World health day is celebrated each year on 7th April and marks the anniversary of the founding of the World Health Organisation. The theme for 2014 is ‘vector-born diseases’.
The Nuffield Department of Medicine spoke to Professor Nick Day, Director of the Mahidol Oxford Tropical Medicine Research Unit (MORU) about his research on scrub typhus, a vector-borne disease that is common in the Asia Pacific region.
Q: What is a vector-borne disease?
Nick Day: A vector-borne disease is an infectious disease that is transmitted by a third party and not directly from human to human or from the environment. Usually these will be arthropods such as ticks, mites, lice and mosquitos. Vector-borne diseases have always been a major cause of human disease and at the moment they are a major problem in tropical climes. However, vector-borne diseases, such as Lyme disease, are present in the UK. The major burden of disease is found in the tropics and one of the major challenges is testing for these pathogens in rural areas with few microbiology labs.
Q: Why is it important that we raise awareness of vector-borne diseases?
ND: There are a very wide-range of vector-borne diseases, with transmission of anything from tiny viruses such as dengue, bacteria, and through to bigger organisms such as the parasites that cause malaria. We study all of these three categories. The one that we are particularly interested at the moment is a bacterium transmitted by trombiculid mites. The larval stage of these mites bites humans and can transmit a small intracellular bacterium called Orientia tsutsugamushi, which causes scrub typhus.
The reason that we are particularly interested in scrub typhus is that it is a very common disease, especially in rural areas, in the Asia Pacific region. It is under-recognised, difficult to diagnose and under-studied. A lot of farmers and other rural workers catch this disease throughout Southeast Asia. In the last 20 years scrub typhus has increasingly been recognised as being a major cause of fever, including complicated fever causing hospitalisation and even death.
After a few days a small sore (called an eschar), often appearing like a cigarette burn, develops at the site where the mite bites the human. This is usually painless, can often be in places which are covered such as the groin area, and for these reasons can easily go unnoticed by doctor. There are lots of causes of similar fever (e.g. malaria, dengue and leptospirosis) making it difficult to identify the cause if an eschar isn’t identified. We need to raise awareness of the disease to increase recognition of symptoms, so that patients receive treatment as soon as possible.
Q: How many people are affected by scrub typhus?
ND: Scrub typhus first came to light in the 19th century. A disease was discovered in Japan that was thought to be caused by a small mite found in fields. People knew that when these mites appeared in fields where they worked they might develop a fever, which they could then die from. They called this tsustugamushi disease – tsutsugamushi means ‘harmful bug’ in Japanese. This disease was studied by Japanese researchers in great detail during the first part of the 20th century. At the same time researchers in Malaya were researching a similar disease that workers in rubber and palm plantations were catching, which they called scrub typhus. It wasn’t until the 1930s that researchers realised that scrub typhus and tsutsugamushi disease were the same disease and that it was actually quite common throughout the whole of Asia.
Scrub typhus was particularly problematic during World War II when lots of American and Japanese soldiers caught it when they were fighting in the jungles of Southeast Asia. In fact, it was second only to malaria in the number of casualties that it caused. It was discovered quite soon after World War II that you can treat it with antibiotics. Chloramphenicol was the first antibiotic used. After that there was much less research done on it but it was still endemic.
We estimate that 1 or 2 billion people are exposed to tsutsugamushi, and that there are several million cases of scrub typhus per year. It is difficult to put exact numbers on it but we know that whenever we investigate cases of fever in rural Asia we find that between 5 and 25% of those fevers are caused by scrub typhus or related Rickettsial diseases. Luckily it is easy to treat, if you give the right antibiotic. It is therefore important that we understand the biology of this arthropod-bacterium lifecycle. We need to work out how we can diagnose it cheaply and accurately and also raise awareness so that doctors use the right treatment. The problem is that if doctors are treating a fever and they decide to give antibiotics, they usually give a penicillin-like antibiotic which is not effective against these bacteria.
A problem is that currently no good widely deployable diagnostic test exists to identify scrub typhus and this is something that we are working on.
Q: What are the problems we need to tackle in order to combat scrub typhus?
ND: Our research focuses on understanding the biology of this bacterium and the human response to it. A lot of people in remote farming populations get exposed all the time but they don't always become ill. We are therefore studying the immune response to work out better ways of diagnosing and preventing scrub typhus. We are looking at how the immune system responds to scrub typhus infection and if there are any markers we can use to confirm infection.
We are also developing tests that can detect the presence of the bacteria in the blood We are hoping to develop simple tests that are accurate but are can also be used in rural areas so that these cases can be recognised in time and appropriate treatment given. Ultimately if we understand the immune response well enough we may be able to develop an effective vaccine, which so far has not been possible.
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