Rickettsial Diseases of Recent Significance in India

Abstract

Rickettsial diseases are considered as re-emerging zoonotic diseases in India caused by rickettsial pathogens. The rickettsial organisms are obligate intracellular vector-borne pathogens causing wide-ranging symptoms. Rickettsia is subdivided into typhus and spotted fever groups. The reports of the disease have been increasing since the start of the 20th century in different parts of India mainly in rural areas with abundant vector populations especially in the post-monsoon season (August-November). The reports of outbreaks are recorded in various states such as Tamil Nadu, Himachal Pradesh, Manipur, Delhi, Puducherry, Uttarakhand, Jammu and Kashmir, Maharashtra, Karnataka, West Bengal and northestern parts of India. The outbreak of Scrub typhus is found to be the major among the diseases. However, the burden of the disease in India remains still unclear owing to the lack of accurate diagnostic facilities. The larval stages of the vectors are vital to the transmission of the disease and cause symptoms such as eschar formation present in 50%–80% of cases, fever, vomiting, headache, abdominal pain, breathlessness and lymphadenopathy. With the advancement in molecular biology, PCR and qPCR and serological assays such as Weil-felix and ELISA are adopted for diagnostic purposes. Surveillance strategies, avoidance of open defecation and urination, and wearing fully covered clothes are key strategies to prevent infection.

1. Introduction

Rickettsial diseases have emerged as one of the major public health threats to mankind. Rickettsial infections are caused by a wide range of obligatory intracellular, Gram-negative pathogens belonging to the genus Rickettsia, Orientia, Ehrlichia, Neorickettsia, Neoehrlichia, and Anaplasma of Alphaproteobacteria. These are the emerging and re-emerging diseases with a peak in reports in recent times with the advancement in diagnostics. The most observed rickettsial infections in India are scrub typhus, murine flea-borne typhus, Indian tick typhus and Q fever. The rickettsial diseases are vectorborne zoonotic diseases, transmitted to humans through the bite of infected mites, lice, fleas and ticks. Rickettsial diseases are difficult to diagnose with a high fatality of 9–70% amongst untreated individuals and are a threat to global public health [1]. Rickettsial diseases remain obscured in many parts of India with lacunae in the infrastructure to diagnose the pathogens at many places and disturbance of the vector habitat with human interventions.

2. Etiology

The rickettsial pathogens are small, gram-negative bacilli, with a unique obligate intracellular parasitism that makes them distinct from other Gram-negative microbes. The microbes are handled in the laboratory with the use of eukaryotic cells (e.g., in susceptible animals, embryonated eggs, cell culture) for isolation to mimic the intracellular lifestyle, unlike other bacteria which can be cultivated with the help of agar enriched with the required nutrients [1]. The rats and mice act as major mammalian reservoirs [2]. The pathogens are acquired by hematophagous arthropods (lice, mites, fleas, ticks), the pathogens multiply in the alimentary canal of ectoparasites and are accidentally transmitted through bites to humans (Table 1) [3].

Diseases	Etiological agent	Vectors	Mammalian reservoirs
Spotted fever group
Indian tick typhus	Rickettsia conorii	Tick	Rodents, dogs
Rocky mountain spotted fever	Rickettsia rickettsii	Tick	Rodents, dogs
Rickettsial pox	Rickettsia akari	Mite	Mice
Typhus group
Epidemic typhus	Rickettsia prowazekki	Louse	Humans
Murine typhus	Rickettsia typhi	Flea	Rodents
Scrub typhus	Orientia tsutsugamushi	Mite	Rodents
Others
Q fever	Coxiella brunetti	Tick	Cattle, sheep, goats
Trench fever	Rochalimaea quintana	Louse	Humans

Table 1.

Details of rickettsial diseases with etiological agents, vector and host.

3. History

The rickettsia groups of pathogens have bombarded the world since the 5th century BC. The description is mainly associated with the louse, though the identification and classification were not there. The typhus was described to be the cause for Napoleon to stop the invasion of Russia in 1812. It caused havoc during the period of world war I with approximately 30,000,000 cases and 3,000,000 deaths [4].

Rickettsial diseases are widely reported across the world. Scrub typhus is the major rickettsial disease reported in India. As far as other rickettsial diseases in the Indian context are concerned, was reported by Megaw (1917) in an European male of febrile case, bitten by a tick on his way from Almora to Lucknow. An epidemic of 12 cases was diagnosed by Weil–Felix reaction by Major E.S. Phipson (1922) at Shimla [2]. The study over the past century is suggestive of greater prevalence of Scrub typhus in India. Non-Scrub typhus Rickettsial Diseases, Spotted fever group and typhus group are not thoroughly studied as Scrub typhus. The Spotted Fever Group include Indian tick typhus, tick-borne Rocky Mountain spotted fever, African tick bite fever caused by Rickettsia conorii, R. rickettsii, R. africae respectively. In addition, Rickettsia akari causes mite-borne Rickettsial pox. The Typhus Group consists of flea-borne endemic typhus (murine typhus) caused by R. typhi and louse-borne Epidemic typhus caused by R. prowazekii [5, 6].

In India reports of scrub typhus date to 1930s in Kumaon region, among the soldiers at the time of the Second World War in Assam, murine and scrub typhus were reported from Jabalpur of Madhya Pradesh and murine typhus was reported from Kashmir. Scrub typhus which is the commonest has been reported in several states of India including J&K, Himachal Pradesh, West Bengal, Meghalaya, Uttarakhand, Bihar, Karnataka, Tamil Nadu Rajasthan, Maharashtra, and Kerala [7, 8, 9, 10]. In a few places, 50% of non-diagnosed fevers presented to the hospital were found to be Scrub typhus [2].

4. Transmission

Rickettsial diseases are zoonotic in nature where humans are accidental hosts in the chain of transmission within ticks, trombiculid mites (chiggers), or fleas and animals (mostly rodents). Commonly found rickettsiosis in India are scrub typhus, Indian tick typhus, murine flea-borne typhus, and Q fever. Orientia tsutsugamushi is transmitted to humans and other vertebrates, with the bite of infected larval trombiculid mites (Chiggers). The mites usually feed upon specific vertebrate hosts, mostly rodents [1].

The rodent and the vector act as reservoirs which perpetuates the pathogen in nature. The vector is found in different ecological niches such as rainforests, semi-deserts, Himalayan region. Endemicity is associated with habitats such as gardens, forest clearings, shrubby fringes of fields and grassy fields. These ecological patches that harbor the regular host of mite vectors are called ‘mite islands [2].

5. Pathogenesis

The pathogen is introduced into the body with the feeding of a tick or mite or through the scratch of fleas or deposition of louse feces onto the skin. These events mark the beginning of infection. Initially, upon entry, CD8+ Dendritic cells or macrophages are the targets. Later the pathogen spreads primarily to infect endothelial cells via the hematogenous route and lymph nodes through the lymphatic channel.

The main lesson that can be observed is Vasculitis, which involves events like activation of endothelium and then spread to other cells, this stimulates cell signaling which releases cytokines and chemokines to produce an innate immune response. The CD4 and CD8 T lymphocytes and macrophages rush to the site, causing a non-occlusive platelet plug and resulting in interstitial edema with increased vascular permeability. The above event leads to the formation of rashes which proceeds to form macules to maculo-papules. The lesion causes life-threatening conditions in various organs, interstitial pneumonia, non-cardiogenic pulmonary edema, and diffuse alveolar damage in the lungs, while in the CNS, meningoencephalitis has glial nodules. Similar damage is observed in various organs that are affected [5].

6. Clinical manifestation

Spotted Fever Group (SFG) and Typhus Group (TG) are two categories for rickettsial illnesses in terms of symptomatology. The three main diseases in SFG are rickettsial pox, Rocky Mountain spotted fever, and Mediterranean spotted fever. In TG, typhus outbreaks and endemic typhus are the primary clinically significant infections. In this population, recrudescent typhus (Brill-Zinsser disease) is a disease of epidemiological significance. A nearly comparable illness brought on by the genus Orientia is scrub typhus. It is crucial to get a thorough history of outdoor activities, recent travel to endemic regions, family pets, tick/mite exposure within 15 days of sickness, and other affected family members. The history would help to make a tentative diagnosis. The bites of ticks and mites are quite painless, and patients might not observe. The incubation period for any of the group diseases is almost 5–15 days. High fever is considered to be the prominent feature among all rickettsial groups. Multisystemic affection may be observed by disseminating the organism to different organs [6].

7. Cutaneous

Within three to five days of the start of a fever, a mild erythematous skin rash appears. Rose-red, distinct, Blanche macules begin at the wrists and ankles and progress (centripetal spread) to the proximal extremities, trunk, and face. Traditionally, the palms and soles are included. The rash in persons with dark skin might not be immediately apparent. The rash in RMSF may occasionally be transient, localized, or completely absent (20%). The rash gradually develops a petechial or maculopapular appearance. As the condition worsens, confluent purpuric lesions may appear, signalling a serious sickness [6, 11, 12].

8. Systemic

High fever with sudden onset occasionally subsides the next day morning. At the time of presentation, fever may also be accompanied by shaking chills, headache, myalgia, non-productive cough, dyspnea, nausea, and vomiting, as well as calf discomfort and tenderness. Frontal headaches are typical. It is a recurring trait and could be serious. Myalgia frequently affects the lumbar region, thigh, and calf muscles, causing pain in the lower back and lower limbs. Headache and injection of the conjunctiva are frequent first symptoms of scrub typhus. Most patients exhibit hepatosplenomegaly and generalized lymphadenopathy [12].

9. Diagnosis

The vague clinical symptoms that mimic viral exanthema make rickettsial illnesses difficult to diagnose in the first few days. Even in endemic areas, between 60 and 75% of patients with RMSF are initially misdiagnosed as suffering from another ailment [12]. The ectoparasite bite is typically painless, therefore the victim may not be aware of an eschar. Given that the patient may be experiencing the early stages of the illness and that fever and even rash may be missing in these patients, the absence of fever or rash in a patient at presentation should not prevent the doctor from diagnosing rickettsial fever [13].

The case definition for rickettsial diseases has been defined by the Indian Council of Medical Research (ICMR) as follows [14].

9.1 Definition of suspected/clinical case

Acute febrile illness for a period of 5 days or more with or without eschar must be suspected as a rickettsial infection (if eschar is found, it should be considered as scrub typhus irrespective of duration of fever). Other features may include headache, rash, lymphadenopathy, multi-organ involvement like liver, lung and kidney and acute respiratory distress.

The differential diagnosis with malaria, pneumonia, dengue, leptospirosis and typhoid should be carried out”.

9.2 Definition of the probable case

A suspected clinical case is the one that shows a titer of 1:80 or more in OX2, OX19, and OXK antigens with weil-Felix reaction and an OD (optical density) > 0.5 for IgM ELISA is considered as positive for typhus and spotted fever classes.

9.3 Definition of the confirmed case

A case is said to be confirmed when (a) Rickettsial DNA is found in eschar or whole blood by PCR, or (b) Rise in the titer of antibody in acute and convalescent serum by indirect immune fluorescence assay (IFA).

10. Immunofluorescence assay (IFA)

This test is regarded as the “gold standard” for rickettsial infection diagnosis. The test’s sensitivity is dependent on how long the illness has been present; typically, it turns positive after 7–10 days. Beyond 14 days of illness, the IFA assay’s sensitivity increases to 94–100%, making it the most sensitive serological test for rickettsial infection. It is recommended to test the matched sera (acute and convalescent) after a gap of two to three weeks to show a more than fourfold increase in IgM and IgG titers. This test requires good infrastructure and is only available in a few centers, and it is useless in the first week after being ill. IgM antibodies fade in RMSF by 3–4 months, while IgG antibody wane by 7–8 months [7]. IgM and IgG antibody detection for rickettsial species may not be very useful in endemic regions because of anti-R. conorii IgM has been shown to cross-react with bacterial antigens including Legionella spp. and Coxiella burnetii, leading to false positive results [8].

11. Enzyme-linked immuno sorbent assay (ELISA)

ELISA is employed to diagnose rickettsial infections. Serum IgM capture assays demonstrate recent infection; however, it is necessary to establish the substantial baseline titer in each endemic area [12].

12. Polymerase chain reaction

Early and quick diagnosis is possible using rickettsial antigen detection by PCR. Blood is not a very good sample for this; instead, a skin or organ biopsy specimen should be used. However, once more, only some laboratories have access to this capability. Doxycycline treatment reduces the sensitivity of PCR [12].

13. Immunoperoxidase test

This is another gold-standard test for the diagnosis of rickettsial fever and is regarded as being equivalent to the IFA assay. It is only occasionally available, though, and is primarily utilized for study [9].

14. Weil-felix reaction

This is an antibody test for heterophiles. The antigens shared by Proteus species (P. vulgaris: OX19, OX2; P. mirabilis: OXK) and Rickettsia species (P. mirabilis: OXK), as well as the presence of agglutinins to these organisms in a ‘suspect case’ of rickettsial fever, form the basis of this test. Given that non-rickettsial infections can also exhibit a high quantity of agglutinins, this test lacks the sensitivity and specificity of an effective diagnostic test [7].

There are lacunae in the diagnosis and improper reporting of disease at different levels of the health care system. The rickettsial disease reported in India is mainly with the weil felix reaction, followed by ELISA. Though the IFA is considered the gold standard it is not economical and not all hospitals have access to this facility. The molecular techniques for the diagnosis are limited, especially the characterization and differentiation of rickettsial species, so there is a lack of epidemiological data for the prevention and control strategies [10].

15. Rickettsial diseases of recent origin

Rickettsial diseases in India have been reported since the time of independence, and there have been numerous reports at different geographical setups. The various rickettsial infections reported from India are Scrub typhus, Indian tick typhus, Spotted fever, Malignant Mediterranean spotted fever, and other flea-borne rickettsial infections. The most commonly reported rickettsial infections in India are Scrub typhus, Indian tick typhus, and Murine flea-borne typhus [15].

The earlier reports of rickettsial infection were reported during World War II in the forest area of the North Eastern states of India such as Assam and West Bengal, then later the reports were also from the Indo-Pakistan border during the 1990s. The recent reports of outbreaks from 2000 are recorded in various states such as Tamil Nadu, Himachal Pradesh, Manipur, Delhi, Puducherry, Uttarakhand, Jammu and Kashmir, Maharashtra, Karnataka, West Bengal and northeastern parts of India [15].

16. Reasons for the re-emergence of Rickettsial disease in India

India is a unique country with varying geographical conditions with a variety of flora and fauna. The rickettsial diseases being vector-borne diseases were observed to be in the area with an abundance of the vector population. The change in human behavior, increasing population, industrialization, and deforestation have led to the interruption of various ecologies and imbalance in nature leading to spillover of the disease. In the past, the vectors were restricted to shrubs in forests and hilly areas, with the intervention of man and the disturbance of harmony of nature, the vectors such as mites, ticks, and louse have entered human dwellings through the rodents into urban areas. When it comes to scrub typhus it spreads through the bite of mite at the time of camping or any recreational activities in the jungles, jogging in parks [16]. The other reason can be the profuse growth of vectors. In the early 70s and 80s the vectors were constrained to some endemic areas and so the diseases were also endemic to that area. Later with indiscriminate use of chloramphenicol and tetracyclines for acute febrile illness patients reduced the cases drastically. With the introduction of insecticides and pesticides for the control of the vector population, the vectors population was bare minimum but since recent times the vectors have emerged with survival strategies and bounced back with increasing and widespread population so there has been an upsurge in the cases of rickettsial disease, breaking the endemicity pattern of disease to epidemics [17, 18].

17. Indian tick typhus

Indian tick typhus is a tick-borne rickettsial infection seen in India. The disease belongs to the spotted fever group with similarity to rocky mountain spotted fever. The causative agent is Rickettsia conorii. For the first time Rickettsia conorii was first found in brown dog tick Rhipicephalus sanguineus in the 1950. The disease has been reported from various states of India i.e., Karnataka, Maharashtra, Tamil Nadu, Uttarakhand, Himachal Pradesh, Rajasthan, Assam, Kerala, Jammu and Kashmir, and West Bengal. It is also transmitted by other species of ticks such as Hyalomma and Haemaphysalis [19].

Indian tick typhus in recent times has been reported in Nagpur, Jabalpur, Sagar, Pune, Lucknow, Bengaluru, Secunderabad. The patients exhibit a wide variety of symptoms. The disease is characterized by sudden onset of fever moderate to high, headache, deep muscle pain, and malaise. The rash begins with maculopapular lesions on 3rd day of fever at the extremities, then it moves centripetally to the rest of the body. The Indian tick typhus can be differentiated from Mediterranean Spotted Fever with characteristic purpuric rash and an inoculation eschar at the site of tick bite which is rarely identified [20].

18. Scrub typhus

Scrub typhus is caused by Orientia tsutsugamushi. Because they lack lipopolysaccharide, peptidoglycan, and the slime layer, members of the genus Orientia may be clearly distinguished from Rickettsia. It affects around a million individuals annually throughout the world, with an approximate 50% fatality rate if left untreated. Approximately one billion people worldwide are at risk of contracting Orientia each year [21].

Due to the vector mite’s presence in equatorial rain forests, subarctic regions, and semi-desert in the Himalayas, scrub typhus first appeared in India during the Second World War and spread to Assam and West Bengal states. Small mammals, including humans, are susceptible to scrub typhus because of heat and humidity. Currently, the disease can be seen in different parts of India i.e., the Sub-Himalayan belt from Jammu to Nagaland; Sikkim, Darjeeling, Himachal Pradesh, Bihar, Rajasthan, Maharashtra; South India-Puducherry, Tamil Nadu, Kerala, Karnataka [15, 22].

The larval stage of the Trombiculidae also known as chigger mite causes disease in both humans and rodents. The mite enters the human body through hair follicles and pores, then feeds for three to five days using a cyclostome, which is also used to inject the salivary secretion that causes the host tissue to lyse. The trombiculid mites transmit O. tsutsugamushi via transovarial and trans stadial transmission [23]. However, the novel vector (mite) Ornithonyssus bacotii for the transmission of the pathogen has also been identified in the Nagpur region; the central part of India [24].

Human scrub typhus typically takes 7–12 days to incubate. Humans exhibit symptoms like pyrexia of unknown origin, headache, body aches, joint pain, and characteristic entry lesion or eschar seen on the biting site due to this infection and activation of endothelial cells that aid in the morbid physiology of scrub typhus. It also affects several organs, including the heart, kidney, pancreas, skin, and brain, and in severe cases, multiple organ failures take place [25].

Scrub typhus can be diagnosed using the eschar-like characteristic lesion and serological methods which includes ELISA, Weil Felix, indirect immunoperoxidase (IIP), immunochromatographic test (ICT), and the gold-standard immunofluorescence assay (IFA). The molecular techniques utilized for diagnosis include polymerase chain reaction (PCR) by targeting a 56 kDa gene, a 47 kDa gene, Karp, Kato, and a gallium gene specific to Orientia tsutsugamushi for PCR. Other molecular techniques include; nested PCR and quantitative real-time PCR [21].

Considering the re-emerging nature of the disease in India more laboratories need to be strengthened for the accurate diagnosis of the rickettsial diseases.