The Past and Present of Sporothrix
The fungal genus Sporothrix (order Ophiostomatales) comprises a group of thermodimorphic
pathogens that cause skin infections in humans and other mammals. Human sporotrichosis
was first described in the Mid-Atlantic United States in 1898 by Schenck [1], followed
shortly by reported animal infections [2]. Sporotrichosis occurs worldwide, with hyperendemic
areas in Brazil, China, and South Africa [3,4]. Clinical sporotrichosis in mammals
results from two major infection routes: animal transmission and plant origin. Both
routes involve trauma to cutaneous and subcutaneous tissues to introduce Sporothrix
propagules into the skin. Cutaneous lesions develop at the inoculation site, and local
dissemination occurs through the lymphatics during the first two to three weeks of
infection [5]. Infections transmitted via either animal or plant vector often escalate
to outbreaks or epidemics.
Over the last decade, molecular phylogeny has revolutionized the taxonomy of pathogenic
Sporothrix species [6], altering our perceptions regarding epidemiology, host-association,
virulence, and drug susceptibility [7–10]. The classical agent Sporothrix schenckii
now comprises several molecular siblings nested in a clinical clade with S. brasiliensis,
S. globosa, and S. luriei (Fig 1) [6]. S. brasiliensis is related to atypical and
more severe clinical manifestations [11]. For decades, feline sporotrichosis in Brazil
appeared only as sporadic, self-limiting clusters. However, the current outbreak of
feline sporotrichosis because of S. brasiliensis in South and Southeast Brazil has
risen to epidemic status, creating a public health emergency of international concern
because of the potential of zoonotic transmission [9,12–15].
10.1371/journal.ppat.1005638.g001
Fig 1
Phylogenetic analyses correlate with ecological behavior of Sporothrix species.
Comparing the clades over the entire tree reveals a consistent decrease of bark beetle
and soil association outside Ophiostoma, with a concomitant increase of vertebrate
infectivity in S. brasiliensis, S. schenckii, and S. globosa. Phylogenetic tree generated
by neighbor-joining analysis using partial nucleotide sequences of the rDNA operon
(ITS1+5.8s+ITS2). Bootstrap value (1,000 replicates) was added to respective branches.
Each species is indicated at its respective position in the phylogenetic tree. Bar
= total nucleotide differences between taxa.
Diverse Ecologies and Strategies of Sporothrix
Our knowledge of the natural ecology of Ophiostomatales derives from studies including
basal lineages of Sporothrix and Ophiostoma, which occupy many niches (plant, soil,
and bark beetles) and usually lack pathogenicity for mammals. Despite the close phylogenetic
relationship with members of Ophiostomatales (Fig 1), the ecology of human-pathogenic
Sporothrix remains enigmatic [3,4,16], particularly regarding the factors driving
population dynamics and distribution ranges in nature. Species tend to appear in the
form of outbreaks, which depend on specific and rare conditions in decaying plant
material [4,17].
Genomic comparisons of S. brasiliensis and S. schenckii have elucidated the emergence
of mammal pathogenicity in the Ophiostomatales. Sporothrix undergoes morphological
transition in response to temperature, developing as filamentous hyphae during its
saprophytic stage at 25°C or as yeast in host tissue at 37°C. Comparisons with other
dimorphic fungi—including Paracoccidioides brasiliensis, Histoplasma capsulatum, Blastomyces
dermatitidis, and Talaromyces marneffei—reveal multiple complex adaptive evolutionary
strategies and that dimorphism in Sporothrix represents convergent evolution [18],
shared with distant human pathogens in Onygenales and Eurotiales. This emphasizes
the importance of this morphological adaptation to infection. Indeed, the global efficiency
of mycelium-to-yeast transition in Sporothrix varies among clades that exhibit different
pathogenic behaviors. For example, S. brasiliensis shows successful development of
numerous yeast-like budding cells, while impaired morphological switching is seen
among true environmental entities, such as S. chilensis and S. mexicana. This phenomenon
could be correlated with attenuated virulence of Sporothrix in the environmental clade
[10,19,20]. Moreover, comparative genomics has enabled identification of a contraction
of plant-degrading enzymes in S. brasiliensis and S. schenckii, which has been interpreted
as adaptation from plants to animals [18]. This may further support the previous epidemiological
perception of Sporothrix making a host jump from plant to animal transmission [9].
Transmission Types in Sporothrix
S. schenckii and S. globosa are cosmopolitan pathogens that generally follow an environmental
transmission route via traumatic inoculation of contaminated plant debris (Fig 2A,
green route). For over a century, this route has affected specific occupational populations,
including agricultural workers and gardeners (Fig 2), and was termed “rose breeders’
disease.” At the other extreme, the highly virulent clonal offshoot S. brasiliensis
is associated with animal infections and zoonotic transmission through deep scratches
and bites from infected cats [14]. The host jump of Sporothrix from plant to animal
transmission [9] is an important feature among the Ophiostomatales (Fig 2A, purple
and red routes), distinguishing cat-transmitted sporotrichosis as an occupation-independent
disease.
10.1371/journal.ppat.1005638.g002
Fig 2
Transmission routes in human and animal sporotrichosis.
The transmissibility between different species of clinical interest is explored based
on epidemiological data. (A)
Sporothrix brasiliensis is associated with large epizooties during animal horizontal
transmission (purple route). This is not an exclusive host association, since S. schenckii
may also infect cats but with lower frequency. Cat-borne sporotrichosis can be transmitted
to humans (zoonoses) via deep scratching and biting, through which high loads of yeast
cells are inoculated into to host tissue (red route). The threat of cross-species
pathogen transmission (purple and red routes) poses the risk of a massive epidemic
for humans in highly endemic areas. Note that Sporothrix schenckii and Sporothrix
globosa cause large sapronoses (green route), while S. brasiliensis is less frequent
during sapronoses. The size of the species’ circumference is proportional to the likelihood
of involvement (high, medium, or low) in each transmission route. (B) In the sapronotic
route (classical pathway), the presence of the etiologic agents of sporotrichosis
in nature can lead to an endemic profile, with fluctuation in the number of transmissions.
However, the infections remain close to the baseline over time. (C) Highly specific
conditions must be met to promote pathogen expansion in plant debris (see [4]). (D)
In the alternative route, feline-borne transmission via deep scratching is highly
effective during animal horizontal transmission and during zoonotic transmission,
placing a larger number of individuals at risk of acquiring sporotrichosis.
Numerous and large outbreaks are described in the literature [4,9,12,15,17,21]. S.
brasiliensis accounts for the majority of infections via felines, with a clonal population
structure observed during outbreaks. Strains with identical genotypes are found at
small geographic distances [9,13]. A slow dispersal vector is likely at work. Since
domestic felines are relatively sedentary, the zoonosis is expected to show slow geographical
expansion (Fig 2A, purple route) [9,12]. S. globosa also shows low degrees of variation,
suggesting clonality [4,6], but a major difference with S. brasiliensis is observed
in that identical genotypes are repeatedly found at large geographic distances, with
identical isolates originating from Colombia, China, and Brazil [4]. A rapid vector
of dispersal may be responsible for the transoceanic spread of S. globosa. Given the
large distances between strains, airborne dissemination via plant debris dust seems
likely. Cats rarely play a role in sapronoses by S. globosa [21]. Conversely, plants
are never observed as sources of infection by S. brasiliensis, and felines are responsible
for continued animal–animal transmission during epizooties and zoonoses in South and
Southeast Brazil (Fig 2A). In this regard, S. schenckii seems ecologically intermediate,
and genetic recombination among different genotypes may have contributed to the evolution
of diversity in the pathogenic clade (Fig 2A) [9].
Drivers of S. brasiliensis Infection: Letting the Cat out of the Bag
Understanding the emergence of new and old fungal agents is critical for promoting
effective public health policies [22]. Host shifts generally result from recent pathogen
introduction into a susceptible host population [23–25]. Felines present a broad spectrum
of clinical sporotrichosis, ranging from single lesions to fatal systemic forms [14,26].
The disease is easily transmitted, with cat-to-cat and cat-to-human transmissions
occurring via deep scratching and biting that inoculates high loads of Sporothrix
(Fig 2A). Phylogenetic data support a recent habitat shift in Sporothrix from plant
to cat, which apparently occurred in southeastern Brazil and is supporting S. brasiliensis
emergence [9]. The cat-borne, highly pathogenic S. brasiliensis depends on its feline
host for its epidemic emergence [9]. Feline sporotrichosis emergence in areas where
the number of cases has remained near baseline for long periods underlines the threat
of cross-species pathogen transmission (Fig 2D) [9,12]. Cats are the main vectors
of S. brasiliensis transmission to humans in Brazil, but the roles of other mammals
(e.g., rats) should also be investigated during S. brasiliensis epizooties. Lutz &
Splendore isolated pathogenic specimens from naturally infected rats [2], confirming
that Rattus norvegicus can develop sporotrichosis. Several reports have proposed other
mammals as potential carriers of Sporothrix propagules, emphasizing their importance
in sporotrichosis transmission, including armadillos, bats, dogs, and squirrels as
well as invertebrates such as mosquitoes, ants, and spiders [4].
The epicenter of the long-lasting outbreak of cat-transmitted sporotrichosis is the
metropolitan region of Rio de Janeiro (Brazil), where more than 4,000 human and 4,000
feline cases were diagnosed at Fundação Oswaldo Cruz between 1998 and 2012 [14]. Similar
epidemics are occurring in Rio Grande do Sul and São Paulo (Brazil), which present
high prevalences of S. brasiliensis infections [12,13,15]. Urban areas with high feline
population densities seem to be important drivers of epizooties because of S. brasiliensis.
Outside these areas, classical transmission types prevail, with subjects mainly infected
via accidental traumatic inoculation, mostly while manipulating contaminated plant
material [9]. Globally, S. schenckii is the major etiological agent transmitted via
the classical route [9], although S. globosa is preponderant in East Asia [4]. In
endemic areas of feline sporotrichosis, early outbreak episodes are followed by massive
continued transmission, suggesting negligence of the disease.
Compared to the alternative route (i.e., via the feline host, as in S. brasiliensis;
see Fig 2D), the classic infection route is expected to be less effective, leading
to scattered sporotrichosis cases in specific occupational groups (Fig 2B). However,
large outbreaks have occurred by the classical route (Fig 2C). Large sapronoses reported
from France [27], the US, [17], South Africa [28], and China [21] indicate that highly
specific conditions are required to promote pathogen expansion in plant debris [4].
In the alternative feline transmission route, deep scratching is highly effective,
placing larger numbers of individuals at risk of acquiring sporotrichosis (Fig 2D)
[9,12,14].
Strategies for Disease Containment
In general, zoonotic pathogens are twice as likely to be associated with emerging
diseases than nonzoonotic pathogens [29]. Here, we face a novel pathogen that emerged
within a highly susceptible feline population and achieved efficient and massive transmission
to humans [12–14]. Moreover, S. brasiliensis virulence is higher than that of other
Sporothrix. Epidemics driven by different transmission routes and agents with deviating
virulence [7,19] and differential susceptibility to antifungals [8] necessitate Sporothrix
diagnosis and identification for guiding public health policies and adjusting antifungal
therapy [5,26,30]. Currently, two rapid assays are available to detect Sporothrix
DNA directly from lesions and identify the pathogen with high sensitivity and specificity:
rolling circle amplification [31] and species-specific PCR [30]. Likewise, serological
assays, such as ELISA and immunoblot based on antigen preparations from Sporothrix,
can detect antibodies to 3-carboxymuconate cyclase (gp60 and gp70) and may aid feline
and human diagnosis [20,26] and patient follow-up [5]. Improvements in early diagnosis
and surveillance systems may facilitate rapid identification and control of future
outbreaks among cats and humans. Tackling an outbreak via the classical route (sapronoses)
requires removal of foci in nature. It is much more difficult to control the alternative
route of transmission via felines [14]. Strategies are needed to educate the population
about cat maintenance, Sporothrix transmission, and animal sterilization, to improve
early diagnosis, treatment, and prophylaxis, and to develop campaigns to prevent random
abandoning of diseased animals and cadavers.