Scrub Typhus

Scrub Typhus is an acute infectious disease. It was first recognised in Japan in 1899. Scrub Typhus is caused by a rickettsia bacteria called Orientia tsutsugamushi, which is transmitted to humans by mites (vector). The host of the vector, in this case, is wild or field rodents. Neither the rodents nor the mites develop infection from Orientia tsutsugamushi - so they can act as carriers for the disease in perpetuation.

The word “scrub” refers to the type of vegetation where the vector lives. “Typhus” is derived from the Greek word meaning “fever with stupor (state of near-unconsciousness or insensibility)”. The “Tsutsuga” in Orientia tsutsugamushi means small and dangerous and “mushi” means insect or mite.

During World War II, Scrub Typhus emerged as a dreaded disease among soldiers in the Far East. In India, scrub typhus infection first broke out in Assam and West Bengal during World War II. Subsequently, the disease became endemic to India.

In India, the disease is more common in the Shivalik ranges from Kashmir to Assam, in the Eastern and Western Ghats, and the Vindhyachal and Satpura ranges in central India.

During monsoon, a large number of larval mites attach themselves to the rodents. This could be why most outbreaks of scrub typhus occur in the rainy season. However, cases of Scrub Typhus have also been reported in other seasons.

Every year in India, Scrub Typhus affects people across age groups. When properly treated, the chances of death are under 1%. Failure to diagnose and treat the disease can, however, raise mortality risk to 50%. Proper treatment depends on the specific strain of O. tsutsugamushi that causes the infection, the duration of the infection, the patient’s overall health, and taking the right antibiotics without delay.

The causative organism of Scrub Typhus is Orientia tsutsugamushi - a gram-negative bacteria of family Rickettsiaceae which is an obligate intracellular parasite. An obligate intracellular parasite is one that cannot reproduce outside the host cell.

Vector: The vector for Scrub Typhus are mites like L. deliense and Leptotrombidium akamushi. Vectors for Scrub Typhus are present in almost all countries of the South-East Asian region and they are endemic in a few regions of India, Indonesia, Maldives, Myanmar, Nepal, Sri Lanka, and Thailand. In some of these endemic regions, there is especially intense transmission of Scrub Typhus from limited areas - these are known as typhus islands.

The infection is transmitted only through the larval mites also known as “chiggers”. This is because it's only at the larval stage that these mites feed on blood.

Host: Small, wild rodents are the natural host for Scrub Typhus. The bacteria does not cause disease in rodents, making them good agent for transferring the disease to humans.

Transmission of disease: The bacteria O. tsutsugamushi survives in mites. It goes from being an egg to the larva or adult stage through a process called transstadial transmission (a mode of gene transfer).  O. tsutsugamushi is not a parasite for mites - mite larvae feed on human blood. 

The mite may attach itself to the body of a person walking through the woods, or lying down in the grass. Rather than biting or piercing the skin, mite larvae usually insert a part of their mouths down the hair follicles or into pores on human skin, thereby causing the disease. A large number of O. tsutsugamushi are present in the salivary glands of the larvae and these are injected into their host or human blood when they feed.

Scrub Typhus usually presents itself with non-specific signs and symptoms. The most common symptom experienced by people with Scrub Typhus is fever. In endemic areas, undiagnosed fever associated with Scrub Typhus is considered as fever of unknown origin. A person bitten by infection carrying mite manifests symptoms after five to 14 days.

Here are a group of signs and symptoms of Scrub Typhus:

• Fever 
• Chills and rigors 
• Headache
• Body ache 
• Muscle pain
• A dark, scab-like region at the site of the chigger (larva of mites) bite, also known as eschar. It usually appears on the groin, axilla, genitalia, or neck region. Though the eschar is the single most important clue for diagnosis and is a pathognomonic sign (specific to Scrub Typhus only), it can appear in as few as 7% of the patients, or as many as 80% of them.
• Mental changes, ranging from confusion to coma
• Regional lymphadenopathy (enlarged lymph nodes)
• Skin rash

Scrub Typhus infection can become severe when the symptoms appear very late, or when there is no diagnosis, late diagnosis or antibacterial resistance. In these cases, Scrub Typhus may become life-threatening and present with the following symptoms:

• Hepatosplenomegaly or swelling in the liver and spleen
• Gastrointestinal bleeding
• Pneumonia 
• Acute respiratory distress

Your doctor will take a detailed history of the disease. After that he will look at the clinical signs of the disease like fever and eschar, which is pathognomonic for the disease. Presence of other clinical signs and symptoms will aid them in making a provisional diagnosis of Scrub Typhus.

For making definitive diagnosis, further diagnostic tests may be recommended. These include:

• Serology, to detect the antibody for Scrub Typhus in the blood. A significant antibody (IgM) titre is seen by the end of the first week of infection. The Weil-Felix (WF) test is widely used to check for Scrub Typhus, as it is cheap and easy to do
• Indirect immunofluorescence is the gold standard diagnostic test for Scrub Typhus. It checks for the presence of Scrub Typhus-specific antibody bound to smears of Scrub Typhus antigen. This can confirm infection before the body forms antibodies against the antigen
• Biopsy of rash with fluorescent antibody staining to detect organisms
• Polymerase chain reaction (PCR)

After confirming the diagnosis, the doctor will prescribe medication to eliminate the infection from the bloodstream. In suspected cases of Scrub Typhus, tetracycline or chloramphenicol remain the most commonly prescribed drugs.

Doxycycline is the first line of treatment for scrub typhus disease in a dose of 2.2 mg/kg/dose, twice a day, taken orally or intravenously. The maximum dose can be 200 mg/day for 7–15 days.

Tetracycline is recommended with a dose of 25–50 mg/kg/day every six hours, given orally. The maximum dose can be 2g/day, for 7–15 days.

Alternative medicine chloramphenicol is given at 50–100 mg/kg/day every six hours, intravenously. The maximum dosage is 3g per 24 hours, or 500 mg qid orally for 7–15 days for adults. If chloramphenicol is prescribed, it should be closely monitored to maintain serum concentrations of 10-30 μg/mL. Chloramphenicol is contraindicated during pregnancy. Reduced dosage is also advised in patients with hepatic impairment (liver problems).

Other antibiotics which seems to be effective in scrub typhus are azithromycin (500 mg orally for 3 days), rifampicin, and roxithromycin. 

Roxithromycin (150 mg, twice a day) was said to be as effective as either doxycycline or chloramphenicol, suggesting a role as an alternative therapy for children or pregnant women.

For prophylaxis against scrub typhus in endemic areas, doxycycline is recommended.

If not treated properly, scrub typhus can be fatal. The complications that can arise due to Scrub Typhus are: 

• Meningo-encephalitis or swelling in one part of the brain
• Hepatitis or inflammation of the liver
• Acute renal failure
• Pancreatitis
• Acute respiratory distress (ARD)
• Multi organ failure

Currently, there is no vaccine for Scrub Typhus. You can reduce your risk of getting Scrub Typhus by avoiding contact with infected chiggers.

• When travelling to endemic places, avoid areas with lots of scrub/bush vegetation
• Use insect repellents specifically meant for chiggers on exposed skin as well as clothing. The insect repellent should be registered under Environment Protection Agency (EPA)         
• You can also ask your doctor to prescribe antibiotics to guard against infection (chemoprophylaxis) by Scrub Typhus