Research Problems

 

 

1. Understanding schistosome diversity

 

In the phylum Platyhelminthes, class Trematoda, subclass Digenea, there are ~2,500 genera, and ~18,000 species of digenetic trematodes worldwide. This represents one of the largest groups of internal metazoan parasites, second only to the nematodes. Digenetic trematodes are obligate adult parasites in most vertebrate definitive hosts, several phyla as second intermediate hosts, and mollusks as first intermediate hosts. One of the most distinctive superfamilies of trematodes is the Schistosomatoidea, which includes the Schistosomatidae in birds and mammals, Spirorchiidae in turtles, Sanguinicolidae in fishes and Clinostomidae in birds. Members of the Schistosomatidae are unique digeneans that are morphologically distinct with a number of features that set them apart from the rest of the subclass. Most notable, they are dioecious, dimorphic and have a 2-host life cycle: bird or mammal and snail. Schistosomes are primarily associated with freshwater habitats and are found in all temperate and tropical regions of the world. Schistosomes are of particular concern as they cause morbidity and mortality in humans that has placed them as a study priority, much of which is focused on genomics, control and drug resistance, but not taxonomy! Human schistosomiasis remains one of the worldÕs great-unconquered, infectious helminth diseases. Morbidity and mortality rates have remained high (200 million infected, 130,000 deaths/year). Another schistosome zoonosis, is cercarial dermatitis, also known as ÒswimmerÕs itchÓ (see point 3 below). Other than the medically important mammalian genus Schistosoma, much remains to be learned about global species diversity, biogeography, patterns of vertebrate and snail host usage, and evolutionary relationships of the other schistosome genera.

 

 

The aim of this project is an ongoing effort to incorporate new sequence and/or morphological data from schistosome samples into the existing phylogeny of the blood flukes. There are many areas in the world where the diversity of schistosome worms, from either snails or vertebrates, has been insufficiently sampled. Thus far, we have samples (marine and freshwater) from across the U. S., Sri Lanka (R.P.V.Jayanthe Rajapakse, Dept. Veterinary Pathology), Nepal (Jeevan Sherchand, Prem Budha, Ramesh Devkota), Australia (David Blair), Kenya (Dr. Gerald Mkoji), Canada, and New Zealand (Norm Davis). So far we have recovered about 30 species, 10 of which are currently undescribed. 

 

 

A major focus is the avian schistosomes. Avian schistosomes comprise the largest and most diverse clade in the family (Brant et al. 2006). These schistosomes are ecologically restricted to waterbirds as definitive hosts, and most use freshwater snails as intermediate hosts. Avian schistosome species commonly cause cercarial dermatitis. Most of the diverse clade of avian schistosomes is poorly known taxonomically and the classification and relationships are ambiguous or unknown. To date we have examined over 400 birds representing about 50 species and many individual snails representing about 25 species and found 15 taxa of avian schistosomes.

 

 

 

 

 

 

 

 

 

 

 

 

Figure from Brant SV, Loker ES. 2005. Can specialized pathogens colonize distantly related hosts? Schistosome evolution as a case study. PLoS Pathogens 1(3): e38.

 

 

 

 

2. Systematics of Trichobilharzia

 

Compared to the mammal schistosomes, bird schistosomes have received less attention, especially Trichobilharzia, the largest genus within Schistosomatidae. There are reported over 40 species of Trichobilharzia world wide, mostly from ducks. Parasites of the avian schistosome genus Trichobilharzia are one of the main genera responsible for. The avian schistosomes responsible for dermatitis outbreaks are poorly characterized in the U.S. and we lack reliable information on the number of species involved, their natural hosts, geographic distributions, and basic epidemiology. The most immediate and pervasive problem with respect to understanding dermatitis-causing schistosomes from the U.S. is that we remain reliant on dated morphological descriptions to differentiate and enumerate species. We lack a similar level of understanding of host usage for the most common species causing dermatitis in freshwater, Trichobilharzia. In many cases, dermatitis-causing cercariae have been identified from snails without experimental or molecular verification of the identity of the corresponding adult worms. It is the goals of this project to use morphology and DNA of both adults and larval stages to identify the diversity and host use of Trichobilharzia in the U.S. The adult worms live in the blood vessels of the intestine in ducks. Because of the remote habitat in the ducks, their small size and difficulty in identifications, very little is know about how many species of these worms exist, especially here in North America. Therefore, more attention is needed towards understanding the diversity of these worms in nature.

 

3. Etiology and Epidemiology of cercarial dermatitis

 

Cercarial dermatitis, also known as ÒswimmerÕs itchÓ, an ailment caused by the penetration of human skin by the cercariae of non-human schistosome parasites, is a common, recurrent phenomenon in freshwater, brackish and marine habitats worldwide (Cort, 1950). Adult schistosomes typically live in the mesenteric blood vessels of birds and mammals, and produce eggs that pass from their hostÕs body in the feces, then hatch and release miracidia that penetrate and develop in snail intermediate hosts. Snail infections culminate in the production of numerous cercariae that are regularly released into the water where they seek to penetrate the skin of a vertebrate definitive host. Cort (1928) working at Douglas Lake, Michigan was the first to associate schistosome cercariae emerging from freshwater snails as the causative agent of a dermatitis known around Michigan lakes as swimmerÕs itch. He and others showed this ailment to be caused by the cercariae of several genera of schistosomes that normally use aquatic birds and non-primate mammals as their definitive hosts. Anyone who has spent summers swimming in freshwater ponds and lakes in North America is aware of this problem and may well have been a victim.

 

 

 

AppleMark

 

 

In addition to their medical impact, these outbreaks can have significant economic repercussions (e.g. tourism, real estate) for affected communities. Avian schistosomes that are responsible for dermatitis outbreaks are poorly characterized; even in the U.S. we lack reliable information on the number of species involved, their natural hosts, geographic distributions, and basic epidemiology. For 80 years, this lack of understanding has limited our ability to determine which species of schistosomes in the U.S. are most responsible for outbreaks, let alone worldwide.

 

 

The Schistosome Group Prague and colleagues have made considerable advances in the systematics and biology of these worms in Europe.

 

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