Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (505 page)

   Laboratory Findings

Viral culture
: Mumps virus may be isolated from saliva, urine, or CSF early in acute disease. Viral culture is usually used for complicated infection or when a virus isolate is needed, as for epidemiologic investigation.

Serology
: Mumps is confirmed by a positive mumps-specific IgM result or a significant change in mumps-specific IgG titer in acute and convalescent (2–4 weeks after acute onset) serum samples. IgM usually peaks at approximately day 7 of acute disease and persists for 6 weeks or longer. IgM response may be blunted in previously immunized patients and a negative result does not exclude mumps infection in this population. Detectable IgG levels usually peak at 2–4 weeks and persist for years. See Mumps Serology Screen (Mumps IgG and IgM) in Chapter
17
, Infectious Disease Assays.

Molecular diagnosis
: Real-time PCR for specific mumps sequences has been shown to improve detection of mumps encephalitis.

Core laboratory
: WBC and ESR are normal in acute infection. There may be a slight relative lymphocytosis. Serum and urine amylase are increased during the first week of parotitis; therefore, increase does not always indicate pancreatitis. Serum lipase is normal.

NOROVIRUS GASTROENTERITIS (NORWALK AGENT)

   Definition

Novovirus
has been identified as the major cause of epidemic and endemic gastroenteritis. The Norwalk agent was discovered by immune EM of stool in patients with diarrhea. These viruses have subsequently been classified by molecular techniques as a member of the family
Caliciviridae
. The virus is nonenveloped with a single positive-strand RNA genome. Genetic and immunologic diversity in clinical isolates is significant. Humans are thought to be the only host for
Norovirus.
Infections occur globally and affect individuals of all ages.

   Who Should Be Suspected?

Disease outbreaks have been associated with a wide variety of exposures, including day care centers, long-term care facilities, cruise ships, and restaurants. Persons living in high-density conditions are at high risk. Clinical disease usually presents with abrupt onset of vomiting and/or diarrhea 10 hours to 2 days after exposure. Abdominal cramping is common. Patients often have nonspecific symptoms, including low-grade fever, headache, myalgias, and fatigue. Disease is self-limited in most patients, spontaneously resolving after several days. Prolonged symptomatic and more severe disease may be seen in young children, the elderly, and immunocompromised patients.

   Laboratory Findings

Because most patients have relatively mild, self-limited disease, they do not require specific diagnostic testing. Diagnostic testing may be necessary for patients with severe diseases or to establish the cause of an outbreak.

Molecular testing
: Real-time PCR has emerged as the most widely used assays for diagnosis of
Norovirus
infection. Virus-specific RNA may be detected for several weeks after onset of illness.

Antigen detection assays
: Use of antiserum reagents elicited against recombinant viral antigens has been described, but the sensitivity of available assays is relatively poor.

PARVOVIRUS B19 (ERYTHEMA INFECTIOSUM, FIFTH DISEASE, TRANSIENT APLASTIC ANEMIA)

   Definition

Parvovirus B19 is a single-strand, nonenveloped DNA virus. It is the cause of the erythematous childhood rash erythema infectiosum (fifth disease). Parvovirus B19 infections occur worldwide, causing endemic and epidemic disease. Humans are the only natural host for the virus, with the bone marrow being the primary target of infection. Serologic surveys demonstrate that infection is common. Infection is usually transmitted by respiratory droplets.

   Who Should Be Suspected?

Parvovirus B19 infections occur most commonly in children. The classic presentation involves a confluent, erythematous rash, especially on the cheeks with circumoral pallor (slapped face appearance) with viral syndrome symptoms, like fever, malaise, myalgias, headache, cough, and pharyngitis. Arthralgias develop in some patients. The facial rash fades in several days, followed by formation of a lacy rash affecting the extremities and trunk. Complications of parvovirus B19 infection are uncommon and include hepatitis, myocarditis, and meningoencephalitis. Adults diagnosed with parvovirus B19 infection present more frequently with viral syndrome and arthropathy, and rash is less common. Complications of parvovirus infection during pregnancy include fetal hydrops and congenital anemia; specific IgM is detected in cord blood. Chronic infection may cause severe anemia in immunocompromised persons. Pure erythrocyte aplasia and persistent infection may develop in patients with immunodeficiency or underlying hemolytic anemias, like sickle cell disease, hereditary spherocytosis, pyruvate kinase deficiency, and beta-thalassemia.

   Laboratory Findings

Serology
: Usual diagnostic method. Specific IgM is formed very early in infection, closely followed by IgG. IgM levels begin to wane after 1–2 months but may be detectable for 6 months after acute infection. IgG antibody typically remains detectable for years.

POLIOMYELITIS

   Definition

Poliomyelitis is caused by poliovirus species (types 1–3), in the genus
Enterovirus
. The transmission of polio has been greatly reduced in areas with effective vaccination programs; however, wild-type virus continues to occur sporadically in developing countries. The attenuated virus used in the oral polio vaccine has caused paralytic disease in immunocompromised patients. It is mandated to report paralytic poliomyelitis in all states in the United States. Public health officials should be contacted as soon as paralytic poliomyelitis is suspected. Public health officials may provide guidance regarding confirmatory testing.