Diseases of the myeloid and lymphoid system

• What are the myeloid systems?

  Cells:

  • Granulocytes (neutrophils, eosinophils, mast cells ect..)
  • Moncytes
  • Macrophages
  • Erythrocytes
  • Thrombocytes
  • Other (e.g. dendritic cells)

  Tissue:

  • Bone marrow
• What are the lymphoid systems?

  Cells:

  • Lymphocytes

  Tissues:

  • Lymph nodes
  • Thymus
  • Spleen
  • Other (e.g. Peyer’s patches).
• Common stimuli of the myeloid and lymphoids

  Physiologic leukocytoasis (fight or flight)

  • Epinephrine

  Physiological stress (stress leukogram)

  • Corticosteroid (including exogenous)

  Iatrogenic antigenic stimulation (vaccines)

  Inflammation

  Infection

  Parasites/ foreign bodies.
• Examples of non-neoplastic disorders

  Tissues:

  • Lymph nodes - Hyperplasia lymphadenitis
  • Spleen - Torsion
  • Thymus - Haemorrhage infarction

  Lymphatics:

  • Major lymphatic vessels - Chylous effusions
  • Intestinal lacteals - lymphangiectasia
• What is chyle?

  A mixture of lymph and chylomicrons

  Chylomicrons = lipids absorbed from intestine - transported via lymphatics.

  Chylous effusions may result from rupture (e.g. trauma) or obstruction (e.g. neoplasia) of thoracic duct or other major lymphatic vessel.

  • Often idiopathic, site of leak not always detected.

  Both effusions (thoracic, abdominal) are rare.
• Chylothorax and chyloabdomen

  Chylothorax is usually a bilateral pleural effusion.

  Chylous effusion = chyle

  • Previously classified as modified transudate (progressing to exudate as inflammatory cells react to its presence).
  • Lipid will interfere with refractometry.

  Treatment may involve ligation of the thoracic duct.
• What is lymphangiectasia

  Lacteals dilation

  Pathophysiology - intestinal lymphatics dilate and lose chyle into the lumen - protein losing enteropathy.

  Aetiology (in most common idiopathic)

  • Congenital - may be inherited
  • Acquired obstruction - e.g. neoplasia

  Thought to be more common in dogs; rare in cats.

  May be managed with low-fat diet +/- immunosuppressive (e.g. prednisolone)
• approach to the investigation of lymphangiectasia?

  History:

  • Gastrointestinal signs
  • Weight loss
  • Diarrhoea (chronic)
  • Vomiting

  Physical exam:

  • May present with poor body condition
  • Ascites

  Biochemistry:

  • Parameters suggestive of PLE (e.g. hypoalbuminaemia)
  • Hypocholesterolaemia (component of chyle).

  Haematology:

  • Lymphopaenia (loss of chyle)

  Imaging:

  • Ultrasound (Hyperchoic lacteals)

  Biopsy:

  • Consider endoscopic vs surgical
• Examples of myeloid neoplasia

  Myeloid leukaemia:

  • Acute (immature cells)
  • Chronic (differentiated cells)
  • Polycythaemia Vera (erythrocytes)

  Myeloid cells

  • Mast cell tumour
  • Histiocytoma (Langerhans cells = macrophages)
  • Transmissible veneral tumour (TVT) (believed to be histiocytic).
• Myeloid neoplasia - transmissible venereal tumour

  Infectious tumour:

  • Usually transmitted during mating
  • Beleived to be histiocytic in origin

  Not endemic in the UK

  • May see more with increased number of imported dogs.

  Can respond well to chemotherapy (e.g. vincristine).
• Myeloid neoplasia - Histiocytoma

  Common neoplasm in small animal practice

  Young dogs (<2 years)

  Classically benign

  Can regress independently over several weeks

  But use caution when advising clients

  • Mast cell tumour can look a lot like Histiocytoma.
• Myeloid neoplasia - mast cell tumour

  Mast cell are a form of granulocyte

  • may be normal to see them in reactive lymph nodes.

  Malignant transformation - MCT

  • Usually older dogs but any age at risk.

  MCTs usually present as skin masses

  • Common - up to 20% of all canine skin tumours.
  • Cytoligy very sensitive - FNA of suspected skin masses recommended.
  • Some breeds more susceptible: boxers, labradors, golden retriever's, Staffordshire bull terriers, Boston terriers, pugs.
• What’s so bad about MCTs?

  Variable grades:

  • Lower grades may be managed by excision and monitoring.
  • Higher grades may spread to lymph nodes, liver, spleen, bone marrow - importance of staging.

  Locally invasive:

  • For high grade, recommendations of 3cm margins and 1-2 fascia plane deep to tumour.
  • Revisional surgery may be required.

  Risk of degranulation:

  • Mass histamine release can produce anaphylaxis.
  • Many sources recommend H1 blockers (e.g. chlorphenamine) if tumour likely to be damaged during biopsy or surgery.
• How do we grade MCTs?

  Two grading systems in common use

  • Grading in histopathology (not fine needle aspirate).

  Evaluation of proliferative activity per high power field:

  • Mitotic index
  • Multinucleated cells
  • Atypical nuclei (shape and size)
  • Guided by anisokaryosis
• MCTs in cats

  Cutaneous form:

  • If well differentiated may act benign

  Visceral form - poorer prognosis

  • Spleen, LN, liver
  • Intestine may be diffuse

  No grading system currently available

  Association between mitotic index and survival time.
• Plasmacytomas

  Extramedullary plasmacytoma - skin, oral cavity, colon/rectal mucosa, solitary osseus (rare).

  Usually benign (but can be locally invasive)

  • Complete excision of solitary lesion should be curative.
  • Cutaneous plasmacytosis (multiple skin tumours) more aggressive.

  Rare in cats.