Scrub typhus: infection reduces viral load in some patients

John S. James, AIDS Treatment News

An accidental discovery in Thailand led researchers at the U.S. Armed Forces Research Institute of Medical Sciences in Bangkok and several other U.S. and Thai medical institutions to conduct additional research which found that the disease scrub typhus can produce unknown HIV suppressive factors, possibly certain antibodies, that can reduce HIV replication.

This is the first infection known which can reduce HIV viral load (most infections increase HIV viral load temporarily). A five-page article on the finding was published recently in The Lancet.

Scrub typhus, caused by bacteria, is common in parts of Asia; it is easily treated with antibiotics, but if untreated, varies in severity from mild to fatal. In this study patients received antibiotics for scrub typhus, but no antiretrovirals. The degree of HIV suppression varied greatly; the average viral load of the patients studied appears to have increased, although not as much as with other illnesses.

However, two of the 10 patients with scrub typhus who were tested in this study had viral loads become temporarily undetectable, and they were also negative for HIV DNA (using a PCR test) – even though all had advanced AIDS, with CD4 counts ranging from 7 to 161 when first admitted to the hospital. And two other patients in that group of ten also had substantial drops in their viral load. [No baseline viral load was available, as patients were already ill with scrub typhus when they came to the hospital, and their viral load may have already been affected; therefore their viral load during the illness was compared with the value at day 28, after their bacterial infection had been successfully treated and the viral load had presumably returned toward its usual set point.]

The great variation in anti-HIV effect suggests that perhaps only some scrub typhus bacteria produce the right antibodies or other factors to inhibit HIV.

A possible mechanism of action for the HIV suppression is the prevention of syncytia, clusters of cells which abnormally merge, become infected, and die. All 10 of the scrub typhus patients has NSI (non-syncytia-inducing) virus, while five of seven comparison patients who did not have scrub typhus has SI (syncytia-inducing) virus.

In another test, serum from an HIV-negative scrub-typhus patient strongly inhibited HIV in a laboratory test. Depletion of antibodies from the serum significantly reduced this effect.

Interestingly, scrub typhus is not more severe in HIV- positive than in HIV-negative persons – and one study found a lower, not higher, rate of positive scrub typhus blood culture in persons who are HIV positive.

An August 8 report by the wire service Agence France-Presse noted that in Thailand, 300,000 people have already died of AIDS, and fewer than 10% of HIV patients can afford the antiviral combinations used in richer countries. Researcher Dr. George Watt, quoted in the report, said that antibodies to scrub typhus could potentially be a very inexpensive treatment. “Every day at the hospital in Chiang Rai we have to tell dying HIV patients, ‘Yes, there is treatment for HIV, but it’s too expensive for you.” So the motivation is there for the scrub typhus work.”


Watt G, Kantipong P, de Souza M, and others. HIV-1 suppression during acute scrub-typhus infection. The Lancet. August 5, 2000; volume 356, pages 475-479.

Source: AIDS Treatment News: Copyright 2000 by John S. James. Subscription and Editorial Office: P.O. Box 411256, San Francisco, CA 94141. 800/TREAT-1-2 toll-free U.S. and Canada 415/255-0588 regular office number.

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