DIAGNOSIS OF SCRUB TYPHUS

INTRODUCTION

Scrub typhus is a condition of acute febrile illness. The patient infected with scrub typhus has a persistent fever for 2-7 days with a body temperature of 38 degrees Celcius or higher. Scrub typhus is caused by the gram-negative bacterium Orientia tsutsugamushi. Scrub typhus is transmitted when infected chiggers in their larval stage bite an individual. The diagnosis of scrub typhus is often challenging because its symptoms are very similar to those of many other acute febrile illnesses.

SOME BASIC INFORMATION ABOUT SCRUB TYPHUS

Scrub typhus is a zoonotic disease, which is an infectious disease transmitted between animals and humans. In this case, the animal is a particular species of trombiculid mites, particularly Leptotrombidium delicense. These animals are abundant in places with heavy scrub vegetation, hence “scrub typhus.” The mite larvae are known as chiggers, ectoparasites of rodents. Contact with infected chiggers causes infection.

The disease is more endemic in regions that form the tsutsugamushi triangle. This triangle covers the Russian Far East, Japan, Northern Australia, and Afghanistan. Although there is evidence to support the presence of the disease in other regions, it is more prevalent in the parts mentioned above.

Now, let us dive into the central focus of our discussion, which is how the diagnosis of scrub typhus is carried out.

DIAGNOSIS OF SCRUB TYPHUS:

The organism that causes scrub typhus is a bacterium named Orientia tsutsugamushi. It is a Gram-negative bacterium. It has at least six strains with various degrees of variability. The strains are Karp, Kato, Kuroki, Kawasaki, Shimokoshi and Gilliam. For example, suppose you have recently visited an area known to be endemic for scrub typhus. You took part in outdoor activities like games, farming, and others. In that case, you might need to go for laboratory tests to confirm whether you have been infected or not.

As stated earlier, the diagnosis of scrub typhus is quite tricky due to the close resemblance of its symptoms to those of other related acute illnesses. The diagnosis requires laboratory techniques such as

• Weil-Felix test
• immunofluorescence assay (IFA)
• immunochromatographic test (ICT)
• polymerase chain reaction (PCR)
• Loop-mediated isothermal amplification (LAMP)
• Enzyme-Linked Immunosorbent Assay (ELISA)
• Immunoperoxidase assay
• The physician can also do a blood test to detect the bacteria’s antibodies.

There are primarily two methods of diagnosing Scrub Typhus:

1. Direct method
2. Indirect method.

In the indirect method of diagnosis, the test involves testing for the presence of antibodies that fight against Orientia tsutsugamushi in the body. The serological assays used are based on detecting the presence of immunoglobulin G (IgG) and immunoglobulin M (IgM) in the patient’s blood. These methods are more popular than direct methods like blood culture and PCR-based assays because the latter requires more sophisticated equipment and highly trained personnel, which might not be readily available in most areas where the disease is endemic. As a result, most of the serological kits available in the market for diagnosing scrub typhus are based on the detection of IgG and IgM.

IgM-based methods can detect acute infection, while IgG-based methods are more useful when the prevalence of the disease is to be determined. Therefore, it is advisable to use paired serum samples in serological assays.

The direct immuno-based methods include the

• Weil-Felix test
• Immunofluorescence assay (IFA)
• Immunochromatographic test (ICT)
• Indirect Enzyme-Linked Immunosorbent Assay (ELISA) and
• Immunoperoxidase assays.

Now, let us elaborate on some of these direct methods.

• Weil-Felix test is an agglutination method used to detect rickettsial infection in the patient’s serum samples. This method detects the patient’s immune response against different Proteus antigens like OX19, OXK, and OX2. It is used chiefly in developed countries. However, it has less specificity and sensitivity because it uses non-rickettsial antigens. Despite these, many small laboratories still use it to diagnose scrub typhus.
• Immunofluorescence assay makes use of fluorescein-linked anti-human reporter antibody for detecting the presence of scrub-typhus-specific antibodies in the serum sample of the patient. Immunofluorescence assay is considered a gold standard for diagnosing scrub typhus due to its high specificity and sensitivity. However, other techniques like ELISA have similar specificity and sensitivity and are cheaper.
• ELISA is ranked best among the serum-based methods for diagnosing scrub typhus due to its high sensitivity and specificity. Most of its methods focus on IgG and IgM antibodies in the serum samples of scrub typhus patients to detect Orientia tsutsugamushi. IgM captured ELISA is a better method for diagnosing scrub typhus in the early stages of infection. This is because IgM antibodies can be seen after the first week of illness, unlike IgG antibodies which appear almost at the end of the second week.
  Some studies have proved immunochromatographic tests (ICT) to be more sensitive and specific for detecting IgM. ICT can be combined with another method to obtain more accurate results. ICT has a higher sensitivity and specificity than Weil-Felix, passive hemagglutination, and immunofluorescence assays.

TO WRAP UP

The diagnosis of scrub typhus is not as easy as other common diseases. Patients often do not present with classic symptoms that indicate it. This is due to the close similarities between its symptoms and other acute febrile illnesses. There is also no officially approved vaccine for Scrub Typhus at the moment. Therefore, it is essential to prevent getting infected by observing necessary precautions as prevention is always better than cure. Avoid traveling or visiting places where the disease is known to be endemic. If you have to visit these places, you should apply insect repellants on your skin and wear long clothes covering your hands and legs.

REFERENCES

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2. CDC. Scrub Typhus [Internet]. Centers for Disease Control and Prevention. 2021 [cited 2022 Feb 28]. Available from: https://www.cdc.gov/typhus/scrub/index.html
3. Kala D, Gupta S, Nagraik R, Verma V, Thakur A, Kaushal A. Diagnosis of scrub typhus: recent advancements and challenges. 3 Biotech [Internet]. 2020;10(9):396. Available from: http://dx.doi.org/10.1007/s13205-020-02389-w
4. Chen J, Zheng X-D, Dai Q-H, Hong W-L, Li Y-P, Chen R, et al. Diagnosis of severe scrub typhus infection by next-generation sequencing:a case report. BMC Infect Dis [Internet]. 2020;20(1):270. Available from: http://dx.doi.org/10.1186/s12879-020-04991-y