A review on emerging parasitic and viral zoonotic diseases in India: Mitigating strategies and one health approach

Introduction

The majority of human diseases that are transmitted between people via arthropods, insects, other invertebrates or vertebrates, have their origins in animals. The socioeconomic progression leading to forest cutting, urbanization on previously unused lands has led to sudden change in ecosystem. These ecological changes are causing alterations in the epidemiology of diseases. The transformation of wild animals' habitats can result in more frequent human-animal encounters, which may pose a health risk to the public due to potential zoonotic diseases. Over the past 20 to 30 years, several diseases that had been largely overlooked or confined to specific areas have resurfaced, spreading widely. In India, there has been a heightened sense of alertness towards the emergence of zoonotic and other re-emerging diseases in recent times (Pavani, 2014) [25] . The emerging zoonotic diseases can either be newly emerged or might have been already existing in the past and are now expanding quickly to new terrains or host due to their evolving ecology, such as Scrub typhus, Malaria, Cutaneous Leishmaniasis, West Nile, Crimean-Congo hemorrhagic fever (CCHF) and Japanese Encephalitis (Taylor et al., 2001) [37] . One Health approach seeks to harmoniously balance and enhance the health of humans, animals, and their interconnected ecosystems. The health of humans, domestic animals, wild animals, plants and the environment are understood to be closely linked (WHO, 2021) [40] . Understanding the pathogen ecology of natural host and human-host interactions are must in order to inform surveillance and public health interventions for preventing and/or mitigating future zoonotic breakouts (Cunningham et al., 2017) [9] .

Zoonotic Diseases

According to WHO, zoonotic infections are those diseases which are naturally transmissible from vertebrate animals to humans through direct or indirect contact (Ryu et al., 2017) [29] . There are 5 stages of spreading zoonotic diseases to humans, ranging from only spread among animals (stage 1) to fully human pathogens (stage 5) (Fig. 1) (Wolfe et al., 2007) [46] . [46] .

It is clear that the relationship between humans, animals and the environment create conditions for the emergence of diseases that affect the public health and socio-economic well-being of the world's population. The incidence of zoonotic diseases increases as humans live in close contact with animals and as they encounter animals in new geographical areas. Examples include Lyme disease transmitted by ticks and Salmonella (CDC, 2017). Zoonotic pathogens can be classified into four types based on the nature of pathogens: bacterial, viral, fungal and parasitic, along with their hosts (Table 1) (Bardhan et al., 2023) [5] . [36] , the first naturally acquired human outbreak of P. knowlesi was reported in Malaysia. The authors also described the first human outbreak of P. knowlesi malaria and demonstrated that wild monkeys in the Kapit Division of Sarawak, Malaysian Borneo, were infected with malaria parasites, including P.

knowlesi. The number of P. knowlesi genotypes per infection in monkeys is much higher than in humans, providing indirect evidence that P. knowlesi malaria is primarily a zoonotic disease. In the Andaman and Nicobar Islands (India), the co-infection rate with P. knowlesi was observed to be 11.9%, of which 5.3% were P. knowlesi mono-infections. In the island, where human habitation is close to forests, it is imperative to understand the role of monkeys in the epidemiology of P. knowlesi malaria in India to support prevention and control measures.

b) Leishmaniasis

Leishmaniasis is caused by various species of Leishmania protozoa. In India, two forms of leishmaniasis are prominent, visceral leishmaniasis (VL) and cutaneous leishmaniasis (CL). Visceral leishmaniasis (VL), commonly known as Kala-azar, is caused by Leishmania donovani. It is mainly prevalent in Bihar, Uttar Pradesh, West Bengal and Jharkhand. Phlebotomus argentipes, a species of sandfly, is vector for this disease (Naik et al., 1979) [21] . In India, the epicenter of visceral leishmaniasis is known to be western Rajasthan (WHO, 2010), where the reservoir of infection is Meriones hurrianae, a desert rodent, and the vector is P. salehi. On the other hand, in urban areas, dogs are reservoir host for Leishmania donovani and P. papatasi or P. sergenti act as vectors.

CL is mainly caused by Leishmania donovani and L. tropica. Humans are accidentally infected with CL while working in the fields. Due to lack of knowledge about the signs and symptoms of CL, the prevalence of the disease is not known, leading to neglect of the problem. The detection of a new outbreak of CL in Himachal Pradesh has highlighted the role of zoonotic reservoirs, increased human-vector contact due to deforestation and construction activities in the area (Sharma et al., 2003) [35] . The recording of stray cases of CL and VL in the Indian states underscores the need to study the role of zoonotic reservoirs in the disease transmission and to conduct an epidemiological investigation of other outbreaks from the ecological perspective.

c) Scrub typhus

Scrub typhus is a Rickettsial disease caused by the Orientia tsutsugamushi and spread by trombiculid mite Leptotrombidium deliense. The term tsutsugamushi is a Japanese term; tsutsuga signifies dangerous, and mushi denotes insect or mite. The geographic distribution is limited to Southeast Asia, including Japan and Korea. In India, the sickness was thought to be widespread among army personnel and was first documented in 1934 (Vivekanandan et al., 2010) [43] . Scrub typhus symptoms include a non-specific high-grade fever lasting 1-2 weeks, eschar/papule at the mite bite site, headache, myalgia, dry cough, lymphadenopathy, hepatosplenomegaly, apathy, dyspnea and myocarditis. The Weil-Felix test, indirect immunofluorescence test, quantitative ELISA and polymerase chain reaction are used for diagnosis (Dhiman, 2014) [10] . Scrub typhus is a zoonotic disease and rickettsia infection persists in small rodents living in recently cut forest/scrub habitats. The nymph and adult mites do not feed on humans, and transmission occurs by accident through the mite's larval stage. When the ecological conditions of a forest are altered owing to deforestation, and humans enter the cleared forest, the larval mite bites man. Scrub typhus is linked to farm worker population density, timber management, recreational forests and natural reserves (Tsai et al., 2013) [41] . Though scrub typhus has been reported to be endemic throughout India (Padbidri and Gupta 1978) [23] , reports of the disease have increased since 2004, particularly in Himachal Pradesh (Sharma et al., 2005) [32] , Darjeeling (Sharma et al., 2009) [34] , Jammu and Kashmir (Kumar et al., 2010) [18] , Andhra Pradesh (Issac et al., 2004) [16] , Uttarakhand (Ahmad et al., 2010) [1] , Pondicherry (Vivekanandan et al., 2010) [43] and West Bengal (Sharma et al., 2014).

Viral zoonotic diseases a) Japanese Encephalitis

One of the major public health concerns in Asia is Japanese encephalitis (JE) (Umenai et al., 1985) [42] . Approximately 25 percent infective cases are fatal, and nearly 50 per cent of the cases reportedly suffer from permanent neuropsychiatric dysfunction (Campbell et al., 2011) [6] . Culex vishnui, subgroup of mosquitoes acts as vector and transmits the causative agent, JE virus (JEV) to humans. Clinical symptoms of JE include high fever, chills, headache, myalgia and confusion; seizures can be experienced by approximately 75 per cent of the affected individuals. Till date, there is no specific treatment for JE, hence its prevention and control only depend upon the vaccination (Scholer et al., 2007) [31] . In India, first recognition of JE was in Nagpur, Maharashtra, in 1952 (Banerjee, 1996) [4] . In 1955, the accumulation of serological evidence in southern India, resulted in rise in the number of cases in approximately 16 States including Andhra Pradesh, Assam, Bihar, Delhi, Goa, Haryana, Karnataka, Kerala, Maharashtra, Manipur, Nagaland, Punjab, Uttarakhand, Tamil Nadu, Uttar Pradesh and West Bengal (Tiwari et al., 2012) [39] . JE has been documented in Tamil Nadu, Andhra Pradesh and Karnataka. In contrast, its cases also have been showing escalating trend of incidence in north India (Saxena et al., 2009) [30] . In 2011, this disease spread to 15 [24] . Pigs and horse have also been found with antibodies against West Nile fever. Clinical manifestations include mild influenza, fever, general body ache, headache, nausea and vomiting including encephalitis syndrome. Transovarial transmission has been reported in Culex spp. Up to 2002, cases of West Nile fever were reported mainly from southern India, but according to a study by Thakare et al., 2002 [38] , the prevalence of West Nile fever from non-endemic (Maharashtra and Rajasthan) and endemic area of JE (Goa and Orissa) were reported. Khan et al., 2011 [17] , reported the West Nile fever from Dibrugarh, Golaghat, Sivasagar, and Tinsukia districts of Assam. West Nile fever may be a cause of acute encephalitis syndrome in JE endemic areas. Its reemergence with severe neuro-invasive pathogenic potential in humans in Assam, Kerala, West Bengal and Tamil Nadu states indicates the need for immediate action to comprehend the dynamics of circulating strains in relation to their environment, hosts, and vector.

c) Chickungunya

Aedes aegypti mosquito spreads the CHIK virus, which causes chikungunya fever (Wimalasiri-Yapa et al., 2019) [15] . In 1824, an illness that resembled chikungunya symptoms was described from India; nevertheless, chikungunya outbreaks were observed in 1963. Manmosquito-man (urban cycle) or animal-mosquito-man (sylvatic cycle) are the two possible transmission cycles for CHIK virus. In Africa, chikungunya is spread by a sylvatic cycle that involves humans, mosquitoes, and monkeys. In India, however, man-to-man transmission via A. aegypti mosquitoes is known to occur. In Africa, A. albopictus has also been identified as a chikungunya vector (Diallo et al., 1999) [11] . In the 1960s, neutralizing antibodies were discovered in wild monkeys in Malaysia, suggesting that the CHIK virus may be preserved in the wild by animals (Apandi et al., 2009) [2] . In India, chikungunya outbreaks were documented during the 1960s and 1970s (Parvi et al., 1963) [26] . In 1965, Banerjee reported that sera obtained from Madras state in 1956 contained antibodies to the CHIK virus. In 2005, there was a resurgence of chikungunya in India, with severe outbreaks recorded throughout the southern region and a gradual spread to the northern region (Banerjee 1956) [3] . Prior to the 2005 epidemics, the CHIK virus was isolated from A. aegypti in Yawat town, Pune district, in 2001. There have been reports of chikungunya illnesses from 13 to 18 Indian states since the outbreak in 2005, with 18,639 cases in 2013 that were clinically suspected (Mourya and Banerjee 1987) [20] . The majority of the papers have detailed the epidemiological profile of cases, but they have not explained the causes of outbreaks from the perspectives of socioeconomic development, ecology and climate. Given the documented importance of the periurban-dwelling A. albopictus mosquito in transmission, there may be a chikungunya sylvatic cycle in India that merits more research. Till, 11th September, 2016 a total of 14,656 clinically suspected cases (including 1724 in Delhi) from 18 states and 2 UT's have been reported

(Chhabra et al., 2008) [14] .

d) Crimean-Congo hemorrhagic fever

Crimean-Congo hemorrhagic fever is caused by Crimean-Congo hemorrhagic fever virus (CCHF). The virus is transmitted by Hyalomma spp. ticks or by direct contact with humans or animals that serve as reservoirs (Mardani et al., 2007) [19] . Since the ticks live close to cattle and other wild animals, people living in rural areas are more likely to get CCHF. There have also been reports of nosocomial outbreaks among medical personnel. The disease, which is found in Africa, Asia, Europe, and the Middle East, was first documented in 1944 in Crimea, Russia. Its symptoms are similar to those of dengue hemorrhagic fever, and during outbreaks, its fatality rate can reach 80% (Mardani et al., 2007) [19] . In 2010, the first recorded case of CCHF occurred in Ahmedabad, India; while an epidemic in a cluster of Gujarati villages of Amreli and Patan area was reported in 2014 (Yadav et al., 2014) [48] . Hyalomma ticks tested positive by RT-PCR, and the animals' IgG antibody positivity reached 43.9%. In order to diagnose, treat CCHF cases promptly and to stop their transmission, public health stakeholders must be made aware of the symptoms, diagnosis and preventive actions.

Factors contributing to emergence of outbreaks:

As listed below, a number of variables contribute to the prevalence of zoonotic diseases in the various regions of the nation (Quigley, 2006) [27]: [12]:  Effective and early diagnosis and prompt treatment  Isolation of disease animals from healthy stock  Vaccination of healthy stock  Chemoprophylaxis including deworming  Awareness campaign  slaughtering/culling of diseased animal

One Health approach

The idea that the health of humans, animals, and plants is interconnected and linked to the health of the ecosystems in which they live is summed up by "One Health" approach (WOAH, 2022) [22] . Indian government approved "National One Health Program" for Prevention and Control of Zoonoses (NOHP-PCZ) in 2013 as central sector scheme with goal of protecting communities and minimizing socioeconomic losses due to emerging and re-emerging zoonotic threats (Jhandai et al., 2023) [13] . By collaborating with medical professionals, veterinarians, ecologists, and others, Center for Disease Control and Prevention (CDC) uses the one health strategy to track and manage public health risks and understand how diseases spread across humans, animals, and the environment.

Conclusion

Zoonoses is a major challenge to public health in view of rapid deforestation, urbanization, population movement and changing climatic scenarios. Chikungunya, scrub typhus, swine flu, dengue, CCHF, and CL are among the unknown and re-emerged diseases that have affected both developed and developing nations. Given that the majority of diseases are life-threatening, it is necessary to comprehend each disease using an eco-epidemiological approach, with a focus on the habitats of infection reservoirs, the micro niche of arthropod/insect vectors, and vulnerable areas from the perspective of climatic determinants for pathogen and vector development. Early action, awareness-raising, and prompt preparation will aid in outbreak containment. It is crucial to investigate and extract the elements that contribute to the epidemic and advance knowledge of the dynamics of zoonotic infections.

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