Introduction

India is the seventh-largest country in the word and geographically located at 28°36.8′N and 77°12.5′E in the northern hemisphere of the globe. It is stretched over an area of 3,287,263 km2 and comprises 29 states and seven union territories. According to the 19th Livestock Census, 2012 (Department of Animal Husbandry, Dairying and Fisheries, Ministry of Agriculture, Government of India), the total population of livestock in India is 512.1 million (119.9 million of cattle, 108.7 of water buffaloes, 135.2 of goats, 65.1 of sheep, 0.6 of horses, 0.4 of dromedary camels and 0.3 of donkeys). In recent years, besides the perennial lentic and lotic surface waters (ponds, dams, rivers, etc.), main drinking water sources for domesticated animals rearing in rural areas or villages are hand pumps and bore wells. Now these are abundant in those hyper-endemic states for fluorosis (Andhra Pradesh, Gujarat, Rajasthan and Telangana state) where dracunculiasis was more prevalent (Choubisa 2002, 2010a). Actually, thousands of underground drinking water sources (hand pumps and bore wells) were dug out in the country to check or break the life cycle of dreaded Guinea worm (human nematode parasite) (Dracunculus medinensis) which was responsible for dracunculiasis in human population.

In India, except a few eastern states fluoride content in varying concentration is reported in groundwater (hand pumps and bore wells) of 23, out of 36 states and union territories including Andhra Pradesh (0.4–29.0 ppm), Arunachal Pradesh (0.0–0.48 ppm), Assam (1.6–23.4 ppm), Bihar (0.2–8.12 ppm), Chhattisgarh (0.2–13.2 ppm), Delhi (0.2–32.46 ppm), Gujarat (1.5–13.0 ppm), Haryana (0.23–48.0 ppm), Jammu and Kashmir (0.5–4.21 ppm), Jharkhand (0.5–14.0 ppm), Karnataka (0.2–7.79 ppm), Kerala (0.2–5.40 ppm), Madhya Pradesh (1.5–4.20 ppm), Maharashtra (0.11–10.0 ppm), Manipur (0.18–0.84 ppm), Odisha (Orissa) (0.60–9.20 ppm), Punjab (0.23–42.5 ppm), Rajasthan (0.10–90.0 ppm), Tamil Nadu (0.1–7.0 ppm), Telangana (0.4–29.0 ppm), Uttar Pradesh (0.2–25.0 ppm), Uttarakhand (1.0–3.0 ppm) and West Bengal (1.1–14.47 ppm) (Unicef 1999; Devi and Kamble 2006; Susheela 2007; Tewari and Pande 2010; Beg et al. 2011; Choubisa 2012a; Giri et al. 2013; Choubisa 2016). Among these states, 70–100% districts are affected with fluoride in Andhra Pradesh, Gujarat, Rajasthan, and Telangana, while in Assam, Bihar, Chhattisgarh, Delhi, Haryana, Jharkhand, Karnataka, Madhya Pradesh, Maharashtra, Punjab, Tamil Nadu and Uttar Pradesh states, 40–70% districts are endemic for fluoride. Distribution of fluoride in drinking waters in different states in India is shown in Fig. 1. Despite the occurrence of high level of fluoride in drinking water sources of these states, studies on hydrofluorosis in diverse species of domestic animals, viz. cattle (Bos taurus), water buffaloes (Bubalus bubalis), sheep (Ovis aries), goats (Capra hircus), horses (Equus caballus), donkeys (Equus asinus) and dromedary camels (Camelus dromedarius) are still meager (Choubisa 2013a, b) as compared to work done so far in the human population (Choubisa et al. 2001; Choubisa 2001) in the country. However, few interesting and significant studies on various aspects of chronic fluoride intoxication in different species of domestic animals rearing in different geographical provinces critically reviewed and highlighted various research gaps or future scope for advance research work on chronic fluorotoxicosis in domestic animals in India. To the best of my knowledge, such review has not been reported previously from India.

Fig. 1
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Map of India showing fluoride (in ppm) distribution in states and union territories. This map was modified or prepared based on published scientific reports (Unicef 1999; Devi and Kamble 2006; Susheela 2007; Tewari and Pande 2010; Beg et al. 2011; Choubisa 2012a; Giri et al. 2013; Choubisa 2016)

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Fluoride and its biomedical significance

Fluoride compounds or fluorides are the resultant or products of chemical combination of fluorine with other elements. Indeed fluorine is the most electronegative and highly reactive diatomic pale yellow-green with pungent and irritable gas and hence never found free in the nature in element form. Readily soluble fluorides are comparatively more toxic and are easily absorbed by alimentary canal of animals. Whatsoever, inorganic forms of fluoride are found relatively more toxic than its organic forms (Adler et al. 1970; Swarup and Dwivedi 2002).

It is well stated that excess intake of fluoride either through ingestion or inhalation for a long tenure is accumulated gradually in biological systems and causes diverse adverse ill or toxic effects. The condition is referred to as fluorosis or fluorotoxicity (Wheeler and Fell 1983). In general, these pathognomonic signs or toxic effects appear in teeth, bones and organs or soft tissues of body and are known as dental, osteal or skeletal and non-skeletal fluorosis, respectively. These pathognomonic signs (fluorosis) generally appeared or developed when drinking waters have fluoride above threshold level of 1.0 or 1.5 ppm (Adler et al. 1970; ICMR 1974; Swarup and Dwivedi 2002).

Fluoride exposure in domestic animals

Natural

The principal sources of natural fluoride exposure to animals are: fluoridated drinking waters and vegetation grown on fluorotic soils and water (Swarup and Dwivedi 2002; Ranjan and Ranjan 2015). In rural India, now hand pumps and bore wells are the major drinking groundwater sources for all kinds of domestic animals. In India, these drinking water sources of 23, out of 36 states and union territories are contaminated with fluoride in varying concentration. Of these, 17 states, namely Andhra Pradesh, Assam, Bihar, Chhattisgarh, Gujarat, Haryana, Jharkhand, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Odisha (Orissa), Punjab, Rajasthan, Telangana, Uttar Pradesh and West Bengal, have fluoride beyond the maximum permissible limit of 1.0 or 1.5 ppm in drinking waters. Besides these, other natural sources of drinking water for domestic animals are perennial small and large ponds and rivers. At present, except from the Rajasthan state (Choubisa et al. 1995, 1996a; Choubisa 1996a, 1997) no scientific reports on fluoride contamination in surface drinking water sources are available from any of fluoride-endemic states in the country. Therefore, more extensive scientific surveys on fluoride distribution in surface water sources of each state are highly needed.

Occurrence of anomalously high fluoride concentration in groundwater of all 23 states of country is not an anthropogenic but is natural and due to the presence of heavy deposition of fluoride-bearing minerals in the host rocks and sediments. The important rocks are basalt, granites and gneisses, shales and clays, limestone, sandstone, phosphorite and coals (ash) and these rocks contained fluoride in the range (ppm) of 20–1060, 20–2700, 10–7600, 0–1200, 10–880, 24,000–41,500 and 40–480, respectively. The average fluoride concentration (ppm) in these rocks is 360, 870, 800, 220, 180, 31,000 and 80, respectively. Some minerals of these rocks such as biotite, phlogopite, lepidolite and muscovite contain a very high amount of fluoride in the range (ppm) of 970–3500, 3300–37,000, 1900–68,000 and 170–14,800, respectively (Keller 1979; Agrawal et al. 1997). Other mineral deposits such as sellaite (MgF2), villiaumite (NF), fluorspar (CaF2), cryolite (Na3 Al F6), bastnaesite (CeLaY) (CO3)F and fluorapatite [Ca5(PO4)3 F] also contain fluoride with concentration (%) of 61, 55, 49, 45, 9 and 3.5, respectively (Ranjan and Ranjan 2015). Their chemical properties such as decomposition, dissociation and dissolution and interaction with water are considered to be the main cause of fluoride in groundwater. But distribution of fluoride in region is under control of regional hydrogeological and climatic condition. Besides hydrogeological setup, the climate and physiography are other important factors that the areas of less rainfall have higher fluoride content as compared to groundwater in high rainfall areas despite having similar hydrogeological formation (Gupta et al. 1993).

The weathering and leaching process, mainly by moving and percolating water, also play an important role in the decreasing or increasing of fluoride concentration in groundwater. Other factors such as the chemical composition, the presence and accessibility of fluoride minerals to water and the time contact between the source of minerals and water also govern the release of fluoride into water (Gupta et al. 1993).

Fluoride is also present in the atmosphere either in gaseous or particulate forms. Its concentration in the atmosphere in unpolluted areas usually varies between 0.02 and 2.0 μg/m3 (USEPA 1980). In gaseous forms include hexafluorosilicic acid, silicon tetrafluoride, hydrogen fluoride and sulfur hexafluoride. Particulate forms include sodium hexafluorosilicate, sodium aluminum fluoride, lead fluoride, calcium fluoride, calcium phosphate fluoride and aluminum fluoride. However, hydrogen fluoride and inorganic fluoride particulates (sodium and calcium fluoride) are the major inorganic fluorides exist in the atmosphere, accounting for nearly 75 and 25%, respectively (WHO 2002).

Anthropogenic

In recent years, another source of fluoride exposure in domestic animals is the industrial fluoride emission. Number of coal burning and industrial activities such as power generating stations, and the manufacturing or production of steel, iron, aluminum, zinc, phosphorus, chemical fertilizers, bricks, glass, plastic, cement and hydrofluoric acid are generally discharging fluoride in both gaseous and particulate/dust forms into surrounding environments which create an industrial fluoride pollution and are an important cause of occupational exposure to fluoride in many states (Adler et al. 1970; Choubisa and Choubisa 2016). An industry-emitted fluoride contaminates not only surrounding soil, air and water but also vegetation, crops and many other biotic communities on which domestic animals are generally dependant on food. Long-time of inhalation or ingestion of industrial fluoride also causes serious health problems in the form of industrial and neighborhood fluorosis (Choubisa 2015; Choubisa and Choubisa 2015, 2016). Industrial fluorosis in animals is generally restricted to particular location or herds.

Besides these fluoride exposure sources, another possible anthropogenic sources are feed supplements such as mineral and phosphate supplements or commercial phosphorous lick to domesticated animals. These supplements also contain high amount of fluoride; therefore, these are also potential sources for the genesis of food-borne fluorosis in animals (Swarup and Dwivedi 2002). Howdever, reports on food-borne fluorosis in domestic animals in India are too scanty. However, few reports are available on this entity (Mehrotra and Singh 1989; Singh and Swarup 1995). Study on food-borne fluorosis in different species of domestic animals is also suggested in the country, since it is not well known among villagers or herdsmen.

Hydrofluorosis in domestic animals

The fluoride-induced toxic effects appeared in the teeth (dental fluorosis), bones (skeletal fluorosis) and organs or soft tissues of body (non-skeletal fluorosis) in both humans (Choubisa and Sompura 1996; Choubisa et al. 1997, 2007; Choubisa 2012a, b) and animals (Choubisa 1999a, b, 2000) due to drinking of fluoridated water for a long tenure; then, they are collectively referred as hydrofluorosis which is natural, more prevalent and wide spread in nature.

In India, for the first time in 1934–1935, farmers in Nalgonda district (earlier in Andhra Pradesh state and now in Telangana state) detected signs of hydrofluorosis (inability to walk due to painful and stiff gait) in a pair of bullocks. Subsequently, the disease was scientifically documented in the report of Veterinary Investigation Officer, Hyderabad as aphosphorosis by Mahajan in 1934–1935 (Rao and Reddy 1977). However, hydrofluorosis in domestic animals (cattle, buffaloes and sheep) was reported for the first time from the Nellore district of Madras Presidency of South India (Viswanathan 1944). Hydrofluorosis in various species of domestic animals included cattle (B. taurus), buffaloes (B. bubalis), horses (E. caballus), donkeys (E. asinus), dromedary camels (C. dromedarius), sheep (O. aries) and goats (C. hircus) living in the same fluoride-endemic areas was identified and reported for the first time from Rajasthan (Choubisa 2013a). Hydrofluorosis in domestic horses and donkeys was reported for the first time in India, and this condition in donkeys was for the first time anywhere (Choubisa 2010a; Spittle 2010). At present, based on available published reports, out of 23 fluoride-endemic states, only in nine states, namely Andhra Pradesh, Chhattisgarh, Gujarat, Karnataka, Madhya Pradesh, Odisha, Punjab, Rajasthan and Uttar Pradesh, endemic hydrofluorosis has been detected and reported in domestic animals. The prevalence of dental and skeletal hydrofluorosis in diverse species of domestic animals in relation to fluoride concentration in drinking water is depicted in Table 1. Hydrofluorosis in domestic animals reported from different fluoride-endemic states is also shown in Fig. 2.

Table 1 Prevalence (%) of dental fluorosis (DF) and skeletal fluorosis (SF) in diverse species of domestic animals in relation to fluoride (F) concentration (ppm) in drinking water of different states based on available research reports
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Fig. 2
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Map of India showing hydrofluorosis in domestic animals reported from fluoride-endemic states

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In India, despite having a number of fluoride-endemic states, only few reports, but nevertheless, interesting on endemic hydrofluorosis in animals are available. Therefore, more epidemiological studies on endemic hydrofluorosis in relation to fluoride concentration in drinking water and other determinants in various species of domestic animals of each fluoride-endemic state are highly needed for the exact assessment of chronic fluoride poisoning and its prevalence status. Such survey studies are very important and helpful in making of health policy and for the mitigation or prevention and control of water-borne hydrofluorosis disease in animals. In rural India, bovines, flocks, equines and camels are economic sources of villagers, farmers and herdsmen; therefore, the preservation of animal health from fluoride poisoning is highly suggestive.

Dental fluorosis

The earliest visible pathognomonic sign or manifestation of all kinds of chronic fluoride toxicosis in both man and animals is the dental mottling or dental fluorosis (Adler et al. 1970). This is the most recognizable, irreversible, sensitive and indexive sign of chronic fluoride poisoning. In India, this abnormality is rampant. In general, this entity is characterized by the presence of bilateral striated, condensed or diffused and varying degree of horizontal light to deep brownish staining on teeth surface (Figs. 3a–c, 4a, b) of domestic animals (Choubisa 2008, 2013a, b). In some cases, unusual dental fluorosis is also appears as light to deep brownish spots, patches and fine dots or granules on the enamel surface of teeth. In advanced condition of dental fluorosis, pronounced loss of teeth supporting alveolar bone with recession and swelling of gingival and excessive wearing of teeth giving a wavy appearance are also due to chronic fluoride exposure/intoxication. In recent study, conducted in the desert of Rajasthan where drinking water have fluoride in the range of 1.5–2.5 ppm teeth of certain bovine calves revealed well-stratified light to deep blackish dental staining (Fig. 5) instead of brownish yellow (Choubisa et al. 2012). The reason behind this difference in staining is still not understood. However, such blackish staining in buffalo calves of non-desert fluoride-endemic provinces had has also been reported (Choubisa 1999a). By contrast, those animals having elongated incisor teeth as found in sheep, goat, camel, horse and donkey animals revealed a different pattern of dental staining characterized with bilaterally, but vertically and homogenously brown yellowish in color has also been reported (Choubisa 2010b).The exact reason for this difference is also not clear. Therefore, more studies are still needed in camels and equines to understand and confirm the exact cause of such pattern of dental fluorosis.

Fig. 3
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Bovine calves under 2 months of age showing appearance of stratified brownish yellow staining in moderate (a, b) and severe (c) form of dental fluorosis

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Fig. 4
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Severe and moderate form of dental fluorosis in a mature cattle (a) and dromedary camel (b) having excessive attrition and irregular wearing teeth stained with brownish yellow

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Fig. 5
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Cattle calf showing dental fluorosis with stratified deep blackish staining instead of brownish yellow

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Whatsoever, dental fluorosis is also important because it reduces the life span of domestic animals. When these dental lesions become severe enough to cause difficulty in grazing and mastication, the animals die at a young age from hunger and cachexia (Adler et al. 1970; Wang et al. 1992). Nevertheless, the death of animals at an early age has economic consequences for villagers/farmers/herdsmen. It is interesting to note that despite economic losses, not only herdsmen/villagers/farmers but also veterinarians in rural India are mostly unaware about the cause and consequences of dental fluorosis (Choubisa 2013a, b).

Skeletal fluorosis

This osteal or bony deformity is more dangerous, more painful and highly significant since it diminishes the mobility of fluorotic men and animals in very early age by producing varying changes in the bones such as periosteal exostosis, osteosclerosis, osteoporosis and osteophytosis (Choubisa and Verma 1996; Choubisa 1996b, 1999b, 2001, 2012b, c). These changes appear clinically in the form of vague aches and pains in the body and joints which are associated with rigidity or stiffness and lameness, stunted growth, palpable bony lesions and snapping sound in feet during walking in animals (Choubisa 1999a). Simultaneously, these progressive and irreversible osteal changes become more severe with advancing of age. The excess accumulation of fluoride in muscles also diminishes or restricts the bone movement which leads to lameness in animals. However, intermittent lameness, enlarged joints, debility, mortality, hoof deformities, wasting of body muscles and bony exostosis or lesions in the mandibles, ribs, metacarpus and metatarsus regions are well recognized in the fluorosed animals (Fig. 6) (Choubisa 1999a, 2013a). Severe form skeletal fluorosis cases of ankylosis in animals are also seen due to chronic hydrofluorosis (Viswanathan 1944). But such severe form skeletal fluorosis cases are rarely detected in India.

Fig. 6
figure 6

Emaciated <2-year-old cow calf afflicted with severe skeletal fluorosis. Note wasting of body muscles and bulging lesions on legs and ribes. Four legs are also slightly bending laterally

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Non-skeletal fluorosis

All the manifestations resultant to toxic effects in various organ systems are referred to as non-skeletal fluorosis. In fluoride-endemic provinces, the most common health complaints such as gastrointestinal discomforts (loss of appetite, bloating, colic pain, constipation, intermittent diarrhea, etc.), frequent tendency to urinate (polyuria)/itching in the region, frequently intake of water (polydipsia), muscles/body weakness, allergic reactions, irregular reproductive cycles, abortion and stillbirth in various species of domestic animals are the resultant of chronic fluoride intoxication (Choubisa 2010a, b, 2013a). Interestingly, all fluoride-induced complaints are reversible after withdrawal of fluoride exposure. However, works on fluorotoxicosis in excretory and male and female reproductive systems in different animal species are not sufficient. Fluoride-induced effects on reproductive organs, endocrine glands, gametogenesis, embryogenesis and brain are also not well studied in domestic animals. Therefore, more research works are needed at molecular level to the understanding of mechanism involved in diverse fluoride-induced changes in different species of domestic animals.

Determinants of hydrofluorosis

The prevalence and severity of hydrofluorosis are greatly varied in animals living in either same or different geographical provinces having almost similar fluoride concentration in drinking waters. This can be attributed to a number of determinants such as concentration of fluoride in drinking water and its duration and frequency of exposure, other chemical constituents in drinking water, age, sex, habits, food constituents, environmental factors, besides the individual susceptibility and biological response or tolerance and genetics of an individual (Choubisa and Sompura 1996; Choubisa et al. 2009, 2010a, 2011b; Choubisa 2012d).

The prevalence and severity of osteodental fluorosis in relation to fluoride concentration, age, sex, chemical constituents in drinking water and food constituents have been well studied in different species of domestic animals in the state of Rajasthan (Choubisa et al. 2009, 2010a, 2011b; Choubisa 2012d). The results obtained from these studies are more useful in the mitigation/prevention and control of hydrofluorosis. However, more studies are still needed for the understanding of correlation between these determinants and the prevalence and severity of hydrofluorosis in different species of animals rearing in the different geographical provinces.

Is hydrofluorosis reversible or irreversible?

Indeed major manifestations of the chronic fluoride intoxication have emerged as a global health problem. Unfortunately, besides considerable outstanding developments in veterinary sciences, a complete cure or reversal of natural genesis of chronic osteodental hydrofluorosis is still not known. However, in experimental conditions some of chemical substances such as molybdenum (Mo), phosphate (P) and sulfate (SO 2−4 ) have been reported to induce gastrointestinal absorption of F while other chemicals such as calcium (Ca), vitamin A, C and D reduce its absorption (Adler et al. 1970).

It is well stated that Ca, ascorbic acid (vitamin C) and vitamin D3 are potentially active to ameliorate the fluoride toxicity (Choubisa et al. 2011c). This was also observed in a large survey study conducted in mature and immature bovines, flocks and camels living in the areas with low fluoride (1.5–1.7 ppm) in drinking waters. Interestingly, the maximum prevalence and severity of dental and skeletal fluorosis were found in grass eaters (e.g., cattle and buffaloes) as compared to the plant eaters (e.g., camels, sheep and goats) ruminants. On the other hand, none of the predominantly plant eating immature ruminants—calves of camels, kids (goats) and lambs (sheep)—showed any manifestations of dental and skeletal fluorosis (Choubisa et al. 2011b; Sheikh 2011). Such natural alleviation of fluoride toxicity in plant eater ruminants living in low and high fluoride-endemic areas has also been well observed (Choubisa 2010c, 2013a, b).

It is well established that fluoride toxicosis is greatly varied between individuals, places and species living in the same fluoride-endemic areas and its severity is also influenced by several determinants besides concentration, duration of exposure and frequency of fluoride intake. In both mature and immature goats and sheep, the severity of fluoride toxicity is relatively less. It indicates, besides the fluoride exposure factor, other factors are also involved to counteract or ameliorate the fluoride effects which are present naturally in their foods. Sheep and goats generally feed on small delicate fresh leaves, pods and small fruits of trees and shrubs. In areas studied where the most common trees and shrubs are anjeer (Ficus carica), bargad (F. benghalensis), ber (Ziziphus jujube, Z. mauritiana and Z. nummularia), babool (Acacia nilotica), imali (Tamarindus indicus), khejri (Prosopis cinearia), kher (Acacia senegal), kikar (Pithecellobium dulce), pipal (F. religiosa), thapadia thore (Opuntia dellenii), thore (Euphorbia cadusifolia), vilayati babool (P. juliflora), etc. on which ruminants are dependent for their food. Most edible parts leaves, pods and small fruits of these plants are very rich in calcium (Ca) and ascorbic acid (vitamin C) nutrients (Choubisa 2010c; Choubisa et al. 2011b; Sheikh 2011). Both these nutrients interfere with the fluoride metabolism and ultimately reduce the fluoride toxicity. Similarly, in dromedary camels fluoride toxicity is also found less due to high content of Ca and vitamin C nutrients in their natural foods (Choubisa 2013c). However, reversibility of dental fluorosis by supplement of Ca and vitamin C in humans is still controversial, but its further advancement can be checked by these supplements (Choubisa et al. 2011a). This indicates that both these nutrients alleviate fluoride effects. Furthermore, although immature animals have greater sensitivity and susceptibility and less tolerance to fluoride, none of the immature goats and sheep studied was found to be afflicted with dental and skeletal fluorosis. It can be concluded that goat and sheep are protected naturally from chronic fluoride poisoning due to the presence of ample amount of calcium and vitamin C nutrients in their feeds (Choubisa 2010c). However, difference in the prevalence of hydrofluorosis between goats and sheep is due to difference in sensitivity to toxic effects of fluoride and in the quantity or frequency of fluoride intake. Sheep are well adapted to desert environment and require relatively less water for their survival. Hence, they have less and irregular fluoride exposure resultant to lower fluoride toxicity as in the case of dromedary camel.

Bio-indicators/biomarkers for hydrofluorosis

Any bio-indicators for hydrofluorosis should have less resistance or tolerance but should have a greater susceptibility to fluoride exposure and give early signs of fluoride intoxication. Recently, a large study was conducted in different animal species residing in the areas having naturally fluoridated drinking water (Choubisa 2014). These animals included both mature and immature buffaloes, cattle, camels, donkeys, horses, goats and sheep. Among these animals, immature ones were found more susceptible to fluoride toxicity (dental fluorosis). However, bovine calves were found to be relatively more ideal bio-indicators as these showed an early signs of dental fluorosis (Choubisa 2014).

A biomarker or biological marker generally refers to a measured characteristic which may be used as an indicator of some biological state. The term occasionally also refers to a substance whose presence indicates the existence of living organisms. These biomarkers are often measured and evaluated to examine normal biological processes, pathogenic processes or pharmacologic responses to a therapeutic intervention. Fluoride content in the environmental samples such as forage and fodder indicates the persistence of fluoride contamination in the environment (Dwivedi et al. 1997; Trangadia et al. 2015). However, fluoride contents in biological (milk, urine, blood serum, nails, teeth, bones, etc.) samples are also better biomarkers for fluoride intoxication in both man and animals in contrast to morbidity and mortality (Samal and Naik 1992; Swarup et al. 2001; Sankhala et al. 2014; Death et al. 2015). Nevertheless, the presence of fluoride in blood serum and urine is the most ideal method for the indication of current status of chronic fluoride intoxication (Patra et al. 2000). However, author suggests that more comparative studies are still required for the consideration of ideal bio-indicators/biomarkers for hydrofluorosis.

Mitigation of hydrofluorosis

The prevention and control of hydrofluorosis in domesticated animals are possible by providing fluoride free drinking water, by giving healthy or qualitative foods and by generating the general awareness in villagers, farmers, herdsmen and veterinarians regarding the preventive measures of fluoride poisoning in animals.

Defluoridation of fluoridated water can be done by adopting appropriate defluoridation techniques. However, several defluoridation techniques are available. However, one of them Nalgonda defluoridation technique is most appropriate, suitable and effective and is also not much expensive. In many fluoride-endemic states, this technique has been adopted, but its success rate is still very poor and at many places it is totally failure due to lack of proper monitoring, maintenance, responsibility and handling. However, author believes that harvesting and conservation of rain water is the most ideal method to get regular fluoride free drinking water for animals. Another effective option is to provide fresh surface waters (ponds, reservoirs, dams, etc.) instead of groundwater (hand-pumped and bore-well water) to domesticated animals which contain fluoride in the range of 0.01–0.3 ppm (Adler et al. 1970; Unicef 1999).

In conclusion, 23 out of 36 states and union territories have fluoride in drinking water beyond the threshold level of 1.0 or 1.5 ppm. Drinking of such fluoridated water for long tenure is highly toxic for health and causes hydrofluorosis in domestic animals. Therefore, a provision for sustainable availability of fluoride free drinking water for animals is highly needed in all fluoride-endemic states. Fluoride analysis of various perennial surface water sources is very important and essential in the rural India as these are also potential sources of fluoride exposure for the genesis of hydrofluorosis in both domestic and wild animals. To know the statewise status of hydrofluorosis in animals, an epidemiological study on various aspects of chronic fluoride intoxication in diverse species of domestic animals of each fluoride-endemic state is highly needed which will be more helpful in the making of the animal health policy. During the survey studies, biological (urine, blood serum, milk, hair, teeth, bones, etc.) and environmental (soils, grass, water, cereals, vegetables, etc.) samples should also be collected and analyzed for evidence of fluoride content which indicates the current status of chronic fluoride intoxication in animals.

The significance of present review not only is to provide the scientific and important information pertaining to endemic hydrofluorosis in diverse species of domestic animals to the researchers but will also be more useful in the preparation of future health plan at state and national level for the mitigation of endemic hydrofluorosis in domestic animals rearing in different fluoride-endemic states of the country.