Differentials
Pneumonia, bacterial or viral
SIGNS / SYMPTOMS
Cough is productive.
INVESTIGATIONS
Chest x-ray usually shows areas of consolidation (increased radiodensity), whereas in human monocytotropic/monocytic ehrlichiosis (HME) and human granulocytotropic/granulocytic anaplasmosis (HGA), chest x-ray might show interstitial pneumonitis simulating a viral infection.
Viral infections are ruled out based on specific laboratory tests (i.e., antigen detection, serology, polymerase chain reaction).
Gastroenteritides, bacterial or viral
SIGNS / SYMPTOMS
Nausea, vomiting, and diarrhoea are more prominent.
INVESTIGATIONS
Direct examination of stool may demonstrate polymorphonuclear neutrophils and/or blood.
Stool cultures and antigen detection confirm diagnosis.
Cholecystitis
SIGNS / SYMPTOMS
Right upper quadrant pain is usually localised.
INVESTIGATIONS
Right upper quadrant abdominal ultrasound may show inflammation and dilation of ducts in the area of the gallbladder.
Sepsis
SIGNS / SYMPTOMS
There are no differentiating signs or symptoms.
INVESTIGATIONS
Positive blood cultures.
Rocky Mountain spotted fever
SIGNS / SYMPTOMS
Frequency of rash is much higher.
INVESTIGATIONS
Positive polymerase chain reaction or immunofluorescence antibody assay specific for rickettsial DNA.
Thrombotic thrombocytopenic purpura
SIGNS / SYMPTOMS
Thrombocytopenia and a purpuric rash.
INVESTIGATIONS
Peripheral smears show schistocytes.
Serum shows elevated lactate dehydrogenase.
Babesiosis
SIGNS / SYMPTOMS
Jaundice is more frequent due to haemolysis.
Can coexist with human granulocytotropic/granulocytic anaplasmosis (HGA) and/or Lyme disease. All 3 aetiological agents are transmitted by the same tick vector.
INVESTIGATIONS
Diagnosed in the acute phase by observing typical intra-erythrocytic parasites in peripheral blood smear.
Polymerase chain reaction is available at research laboratories and is positive for Babesia microti DNA/RNA.
Serological diagnosis is possible in convalescent period. Antibodies against B microti present.
Malaria infection
SIGNS / SYMPTOMS
Travel history to endemic areas. Presence of cyclical fevers.
Jaundice is more frequent due to haemolysis.
INVESTIGATIONS
Diagnosed in the acute phase by observing typical intra-erythrocytic parasites in peripheral blood smear.
T-cell lymphoma
SIGNS / SYMPTOMS
Lymphadenopathy is not short lived.
INVESTIGATIONS
Circulating lymphocytes do not start decreasing by third week of illness.
Tests for clonality: T-cell receptor re-arrangements. However, in sporadic cases, lymphocytosis is severe enough to cause false-positives with this test, and follow-up testing is indicated.
Typhoid infection
SIGNS / SYMPTOMS
May be difficult to distinguish; however, gastrointestinal symptoms are more frequent.
INVESTIGATIONS
Blood and/or stool cultures positive for Salmonella enterica serovar S typhi or S paratyphi.
Lyme disease
SIGNS / SYMPTOMS
Presence of erythema migrans.
Can coexist with human granulocytotropic/granulocytic anaplasmosis (HGA) and/or babesiosis. All 3 aetiological agents are transmitted by the same tick vector.
INVESTIGATIONS
Serological diagnosis is possible in convalescent period. Antibodies against Borrelia present.
Western immunoblotting shows presence of Lyme-specific IgM and IgG.
Polymerase chain reaction positive for Borrelia.
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