Mammalian biodiversity is poor in the volcanic islands of the western Indian Ocean and all mammals currently present, except bats, have been introduced. However, Madagascar is one of the most striking hotspots of biodiversity on Earth where most of the fauna is endemic 14 . Table 2 summarizes the various Leptospira hosts studied in the western Indian Ocean region. Leptospirosis is probably endemic in the mammalian species of the western Indian Ocean islands Table 2 15 16 17 . In Mayotte and Reunion Island, serological surveys showed a high seroprevalence of leptospirosis in non-vaccinated (stray and domestic) dogs 15 17 and dogs have been demonstrated to be renal carriers and urinary shedders of Leptospira on the Reunion Island 17 . In Reunion, the seroprevalence of the disease in dogs has not varied substantially since 1980, and the Canicola serogroup has been described as the main serogroup infecting dogs; Icterohaemorrhagiae is the second most frequent in dogs 17 18 . By contrast, in Mayotte, the serogroup Mini (the main serogroup implicated in human cases) has most frequently been identified by serology in stray and domestic non-vaccinated dogs 15 .

⁽¹⁾ The animal is infected and shows clinical signs of the disease.

⁽²⁾ The animal is infected without clinical signs of the disease.

* These species are generally asymptomatic, but some Leptospira strains can induce acute forms of the disease (e.g. Icterohaemorrhagiae in dog or in horse).

Nd: no data.

Leptospirosis has been known since 1980 to be a major infectious disease in cattle in Reunion Island 19 and in 2003, a study showed that serogroups Sejroe and Hebdomadis were major causes of abortion in dairy cattle 39 . Sejroe was reported to be the main serogroup circulating in beef and dairy cattle in Reunion Island in 2009 17 , whereas in Mayotte, cattle are mostly infected by serogroup Mini (National Reference Center for Leptospirosis, France, unpublished data). The original Hebdomadis serogroup is divided into three separate serogroups according to their serological affinities: Hebdomadis, Sejroe and Mini 43 . Given the absence of isolation data and the antigens used in serological studies, the seroreactivity to Sejroe, Mini, and Hebdomadis serogroups in cattle could be cross-reactions hiding a serological response to only the serovar Hardjo, for which cattle are the maintenance hosts 7 .

Bovine and pig leptospirosis has long been suspected in Madagascar 44 but the renal carriage of Leptospira has never been documented in these species 40 . Nevertheless seroprevalence in apparently healthy cattle and pigs was reported in 1968 in the south region of the island, with the predominance of the Grippotyphosa serogroup 20 . Desvars et al. 17 reported that in Reunion Island, serogroup Pyrogenes and Panama are the most prevalent serogroups found by MAT in goats, Rusa deer, and pigs whereas they are rarely reported in diagnosed patients 45 .

Serological surveys of the insectivorous tenrec (Tenrec ecaudatus) in Reunion Island report seroprevalences of between 13.2% (5/38) 17 and 92% (34/37) 38 in this species, probably depending on the geographical area of sampling and/or age of the animals. Leptospira could not be evidenced in tenrec kidney or urine, suggesting that this species is probably not a chronic reservoir host for the disease 17 .

The epidemiological role of bats in the transmission of Leptospira attracts more and more scientific interest 46 47 48 49 50 51 52 53 . In Madagascar, antibodies to Leptospira could not be evidenced in the fruit bat Pteropus rufus 54 55 , but recently, pathogenic Leptospira spp. were found in bats, in Madagascar and Union of Comoros 42 . In Mayotte, a recent study reported a seroprevalence of 10.2% (5/49) in Pteropus seychellensis with Pyrogenes and Grippotyphosa as infecting serogroups 15 . The free-tailed bat (Mormopterus francoismoutoui, family Molossidae) is a urinary shedder of Leptospira 17 . Nevertheless, the zoonotic role of bats species in the transmission of leptospirosis to humans remains uncertain.

Serological evidence of leptospirosis has been reported in lemurs from Mayotte but at a low seropositive rate (2%, 1/50) 15 . This low rate is probably due to the arboreal lifestyle of these animals minimizing their contact with contaminated water or soil. Sensitivity to leptospirosis differs greatly between non-human primates 56 57 58 59 . We could also hypothesize that lemurs are highly sensitive to leptospirosis such that infected animals die (as demonstrated in the squirrel monkey, Saimiri sciureus, and the marmoset, Callithrix jacchus 56 58 59 ). Thus the contribution, if any, of primates in the transmission of leptospirosis in Mayotte, Anjouan, Moheli, and Mauritius still needs to be elucidated.

All published studies confirm that the black rat is the major reservoir host for Leptospira in Europa and Juan de Nova 60 , Mayotte (where the Norway rat is absent) 15 , Reunion Island 17 , and Madagascar 36 . In Reunion Island and Madagascar, the Norway rat, the shrew, and the domestic mouse have also be shown to be renal carriers and/or urinary shedders of Leptospira 17 36 . In Reunion Island, the major serogroup identified in the rat is Icterohaemorrhagiae, but other serogroups also seem to circulate (Canicola, Sejroe) 17 . In Mayotte, the Mini serogroup is the main circulating serogroup in R. rattus and there is strong evidence that the black rat population is the major reservoir of Leptospira and source of its transmission to humans 15 .

Acute leptospirosis has never been described in Madagascar and investigations conducted locally have failed to show the presence of the bacterium in humans. Lhuiller et al. 55 reported a low seroprevalence rate among Antananarivo inhabitants. The only autochtonous clinical case (identified in the 1950’s) to be confirmed serologically had antibodies to serogroup Australis 54 61 . In 1968, Silvérie et al. 20 identified that Tarassovi, Grippotyphosa, Australis, and Hebdomadis were the most prevalent serogroups in the human population in the region of Toliara. They reported a seroprevalence of 50.8% (33/65) 20 which seems surprisingly high in a country where no clinical cases have been reported. Recently, one human case was diagnosed at the hospital of Mamoudzou (Mayotte) involving Leptospira kirschneri serogroup Mini and was suspected to have been imported from Madagascar 21 .

The first confirmed case of human leptospirosis in the Maldives was reported in November 2000. Since then, the disease has been under national surveillance 62 .

The disease has been very occasionally reported in Mauritius, but there is probably underreporting, since the epidemiological conditions are very similar to those in the neighboring Reunion Island 63 . Simon et al. 22 recently described a case of leptospirosis in a French patient who had traveled to Mauritius.

The Seychelles presents the highest incidence of leptospirosis in the western Indian Ocean area and one of the highest incidences in the world 23 . Between 1988 and 1990, the annual incidence was 60 cases per 100 000 inhabitants and serogroups Icterohaemorrhagiae and Autumnalis were most frequently identified during this period 64 . In 1995–1996, the incidence of leptospirosis was estimated to be 101 per 100 000 23 and eight serogroups were identified, with Icterohaemorrhagiae and Hurstbridge (the latter commonly considered as non-pathogenic) being the main circulating serogroups.

The first publication describing human leptospirosis in Mayotte was in 1990 24 . It reported 42 cases between 1984 and 1989 and an annual incidence of 3.83 per 100 000 24 . Recent efforts in the detection of the disease 21 25 has led to the annual incidence of leptospirosis being re-evaluated, as 25 per 100 000. Mini is the major serogroup responsible for human clinical cases in Mayotte whereas Icterohaemorrhagiae has never been isolated from patients in Mayotte which represents a unique epidemiological situation 21 ; the other Leptospira serogroups identified in patients are Pyrogenes, Grippotyphosa, and Pomona.

In Reunion Island, the annual number of human cases of leptospirosis has varied little since 1970, with an average of 40 cases per year between 1970 and 1979 and an average of 55.7 cases between 1998 and 2008 65 (probably due to improved diagnosis of this disease). The incidence of leptospirosis was 15.13 per 100 000 in 2010 45 . Between the 1970s and now, serogroup Icterohaemorrhagiae has been the main serogroup involved in clinical cases on Reunion Island 26 27 28 45 65 .

In humans, clinical leptospirosis has protean manifestations but generally causes a febrile illness that often, in its early phase, cannot be differentiated from other acute influenza-like fevers such as dengue, influenza, chikungunya, or malaria 5 8 (Table 2). In the islands of the western Indian Ocean where human leptospirosis is found, less than 10% of hospitalized leptospirosis patients die 21 29 66 . In the Seychelles, about one-third of cases of leptospirosis are mild forms; two-thirds have a more severe presentation with jaundice (without liver failure) and/or acute renal failure and/or pulmonary hemorrhage 23 . In the Seychelles, during a 12-month period in 1995–1996, 8% (6/75) of the patients with acute leptospirosis died 29 ; autopsies showed that diffuse bilateral pulmonary haemorrhage was the main cause of death 29 . In Reunion Island, clinical forms of the disease in hospitalized patients are often severe 26 and pulmonary manifestations are frequent 28 67 68 69 . In Mayotte, 3.2% of the hospitalized cases are fatal 21 .

Leptospirosis in livestock is generally associated with reproductive failure (Table 2) 70 71 72 but the studies published to date do not permit evaluation of the true losses caused by the disease in dairy and beef cattle, pigs, goats, sheep, and Rusa deer in the different islands. Economic data of this type may encourage taking this disease into account in the management of reproduction and health problems within herds. In dogs, leptospirosis can, in some cases, cause an acute disease similar to Weil disease in humans, but mostly, the disease is asymptomatic in dogs (Table 2) 73 .

The risk factors for acquiring leptospirosis in animals have not been studied in the western Indian Ocean region. Nevertheless, risk factors for contamination of both individual animals and herds are most probably similar to those identified in other tropical countries. 74 75 76 77 78 79 .

In humans from Reunion Island and Mayotte, the risk of contamination is the highest for the 20 to 40 year-old age group 21 26 , and in the Seychelles, the mean age of cases is 34 years (and 53 years in fatal cases) 23 . Risk factors for acquiring leptospirosis are similar in all tropical islands. One common feature of the disease is that leptospirosis is more frequent among men than women 21 23 28 . This sex difference, is usually attributed to occupational and behavioral factors 13 80 . In the Seychelles, significant associations have been found between leptospirosis cases and activities in the forest, gardening, and refuse not collected by public services (which favors high rat population density) 23 30 . Washing clothes or bathing in the river, and walking barefoot, favor contamination by Leptospira 23 30 and an association between the disease and wet soil around homes has been reported in the Seychelles 23 and Reunion Island 28 .

The incidence of leptospirosis in humans is affected by rainfall, particularly in tropical regions 81 82 83 . In Reunion Island, cases of leptospirosis are reported all year round, but epidemics mostly occur during the rainy season 26 28 31 84 . Therefore, despite popular belief, the peak incidence of leptospirosis in Reunion Island is not during the sugar cane harvest (July to December) 84 . This seasonality in leptospirosis cases is also observed in Mayotte where almost all cases are reported during the rainy season 21 . In the Seychelles, the relationship between rainfall and leptospirosis cases is weaker than in Mayotte or Reunion Island, probably because the rainy season is not well-defined 30 . A prediction model could be a useful tool for the identification of cases and, obviously, for predicting epidemics; this may facilitate improved diagnosis and treatment of leptospirosis to reduce lethality rates 85 86 . Statistical modeling should include the number of cases, meteorological factors, and data on animal and human population density (Figure 2).