PARASITIC DISEASES

AMEBIASIS (Entamebiasis)

Infection of the colon with Entamoeba histolytica, which is commonly asymptomatic but may produce clinical manifestations ranging from mild diarrhea to severe dysentery.

Etiology and Pathogenesis 

Amebiasis is a protozoan infection of the lower GI tract. E. histolytica exists in two forms: the trophozoite and the cyst.

Two species of Entamoeba are morphologically indistinguishable: E. histolytica is pathogenic and E. dispar harmlessly colonizes the colon. Amebas adhere to and kill colonic epithelial cells and cause dysentery with blood and mucus in the stool. Amebas also secrete proteases that degrade the extracellular matrix and permit invasion into the bowel wall and beyond. Amebas can spread via the portal circulation and cause necrotic liver abscesses.

Symptoms and Signs 

Most infected persons are asymptomatic but chronically pass cysts in stools. Symptoms that occur with tissue invasion include intermittent diarrhea and constipation, flatulence, and cramping abdominal pain. There may be tenderness over the liver and ascending colon, and the stools may contain mucus and blood.

Amebic dysentery, common in the tropics but uncommon in temperate climates, is characterized by episodes of frequent (semi)liquid stools that often contain blood, mucus, and live trophozoites.

Chronic infection commonly mimics inflammatory bowel disease and presents as intermittent nondysenteric diarrhea with abdominal pain, mucus, flatulence, and weight loss.

Metastatic disease originates in the colon and can involve any organ, but a liver abscess, usually single and in the right lobe, is the most common
 

Blood-Lymphatic Pathology

Disorders of primary hemostasis

1. General characteristics of disorders of primary hemostasis (due to problems of blood vessels or platelets):

a. Occur early in life.

b. Unlike secondary hemostasis, bleeding occurs in more superficial areas such as skin and mucous membranes rather than in secondary hemostasis.

c. Signs include petechiae.

d. Can be caused by vascular and platelet abnormalities or alterations in the plasma proteins required for adhesion of platelets to vascular subendothelium.

e. Laboratory findings include prolonged bleeding time, as seen in platelet disorders.

2. Vascular abnormalities

Scurvy

(1) Caused by a vitamin C deficiency leading to decreased synthesis of collagen. Note: vitamin C is necessary for the formation of collagen via hydroxylation of lysine and proline.

(2) Symptoms include:

- Delayed wound healing.

- Petechiae and ecchymosis.

- Gingival bleeding, swelling, and ulcerations.

3. Platelet abnormalities

a. Thrombocytopenia

(1) Characterized by a decreased number of platelets.

(2) The most common type of bleeding disorder.

(3) Can be caused by a number of diseases, such as irradiation, acute leukemia, disseminated intravascular coagulation (DIC), or idiopathic thrombocytopenic purpura (ITP).

b. Thrombocytopenic purpura

(1) Idiopathic: An autoimmune disease characterized by the presence of autoantibodies against platelets, resulting in the removal of platelets by splenic macrophages.

(2) May also be drug-induced.

Disorders of secondary hemostasis

1. General characteristics of disorders of secondary hemostasis (due to problems with clotting factors):

a. Symptoms occur later in life.

b. As compared to disorders of primary hemostasis, bleeding occurs in deeper areas and larger vessels (i.e., joint spaces).

c. Laboratory findings include abnormal:

- Partial thromboplastin time (PTT)—measures the intrinsic and common clotting pathway (i.e., tests all coagulation factors except factor 7).

- Prothrombin time (PT)—measures the extrinsic pathway.

- Does not affect the bleeding time.

Hemophilia

a. Caused by a deficiency of particular clotting factor(s).

b. All types of hemophilia affect the intrinsic pathway of the clotting cascade.

c. Signs and symptoms include:

- Prolonged PTT.

- Continuous bleeding from cuts or trauma, which can lead to excessive blood loss.

- Bleeding into joint cavities (hemarthroses) and muscle.

Two types:

(1) Hemophilia A (classic hemophilia)

- Caused by a deficiency of factor 8 (antihemophilic factor).

- Transmission: sex-linked recessive—only occurs in males; however, females can be carriers.

(2) Hemophilia B (Christmas disease)

- Caused by a deficiency of factor 9 (plasma thromboplastin).

- Transmission: sex-linked recessive—only occurs in males; however, females can be carriers.

- Lower incidence rate than hemophilia A.

(3). Vitamin K deficiency

- Causes include malnutrition and malabsorption of fats.

- A decrease in clotting factors 2, 7, 9, and 10 and prothrombin is observed.

- Prolonged PT.

Disorders of both primary and secondary hemostasis

1. von Willebrand’s disease

a. Characterized by a defective von Willebrand’s factor (vWF). Defective vWF affects both primary hemostasis by affecting platelet adhesion to

endothelium, and secondary hemostasis, by a defective factor 8.

b. Genetic transmission: autosomal dominant.

It is the most common hereditary bleeding disorder.

2. Liver disease—disease of the liver results in a decreased production of coagulation factors and therefore can lead to problems with hemostasis.

3. Disseminated intravascular coagulation a condition in which clots form throughout the vasculature. This uses up all available clotting factors and platelets, resulting in problems with bleeding.

Roseola
 - alias exanthem subitum; caused by Herpes virus type 6.
 - children 6 months to 2 years old; spring and fall; incubation 10-15 days.
 - sudden onset of a high fever with absence of physical findings; febrile convulsions are particularly common.
 - fever falls by crisis on the 3rd or 4th day → 48 hours after temperature returns to normal macular or maculopapular rash starting on the trunk and spreading centrifugally.

Mycobacterium leprae 

- tuberculoid type has intact cellular immunity
 - forms granulomas and kill the organisms (very few present).
 - evokes a positive lepromin skin test
 - localized skin lesions that lack symmetry
 - nerve involvement (organisms invade Schwann cells) that dominates the clinical picture and leads to skin anesthesia, muscle atrophy and autoamputation.
 - lepromatous leprosy patients lack cellular immunity
 - no granulomas
 - organisms readily identified
 - negative lepromin skin test
 - Bacteremia disseminates to cooler areas like the digits.
 - symmetrical, skin lesions that produce the classic leonine facies; biopsy reveals grentz zone in superficial dermis and then organisms in macrophages.
 - neural involvement is a late feature of the disease.
 - lepromin skin test is to determine host immunity; not a diagnostic test.
 - treatment: dapsone + rifampin

Hepatic failure 
Etiology. Chronic hepatic disease (e.g., chronic active hepatitis or alcoholic cirrhosis) is the most common cause of hepatic failure although acute liver disease may also be responsible.

- Widespread liver necrosis may be seen with carbon tetrachloride and acetaminophen toxicity. Widespread steatosis is seen in Reye's syndrome, a cause of acute liver failure most often seen in children with a recent history of aspirin ingestion for an unrelated viral illness. 
- Massive necrosis may also be seen in acute viral hepatitis, after certain anesthetic agents, and in shock from any cause. 

Clinical features. Hepatic failure causes jaundice, musty odor of breath and urine, encephalopathy, renal failure (either by simultaneous toxicity to the liver and kidneys or the hepatorerial syndrome), palmar erythema, spider angiomas, gynecomastia , testicular atrophy 

STOMACH 
Congenital malformations

1. Pyloric stenosis 

Clinical features. Projectile vomiting 3-4 weeks after birth associated with a palpable "olive" mass in the epigastric region is observed. 
Pathology shows hypertrophy of the muscularis of the pylorus and failure to relax. 

2. Diaphragmatic hernias are due to weakness in or absence of parts of the diaphragm, allowing herniation of the abdominal contents into the thorax. 

Inflammation 

1. Acute gastritis (erosive)

Etiology. Alcohol, aspirin and other NSAIDs, smoking,  shock, steroids, and uremia may all cause disruption of the mucosal barrier, leading to inflammation. 
Clinical features. Patients experience heartburn, epigastric pain, nausea, vomiting, hematemesis, and even melena. 

2. Chronic gastritis (nonerosive) may lead to atrophic mucosa with lymphocytic infiltration. 

Types 

(1) Fundal (Type A) gastritis is often autoimmune in origin.  It is the type associated with pernicious anemia and, therefore, achlorhydria and intrinsic factor deficiency. 
(2) Antral (Type B) gastritis is most commonly caused by Helicobacter pylori and is the most common form of chronic gastritis in the U.S. H. pylori is also responsible for proximal duodenitis in regions of gastric metaplasia.

Clinical features. The patient may be asymptomatic or suffer epigastric pain, nausea, vomiting, and bleeding. Gastritis may predispose to peptic ulcer disease, probably related to  H. pylori infection.

3. Peptic ulcers

Peptic ulcers are usually chronic, isolated ulcers observed in  areas bathed by pepsin and HCI; they are the result of mucosal breakdown

Common locations are the proximal duodenum, the stomach, and the esophagus, often in areas of Barrett's esophagus. 

Etiology. There are several important etiologic factors. 
Duodenal ulcers occur predominantly in patients with excess acid secretion, while gastric ulcers usually occur in patients with lower than average acid secretion. 

Other predisposing conditions include smoking, cirrhosis, pancreatitis, hyperparathyroidism, and H. pylori infection. Aspirin, steroids, and NSAlDs are known to be assoicated with peptic ulcer disease. Next to H. pylori colonization, aspirin or NSAID ingestion is the most common cause of peptic ulcer. 

Clinical features. Patients experience episodic epigastric pain. Duodenal and most gastric ulcers are relieved by food or antacids. Approximately one-fifth of gastric ulcer patients get no relief from eating or experience pain again  within 30 minutes.

Pathology. Benign peptic ulcers are well-circumscribed  lesions with a loss of the mucosa, underlying scarring, and sharp walls. 

Complications include hemorrhage, perforation, obstruction, and pain. Duodenal ulcers do not become malignant .Gastric ulcers do so only rarely; those found to be ma1ignant likely originated as a cancer that ulcerated.

Diagnosis is made by upper gastrointestinal Series , endoscopy, and biopsy to rule out malignancy or to demonstrate the presence of H. pylori. 

4. Stress ulcers 

are superficial mucosal ulcers of the stomach or duodenum or both. Stress may be induced by burns, sepsis shock, trauma, or increased intracranial pressure. 

Tumors 
1. Benign 

a. Leiomyoma, often multiple, is the most common benign neoplasm of the stomach. Clinical features include bleeding, pain, and iron deficiency anemia. 

b. Gastric polyps are due to proliferation of the mucosal epithelium. 

2. Malignant tumors 

a. Carcinoma 

Etiology. Primary factors include genetic predisposition and diet; other factors include hypochlorhydria, pernicious anemia, atrophic gastritis, adenomatous polyps, and exposure to nitrosamines. H. pylori are also implicated. 

Clinical features. Stomach cancer is usually asymptomatic until late, then presents with anorexia, weight loss, anemia, epigastric pain, and melena. Virchow's node is a common site of metastasis. 

Pathology. Symptomatic late gastric carcinoma may be expanding or infiltrative. In both cases the prognosis is poor (approximately 10% 5-year survival), and metastases are frequently present at the time of diagnosis. 
Adenocarcinomas are most common. 

b. Gastrointestinal lymphomas may be primary In the gastrointestinal tract as solitary masses. 

c. Sarcoma is a rare, large, ulcerating mass that extends into the lumen. 

d. Metastatic carcinoma. Krukenberg's tumor is an ovaria metastasis from a gastric carcinoma. 

e. Kaposi's sarcoma. The stomach is the most commonly involved GI organ in Kaposi's sarcoma. It primarily occurs in homosexual men, appearing as hemorrhagic polypoid, umbilicated nodular lesions, typically in a submucosal location. It rarely causes symptoms