Biology Reference
In-Depth Information
fish. Brown trout (
Salmo trutta
), native to Europe, are carriers of the dis-
ease, but rarely suffer serious effects. Some native North American
salmonids, especially the rainbow trout (
Oncorhynchus mykiss
), are severely
infected, especially in some parts of the Rocky Mountain region.
Many species of trout and salmon can be infected by the whirling dis-
ease parasite, but different species and subspecies show varying degrees of
susceptibility to infection. Among cutthroat trout (
Oncorhynchus clarki
)
subspecies, for example, the subspecies
O. c. utah
from Bear Lake, on the
border of Utah and Idaho, shows lower susceptibility to infection than
other subspecies (Wagner et al. 2002). Fry of this form of the cutthroat
trout tend to grow rapidly in length and to switch from feeding on plank-
ton to feeding on other fish earlier in life than do other forms of cutthroat
trout.The reasons for this difference are unclear.
In North America, some evidence of evolutionary adaptation exists for
both tubifex worms and rainbow trout. One
Tubifex
lineage from
Ontario, Canada, for example, appears to be highly resistant to infection
by
Myxobolus
(Beauchamp et al. 2001). This lineage, however, may really
be one of several cryptic species of
Tubifex
that cannot be distinguished
by morphology. More recently, Beauchamp et al. (forthcoming) found
that in Colorado two
Tubifex
lineages appeared to consist of individuals
largely resistant to infection and two others to contain both resistant and
susceptible individuals. Thus, a considerable potential for evolutionary
adjustment by
Tubifex
forms to
Myxobolus
appears to exist.
Rainbow trout imported into Germany about 120 yr ago recently
were found to show resistance to whirling disease (El-Matbouli et al.
2002). Development of this resistance was presumed to be the result of
growth and reproduction of rainbow trout under conditions of continu-
ous exposure to the parasite in hatcheries that use river water and mud-
bottomed rearing ponds. Laboratory tests of the ability of rainbow trout
from North America and Germany to resist infection by
Myxobolus
were
conducted by Hedrick et al. (2002). These tests showed that trout from
Germany were much more resistant than any North American strain that
has been tested.
Avian malaria, caused by the blood parasite
Plasmodium relictum
,was
brought to Hawaii by the introduction of alien birds and the mosquito
Culex quinquefasciatus
, which serves as a vector (van Riper et al. 1986).
Native Hawaiian songbirds initially lacked resistance to avian malaria,
which has evidently contributed substantially to their decline. Most native
land birds no longer occur at lower elevations to which
Culex
mosquitoes
and the malaria parasite are restricted (Jarvi et al. 2001).
