Abstract
Strongyloides venezuelensis, an intestinal nematode of rat has
recently been used as a model for the understanding of
host-parasite interaction of human and/or animal strongyloidiasis.
The present studies were conducted to provide information on
biology of S. venezuelensis for the maintenance of the parasite as
bio-bar system to facilitate scræning tests for potential new
chemicals and anthelmintics, and also to determine the protective
responses of rats to S. venezuelensis infection for the
understanding of host-parasite interaction in strongyloidiasis.
An in vitro culture method was developed for harvesting the
filariform larvae (La) of S. venezuelensis using a nutrient broth as
substitute for rat-feces in polyvinyl culture bag (PCB) (IOX 12 cm).
A significantly higher (p<0.05) development rate (20.6%) of L3 was
observed in 0.12% nutrient broth concentrations, incubated at 20t
for 5 days. Yields of were observed relatively high when the
culture medium was incubated at higher temperatures which peaked
at 25 C (20.0%) than at lower temperatures, 15 C (10.9%) and 20 C
(18.1%). The highest yield (19.2%) of L3 was obtained from PCB
inoculated with eggs in 0.12% nutrient broth and incubated
at 25t for four . days. The present culture technique resulted in
clean larval suspension is useful for screening tests for new
chemical products and anthelmintics, and for parasitological studie with Strongyloides species.
Viability of S. venezuelensis different stages (e.g., eggs, L3 and
adults) exposed to various conditions was determined for in Ditro
maintenance. Eggs in the feces remained viable for about 25 days
at 4 C but very sensitive to air drying, and rapidly lost their
viability (S 12 hr). The L.3 survived for a maximum period of 45
days in fecal suspension and 28 days in 0.12% nutrient broth in
PCB maintained at 20 C. The highest survival period of the adult
worms (A W) was 9 days in serum supplemented (10% rat-serum)
nutrient broth at 37 C. These findings may provide useful
information
for biological
and biochemical studies
on
strongyloidiasis.
The effects of in vitro culture methods on morphological
development and infectivity of S. venezuelensis to rats were
examined. A significantly higher (p<0.05) development of body
length was observed in 1-3 from filter paper culture (FPC) (597.3±
32.2 gun) than those in fæal culture (FC) (509.9±35.0 VIII) and
nutrient broth culture (NBC) (503.3±31.0 gm). Larval infectivity was
assessed by exposing rats to L3 harvested from each culture,
and migrating stage larvae (MSL3) from the lungs on day 3
post-infection (Pl) and AW from the small intestine on day 7 Pl
were recovered. Recovery rate of these MSL3 and AMT did not
show any significant difference according to culture methods. A
significantly increased (p<0.05) body length of AW were recorded in those corresponding to the La obtained frolll FPC (2777.5±204.4
Pill) and NBC (2732.5±169.8 m) than those to the L3 from FC
172.4 gal). Although eggs per gram feces (EPG) and worm
fecundity found to differ among culture methods but the time
course Of egg discharge and persistence of infection did not. This
study suggests that the culture methods have a significant effect
on the morphological development of the larvae to the stage but
do not influence their infectivity to rats.
Characteristics of an initial infection with S. venezuelensis in
rats and the protective response against a homologous challenge
infection were assessed for the understanding of host-parasite
interaction in strongyloidiasis. After an initial infection with 1,000
in normal rats, the populations of MSL3 in the lungs on day 3
Pl and that of AW in the small intestine on day 7 Pl were 180.8±
14.5 and 336.8±70.7, respectively. After an initial infection, rats
developed strong immunity against a homologous challenge
infection, which was manifested by marked reduced development of
worms with reduced body length, rapid expulsion of worms and
damage to reproductive organs. Dexamethasone treatment of
immunized rats resulted in immunosuppression and leads to an
increasing developmenL of MSL3 (13.6±5.7) and AW (86.2±41.7).
Worm expulsion from normal and immune rats was preceded by a
significantly increased (p<0.05) peripheral blood eosinophilia (PBE).
These findings suggest that host's protective immunity to S. venezuelensis infection could suppress worm reproduction
completely and that PBE may play an important role in worm
expulsion mechanisms.
Rats were immunized through an initial infection with L3 of
Nippostrongylus brasiliensis and challenged with the same infective
dose of S. venezuelensis Lg to examine whether cross-resistance
(CR) developed against a heterologous parasite. In this study, CR
exptessed partially against MSL3 and AW of S. venezuelensis.
Populations of MSL3 were significantly lower (p <0.05) in
immunized rats (22.0±7.4) with reduced body length compared to
controls (105.0±27.6). Migration of these larvae also delayed in
immunized rats which was evident by recovering the peak
populations of AW on day 14 post—challenge (PC) (214.0±33.9),
while in controls, that of AW peaked on day 7 PC (229.4±59.7).
However, kinetics of AW from day 14 PC did not show
significant difference and won-ns were expelled towards day 49 PC
from both imrnunized rats and controls. Although EPG counts and
worm fecundity were significantly decreased (p<O.05) in immunized
rats at initial stage but from day 14 PC they showed almost
similar kinetics to those in controls. PBE were significantly
decreased 0.05) on day 7 PC, then gradually increased and
peaked on day 42 PC when most of the worms were expelled
indicating that PBE were strongly involved in worm establishment
and expulsion mechanisms.