[5] In its more advanced form, malabsorption (insufficient absorption of nutrients from the diet) leads to wasting and the enlargement of lymph nodes in the abdomen.

Chronic malabsorptive diarrhea leads to the poor absorption of fat, causing steatorrhea (fatty, offensive-smelling stool), flatulence, and abdominal distension.

Weakness and poor coordination of part of the body, headaches, seizures, and a number of more uncommon neurological features are present in some cases.

[5] Individuals who are most susceptible to the disease are those with decreased ability to perform intracellular degradation of ingested pathogens or particles, particularly within macrophages.

Several studies indicate that defective T-lymphocyte (particularly TH1 population) function may be an important predisposing factor for the disease.

[8] Weight loss and diarrhea are the most common symptoms that lead to identification of the process, but may be preceded by chronic, unexplained, relapsing episodes of nondestructive seronegative arthritis, often of large joints.

[9] Endoscopy of the duodenum and jejunum can reveal pale yellow shaggy mucosa with erythematous eroded patches in patients with classic intestinal Whipple's disease, and small bowel X-rays may show some thickened folds.

Other pathological findings may include enlarged mesenteric lymph nodes, hypercellularity of lamina propria with "foamy macrophages", and a concurrent decreased number of lymphocytes and plasma cells, per high power field view of the biopsy.

[citation needed] Diagnosis is made by biopsy, usually by duodenal endoscopy, which reveals PAS-positive macrophages in the lamina propria containing nonacid-fast, Gram-positive bacilli.

[13] In the United States and some other countries, it occurs predominantly in those of Caucasian ethnicity, suggesting a possible genetic predisposition in that population.