Hemolymphatic System: Thymic and Bursal Diseases Flashcards
1
Q
Thymic Anatomy:
Summary
A
- Multilobulated organ composed of Epithelial cells and lymphocytes that functions to select lumphocytes that are not directed against self-antigens and post-puberty undergoes normal physiologic involution
2
Q
Thymic Anatomy:
Embryology
A
Endoderm form the 3rd pharyngeal pouch gives rise to the epithelial cells and combines with thymic T lymphocytes.
- In most mammals, the thymus is bi-lobed found in the cranial mediastinum, adjacent to the pericardium and often bulges out through the throacic inlet
- Ruminants and Pigs:
- thymic tissue extends diffusely ventrolateral to the trache in the neck.
- Birds:
- have 4-8 thymic lobes, bilaterally that extend lateral toa the trache from the clavicle/crop to the laryngeal mound
- Reptiles:
- don’t have thymic tissue
- Maximal development into puberty and will involute as the animal becomes an adult leaving small, often difficult to find remnants
3
Q
Thymic Anatomy:
Histology
A
- Each lobe consists of many lobules that depict a distinct cortex and medulla of porgenitor T lymphocytes with a lattice of stellate epithelial cells and clustered epithelial cells forming Thymic (Hassall’s) corpuscles within the medulla
4
Q
Thymic Anatomy:
Function
A
- Thymic T lymphocytes maek their way to the thymic cortex and following differentiation and selection become mature naive T lymphocytes when they enter the thymic medulla. T cells that recognize self-molecules but not self-antigen are selected and enter circulation and reside in secondary lymphoid tissue
5
Q
Thymic Diseases:
Thymic Atrophy
A
- A diagnosis of atrophy presumes the thymus was normal at birth then decreased in size more rapidly than physiologic involution
- Physiologic involution:
- is a pre-programed, age-associated decreased in thymic cellularity, weight, and overall size. Involution is the normal fate of the thymus
6
Q
Thymic Disease:
Thymic Atrophy:
Stress
A
- There are numerous forms of strss but ultimately, this is defines as “stressors” that lead to increased endogenous glucocorticoid release.
- Regardless of cause, impaired T lymphocyte producted will also mainifest as immunosuppression with increased susceptibility to disease.
- Endogenous corticosteroids induce lymohocytolysis resulting in decreased thymic size and weight.
7
Q
Thymic Disease:
Thymic Atrophy:
Inadequate Nutrition
A
- Also a type of stress that can result from many causes, resulting in accelerated thymic atrophy, in part due to endogenous corticosteroids but also related to improper nutrition, including:
- severe cachexia
- Deficiencies of VitB6, zinc, and fatty acids
8
Q
Thymic Disease:
Thymic Atrophy:
Viral Infections:
A
- In general, viral infections may result in accelerated thymic atrophy related to death of thymic lymphocytes
- canine parvovirus-2
- Canine distemper virus
- Felin panleukopenia
- Feline leukemia virus
- Feline immunodeficiency virus
- Equine herpevirus
- Porcine reproductive and respiratory syndrome virus
- Classical swine fever virus
- Bovine viral diarrhea virus
9
Q
Thymic Hemorrhage/hematoma:
Signalment
A
- Young dogs ~ less than 2 yrs
- +/- respiratory distress
- hypovolemia
- Sudden death
- Volume of hemorrhage/hematoma is large, that dog may manifest generalized pallor
10
Q
Thymic Hemorrhage/hematoma:
Cause:
Intoxicants
A
- Anticoagulants (rodenticides): anticoagulants must be your top differential for otherwise unexplained hemorrhage within the thymus in dogs.
- Once proven not to be an anticoagulant, then there are many otheres to rule in or out.
- Anticoagulant intoxication should be the first possible cause to rule in
11
Q
Thymic hemorrhage/hematoma:
Consequence of involution
A
- Thymus normally involutes over time, most noticeable after puberty.
- Blod vessels, specifically veins and venules, become thin-walled and are possibly weakened due to more loose connective tissue in the interstitium.
- THese weakened blood vessels in loose stroma may be more susceptible to traumatic injury and may rupture more easily leading to hemorrhage
12
Q
Thymic hemorrhage/hematoma:
Severe Neck trauma/streching
A
- Documented in the study related to abrupt stopping dogs by tugging on a leash while running.
- Symptoms of intrathoracic hemorrhage were observed shortly after the severe leash tugging on the neck.
- Severe trauma due to blunt force trauma has been associated with lung hemorrhages and rib fractures
13
Q
Thymic hemorrhage/hematoma:
Aortic Aneurysm
A
- Documented in 4/20 dogs in a study and ¾ were a single breed. Bouvier Des Flandres.
14
Q
Thymic hemorrhage/hematoma:
Idiopathic
A
- Recall from discussion of consequences of involution, therefore subtle or unknown trauma may rupture thymic blood vessels.
- Microtrauma within the thin-walled veins in the involuting thymus was observed microscopically
- Hemorrhage was observed within the medulla but the cortex was not involved.
- Another mechanism that be sudden increases in blood pressure - hemorrhage due to a lack of counter pressure form the loose interstitium in an involuting thymus
15
Q
Thymic Hemorrhage/hematoma:
Diagnostic Approach:
A
- Take a thourough history. Evidence of knowledge of previous trauma? Recent of recurrent abrupt jerking of leash attached to a collar? What breed is the dog? Could it be an aortic aneurysm
- Perform a complete physical Exam
- Rule in anticoagulant rodenticides
- 1st generation rodenticides deplete Vit K dependent clotting factor synthesis by blocking a synthesizing enzyme and halting recycling of Vit K.
- Vit K-dependent clotting factors are depeted resulting in an animals prone to hemorrhage
- Diagnosis:
- assay for known intoxicants using stomach contents, liver at postmortem, in live animals perform prothrombin time and activaed partial thromboplastin time. TImes become prolonged related to overall depletion of clotting factor
- If nothing else is ruled in, this is likely a case of idiopathic thymic hemorrhage.
