Rickettsial infections are unknown, yet surprisingly common

The Star

26-06-2025

Rickettsial diseases, or rickettsioses, are a diverse group of bacterial infections that are spreading and/or reappearing, especially in South-East Asia, including Malaysia.
These diseases are mainly transmitted through the bites of ectoparasites such as ticks, fleas, lice and mites.
They accounted for approximately 26% of all reported infectious pathogens associated with fever-causing illness in South-East Asia, second only to the dengue virus.
Yet, they remain under-recognised by both the general public and healthcare authorities.
And this is despite the fact that delayed or untreated cases can result in death rates as high as 30%.
These bacteria are commonly found in a variety of animals, including rats, companion animals such as dogs and cats, as well as livestock.
They pose both health and economic burdens, particularly in rural and farming communities.
Infected animals may not always show symptoms, making detection and control more challenging.
It is important to highlight that zoonotic transmission – the spread of disease from animals to humans – can occur through the bite of the tiny ectoparasites that infest these animals.
Types of rickettsial diseases
Scrub typhus, one of the most common rickettsial diseases in humans, is caused by the bacteria Orientia and is spread by tiny mites known as chiggers.
This is a life-threatening illness affecting approximately one billion people.
It is also one of the most underdiagnosed and underreported fever-causing illnesses requiring hospitalisation in Asia.
Its non-specific symptoms are often similar to those of other tropical diseases such as dengue, malaria and leptospirosis.
Limited access to diagnostic tests and low clinical awareness, particularly in rural or resource-limited settings further contribute to misdiagnosis and underreporting.
Other common human rickettsioses in our region include murine typhus, carried by fleas from rats; and spotted fever, transmitted by fleas and ticks.
Anaplasmosis and ehrlichiosis transmitted by ticks, on the other hand, are common diseases in animals, particularly in dogs.
They can lead to severe illness, or even death, in the affected animals if left untreated.
Zoonotic transmission to humans is also frequently reported worldwide, making them a significant public health concern.
Another often-overlooked disease is neorickettsiosis.
This infection remains largely unknown, not only to the general public, but also within the scientific community.
Its causative agent Neorickettsia is transmitted through flatworms found in fish, making its life cycle distinct from other rickettsial diseases.
Human infection has been reported in several parts of Asia, including Japan, Malaysia, Laos and Thailand.
It is typically acquired through the consumption of raw or fermented fish.
The spotted fever-causing Rickettsia asembonensis , commonly found in cat fleas, is also worth highlighting.
It was first detected in monkeys and later identified in a 15-year-old human patient in 2013 in Malaysia, marking the first report of human infection.
It has since been reported in humans from other countries and found in additional animals, including goats, sheep, dogs and cats, suggesting a broader host range and the potential for widespread distribution.
Rickettsial diseases often affect rural and farming communities due to close contact with wildlife or livestock, and exposure to infected bugs.
However, there is a growing trend of human infections in urban areas, particularly murine typhus and spotted fever, which are linked to the rats and pet animals such as dogs and cats in urban areas.
Individuals exposed to flea-infested cats, dogs or rats are especially at risk of infection.
A typical eschar observed in a patient with scrub typhus. — The American Journal of Tropical Medicine and Hygiene
Difficulty in diagnosis
Recognising rickettsial infections can be challenging because the symptoms and signs are non-specific and often similar to other fever-causing illnesses such as leptospirosis and dengue.
Common symptoms include cough, vomiting, headache, diarrhoea, muscle pain, skin rash, swollen lymph nodes and eschar (a dark scab at the site of a bite).
While the presence of eschar is the key sign of rickettsioses, especially scrub typhus, it may not be present in many infected patients, making diagnosis more difficult in most cases.
These bacteria are genetically diverse, and various new strains, especially within the spotted fever group, have been reported in our region.
Many test kits developed elsewhere fail to detect pathogens specific to our region as they are often based on strains that are not native to this part of the world.
Furthermore, some test kits can only indicate that the infection belongs to a broader group, such as scrub typhus or spotted fever, rather than identifying the specific germ that causes the illness.
This highlights the need for locally-tailored diagnostic tools to ensure that these infections are not missed, allowing for timely and accurate treatment.
While the antibiotic doxycycline remains the common treatment, a specific diagnosis is essential as the duration, choice of alternative agents, potential doxycycline resistance and clinical severity can all influence the treatment regimen.
Prevention and awareness
To fight these diseases, correct use of insect repellents and awareness are keys to preventing bites, especially in high-risk areas or outdoor environments where insects are abundant.
Wearing long sleeves and pants also provides an effective physical barrier against insect bites.
This applies not only to rickettsioses, but also to more common illnesses such as malaria and dengue, which are widespread in Malaysia.
Regular inspection of pets for ectoparasites is also important, and infestations should be managed promptly, using appropriate measures such as medicated shampoos, anti-parasitic collars or spot-on treatments.
To raise awareness and strengthen regional collaboration in combating rickettsial diseases, Malaysia will be hosting the 4th Asia Pacific Rickettsial Conference (APRC4) for the first time from Sept 29 to Oct 1 (2025) in Penang.
This conference will serve as a key platform for researchers, healthcare professionals and experts from diverse fields, including medicine, veterinary science and pest control, to come together.
By adopting a One Health approach that emphasises collaborative efforts across multiple disciplines at local, national and global levels to achieve optimal health for people, animals and the environment, APRC4 aims to bring together stakeholders to collaborate on strategies for the prevention, diagnosis and management of rickettsial diseases in the region.
Associate Professor Dr Lucas Low Van Lun is the executive director of Universiti Malaya's Tropical Infectious Diseases Research and Education Centre and president of the Malaysian Society of Parasitology and Tropical Medicine. For more information, email [e-mail]. The ­information provided is for educational and communication purposes only, and should not be considered as medical advice. The Star does not give any warranty on accuracy, completeness, ­functionality, ­usefulness or other assurances as to the content appearing in this article. The Star disclaims all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.