heavily and how frequently particular host species are infected. Host
specificity takes intensity and prevalence (frequency) of infection into
consideration. Rohde (1980) has developed specificity indices that
measure host specificity. He defined the host specificity index based on
intensity of infection as:
Si(intensity) =Σ(xij/nihij)/Σ(xij/nj)
whereSi= host specificity in the ith parasite species,xij= number of
parasite individuals of theith species in thejth host species,nj= number
of host individuals of thejth species examined, andhij= rank of host spe-
ciesjbased on intensity of infection. If all parasite species of a community
are considered, the specificity index of the parasite community can bedefined asSi(intensity) =ΣS
ni
p, wherenp= number of parasite species inthe community.
The index can be modified to take frequencies (prevalences) of
infection instead of intensities into consideration. Parameters in the
formula are now changed as follows:xij= number of host individuals
of jth species infected with parasite species i, nj= number of host
individuals ofjth species examined, andhij= rank of host species based
on frequency of infection. One can also make use of intensities as well
as frequencies of infection, for instance by using a combined indexSi
(intensity) +Si(frequency).
Numerical values for the indices vary between 0 and 1: the closer to 1,
the higher the degree of host specificity, the closer to 0, the lower. For
example, many Digenea from the White Sea have a wide host range, i.e.
they infect many host species, but few of them heavily (Shulman and
Shulman-Albova, 1953):Lecithaster gibbosuswas found in 12 of 31 fish
species examined, but the vast majority of all parasites of this species
were found in a single host species, and itsSi(intensity) is consequently
0.99. An example of a parasite with very wide host range and low
host specificity isToxoplasma gondii, which infects many species of
mammals; examples of parasites with very high host specificity, infecting
a single host species, are many monogeneans:Polystomoides malayihas
been found only in the freshwater turtleCuora amboinensis, and some
other species of the same genus are restricted to single host species as
well. Rohde (1978) has shown that host ranges of marine monogeneans are
small at all latitudes, but that host ranges of marine digenean trematodes
show a gradient of decreasing host ranges from cold to warm seas.
In contrast, host specificity is similarly great at all latitudes in both
groups.Restriction to host individuals (aggregation within host populations)
Almost as universal as restriction to certain host species is aggregation
within host populations, i.e. some host individuals are much more172 K. Rohde