Chytridiomycosis in Native Arizona Frogs

The chytridiomycete fungus Batrachochytrium dendrobatidis (Bd) colonizes mouthparts of amphibian larvae and superficial epidermis of post-metamorphic amphibians, causing the disease chytridiomycosis. Fungal growth within host cells has been documented by light and transmission electron microscopy; however, entry of the fungus into host cells has not. Our objective was to document how Bd enters host cells in the wood frog Lithobates sylvaticus, a species at high mortality risk for chytridiomycosis, and the bullfrog L. catesbeianus, a species at low mortality risk for chytridiomycosis. We inoculated frogs and documented infection with transmission electron microscopy. Zoospores encysted on the skin surface and produced morphologically similar germination tubes in both host species that penetrated host cell membranes and enabled transfer of zoospore contents into host cells. Documenting fungal and epidermal ultrastructure during host invasion furthers our understanding of Bd development and the pathogenesis of chytridiomycosis. KEY WORDS: Chytridiomycosis · Morphology · Transmission electron microscopy · Lithobates catesbeianus · Lithobates sylvaticus Resale or republication not permitted without written consent of the publisherDis Aquat Org 100: [201][202][203][204][205][206][207][208][209][210] 2012 scopically, excessive skin sloughing, lethargy, anorexia, loss of righting reflex, and abnormal posture may be observed (Nichols et al. 2001, Daszak et al. 2004) but often not until immediately before death (Bradley et al. 2002, Parker et al. 2002. Proximate cause of death is inhibition of electrolyte transport across epidermis and subsequent disruption of cardiac electrical activity (Voyles et al. 2009).The asexual life cycle of Bd consists of a motile zoospore stage and a stationary thallus stage (Berger et al. 2005). Zoospores are flagellated and are not bounded by a cell wall. After a motile period, usually lasting ≤24 h, a zoospore encysts (i.e. attaches to a substrate, retracts the flagellum, and forms a wall of chitin around the spore body). Morphology after encystment depends on substrate type. On nutrient media, an encysted zoospore develops into a single or colonial (internally divided) zoosporangium (endogenous growth); root-like rhizoids originating from 1 or more points on the zoosporangium may be numerous . In an amphibian host, cellular contents of a superficially encysted zoospore leave the cyst and enter a host cell (exogenous growth), inside which a single or colonial zoosporangium develops; rhizoids are usually less abundant than on nutrient media (Berger et al. 2005). The specifics of amphibian host invasion by Bd (i.e. the morphological process by which zoospore contents are transferred from a cyst into a host cell) have not been reported. In other chytrid species with known developmental morphologies (saprophytes and patho gens of plants, algae, protists, and invertebrates), endobiotic (i.e. intracellular) growth of a thallus is initiated when a protrusion of the zoospore cyst wall e...

Section: Resale or Republication Not Permitted Without Written Consenmentioning

confidence: 99%

Host invasion by Batrachochytrium dendrobatidis: fungal and epidermal ultrastructure in model anurans

Greenspan¹,

Longcore²,

Calhoun³

2012

“…Chytridiomycosis has been recorded from Australia, New Zealand, Europe, Africa, and South, Central and North America, from a broad range of habitats , Lips 1999, Mutschmann et al 2000, Bosch et al 2001, Fellers et al 2001, Speare et al 2001, Bradley et al 2002, Weldon et al 2004. Mortality rates of up to 100% occurred during natural outbreaks in captivity and in transmission experiments in captive amphibians of susceptible species (Berger et al 1998, Nichols et al 2001, although other species can survive infection (Ardipradja 2001, Davidson et al 2003.…”

Section: Introductionmentioning

confidence: 99%

Life cycle stages of the amphibian chytrid Batrachochytrium dendrobatidis

Berger¹,

Hyatt²,

Speare³

et al. 2005

276

288

An overview of the morphology and life cycle of Batrachochytrium dendrobatidis, the cause of chytridiomycosis of amphibians, is presented. We used a range of methods to examine stages of the life cycle in culture and in frog skin, and to assess ultrastructural pathology in the skin of 2 frogs. Methods included light microscopy, transmission electron microscopy with conventional methods as well as high pressure freezing and freeze substitution, and scanning electron microscopy with critical point drying as well as examination of bulk-frozen and freeze-fractured material. Although chytridiomycosis is an emerging disease, B. dendrobatidis has adaptations that suggest it has long been evolved to live within cells in the dynamic tissue of the stratified epidermis. Sporangia developed at a rate that coincided with the maturation of the cell, and fungal discharge tubes usually opened onto the distal surface of epidermal cells of the stratum corneum. A zone of condensed, fibrillar, host cytoplasm surrounded some sporangia. Hyperkeratosis may be due to (1) a hyperplastic response that leads to an increased turnover of epidermal cells, and (2) premature keratinization and death of infected cells.KEY WORDS: Batrachochytrium dendrobatidis · Chytridiomycosis · Fungus · Morphology · Ultrastructure · Transmission and scanning electron microscopy · Pathology · Amphibian Resale or republication not permitted without written consent of the publisherDis Aquat Org 68: [51][52][53][54][55][56][57][58][59][60][61][62][63] 2005 its occurrence on an amphibian host and small subunit ribosomal DNA (ssu-rDNA) sequence of B. dendrobatidis indicate that this fungus is distinctly different from other known chytrids (Berger et al. 1998, James et al. 2000. It is the only member of this phylum to cause disease in a vertebrate.The life cycle of Batrachochytrium dendrobatidis is a simple progression from zoospore to the growing organism, called a thallus, which produces a single zoosporangium (= container for zoospores). The contents of the zoosporangium (also known as a sporangium) cleave into new zoospores which exit the sporangium through one or more papillae. Sexual reproduction has not been observed. Colonial development resulting from the formation of more than 1 sporangium from 1 zoospore is the only known variation of the cycle ). The life cycle seems to be the same in culture and in skin. The duration of the life cycle in vitro is 4 to 5 d at 22°C and is assumed to be the same in amphibian skin, although this has not been tested.Batrachochytrium dendrobatidis discharges zoospores through an inoperculate opening and exhibits monocentric or colonial growth . Longcore et al. (1999) comprehensively described the taxonomic features of B. dendrobatidis and gave detailed morphology based on light microscopic observations of cultures and ultrastructure of serially sectioned zoospores, but did not describe the appearance of life cycle stages using scanning electron microscopy (SEM).Significant ultrastructural differences were not obs...

“…Currently, approximately 43% of all known amphibian species are considered to be declining (Stuart et al 2004). Many of these declines are thought to be due to the amphibian disease chytridiomycosis (Berger et al 1998, Lips et al 2006, which can be fatal to many species and is implicated in mass mortalities, population declines and extinctions in Australia, New Zealand, Central and North America, Europe and Africa (Berger et al 1998, Lips 1999, Bosch et al 2001, Bradley et al 2002, Muths et al 2003, Bell et al 2004, Weldon & Du Preez 2004, Lips et al 2005.…”

Section: Introductionmentioning

confidence: 99%

Retreat sites of rain forest stream frogs are not a reservoir for Batrachochytrium dendrobatidis in northern Queensland, Australia

Rowley¹,

Skerratt²,

Alford³

et al. 2007

Chytridiomycosis is a potentially fatal disease of amphibians caused by Batrachochytrium dendrobatidis, and is implicated in declines and extinctions of amphibian populations and species around the world. To cause local host extinction, a disease organism must persist at low host densities. One mechanism that could facilitate this is the ability to persist in the environment. In the laboratory, B. dendrobatidis spreads by both frog-to-frog and environment-to-frog transmission, and can persist on a number of biotic substrates. In the field, B. dendrobatidis has been detected on environmental samples taken during an epidemic, but it is not known if it persists in the environment when endemic. Retreat sites of 2 species of Australian rain forest stream frogs Litoria lesueuri and L. nannotis were sampled 0 to 3 d after occupation during the wet and dry seasons in northern Queensland, Australia, where chytridiomycosis has been endemic for at least 10 yr. The intensity and prevalence of infection in frogs during sampling were comparatively low compared with epidemics. Diagnostic quantitative polymerase chain reaction did not detect B. dendrobatidis in any retreat site samples. It thus appears that retreat sites are not a major environmental source of infection when B. dendrobatidis occurs at low prevalence and intensity on frogs. This suggests that control efforts may not need to eliminate the organism from the environment, at least when prevalence and intensity of infection are low in frogs. Simply treating hosts may be effective at controlling the disease in the wild. KEY WORDS: Batrachochytrium dendrobatidis · Amphibian chytrid fungus · Chytridiomycosis · Environmental reservoir · Disease transmission · FrogsResale or republication not permitted without written consent of the publisher Dis Aquat Org 74: 7-12, 2007 B. dendrobatidis is the only chytrid known to cause disease in vertebrate hosts (Berger et al. 1998), presumably breaking down keratin that occurs in the epidermis of adult amphibians and the mouthparts of larval anurans. Chytridiomycosis can cause rapid mortality (Nichols et al. 2001), with infected frogs of susceptible species dying within 3 wk of infection in the laboratory (Berger et al. 1998(Berger et al. , 2004. In the laboratory, the disease is highly contagious (Nichols et al. 2001), spreading within and among individuals via motile, waterborne zoospores .One of the unusual aspects of chytridiomycosis is that it drives many host species to local extinction during outbreaks (Berger et al. 1998, Lips et al. 2006. For a pathogen to cause local host extinction it must be capable of persisting and infecting new hosts even at very low host population densities (Anderson & May 1986, Dobson & May 1986. One potential mechanism for this is the persistence and growth of Batrachochytrium dendrobatidis in tadpoles and adults that do not die from infection (Berger et al. 1998, 1999. The broad host range of B. dendrobatidis also provides a reservoir of infection, enabling the species most susceptib...