Niels Ørnbjerg Christensen

Heterologous antagonistic and synergistic interactions between helminths and between helminths and protozoans in concurrent experimental infection of mammalian hosts.

Experimental concurrent infection with two or more parasite species in mammalian host models may result in heterologous antagonistic and synergistic interactions ranging in magnitude from reduced/enhanced growth and fecundity to blockage/enhancement of establishment/expulsion. With some exceptions only, there is a reasonable correlation between the levels of interaction monitored by parasitological and by clinico-pathological parameters. Heterologous antagonistic interactions mediated by functional and specific immunological cross-reactivity occur between closely related parasite species exhibiting a marked immunobiological similarity. In contrast, antagonistic interactions between antigenetically more remote species of helminths, protozoan-induced resistance to helminth infection and helminth-induced suppression of concurrent protozoan infection generally appear mediated by immunologically non-specific factors like macrophage activation and inflammatory reactions. Synergistic heterologous interactions between helminths, helminth-induced enhancement of concurrent protozoan infection and interference with the development and maintenance of resistance to helminth infection in response to concurrent protozoan infection are generally thought to be mediated by non-specific parasite-induced immunosuppression. Concurrent experimental infection is very complex. There are problems and limitations in extrapolating from experimental studies on concurrent infection in laboratory animals to natural polyparasitism. This fact, coupled with the complex influence of ecological factors on the pattern and frequency of concurrent natural infection means that major consequences of natural concurrent parasite infection have not been definitively demonstrated. Appropriately planned and controlled field studies and further laboratory experiments on primate and domestic animal models are imperative for elucidation of the importance of heterologous interactions in concurrent parasite infection for the disease pattern in man and domestic stock. Experimental studies hitherto conducted on concurrent parasite infection pointing to natural heterologous interactions may be a valuable starting point for further studies.

Schistosoma japonicum infection in the pig as a model for human schistosomiasis japonica

Valuable information on human schistosomiasis japonica has been provided using primates and experimental rodent hosts. However, major drawbacks such as high costs and ethical concerns for the primate models and large biological deviations for the rodent models have led to the search for more appropriate models. Recent data on the pig indicate that this natural host for Schistosoma japonicum might be a realistic alternative. As only very few research groups have investigated the S. japonicum/pig model, the present review mainly deals with the experimental methods and the major host/parasite findings obtained from the authors own research group. With emphasis on a critical evaluation of the work, the results are compared to the scarce information existing on human schistosomiasis japonica. Like in humans, S. japonicum establishes mainly in the large intestinal veins, with high faecal egg counts during the acute phase of infection, which varies greatly within and between days. Concomitant resistance is another shared feature, but studies in pigs have indicated that the phenomenon is more complex than generally thought. Clinical signs as eosinophilia and diarrhoea with mucus and blood in the acute phase of infection and hepatomegaly, increased portal diameter, periportal fibrosis and ascites in chronic infections are common findings in both humans and pigs. Low protein diet aggravates the disease in pigs by increasing the establishment rates, the faecal egg excretion and the morbidity. A 100% cure rate is achieved when treating S. japonicum infected pigs with praziquantel at 40 mg/kg, and 4 weeks post treatment pigs remain resistant to reinfection. Lastly, human congenital S. japonicum infection has been confirmed in pigs but the implications of such infections for the pathogenesis of schistosomiasis japonica remain to be investigated.

Schistosoma mansoni and Fasciola hepatica: cross-resistance in mice.

Abstract Cross-resistance in Schistosoma mansoni and Fasciola hepatica infections were studied in mice. A primary infection of S. mansoni, 7 to 28 days old, did not stimulate a significant level of resistance to heterologous challenge with F. hepatica. In contrast, in older S. mansoni infections (54–65 days old) there was a significant level of resistance to a challenge with F. hepatica. The F. hepatica worm burden was reduced by 34.0 to 72.5% in separate experiments. Challenge infection with F. hepatica did not influence the number of S. mansoni in primary infections. No heterologous resistance to S. mansoni was found in mice with 7- and 23-day-old F. hepatica infections. However, primary infections with F. hepatica, 28, 32, 42, and 50 days old, conferred significant resistance to a heterologous challenge with S. mansoni. The established schistosome worm burden was reduced by 41.5 to 50.4%. In no case was the primary F. hepatica burden reciprocally influenced by challenge infection with S. mansoni.

Worm establishment and egg production of Schistosoma japonicum in pigs infected by percutaneous methods or intramuscular injection.

Worm establishment and egg production were compared in Landrace/Yorkshire crossbred pigs infected with Schistosoma japonicum using 3 different infection techniques. S. japonicum worm establishment and overall egg production following intramuscular injection of cercariae suspended in Iscoves medium exceeded that in pigs infected by the leg immersion or the coverslip percutaneous technique. However, the egg production per mature female was similar for all three infection methods. The results support the use of intramuscular injection of medium-suspended cercariae as an effective, safe, rapid and convenient infection method for studies on the population biology of S. japonicum in pigs when dermal cercarial penetration is not required.

Homologous and heterologous resistance in Echinostoma revolutum infections in mice.

Homologous and heterologous resistance in Echinostoma revolutum infections was studied in mice. A high level of homologous resistance was demonstrated in mice harbouring a 13-day-old primary E. revolutum infection with 9--10 and 11--15 worms, corresponding to a 70.0 and 66.7% reduction in the size of the established worm burden as compared with that of the challenge control group. A 14- and 20-day-old primary infection with 3--4 worms induced a level of resistance of 61.7 and 81.8% respectively, while higher worm levels of 9--10 and 11--15 induced almost complete resistance corresponding to a 95.1--100% reduction in the size of the established worm burden. Complete resistance was also demonstrated in mice challenged 8 days after elimination of a 20-day-old primary infection with 11--15 worms by anthelmintic treatment. A primary 43-day-old Schistosoma mansoni infection induced a 73.1% reduction in the size of the established E. revolutum challenge infection while infections of an age of 79 and 99 days conferred complete resistance to heterologous challenge with E. revolutum. Primary pre-patent S. mansoni infections and a patent S. bovis infection of an age of 56 days did not induce any resistance to challenge with E. revolutum. A primary 14- and 21-day-old infection of E. revolutum did not stimulate any significant level of resistance to heterologous challenge with S. mansoni.

A method for the in vivo labelling of Schistosoma mansoni and S. intercalatum cercariae with radioselenium

SummaryIn order to develop a standard technique for in vivo labelling of Schistosoma mansoni and S. intercalatum cercariae by incubation of infected host snails with radioselenium, studies were conducted on the relationship between mode of administration, dose size (μCi), and time of administration of the radioisotope on the one hand and the production of cercariae and the amount of cercaria-bound radioactivity obtained on the other.The ability to take up radioactivity by B. glabrata snails was influenced by the following factors: living/heat-killed snails, size of snails, length of exposure period, volume of exposure medium, and the amount of radioactivity per snail in the exposure medium. A linear proportionality existed between the amount of radioactivity in the exposure medium and the subsequent snail-bound radioactivity. The rate of uptake and the final level of snail-bound radioactivity reached was increased by reducing the volume of the exposure medium to a minimum. Thus, at exposure to 75Se-methionine in 1 ml exposure medium the maximum snail-bound radioactivity (70–85% of available amount) was obtained after 5 h. Therefore, the labelling procedure used comprised an individual exposure of snails in 1 ml for 20 h to radioselenium. Irrespective of the exposure level (μCi) used the labelling of infected B. glabrata prior to or 3 days after the start of shedding of cercariae resulted in a significant interference with the production of cercariae. The same interference was observed when snails were labelled 12 days after patency using exposure levels at or above 6.6 μCi/snail. However, a dose level of or below 5.5 μCi/snail administered 12 days after the start of shedding cercariae did not interfere with the host-parasite relationship, i.e. a normal and continuous production of labelled cercariae was obtained during a period of at least 54 days.The maximum cercaria-bound radioactivity obtained, using non-influential dose levels, was obtained 3 days after exposure of snails to the isotope. The maximum amount obtained was roughly comparable when exposure levels of 3.0–19.5 μCi per snail were used after the start of shedding of cercariae.The labelling procedure did not interfere with the biological characteristics of the labelled larvae. Thus their behavioural activity, longevity, and infectivity to mice were comparable with those of unlabelled cercariae.The possible application of labelled larval schistosomes to various problems in research on schistosomiasis is discussed.

Morbidity assessment in urinary schistosomiasis infection through ultrasonography and measurement of eosinophil cationic protein (ECP) in urine.

Summary In a Schistosoma haematobium‐endemic village in western Madagascar we evaluated ultrasonography and Eosinophil Cationic Protein (ECP) in urine as means to detect the associated urinary tract pathology. 192 individuals were matched according to age and sex, and grouped into infected persons with bladder and, if present, kidney pathology (n= 96); infected persons without pathology (n= 48) and noninfected persons without pathology (n= 48). The median urinary egg count was significantly higher in individuals with ultrasonographically detectable urinary tract pathology (115 eggs/10 ml urine) than in infected persons without (45 eggs/10 ml of urine). At 136 ng/ml, the median ECP level was significantly higher in the 144 infected individuals than in the 48 noninfected persons (0.35 ng/ml). Egg excretion correlated positively with ECP level. The median ECP level was significantly higher in the group with ultrasonographically detectable urinary tract pathology than in the group without (183 ng/ml vs. 67 ng/ml). The results suggest that minor degrees of pathology, particularly at an early stage of infection with S. haematobium, might be overlooked by ultrasonography despite the presence of marked inflammation, as indicated by markedly increased urinary ECP levels in infected individuals without ultrasonographically detectable urinary tract pathology. ECP may therefore provide important information on the evolution of S. haematobium‐associated urinary tract morbidity.

Echinostoma revolutum: Resistance to secondary and superimposed infections in mice

A complete or almost complete resistance (94-100%) to a superimposed Echinostoma revolutum infection existed in mice harboring 20-, 30-, and 40-day-old infections in the range of 2-4 to 30-35 worms, but no resistance was found at challenge Day 10. A similar high level of resistance (85-100%) also existed in mice for at least 6 weeks after natural expulsion of a primary 6 metacercarial infection and for at least 5 weeks after anthelmintic termination of a 30-day-old 20 metacercarial infection. Thymus-deficient nude mice failed to develop resistance to a superimposed infection, and the resistance in normal mice was inhibited by corticosteroid treatment. These findings are all in favor of a host immune response being responsible for the resistance against both a secondary and a superimposed infection. Nearly all the worms of a superimposed infection were, in resistant mice, expelled prior to 24 hr following infection (rapid expulsion), and the few worms circumventing this early expulsion persisted for at least 8 days. Newly excysted juvenile worms implanted intraduodenally into resistant mice were rejected to the same degree as juvenile worms from an oral metacercarial infection indicating that the newly excysted juvenile worms are the target of the host immune response. However, 7-day-old worms implanted intraduodenally into resistant mice survived indicating that adaptation to the host immune response had occurred. In conclusion, this host-parasite model is an example of concomitant immunity because the immunological mechanism responsible for the expulsion of the superimposed infection had no effect on the number of primary worms present.

Echinostoma population regulation in experimental rodent definitive hosts

Echinostoma population regulation in the experimental rodent host is governed by the capacity of the latter to express an effective regulatory response. Parasite establishment, survival and fecundity are affected by host-related factors such as species, strain and age and by parasite-related factors such as species, age and the burden of infection. The genetic heterogeneity in the regulatory response to infection is marked. The most intensively studied host/echinostome combinations compriseE. caproni andE. trivolvis in the mouse host, for which a range of interesting host-parasite relationships has been demonstrated, including concomitant immunity with rapid expulsion of superimposed infections, a long-lasting resistance to secondary infection, a negatively dose-dependent pattern of expulsion of primary infections, a positively dose-dependent reproductive potential, an infective-dose independency of primary worm establishment, and a range of heterologous antagonistic and synergistic interactions in concurrent infections with related and unrelated parasite species. TheEchinostoma/rodent model is highly suitable for studying aspects of parasite population regulation in intestinal trematode infections.

HOMOLOGOUS IMMUNOTOLERANCE AND DECREASED RESISTANCE TO SCHISTOSOMA MANSONI IN ECHINOSTOMA REVOLUTUM-INFECTED MICE

Homologous immunotolerance was demonstrated in Echinostoma revolutum infections in the mouse. High-level infections of about 15 worms were not expelled for a period of at least 61 days following infection, whereas low-level infections of about 10 to 12 worms or below were eliminated completely between day 47 and 61 after infection. A primary, 20- to 33-day-old, heavy E. revolutum infection in the mouse produced a statistically significant increase in the number of Schistosoma mansoni recovered in challenge infections. Worm burdens increased by 65.7 to 90.6% as compared with those of the respective S. mansoni challenge control groups.