The diagnostic panel

Type of pathogen:

Parasite

Viruses

Bacteria

Disease:

Malaria

Dengue

Chikungunya

Typho

Pneumoniae

Malaria is one of the most widespread infectious diseases, in terms of both geographical distribution and infected population. Malaria occurs in over 100 countries but is mainly confined to poorer, tropical areas of Africa, Asia and Latin America. Based on data from the World Health Organization, an estimated 200 million people became infected with malaria in 2010, 80% of whom were in Africa. The disease is caused by a protozoan parasite of the genus Plasmodium, of which four species cause disease in humans (P. falciparum is the most virulent). The infection is transmitted to humans through the bite of female anopheline mosquitoes. The infected suffer from a high fever and the disease is often lethal – particularly in tropical developing countries with insufficient health care services.

For malaria, light microscopy of thick and thin stained blood film smears remains the standard method routinely used for parasitological diagnosis. However, a major drawback of light microscopy is its high cost, and the requirement for well-trained, skilled staff to use it, and is definitely not a point-of-care method. Other, non-microscopic method is based on a number of immunochromatographic (strip) tests available (Rapid Diagnostic Tests – RDTs), which detect parasite-specific antigens or enzymes and have some certain ability to differentiate species. Rapid diagnostic tests provide quick results, they are relatively simple to perform and to interpret, and they do not require electricity or special equipment. Although they are true point-of-care tests, the RDTs lack multiplicity in analysis and often show low sensitivity and specificity.


Chikungunya
occurs in Africa, Asia and the Indian subcontinent and is a mosquito-borne viral disease. In 1999-2000 there was a large outbreak in the Democratic Republic of the Congo, and in 2007 there was an outbreak in Gabon. A large outbreak of chikungunya in India occurred in 2006 and 2007. In 2007 transmission was reported for the first time in Europe, in a localized outbreak in north-eastern Italy.

Various methods can be used for diagnosis. Serological tests, such as enzyme-linked immunosorbent assays (ELISA), may confirm the presence of IgM and IgG anti-chikungunya antibodies. IgM antibody levels are highest three to five weeks after the onset of illness and persist for about two months. The virus may be isolated from the blood during the first few days of infection. Various reverse transcriptase–polymerase chain reaction (RT–PCR) methods are available but are of variable sensitivity. Some are suited to clinical diagnosis. RT–PCR products from clinical samples may also be used for genotyping of the virus, allowing comparisons with virus samples from various geographical sources.


Dengue
is the most rapidly spreading mosquito-borne viral disease in the world. An estimated 50 million dengue infections occur annually and approximately 2.5 billion people live in dengue endemic countries. In the last 50 years, incidence has increased 30-fold with increasing geographic expansion to new countries and, in the present decade, from urban to rural settings. Despite poor surveillance for dengue in Africa, it is clear that epidemic dengue fever caused by all four dengue serotypes has increased dramatically since 1980, with most epidemics occurring in eastern Africa, and to a smaller extent in western Africa, though this situation may be changing in 2008.

Of particular significance is the 2005 World Health Assembly resolution WHA58.3 on the revision of the International Health Regulations (IHR), which includes dengue as an example of a disease that may constitute a public health emergency of international concern with implications for health security due to disruption and rapid epidemic spread beyond national borders. For dengue diagnosis, RNA detection gives results in 24-48 h, identifies serotype and genotype but requires acute samples (0-5 days post onset) and is potential to false positives due to contamination. Antigen detection is easier and less expensive to perform than virus isolation of RNA detection, but less sensitive than the latter. Serological tests allow the IgM or IgG detection which is the easiest and cheapest method, although IgM levels can be low in secondary infections, and confirmation requires two or more serum samples.


Typhoid fever
is a common worldwide bacterial disease, transmitted by the ingestion of food or water contaminated with the feces of an infected person, which contain the bacterium Salmonella typhi. It remains an important cause of disease in developing countries. In 2002, it caused an estimated 400 000 episodes of illness in Africa. Salmonella Typhi, the causative agent, is most frequently isolated from blood during the first week of illness but can also be isolated during the second or third week of illness, during the first week of antimicrobial therapy and during clinical relapse.

For diagnosis, isolation of S. typhi from bone marrow is the current gold standard method for confirming a case of typhoid fever. However, this requires equipment, supplies and trained laboratory personnel seldom found in primary health-care facilities in the developing world. As a result, diagnosis may be delayed or overlooked and patients without typhoid fever may receive unnecessary and inappropriate antimicrobial treatment. For this reason, in developing countries typhoid rapid antibody tests can facilitate diagnosis and disease management.


Pneumonia
is a disease that occurs worldwide and is spread by close contact with persons who are ill or who carry the bacteria in their nose or throat. The reported rates are higher in developing than in industrialized countries. Worldwide, Streptococcus pneumoniae remains the most common cause of community-acquired pneumonia (CAP), bacterial meningitis, bacteremia, and otitis media. Because of pneumococcus based pneumonia and meningitis S. pneumoniae is considered as number 4 of individual micro-organism infections leading to death. The risk for pneumococcal disease is generally highest among young children, the elderly, and people of any age who have chronic medical conditions. Healthy travellers of any age can develop pneumococcal pneumonia while travelling.

A definitive diagnosis of pneumococcal infection can be made by isolating the bacterium from blood or other normally sterile body sites, such as cerebrospinal fluid. Several suppliers offer latex reagents for the detection of pneumococcus antigens in CSF. These tests are not suited for other specimen due to low sensitivity because they are directed to capsular antigens (K-antigens) of most invasive pneumococci while for respiratory diseases also pneumococci with other capsular antigens play an important role.