Diseases of the thymus Flashcards
What si the main role of the thymus
The main role of the thymus is during development of the immune system in neonates where it is responsible for T-cell production and the ability to recognize self-antigens
After this time the thymus involutes
- so that it is inapparent on gross necropsy of mature animals
What are the causes of thymic disease in the cat
Causes of thymic disease in the cat:
- thymic lymphoma
- thymoma
- thymic aplasia in Birman kittens (rare)
- thymic branchial cysts (uncommon)
What is the characteristic manifestation of thymic disorders
Thymic disorders usually manifest as a mass effect in the cranial mediastinum
They may or may not be accompanied by pleural effusion
What is thymic aplasia
Thymic aplasia, and consequent cell-mediated immunodeficiency, can occur in kittens infected with FeLV
This has also been reported as a congenital problem in Birman cats
What are thymic cysts
Benign cranial mediastinal cysts (as opposed to cystic thymoma) have been reported in cats, and may be a result of congenital malformations of the branchial arches
What is primary immunodeficiency
Primary immunodeficicency results from an inherited or congenital defect in the innate, cell-mediated immunity and/or humoral immune responses
How frequent is primary immunodeficiency in cats
Primary hereditary immunodeficiencies are extremely rare in cats
Congenital defects may affect neutrophils, lymphocytes or other components of the innate or acquired immune system
When can you suspect primary immunodeficiency
Primary immunodeficiency can be suspected when sveeral related kittens die in utero, fail to thrive after birth, or succumb to early infectionn
- these defects become apparent once the protective effect of maternal antibodies starts to wane
Other clues to the presence of primary immunodeficiency are:
- the presence or recurrence of infections in young animals
- atypical infections
- failure to respond to vaccination
- hypoglobulinemia
- persistent leukopenia
What is Pelger-Huët anomaly
Pelger-Huët anomaly is an uncommon, benign congenital defect of leukocyte development inherited in an autosomal dominant manner
- affected cats are heterozygous because the homozygous defect is lethal in utero
The disorder is characterized by hyposegmentation of the nuclei of neutrophils and eosinophils
- cell function is normal
- no treatment is needed
Affeceted neutrophils resemble immature band cells but have mature, clumped chromatin (absence of toxic changes and normal WBC count)
Affected cats need to be differentiated from those with “pseudo-Pelger-Huët” due to severe infection or inflammation with a marked degenerative left shift, to conditions that alter WBC maturation such as FIV and FeLV or myeloid neoplasia
What is Chediak-Higashi syndrome
Chediak-Higashi syndrome is an autosomal recessive trait of Persians, with all affected cats having a pale grey coat (“blue-smoke” due to abnormal pigmentation) and pale yellow iris pigmentation
- the disease is characterized by abnormal granule formation in melanocytes, leukocytes and platelets
Affected cats have an increased incidence of infection and are prone to bleeding after invasive procedures (prolonged buccal mucosal bleeding time)
These cats also tend to be photophobic and are prone to cataract formation
Affected cats can respond to canine granulocyte colony stimulating factor, however definitive treatment is a bone marrow transplant
What is secondary (acquired) immunodeficiency
Most immunocompromised cats have a secondary immunodeficiency
The causes arre varied and may impact a single aspect of immunity (e.g., immune-mediated neutropenia affects only innate phagocytic immunity) or several components of immunity (e.g., malnutrition affects innate, cell-mediated immunity, and humoral immunity)
What are the main causes of secondary immunodeficiency
Retroviral infections
Coronavirus infection
Feline panleukopenia virus infection
Drugs (e.g., chemotherapy, immunosuppressive agents)
Chronic disease
How FIV affects immune response
FIV infects helper T lymphocytes (CD4+), B lymphocytes and activated macrophages
There is an initial decline in CD4+ cell numbers during the viremic phase
- this is directly attributable to a viral-induced cytopathic effect
CD4+ cell numbers then recover as viremia is suppressed during the “latent” period
Over time the number of CD4+ gradually declines and the cells become less responsive to normal immunological stimuli via alterations in expression of cell surface molecules
What are the most common opportunistic infections seen with feline “AIDS”
The most common opportunistic infections seen with feline AIDS are:
- chronic bacterial stomatitis
- bacterial pyoderma
- persistant viral upper respiratory tract infections
- immune-mediated glomerulonephritis
- hemoplasmosis
- toxoplasmosis
- demodecosis
FIV-positve cats are also five times more likely to develop lymphoma
How FeLV infection can result in immunodeficiencies
FeLV-T is particularly associated with immunosuppression due to its T-lymphocyte tropism
FeLV infection can result in immunodeficiencies via bone marrow failure (e.g., due to leukemia, myelodysplasia) or direct immunosuppression due to the expression of the trans-membrane viral protein p153 which inhibits T- and B-lymphocyte function and alters monocyte distribution and function
How coronavirus infection can lead to immunosuppression
FIP infection is associated with a marked depletion of helper T lymphocytes (CD4+) and cytotoxic T lymphocytes (CD8+)
- it is thought that because the virus targets a type of antigen presenting cell (i.e., monocytes and macrophages) the normal T-cell response is subdued as a secondary phenomenom
How parvovirus infection can lead to immunosuppression
After initially undergoing replication in the tonsils and other oropharyngeal lymphoid tissue, a subsequent viremia spreads the virus systemically
Peripheral lymphoid tissue undergoes necrosis (resulting in lymphopenia) and bone marrow precursors are affected, resulting in the characteristic panleukopenia
White blood cells, particularly neutrophils, are also consumed at a higher rate due to sequestration in the damaged GIT
Because of the breakdown of the normal barrier in the gut, bacterial translocation, plus a systemic immunosuppression may result in bacteremia and subsequent sepsis
