Mites are very tiny, hemimetabolous microscopic parasites. Adults and nymphs are showing with four pairs of legs, but larvae have three pairs. Globally, more than 48,000 different mite species were recorded which are harmless to humans and found beneficial to the ecosystem. On the other hand, approximately 250 mite species are found to have public health importance. The life cycle of many mite species requires less than four weeks and can be as short as eight days. Scrub typhus is transmitted by the mite Leptotrombidium deliense and the rodents are the reservoirs that support the spread of this disease. Sarcoptes scabiei or the itch mite causes scabies which is a common skin infection in India. Dermatitis, trombiculosis, and mite dust allergy are common mite-borne diseases transmitted by Pyemotidae family. Demodex folliculorum and Demodex brevis are two major mite species found in humans causing a disease; “Demodicosis”, common in India. Dermanyssus gallinae, fowl mite causes pruritis to poultry birds, transmits infections to poultry workers when they come into contact with the birds. Dust mites are extremely small, live in house dust, feeding on the dead skin cells that people regularly shed, known for causing allergies. Thus, many mite species are responsible for the spread of diseases like itching, pulmonary and intestinal acariasis, urinary, allergic diseases, and dermatitis in humans.
Three major mite infestations and infections were recorded, (1) human mites dermatoses (2) animal and plant, and plant-insect mite infestations and, (3) bacterial pathogens infections. Scrub typhus was reported in India for a long time. Since this mite-borne scrub typhus infection is a resurfacing disease in India, this review updates the current knowledge in India which will be helpful to the researchers and public health officials to undertake effective control strategies.
MATERIAL & METHODS
The online database PubMed of National Library of Medicine, USA was used for the retrieval of references and abstracts subjected to search terms “mites” and “mite-borne diseases” listed in life sciences and biomedical topics. MeSH terms and subheadings like mites-borne diseases, the biology of mites; and mites-borne infections were used for literature review. For the management of reference information, EndNote Version 9.0.1©1998-2005 of Thomson, Mendeley Reference Manager Version 2.44.0©2020 of Mendeley Ltd was used. Journals, books, book sections, conference proceedings, web pages, and local reports were the different types of documents used in the literature review
Based on mite-borne diseases clinical data and outbreaks, spot maps were created by using EpiMap in EpiInfoTM 22.214.171.124 website of CDC, Atlanta, USA.
Mites and mite-borne diseases
Scrub typhus and Trombiculidae mites
A billion people present in the Asia-Pacific region are at risk and about a million people are affected with scrub typhus every year. In South-East Asian region, the dominant vectors of scrub typhus are Leptotrombidium deliense and L. akamushi. In India, L. deliense is dominant and the only vector for the spread of scrub typhus.
Scrub typhus is distributed in an uneven, irregular pattern of distribution in an environment[9,10]. Larval Trombiculid mites (or) chiggers are aggregated in the groups termed as “mite islands”, that provide a suitable habitat (vegetation structure such as abandoned plantations, gardens, or rice fields, overgrown forest clearings, shrub fields, and forests, riverbanks, and grassy fields) and climatic conditions (temperature, humidity, and rainfall) for their survival[6,9,10,11]. Within the mite islands, there may be a limited area of intensive transmission of rickettsiae called “typhus island”. Chiggers are usually found on parts of the animal hosts that have been in contact with the ground. They are also localized in and around the ears[13,14]. Chiggers are generally considered habitat- and host-specific, and sometimes the larval stage of mites can occur on multiple host taxa. The “tsutsugamushi triangle” is the endemic region of scrub typhus that comprises of major Asia-Pacific region that varies with 0 to 30% mortality depending on the geographical regime.
In India, 204 chigger species under 28 genera of the family Trombiculidae, have been described, significant in public health “scrub typhus”, an acute febrile disease caused by the bacterial pathogen Orientia tsutsugamushi (formerly Rickettsia tsutsugamushi) (Family: Rickettsiaceae), transmitted by the biting of larval mites of Leptotrombidium species (chiggers) coming under the Trombiculidae family. Mites carry the bacterium from larval stages to adults and their progenies through transmedial and transovarial transmission.
Medically important scrub typhus (or) tsutsugamushi disease is the vector-borne rickettsial disease transmitted by Trombiculid mites (Acari: Trombiculidae), distributed worldwide (except in the continent of Antarctica), diverse in the subtropical, tropical, and southern temperate zones, and act as vectors for scrub typhus[7,9,11]. Trombiculidae is one of the largest families in the Acari group that includes more than 3000 species of mites. Scrub typhus is caused by a gram-negative bacterium, Orientia tsutsugamushi, and is transmitted by the larval stage of mites (“chiggers”) in the family Trombiculidae. Scrub typhus was described first in Japan in 1899, caused by the pathogen α-proteobacteria, O. tsutsugamushi (earlier called Rickettsia orientalis or R. tsutsugamushi). In nature, more than 45 species of trombiculid mites are known to be infected with O. tsutsugamushi. Humans are accidental hosts in the transmission of the disease.
The earlier outbreaks had been associated with the predominance of the vector L. deliense in India. But, in the recent outbreaks, L. deliense was missing as a vector, and the emergence of Schoengastiella ligula as the primary vector in the outbreak of Kurseong district, West Bengal, India. Scrub typhus is most prevalent in the hilly forest belt which is full of thick vegetation, suitable habitat for the mite to survive in the grassy fields, shrubby areas, forests, and cleared forests. The district of Darjeeling, West Bengal has also been historically considered as one of the scrub typhus endemic areas in the country with scrub typhus outbreaks reported until the 1960s. Thereafter, for a long time, no outbreaks were reported which continued till 2005. The outbreaks of scrub typhus were again reported in 2005.
Recently, we undertook the surveillance for the scrub typhus chigger mite vector on the rodents in Vellore, Krishnagiri, Dharmapuri and Madurai districts in Tamil Nadu state and Thiruvananthapuram district in Kerala. Our studies conducted in Madurai recorded four species of trombiculid mites[25,26,27]. Similar studies conducted by us in Vellore district, Tamil Nadu showing 18 species of trombiculid mites. Our survey conducted in the scrub typhus endemic Thiruvananthapuram district, Kerala reported 23 species of trombiculid mites. First record of 19 species of Trombiculidae species and 1 species of Laelaptidae were reported from our studies in Kerala. Similarly, we recorded for the first time 14 species of Trombiculidae and 3 species of Laelaptidae from Tamil Nadu. A rapid protocol for clearing, staining, and mounting of Arthropoda: Trombiculidae, Pediculidae, and Pulicidae was developed. Detected Orientia tsutsugamushi in novel Trombiculid mite species in northern Tamil Nadu, India. Three species of dust mites and 5 species of storage mites were recorded in different Indian cities/states. In Kolkata, West Bengal, a study conducted during 2017 showed a total of 51 species belonging to 34 genera and 17 families.
Scrub typhus outbreaks in India
Scrub typhus was recognized in India as rickettsial typhus fever during 1917. In 1932, the first cases of scrub typhus were reported in India and the case was believed to be a tick-borne disease, due to the lack of serological tests. Similarly, in 1934, scrub typhus was reported among personnel from Shimla hills, India. An investigation has reported the presence of Trombicula deliensis (L. deliense) on rodents and shrews in the Shimla hills for the transmission of scrub typhus. During 1935, the presence of typhus was serologically confirmed by Weil–Felix OXK, but no positive cases were showing with eschars in India reported from two outbreaks of scrub typhus occurred during 1937–1938 and 1939–1942, was confirmed by Weil–Felix serology tests. A major outbreak of scrub typhus occurred at the Assam-India-Myanmar border and West Bengal among the military personnel during World War II, and another major outbreak also occurred during 1965 during the Indo-Pak war[22,42]. The disease also resurged at the Pakistan border of India in 1990.
In the pre-antibiotic era, mortality rates were as high as 40–45% due to scrub typhus. Changes in human lifestyle patterns, modern drug developments, and the widespread use of insecticides to control the vectors and also decrease the spread of disease incidence in India[22,44]. Epidemiology and entomological studies also confirm the occurrence of scrub typhus all over India.
Scabies and Sarcoptidae mites infection
Scabies is an infestation caused by the mite Sarcoptes scabiei. Sarcoptes scabiei (or) the itch mite is a parasitic mite of the Sarcoptidae family of mites that burrows into the skin and causes the disease scabies. This mite is distributed throughout the world. Humans are not the only mammals that can become infected. Sarcoptes scabiei is a microscopic mite that burrows in the epidermis and infests the skin of its host with over 100 described species.
Every year more than 300 million cases of scabies are recorded in the world. It is endemic in some Indian villages and reported more in developing countries, where the prevalence is up to 100%. In most of the developed countries, scabies is reported in hospitals, nursing homes, and long-term care facilities in epidemic proportion. The highest rates of infections are found in countries with hot, tropical climates, where the infestation is endemic.
In India, scabies is more prevalent in overcrowded communities with low socioeconomic conditions. 81% of inmates of an orphanage in the rural area of Maharashtra, India, were found to suffer from scabies and all inmates were successfully treated with mass scabies treatment and health education. In the rural community the prevalence rate of scabies is 13%, and topical permethrin and oral ivermectin drugs are used to control human scabies. The mite Sarcoptes scabiei var hominis is an obligate human parasitic mite of about 300–400 microns in size, causes sarcoptic mange (scabies) in humans and other mammals. Humans were the original host of Sarcoptes, and all other hosts were secondarily infested. Scabies transmission is by direct skin-to-skin contact. Crusted scabies is a rare type of scabies characterized by uncontrolled proliferation of mites in the human skin caused primarily by female scabies mite, Sarcoptes scabiei var hominis, common in HIV-infected patients. Scabies is an important and commonly reported public health problem in India which can affect the sleep and cause psychosocial problems. It is also a widely common contagious skin disease affecting all domestic animals[54,55].
Other mite infections
Demodicosis or demodicosis are skin disorders in which increased amounts of demodex mites are seen. Demodicosis was first reported by. Demodex folliculorum and D. brevis are two major mite species found on humans causing a disease “Human demodicosis”, common in India. Severe mite dust allergy leads to human dermatoses, caused by the mites belonging to the Pyemotidae family.
Liponyssoides sanguineus, mite of the Dermanyssidae family transmits the disease, rickettsial pox, caused by the pathogen Rickettsia akari. R. akari parasitizes the house-mouse, Mus musculus, and transmits the disease to man. Scrub typhus chiggers and house-mouse mites are found common and associated with rodent zoonosis. Nasobronchial allergic complaints frequently occurred in the dust areas, due to inhalation of mites which are present in the house dust, reported as dust mites. These mites were reported earlier in house dust dating back to the seventeenth century. More than 130 species of mites have so far been isolated from house dust samples all over the world since 1983. The first detailed information on the mites in Indian house dust was made by Rao et al., 1973. Mites of the family Pyroglyphidae, particularly the genus Dermatophagoides are considered to be the most potent allergen in house dust responsible for asthma and other allergic manifestations[60,61,62].
Trombiculiasis is an ectoparasitic disease caused directly by the bite of ectoparasite trombiculid mites caused by the salivary secretion of biting chiggers and do not survive due to the adverse host reaction.
Trombiculiasis is not common in India. Trombiculosis is the sign and symptoms caused by the bite of the mites as irritation, and active self-grooming in response to intense pruritus.
Ethical statement: Not applicable
In the 1960s and 1970s, scrub typhus was endemic and prevalent in different regions of India. But it disappeared due to the vector control methods adopted. However, in recent years, the resurgence and re-emergence of the disease has occurred, due to the migration of chigger mites from one place to another. Mites were found mainly in the shrubs in hilly and forest terrains. Nowadays this disease is reported from urban areas since rodents carrying the mite are transmitting the disease.
In India, scrub typhus was reported first in the Himalayan foothills and then it spread to different parts of south India[44,64,65,66,67,68] and north and northeastern parts of the country[69,70,71] and some minor cases were reported from Goa, western India. Some localized outbreaks have still occurred and further diagnoses are confirming the prevalence of the disease in India. In the past twenty years, Puducherry has recorded a cluster of 50 cases of scrub typhus during 2006–2008, followed by scrub typhus meningitis in 2011–2012. Numerous publications have shown the outbreak of scrub typhus throughout India[73,74,75]. Distribution of a consolidated total and average hospitalized, serological, and outbreak cases of scrub typhus cases in India from 1934 to till date are given in Table 1. State- and region-wise hospitalized, serological, and outbreaks reported in India is given in Table 2.
Historical background of outbreaks in various parts of the country from 1932 onwards with main outbreaks are shown in Figure 1A & B. The increasing trend in the distribution of scrub typhus in Indian states every five years (2001–2020) is given in [Figure 1]C. The number of states and union territories affected by the cases from 2001 to 2020 is shown in [Figure 2].
Scrub typhus is prevalent till now with historical records in India, distributed in all regions in India (Table 2). Localized outbreak/ clinical cases occur periodically. This is confirmed by serological evidence. Cool climate favors scrub typhus. The proliferation of rodent population increases mite density and there is evidence of increasing scrub typhus cases. In India, historical evidence proved that the Himalayan and sub-Himalayan region climate and vegetations favor scrub typhus via the proliferation of mites and outbreak of the disease.
Recent outbreaks of scrub typhus indicate a favorable situation for rodent-vector relationships. Bloodsucking mites (Ornithonyssus sp., Dermanyssus gallinae, Trombiculids, etc.) affect humans. As a vector, by the transfer of pathogenic agents, Trombiculids are more medically important and Oribatid mites act as vectors of herbivore tapeworms. By causing strong allergic reactions in humans, pets and livestock. Dust mites, stored product mites and predisposition to secondary disease (Demodex for Staphylococci), mites are important ectoparasites in human public health.
Worldwide constant climate pattern variations followed by frequent changes experienced by humans in countries like India created more infections within the rodents which harbored the vector species. The present rate of human urbanization, compounded by climate change and inefficient rat control led to a proliferation of rodent-related risks. The changing ecological patterns resulted in agricultural development in rural areas and urban development controlled the composition of rodent populations play a key role in the present condition of mite-borne disease in India.
Mite-borne infections are expanding all over the world and the distribution of the vector is shifting and expanding. This work reviews the prevalence of mite-borne infections throughout India and its public health burden which will be a prerequisite for public health authorities to ensure intervention. Thus it is emphasized that there should be a need for the establishment of a permanent mite surveillance system with improved mite-borne disease diagnostics to initiate effective vector control efforts to stop the transmission of mite-borne diseases in the future. Controlling rodent hosts and associated vectors helps to control the outbreak of mite-borne diseases.
Conflict of interest: None
We are thankful to our Director, ICMR-Vector Control Research Center, Puducherry for providing all the necessary facilities, constant encouragement, guidance, and useful suggestions for undertaking this study. We want to express gratitude to all our departmental colleagues of ICMR-VCRC Field station (erstwhile ICMR-CRME), Madurai, Tamil, Nadu, India.
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