DEFINITION
Toxoplasma gondii—an obligate intracellular coccidian protozoan parasite that infects nearly all mammals; Felidae the definitive hosts; all other warm-blooded animals are intermediate hosts.
PATHOPHYSIOLOGY
Severity and manifestation—depend on location and degree of tissue injury caused by tissue cysts
Infection—acquired by ingestion of tissue cysts or oocysts; organisms spread to extraintestinal organs via blood or lymph; results in focal necrosis to many organs (heart, eye, CNS)
Acute disseminated infection rarely fatal
Chronic disease—tissue cysts form; low-grade disease; usually not clinically apparent unless immunosuppression or concomitant illness allows organism to proliferate, causing an acute inflammatory response
Clinical disease—often associated with other infections that cause severe immunosuppression (e.g., canine distemper, FIP, and FeLV).
Sunday, April 3, 2011
Thyroid Hormones
DEFINITION
Serum concentrations of T4, T3, free thyroxine, or endogenous canine TSH outside the normal range
PATHOPHYSIOLOGY
The thyroid gland regulates basal metabolism; two molecules, tyrosine and iodine, are important for thyroid hormone synthesis.
The tyrosyl ring can accommodate two iodide molecules; if one iodide attaches, it is called monoiodotyrosine (MIT); if two iodide molecules attach to the tyrosyl ring, it is called diiodotyrosine (DIT).
Two DIT molecules form T4; one MIT coupled with one DIT molecule forms T3.
T4 is the major storage form of thyroid hormone; T3 is the active form of the hormone; most T3 is formed outside the thyroid gland by deiodination of T4.
Another type of T3 is formed when an iodide molecule is removed from the inner phenolic ring of T4; this compound is called reverse T3 and increases in nonthyroidal illness.
Thyrotropin, or TSH, is the most importantregulator of thyroid activity.
TSH secretion is regulated by thyroid hormones via negative feedback inhibition of the synthesis of TRH at the level of the hypothalamus and by inhibition of the activity of TSH at the level of the pituitary.
With thyroid gland failure, the pituitary gland senses decreases in serum free thyroxine (FT4) and TT4, resulting in increased serum endogenous TSH concentration.
The use of endogenous TSH alone is not recommended as a method of assessing thyroid function.
FT4 concentrations are measured by equilibrium dialysis (gold standard) or analogue immunoassays.
Serum concentrations of T4, T3, free thyroxine, or endogenous canine TSH outside the normal range
PATHOPHYSIOLOGY
The thyroid gland regulates basal metabolism; two molecules, tyrosine and iodine, are important for thyroid hormone synthesis.
The tyrosyl ring can accommodate two iodide molecules; if one iodide attaches, it is called monoiodotyrosine (MIT); if two iodide molecules attach to the tyrosyl ring, it is called diiodotyrosine (DIT).
Two DIT molecules form T4; one MIT coupled with one DIT molecule forms T3.
T4 is the major storage form of thyroid hormone; T3 is the active form of the hormone; most T3 is formed outside the thyroid gland by deiodination of T4.
Another type of T3 is formed when an iodide molecule is removed from the inner phenolic ring of T4; this compound is called reverse T3 and increases in nonthyroidal illness.
Thyrotropin, or TSH, is the most importantregulator of thyroid activity.
TSH secretion is regulated by thyroid hormones via negative feedback inhibition of the synthesis of TRH at the level of the hypothalamus and by inhibition of the activity of TSH at the level of the pituitary.
With thyroid gland failure, the pituitary gland senses decreases in serum free thyroxine (FT4) and TT4, resulting in increased serum endogenous TSH concentration.
The use of endogenous TSH alone is not recommended as a method of assessing thyroid function.
FT4 concentrations are measured by equilibrium dialysis (gold standard) or analogue immunoassays.
Thrombocytosis
DEFINITION
A platelet count above the upper end of the reference range
Purdue University Teaching Hospital reference ranges—dogs, 200,000–900,000/mL; cats, 300,000–700,000/mL
PATHOPHYSIOLOGY
Can be caused by overproduction of platelets, decreased clearance of platelets, and decreased sequestration of platelets.
Overproduction occurs secondary to bone marrow stimulation by thrombopoietin and factors such as IL-1, IL-3, IL-6, and IL-11.
For most associated diseases, the exact mechanisms are not well documented.
A platelet count above the upper end of the reference range
Purdue University Teaching Hospital reference ranges—dogs, 200,000–900,000/mL; cats, 300,000–700,000/mL
PATHOPHYSIOLOGY
Can be caused by overproduction of platelets, decreased clearance of platelets, and decreased sequestration of platelets.
Overproduction occurs secondary to bone marrow stimulation by thrombopoietin and factors such as IL-1, IL-3, IL-6, and IL-11.
For most associated diseases, the exact mechanisms are not well documented.
Thrombocytopenia
DEFINITION
Platelet count below the lower limit of the reference range
Purdue University Teaching Hospital reference range—200,000–900,000/mL, dogs; 300,000–700,000/mL, cats
PATHOPHYSIOLOGY
Platelets are produced by megakaryocytes in the bone marrow, released into bloodstream, and circulate for a few to several days.
In the normal state, the platelet count remains stable, because production of platelets is equivalent to the removal of platelets from the circulation and the spleen holds a fairly large reserve.
Caused by decreased production, sequestration, increased destruction, or increased use of platelets.
Platelet count below the lower limit of the reference range
Purdue University Teaching Hospital reference range—200,000–900,000/mL, dogs; 300,000–700,000/mL, cats
PATHOPHYSIOLOGY
Platelets are produced by megakaryocytes in the bone marrow, released into bloodstream, and circulate for a few to several days.
In the normal state, the platelet count remains stable, because production of platelets is equivalent to the removal of platelets from the circulation and the spleen holds a fairly large reserve.
Caused by decreased production, sequestration, increased destruction, or increased use of platelets.
Tetanus
OVERVIEW
Clostridium tetani—an obligate, anaerobic, spore-forming, gram-positive rod found in soil and as part of the normal bacterial flora of the intestinal tract of mammals with a predilection for contaminated, necrotic, anaerobic wounds (puncture, surgery, lacerations, burns, frostbite, open fractures, abrasions)
Germi-nating spores—in wounds produce potent exotoxin tetanospasmin (tetanus toxin); resistant to disinfectants and to the effects of environmental exposure
Found worldwide, especially in the tropics.
Clostridium tetani—an obligate, anaerobic, spore-forming, gram-positive rod found in soil and as part of the normal bacterial flora of the intestinal tract of mammals with a predilection for contaminated, necrotic, anaerobic wounds (puncture, surgery, lacerations, burns, frostbite, open fractures, abrasions)
Germi-nating spores—in wounds produce potent exotoxin tetanospasmin (tetanus toxin); resistant to disinfectants and to the effects of environmental exposure
Found worldwide, especially in the tropics.
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