Discussion
This study stands as the most comprehensive investigation ofLeptospira spp. infection in domestic and wild animals from
Fernando de Noronha archipelago, also providing the description of the
first leptospiral strain ever isolated from an insular setting in
Brazil. Overall, most animals reacted against serogroup
Icterohaemorrhagiae, corresponding to almost 80% of all positive
samples that the most likely infecting serogroup could be determined by
MAT. The presence of anti-Leptospira antibodies in animal species
from Fernando de Noronha have already been described in 2018 by Moraiset al . (2018), who also found Icterohaemorrhagiae as the main
serogroup infecting cattle and rodents in the island. Although
prevalence of renal carriage has not been accessed in the present study,
the identification of Icterohaemorrhagiae as the most prevalent
serogroup in practically all species strongly suggests that rodents play
a major role in leptospiral transmission in Fernando de Noronha. Rodents
are recognized as the most important reservoir of leptospires worldwide,
and renal carriage of different pathogenic serovars causing disease in
humans and other animals have been frequently reported (Boey et al.,
2019).
Invasive rodents in Fernando de Noronha are subject of major concern,
once they can rapidly reproduce and adapt to human activity (Benavidez
et al., 2019). Furthermore, invasion of rodents induced devastating
effects on local biodiversity, leading to extinction of native wildlife
(Russell et al., 2018). These ecosystem disruptions are frequently
associated to diminished diversity of species within ecological
communities, potentially leading to increased incidence of leptospirosis
in insular environments (Derne et al., 2011). Rats, especially black
rats, are widely distributed in Fernando de Noronha and density is
currently estimated at 3,700 individuals/km2 in the
main island (Russell et al., 2018). In our study, both R. rattusand R. norvergicus presented agglutinins exclusively against
Icterohaemorrhagiae serogroup. Icterohaemorrhagiae have also been
identified as the most prevalent serogroup among rodent populations from
numerous insular locations (Boey et al., 2019) and strains belonging to
this serogroup are considered the main pathogen causing human
leptospirosis in the Reunion Islands (Guernier et al., 2016) and French
Polynesia (Guernier et al., 2017).
Interestingly, even in face of the high exposure to Icterohaemorrhagiae,
the characterization of the leptospiral strain recovered from a black
rat revealed infection caused by L. interrogans serogroup
Pyrogenes. Pyrogenes strains have been isolated from humans in New
Caledonia (Thibeaux et al., 2017), Philippines (Villanueva et al.,
2014), Puerto Rico (Valverde et al., 2013) and Mayotte islands (Bourhy
et al., 2010). In the Brazilian mainland, two strains belonging to the
Pyrogenes serogroup were recovered from a grey opossum (Phylander
opossum ) and water rats (Nectomys squamipes ) in previous
attempts to isolate leptospires from wildlife (Santa Rosa et al., 1975,
1980).
Despite no novel STs was revealed through the MLST analysis, further
whole genome sequencing should be performed in future studies for more
accurate phylogenetic inference and for a more comprehensive
understanding on the pathogenicity of the isolated strain, including key
aspects related to host-serovar adaptation. Our results suggest that
Fernando de Noronha might present a mixed pattern of leptospiral
diversity, where serogroups probably brought along in more recent
introduction of rodents coexist with older lineages, as proposed by
Guernier et al . (2018) and Dietrich et al. (2018) in
Western Indian Ocean islands. Nevertheless, further studies focusing on
more in-depth analysis of the leptospiral strains circulating locally
should be conducted to fully elucidate the epidemiology of leptospirosis
in Fernando de Noronha.
From the 18 seroreactive bovines, 14 had the highest titers against
Icterohaemorrhagiae serogroup. These results are markedly different from
those found in Brazilian mainland and most parts of the world, where
seroreactivity is predominantly associated with serogroup Sejroe,
particularly serovar Hardjo (da Silva Pinto et al., 2016). Acute and
incidental infection is rare and has been associated with serovars
Pomona, Grippotyphosa and Icterohaemorrhagiae (da Silva Pinto et al.,
2016). Nine out of the 14 bovines seroreactive for Icterohaemorrhagiae
had titers ranging from 400 to 1.600, thus indicating recent exposure to
the pathogen. Similarly to bovines, most seroreactive sheep and goats
showed the highest titers against Icterohaemorrhagiae, which also
diverges from serological data found in continental Brazilian territory,
where leptospirosis in small ruminants is also historically associated
with serogroup Sejroe, particularly serovar Hardjo (Martins and
Lilenbaum, 2014). Horses had the highest seroprevalence found and
presented the highest titers mostly against Icterohaemorrhagiae.
Although serological surveys in Brazil have confirmed infection caused
by serogroup Australis, including horses from Northeast region (Morais
et al., 2019), Icterohaemorrhagiae infection has been consistently
demonstrated in numerous other studies, currently categorizing this
serogroup as the most prevalent among horses in Brazil (Pinto et al.,
2017).
Only a small proportion of cats were positive by MAT, and most
individuals had titers against serogroup Icterohaemorrhagiae. Curiously,
none of the 200 cats previously surveyed in Fernando de Noronha by
Morais et al. , (2018) presented titers against Leptospira .
The epidemiology of feline leptospirosis is yet to be determined, and
serovars infecting cats vary greatly across different environmental
conditions. Overall, seroreactivity is low, and Pomona has been
frequently attributed to leptospiral infection in cats, however
infection by serovars from Autumnalis and Ballum serogroups can also
occur (Murillo et al., 2020). Under insular conditions, an even wider
range of serovars can be found in cats, such as Bataviae in Malaysia
(Alashraf et al., 2020), Semaranga in Thailand (Sprißler et al., 2019)
and Cynopteri in Saint Kitts (Pratt et al., 2017). Cats have long been
believed to control rodent populations, and low proportion of
seroreactive cats in Fernando de Noronha requires further investigation,
especially considering the high density of rodents found in the island.
Still, these findings may indicate naturally acquired immune response to
leptospiral infection. Alternatively, most of the cats sampled were
found around the inhabited areas and under supervised care by local
inhabitants, and although not specifically kept as pets, cats were
predominantly subsidized by human feeding, which might attenuate
predatory behavior and consequentially lead to less exposure to
pathogenic Leptospira .
It is noteworthy that dogs had the most variable serological response in
this study. Although Icterohaemorrhagiae was indeed the predominant
serogroup found in the canine population, dogs with the highest titers
(≥400) may have possibly presented infection by different serovars such
as Whitcombi, Autumnalis and Australis, with the former being the
serogroup with the highest titration among all seroreactive animals in
the study. These findings are consistent with former studies that also
found highly diverse serological response in dogs living on insular
settings (Pratt et al., 2017). Such repertoire might be influenced by
immunization, which tends to promote paradoxical reactions found in MAT,
leading to titration against multiple serovars unrelated to infection.
Nevertheless, the dogs included in the study were mostly supervised pets
with unrestricted movement and no history of recent immunization, and
serovar diversity infecting dogs may reflect intense environmental
exposure to leptospires or contact with livestock and peridomestic
wildlife reservoirs. Further studies should be addressed to clarify the
actual role of dogs in the epidemiology of leptospirosis in Fernando de
Noronha, and serological findings may at least indicate dogs as
important sentinels for human infection in the island.
Few birds (Gallus domesticus and Bubulcus ibis ) had
antibody titers detectable by MAT. The only three seroreactive cattle
egrets presented titers against Icterohaemorrhagiae serogroup.
Icterohaemorrhagiae could also be detected in one out of the four
chickens positive by MAT. Little is known about leptospirosis in birds,
but previous reports have also shown low seropositivity in domestic and
wildlife birds (Brasil et al., 2013; Acosta et al., 2019). The
prevalence found in tegu lizards was relatively high, with four out of
the 13 samples reacting against serogroups Icterohaemorrhagiae and
Autumnalis, and although the small sample size, similar high prevalence
is usually reported in Brazil (Biscola et al., 2011; Paz et al., 2019;
Miranda et al., 2020). Still, the role of reptiles as reservoirs of
leptospirosis and their risks for public health is debatable (Fornazari,
2017). Few studies have tackled infection and pathogenesis in avian and
reptile species, and serological findings in the present study indicate
exposure to environmental leptospiral strains circulating locally.
Overall, the serological pattern found in all species highlight the
importance of rodents for leptospiral transmission in the island. Rodent
control is currently limited to poison bait stations around areas with
more infrastructure, which are checked and re-stocked at regular
intervals. No rat surveillance in the in the bay area is taken, and
invasion from shipped cargo is neglected. Therefore, there is an urgent
need for the implementation of a systematic rodent control program.
Likewise, the poor sanitary infrastructure conditions found in the
island also poses as risk of Leptospira spp. transmission not
only to the local human population but also for tourists. The
improvement of the sanitary conditions in the archipelago is however a
logistical challenge to local managers, since the long distances from
the continent and high freight costs make any civil construction
intervention very difficult to implement. Although no permanent water
stream is found in the archipelago, most of the small farms are located
around one large water reservoir, which was used for human consumption
in the past. This water reservoir is probably not only associated with
the transmission of Leptospira spp. between farm animals, but
also wild animals. Cattle egrets, for example, were found at the airport
strip, which is located between this water reservoir and the archipelago
garbage dump.