Scrub Typhus: Causes, Symptoms, Treatment, Diagnosis

Scrub typhus, also known as bush typhus, is a disease, which is caused by a mite and produces symptoms such as primary lesion, fever, lymphadenopathy and macular rash. Scrub typhus is a type of typhus, which is caused by Orientia tsutsugamushi, an intracellular parasite and a Gram-negative alphaproteobacterium belonging to Rickettsiaceae family.

Although Scrub typhus has symptoms which resembles other types of typhus, its pathogen does not belong to genus Rickettsia anymore, but belongs in Orientia. For this reason, Scrub typhus is often categorized separately from the other typhi.

Treatment of Scrub Typhus comprises of antibiotics, such as doxycycline and chloramphenicol.

The species which transmits Scrub typhus are found in areas which have heavy scrub vegetation. Orientia tsutsugamushi gets transmitted through the bite of trombiculid mites. These mites feed on rural and forest rodents, including voles, rats and field mice. A person develops infection after the bite of the mite larva. When a person gets bitten by this mite, it leaves a characteristic black colored eschar which helps with the diagnosis.

Scrub Typhus has an incubation period of 6 days to three weeks, after which the patient starts to experience abrupt fever, headache, chills and generalized lymphadenopathy. There is also development of an eschar at the site of the bite. The lesion appears as a red, indurated lesion which is around 1 cm in diameter. Later on, there is vesiculation and the lesion ruptures after which it is covered with a black scab. There is enlargement of regional lymph nodes with increase in the patient’s fever during the first week. Patient experiences severe headache, cough and conjunctival injection. There is development of a macular rash on the trunk from 5th to 8th day of fever. This rash spread to the arms and legs and can either disappear rapidly or can turn maculopapular and intensely colored.

In severe cases of scrub typhus, there is increase in the pulse rate and decrease in the blood pressure of the patient. Patient has stupor, delirium and muscular twitching. Interstitial myocarditis occurs more commonly in scrub typhus than other rickettsial diseases. If scrub typhus is left untreated, then patient may have persistent high fever for more than two weeks and it gradually drops over a period of several days. With treatment, the fever starts to abate within 36 hours.

Other symptoms of scrub typhus consist of headache, muscle pain, gastrointestinal symptoms and cough . Strains of O. tsutsugamushi which are more virulent can cause intravascular coagulation and hemorrhaging. Other signs include eschar, Morbilliform rash, lymphadenopathies and splenomegaly. Patient will also commonly have abnormal liver function tests and leucopenia in the early stage of the disease. Encephalitis, pneumonitis and myocarditis occur in the late phase of illness.

Physical exam, medical history and the symptoms of the patient will help with the diagnosis. In endemic regions, diagnosis is usually made on clinical grounds. There are also chances of a misdiagnosis, as the symptoms also resemble other infectious diseases, such as paratyphoid, dengue fever and fever of unknown origin (FUO). Presence of the eschar is quite a definitive diagnosis of scrub typhus; however, this can be quite unreliable in people with dark skin, as the lesion is not clearly defined. The eschar is often present in covered areas of the body, so there is a great likelihood that it can be missed. Patient is also not able to recall the mite bite, as the mites are very small and cannot be seen by a naked eye and the bite does not cause pain. In most patients, scrub typhus is often classified as FUO.

Serological tests, such as Weil-Felix test can be done. Rapid bedside kits also help with diagnosis. Biopsy of the rash can be done with fluorescent antibody staining to help detect the organisms. Culture and polymerase chain reaction also can be done.

Scrub typhus can be fatal if it is not treated. With treatment, the prognosis is good. The most commonly used drug to treat scrub typhus is doxycycline; and chloramphenicol can also be given as an alternative. Doxycycline 200 mg is given by mouth once and then doxycycline 100 mg is given until there is improvement in the patient, such as no fever for 48 hours and treatment should last for at least a week. Second line of treatment is chloramphenicol 500 mg orally or IV for seven days. Rifampin and azithromycin are other drugs, which can be used as alternatives. Ciprofloxacin cannot be safely used in pregnant women. Azithromycin can be used in pregnant women and children with scrub typhus and when doxycycline-resistance is suspected. It is not recommended to combine treatment with doxycycline and rifampicin, as there is a risk of possible antagonism.

No vaccines are available as of now. Clearing the brush and using residual insecticides to spray the infested areas help in decreasing and eliminating mite population. Insect repellants should be used when the patient is at risk for exposure.