ABSTRACT. The effects of male and female sex hormones on the protective capacity of Wistar rats against infection with Strongyloides venezuelensis were investigated. Male rats were more susceptible than females in terms of worm recovery from the lungs. Orchidectomy of male animals significantly reduced the plasma testosterone concentration and increased host resistance to the migratory stages of S. venezuelensis larvae. In contrast, ovariectomy of female animals significantly decreased host resistance in association with a significant reduction of estrogen levels. To examine the direct effect of sex hormones, exogenous testosterone and estrogen were implanted into animals. Susceptibility significantly increased or decreased in ovariectomized females given testosterone or estrogen, respectively. These results suggest that male and female sex hormones are important in the down-and up-regulation of host resistance against S. venezuelensis in Wistar rats. KEY WORDS: estrogen, sex difference, Strongyloides venezuelensis, testosterone, Wistar rat.J. Vet. Med. Sci. 64(6): 457-461, 2002 Interest in host sex and host hormones, that influence parasitic infection, dates back to the early 1940's, and the influence of hormones was first widely reviewed by Solomon [27]. However, many of the hypotheses and theories proposed still remain to be moot. Sex differences are usually attributed to either ecological (sociological in humans) or physiological causes, the latter usually being hormonal in origin [37]. Testosterone has immunosuppressive properties [9,10,15,37]. The prevalence and intensity of parasitic infections are greater in males of many mammalian species than in females [5, 14, 17, 19-21, 30, 36], especially in abnormal hosts with a primary infection, but not always in normal hosts.Our most recent study [24] demonstrated a significant sex difference in the susceptibility of sexually mature Wistar rats (Rattus norvegicus) to infection with S. venezuelensis. As reported [23], the strain of S. venezuelensis used in our studies was originally isolated from wild R. norvegicus in Okinawa Prefecture, Japan [11], indicating that laboratory rats including the Wistar strain are within the same group of natural/normal hosts for S. venezuelensis.Since sex differences in susceptibility to a parasite are rare in a normal host, the present study examines whether host sex hormones affect the susceptibility of Wistar rats to infection with S. venezuelensis. MATERIALS AND METHODS Animals:Wistar rats (R. norvegicus) at 4 weeks of age were purchased from Seac Yoshitomi, Ltd. (Fukuoka, Japan). These rats were housed in clean metal cages and fed with a standard diet and tap water ad libitum in an air-conditioned room (23 ± 1°C), under conventional conditions with a 12:12 hr light/dark cycle. They received human care as outlined in the "Guide for the Care and Use of Laboratory Animals" by The Faculty of Agriculture, Miyazaki University.Parasitological techniques: A strain of S. venezuelensis was originally isolated from wild brown rats (R...
ABSTRACT. The effects of host age and sex on susceptibility to Strongyloides venezuelensis in Wistar rats were examined by counting larvae recovered from the lungs of animals 3 days after infection. The susceptibility of female rats to S. venezuelensis rapidly decreased with age and elevated estrogen. Resistance in female rats inoculated at 6 and 10 weeks of age was nine and twenty-fold higher, respectively than that in the youngest group (3 weeks). In contrast, the susceptibility of male animals was lowest in the youngest group, then increased with age and elevated testosterone. Sex differences in susceptibility were not evident in the youngest group, but became apparent with age. KEY WORDS: Strongyloides venezuelensis, susceptibility, Wistar rat.J. Vet. Med. Sci. 64(6): 519-521, 2002 Innate factors of host origin that influence helminth burdens include age, sex and genetic background. The influence of age and sex on helminth burdens appears to be largely hormonal [25]. S. venezuelensis is a good helminth model that has been used worldwide to investigate mucosal mast cell-dependent protection in the intestinal tract of host [18,21].We previously examined a strain of S. venezuelensis [18] that was originally isolated from wild brown rats in Naha, Okinawa prefecture, Japan [12]. During the maintenance of this strain using Wistar rats, female animals were resistant to infection with this strain of parasite and have seldom been used for the maintenance. However, this phenomenon and/ or the underlying mechanisms have not yet been investigated.The present study therefore examines the influence of host age and sex on the susceptibility to infection with S. venezuelensis and the relationship with sex hormones is discussed.Wistar rats were purchased from Seac Yoshitomi, Ltd. (Fukuoka, Japan) and housed in our laboratory under conventional conditions. The study included three groups of male (n=6) and female (n=6) rats aged 3 (40-60 g body weight), 6 (140-160 g for females and 160-180 g for males) and 10 weeks (220-250 g for females and 320-350 g for males).The strain of S. venezuelensis used in this study was originally isolated from wild brown rats in Okinawa, Japan in 1988 [12] and maintained in our laboratory by serial passage in Wistar rats and/or Mongolian gerbils (Meriones unguiculatus). Third stage infective larvae (L 3 ) were prepared from the cultures of infected feces using filter paper methods.Rats were infected with 3000 infective larvae (L 3 ) of S. venezuelensis each by subcutaneous inoculation into the groin. The protective capacities of male and female rats at various ages were measured by larval recovery from the lungs 3 days after infection. Infected animals were killed and both lungs removed, cut into small pieces using a motor driven disperser, and incubated in petri dishes with saline at 37°C for 3 hr. The number of larvae that emerged was determined by microscopy.To measure plasma estrogen and testosterone levels, rats were killed by decapitation and trunk blood collected into heparinized plastic...
ABSTRACT. The protective capacity of the cotton rat, Sigmodon hispidus, against the migratory and intestinal phases of Strongyloides venezuelensis infection was examined. After subcutaneous infection with infective larvae (L 3 ), adult worm recovery rates from male and female animals on Day 71 were only 0.10% and 0.06% of initial dose, respectively. To determine whether this enhanced protection was expressed during the migratory phase or the intestinal phase, larval recovery from the lungs of cotton rat was evaluated 3 days after subcutaneous L 3 infection. The larval recovery rate was only 0.5% of initial dose and about 40-fold lower than that from control mice. Protection in the intestine was also evaluated after intraduodenal implantation of adult worms. About 30% of implanted worms became established and worm burden remained constant until Day 28. Despite a high worm burden on Day 28, EPG was about 25-fold lower than the peak count. To evaluate expulsive capacity and monitor the cellular responses in the intestine of cotton rats, adult Nippostrongylus brasiliensis worms were implanted in addition to S. venezuelensis. Cotton rats were unable to expel adult S. venezuelensis worms, even after 21 days of observation. Although the number of mucosal mast cells increased significantly, the intraepithelial migration of mast cells was not observed. In contrast, N. brasiliensis was expelled by Day 6 in association with goblet cell hyperplasia. These results suggest that in contton rats, the defective intestinal protection against adult S. venezuelensis worms results from dysfunction of mucosal mast cells.
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