The role of host hormones in controlling survival and development of Schistosoma mansoni

Internal chemical communication within flatworms

An understanding of reproductive function is important for control of parasitic helminths. In cestodes and trematodes virtually nothing is known about regulatory and coordinating mechanisms that control maturation, gamete formation, egg production, and related processes. Neurosecretory neurons have been reported in various species but specific modes of action of neurohormones have yet to be demonstrated. The role of ecdysone is being investigated.

Environmental influences on egg production by the monogenean Protopolystoma xenopodis

A continuous succession of egg capsules is formed in the ootype of Protopolystoma xenopodis and released into the urinary bladder of the host, Xenopus laevis. Since egg capsules are passed out with the aquatic toad's urine several times a day, the rate of parasite egg production can be recorded accurately by direct counts. Temperature has a marked influence on the rate of egg output which doubles in response to a rise from 17 to 25 °C. Under constant temperature conditions regular egg collections from isolated toads infested with single Protopolystoma revealed that rates of production remain stable for periods of several weeks. The overall mean rate of egg production was 8·9 eggs/worm/day (e.w.d.) at 20·0±0·5 °C (based on 5 consecutive egg collections from 58 individual parasites). Significant differences were observed, however, between the mean rates of output from different hosts, ranging from 2·1 e.w.d. (s.d. = 0·6) to 17·7 e.w.d. (S.D. = 3·2). Neither differences in worm age nor host-factors could explain the observed range in output. Worms transferred to new hosts resumed egg production at pre-transfer rates, suggesting inherent differences in the capacity for egg production by individual parasites. Total production increased with increasing worm burden, but output per parasite was significantly reduced in groups of 2, 3 or 4. Whilst competition for some finite resource provides one interpretation, this observation could equally reflect a statistical phenomenon associated with the reduced chance of encountering groups of individuals which are heterogeneous for egg production as worm burden increases and sample size decreases.

Natural infection with schistosomes of the Schistosoma haematobium group in a dog in Zambia

Post-mortem examination of an adult male Jack Russell dog from Zambia revealed that it was heavily infected with schistosomes. The dog had been admitted, with a history of retching, 4 days before its death. At necropsy, the liver was found to be enlarged, with multiple pin-point yellowish-white foci scattered diffusely throughout the organ. Multiple pin-point recent and old haemorrhages were seen on the mucosal surface of the gastrointestinal tract, particularly in the stomach and proximal duodenum. Large numbers of schistosome worm pairs and eggs were found in all mesenteric, gastric and hepatic veins. Histological examination of the intestines, mesenteric lymph nodes, liver, spleen, pancreas, stomach and lungs revealed numerous strongly fibrotic, encapsulated, epithelioid-giant cell granulomata containing dead, degenerating and viable eggs. A few examples of the Splendore-Hoeppli phenomenon were also detected. The eggs collected at necropsy had a terminal spine and an average length and breadth of 187.6+/-14.1 microm and 57. 3+/-4.1 microm, respectively. DNA analysis of female worms indicated that the schistosomes were either Schistosoma haematobium or a hybrid of Schistosoma mattheei and S. haematobium.

The infection characteristics of the antelope schistosomes, Schistosoma margrebowiei and S. leiperi, in inbred BALB/c mice and in Mastomys coucha

Detailed studies were carried out to assess the infection characteristics of the antelope schistosomes in BALB/c mice and Mastomys coucha. Both schistosome species reached sexual maturity in these hosts. However, they differed markedly in terms of egg production rates and preferred sites of tissue egg deposition. Tissue egg deposition commenced between 4 and 5 weeks in the case of Schistosoma margrebowiei and between 6 and 7 weeks in the case of S. leiperi. Tissue egg accumulation rates of S. leiperi were calculated to be 334 and 311 eggs/gravid worm pair/day in mice and M. coucha, respectively. The fecundity of S. margrebowiei was found to be substantially greater (approximately 5 times; daily output ranging from 1600-1900 eggs/gravid worm pair). While S. leiperi infections remained relatively stable in both hosts during the study period (i.e. up to 15 weeks after onset of egg laying), S. margrebowiei infections were observed to deteriorate rapidly in M. coucha.

Schistosoma mansoni: insulin independence

Several authors have reported that schistosomes are affected by host hormones, including insulin. We found that insulin in a 10,000-fold range of concentrations failed to affect glucose consumption of males, females, or pairs of Schistosoma mansoni incubated for periods up to 24 hr. Insulin, vasopressin, or tetradecanoyl phorbol acetate (an insulin mimic) did not affect the uptake and incorporation of [14C]glucose and [14C]deoxyglucose over the active transport range. Competitive binding studies using 125I-insulin and 125I-insulin-like growth factor II, with varying concentrations of unlabeled hormones, demonstrated only nonspecific binding to intact worms or subcellular fractions. This generalized nonspecific uptake of label was also shown by autoradiography, suggesting that specific insulin receptors are absent from S. mansoni. We conclude that plasma glucose levels are sufficient to supply the metabolic requirements of schistosomes by diffusion, and that no insulin-dependent mechanisms have evolved in these parasites.