood count. They can, however, also be requested individually. Additionally, you should also always prepare a blood smear directly before or after platelet analysis. This blood smear can be used to look for aggregates when platelet counts are low or to confirm thrombocytopenia. To evaluate the blood smear, it is air-dried and then stained. You can, for example, use Wright-Giemsa or Diff-Quik for staining (Figures 2a and 2b).
For further evaluation, a detailed clinical history (e.g. contact with viral, bacterial, parasitic infectious agents? Fever? Gastrointestinal signs? Respiratory signs?) and a thorough examination of the patient are necessary.

Microscopic examination of the platelets is first carried out at 100x magnification (10x objective). The feathered edge is examined for platelet aggregates. Platelets are then counted in the monolayer at 1000x magnification (100x objective). In order to calculate the approximate platelet count, the average of 10 visual fields is multiplied by 15 – 20. Any aggregates in the feathered edge must also be taken into account. This blood smear can be used to look for aggregates (Fig. 3) when platelet counts are low or to confirm thrombocytopenia.  For example: In the monolayer, 98 platelets are counted in 10 visual fields. The average is 9.8 platelets per visual field. The number is rounded up to 10 and then multiplied by 15.
Thus, the estimated platelet count of the horse is 150 G/l. Rough estimates of any aggregates must be added to this figure.

If thrombocytopenia is confirmed by the blood smear, further examinations are recommended depending on the clinical history and the suspected diagnosis, e.g.

• • PCR tests: e.g. Babesia, Anaplasma
  • coagulation tests: PT, aPTT, thrombin time, fibrinogen
  • serological tests: e.g. equine infectious anaemia (EIA), equine herpesvirus (EHV)
  • haematology: antiplatelet antibodies
  • bone marrow biopsy and cytology, if necessary

Thrombocytopenia

In most cases, thrombocytopenia is an incidental finding in horses. Thrombocytopenia is a decrease in platelet numbers below the species-specific norm.⁹ Thrombocytopenia usually indicates a pathological process which can lead to coagulation disorders when platelet count is significantly reduced.¹⁰ There is often more than one cause for thrombocytopenia.¹²
A distinction must be made between pseudothrombocytopaenia and true thrombocytopenia. Pseudothrombocytopenia occurs if not all the platelets present are counted in the platelet count. Manual microscopic examination is therefore always recommended as a first control mechanism in thrombocytopenia.¹⁰
Reasons for true thrombocytopaenia are reduced production in the bone marrow, increased consumption, destruction of platelets or increased sequestration (Table 1).^{10, 11}

Table 1: Summary of causes of thrombocytopenia ^{10, 11}

Cause
Thrombocytopenia	pseudothrombocytopenia	limitations of measurement platelet aggregation due to platelet activation macroplatelets EDTA-induced (very rare)
	reduced production in the bone marrow	bone marrow aplasia megakaryocyte leukaemia myelofibrosis myelonecrosis metastases medication toxins radiation genetic
	increased consumption	severe bleeding disseminated intravascular coagulation (DIC) haemolytic uraemic syndrome (HUS) neoplasia
	destruction of platelets	immune-mediated destruction primary immune-mediated thrombocytopenia secondary infectious diseases neoplasia medication alloimmune thrombocytopenia Evans syndrome non-immune-mediated destruction infectious diseases viruses medication heparin-induced toxins snakebite
	sequestration	splenomegaly severe hypothermia endotoxaemia

 

Clinical signs of thrombocytopenia
In most horses, thrombocytopenia occurs secondary to a variety of underlying diseases.¹²
The clinical picture usually corresponds to the causative disease. In general, signs are rare in low to moderate thrombocytopenia (90 – 30 G/l) as long as secondary haemostasis is not impaired.
Severe thrombocytopenia (< 30 G/l) leads to changes in primary haemostasis.^{13, 14} Patients show petechiae and ecchymoses, which are mainly seen on the mucous membranes and the sclerae. Increased epistaxis has also been described. There is prolonged bleeding after blood sampling.

Thrombocytosis

Thrombocytosis is defined as an increase in the number of platelets circulating in the peripheral blood above the species-specific reference range. As with thrombocytopenia, a distinction can be made between pseudothrombocytosis and true thrombocytosis (Table 2). Pseudothrombocytosis occurs when other cells are counted as platelets in the platelet count. This can include small erythrocytes or lysed erythrocytes. The evaluation of scatterplots and blood smears helps here.

Mild, physiological thrombocytosis often occurs during and after exercise or agitation (e.g. stress during blood collection). This is caused by splenic contractions which release stored platelets into the circulation.^{18, 19}
True thrombocytosis can be classified into primary and secondary thrombocytosis. In most cases, platelet count increase in secondary thrombocytosis is only mild to moderate (300 – 500 G/l), whereas it is high in primary thrombocytosis. Furthermore, primary thrombocytosis is more frequently associated with clinical bleeding and thrombosis.^{20, 21, 22}
Secondary thrombocytosis mostly occurs in inflammation and infections.²⁴

Table 2: Summary of causes of thrombocytosis ^{11, 22, 23}

Cause
Thrombocytopenia	pseudothrombocytosis	limitations of measurement lysed erythrocytes (“ghost cells”)
	redistribution of platelets	exercise catecholamines
	increased production in the bone marrow	inflammation infection immune-mediated surgical intervention trauma non-haematopoietic neoplasms iron deficiency recovery from thrombocytopenia (rebound) splenectomy (post) haemorrhage
	neoplasia	primary (essential) thrombocythemia acute megakaryoblastic leukaemia (as of 2021 not described in horses) chronic myeloproliferative neoplasms

Conclusion

Platelet counts are part of almost every blood count and may provide an indication of an underlying disease. In any case, their plausibility must first be checked manually under the microscope. The cause of thrombocytopenia or thrombocytosis must then be found based on a good medical history and clinical examination.

Dr. med. vet. Annemarie E. Baur-Kaufhold