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.
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.