DETAILS OF RESEARCH PROJECTS AND COLLABORATORS

 

 

1. Increasing our knowledge of 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. As a result these parasites inhabit a staggering number of phyla and classes of animals, sometimes even plants, and have occupied nearly every organ in its host. This fact has attracted the attention of researchers across a broad range of disciplines, especially since many of these schistosomes are etiological agents of human and domesticated animal diseases. 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. The monophyly of the family is well-supported and includes about 90 species in 15 genera and 4 subfamilies. 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 world's great-unconquered, infectious helminth diseases. Morbidity and mortality rates have remained high (200 million infected, 130,000 deaths/year). Another schistosome zoonosis, cercarial dermatitis, known as "swimmer's itch", is an ailment caused by the penetration of human skin by the cercariae of non-human schistosome parasites is a common, recurrent phenomenon in aquatic habitats worldwide. Most cases of dermatitis originate from schistosome species of waterfowl. 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. We need a precise determination of species that is valuable not only from a biological view, but also as a tool to predict health risks.

 

 

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. Further illumination of schistosome biology will also be provided by more complete sampling of other blood fluke families, the Spirorchiidae from turtles generally considered the sister family of Schistosomatidae, and the Sanguinicolidae, the blood flukes of fishes. Thus far, we have collected samples from Sri Lanka (R.P.V.Jayanthe Rajapakse, Dept. Veterinary Pathology), Australia and Kenya (Dr. Gerald Mkoji).

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 avian schistosomes with a focus on Trichobilharzia

 

Why 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 in all parts of the world except Antarctica. 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. Much of our knowledge of dermatitis in North America is severely out of date. Most of the diverse clade of avian schistosomes is poorly known taxonomically and the classification and relationships are ambiguous or unknown.

 

Why 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 genera responsible for a common and recurrent ailment, cercarial dermatitis, caused by the penetration of human skin by the cercariae of non-human schistosome parasites, in both shallow freshwater and marine habitats worldwide. 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.

 

Swimmer's itch

 
Adult worms of Trichobilharzia in ducks release eggs into the environment where the newly hatch larvae find and penetrate a snail. In the snail, the worm undergoes many clonal generations until swimming larva are released into the environment. These infective larvae will complete the life cycle by penetrating the foot/legs of ducks or geese and develop into adults. This swimming stage has been the cause of cercarial dermatitis, or swimmer's itch in people. The adult worms live in the blood vessels of the intestine in ducks and geese. 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.

 

 

Eggs in feces

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


There are three goals for this project:

 

1) To collect waterbirds globally to document the species of schistosome, their hosts and their distributions. Such information will be useful not only to the general scientific community, but to State Fish and Game departments and waterfowl groups.

 

2) To collect freshwater and marine snails globally to look for avian schistosome cercariae and try to match those with what we know about the adult worms. To document the complete life cycle of these worms, we need to know not only the species of adults we may find in the waterbirds, but also which snails and how many different kinds, are necessary to complete the life cycle.

 

3) Once we have all of this information, we will use the DNA sequence data from these worms to determine the following;

-What are the phylogenetic and phylogeographic relationships?

-Do worms remain in the ducks during migration north and/or south? or

-Are worms purged during fall migration, and ducks acquire new infections in the spring? –or do the worms go into remission during fall/winter, and commence egg production in the spring when they are in their breeding grounds?

-Are there particular genetic groups more prone to cause cercarial dermatitis?

-How many of the species of Trichobilharzia in North America cause disease in ducks?

-What is the distribution of snail host use?

 

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.

 

The Schistosome Group Prague has made significant advances in the systematics and biology of these worms in Europe.

 

 

Collaborators:

Dr. Scott D. Snyder

Schistosome Group Prague

 

Back to home page of Sara Brant