1
|
Ogongo P, Nyakundi RK, Chege GK, Ochola L. The Road to Elimination: Current State of Schistosomiasis Research and Progress Towards the End Game. Front Immunol 2022; 13:846108. [PMID: 35592327 PMCID: PMC9112563 DOI: 10.3389/fimmu.2022.846108] [Citation(s) in RCA: 20] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2021] [Accepted: 03/21/2022] [Indexed: 12/14/2022] Open
Abstract
The new WHO Roadmap for Neglected Tropical Diseases targets the global elimination of schistosomiasis as a public health problem. To date, control strategies have focused on effective diagnostics, mass drug administration, complementary and integrative public health interventions. Non-mammalian intermediate hosts and other vertebrates promote transmission of schistosomiasis and have been utilized as experimental model systems. Experimental animal models that recapitulate schistosomiasis immunology, disease progression, and pathology observed in humans are important in testing and validation of control interventions. We discuss the pivotal value of these models in contributing to elimination of schistosomiasis. Treatment of schistosomiasis relies heavily on mass drug administration of praziquantel whose efficacy is comprised due to re-infections and experimental systems have revealed the inability to kill juvenile schistosomes. In terms of diagnosis, nonhuman primate models have demonstrated the low sensitivity of the gold standard Kato Katz smear technique. Antibody assays are valuable tools for evaluating efficacy of candidate vaccines, and sera from graded infection experiments are useful for evaluating diagnostic sensitivity of different targets. Lastly, the presence of Schistosomes can compromise the efficacy of vaccines to other infectious diseases and its elimination will benefit control programs of the other diseases. As the focus moves towards schistosomiasis elimination, it will be critical to integrate treatment, diagnostics, novel research tools such as sequencing, improved understanding of disease pathogenesis and utilization of experimental models to assist with evaluating performance of new approaches.
Collapse
Affiliation(s)
- Paul Ogongo
- Division of Experimental Medicine, Department of Medicine, University of California, San Francisco, San Francisco, CA, United States.,Department of Tropical and Infectious Diseases, Institute of Primate Research, Nairobi, Kenya
| | - Ruth K Nyakundi
- Department of Tropical and Infectious Diseases, Institute of Primate Research, Nairobi, Kenya
| | - Gerald K Chege
- Primate Unit & Delft Animal Centre, South African Medical Research Council, Cape Town, South Africa.,Department of Pathology, University of Cape Town, Cape Town, South Africa
| | - Lucy Ochola
- Department of Tropical and Infectious Diseases, Institute of Primate Research, Nairobi, Kenya.,Department of Environmental Health, School of Behavioural and Lifestyle Sciences, Faculty of Health Sciences, Nelson Mandela University, Gqeberha, South Africa
| |
Collapse
|
2
|
Hoekstra PT, van Esbroeck M, de Dood CJ, Corstjens PL, Cnops L, van Zeijl-van der Ham CJ, Wammes LJ, van Dam GJ, Clerinx J, van Lieshout L. Early diagnosis and follow-up of acute schistosomiasis in a cluster of infected Belgian travellers by detection of antibodies and circulating anodic antigen (CAA): A diagnostic evaluation study. Travel Med Infect Dis 2021; 41:102053. [PMID: 33823289 DOI: 10.1016/j.tmaid.2021.102053] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2020] [Revised: 03/26/2021] [Accepted: 03/29/2021] [Indexed: 11/28/2022]
Abstract
BACKGROUND In order to evaluate the diagnostic value of schistosome circulating anodic antigen (CAA) detection, serum and urine CAA-levels were determined in a single cluster of 34 Belgian tourists at three timepoints within a period of 14 weeks following proven Schistosoma exposure in South Africa and compared with two in-house antibody assays. METHODS Samples were collected 4-5 and 7-8 weeks post-exposure and subsequently 5-6 weeks following praziquantel treatment. Schistosoma antibodies were detected by an adult worm antigen-immunofluorescence assay (AWA-IFA) and a soluble egg antigen-enzyme-linked immunosorbent assay (SEA-ELISA), while CAA concentrations were determined by the Up-Converting reporter Particle labelled Lateral Flow (UCP-LF) test. RESULTS Antibodies were detected in 25/34 (73%) travellers pre-treatment and in 27/34 (79%) post-treatment, with the AWA-IFA showing better performance than the SEA-ELISA. Pre-treatment, CAA was detected in 13/34 (38%) and 33/34 (97%) of the travellers in urine and serum, respectively. Post-treatment, all except one traveller became serum CAA negative. This in contrast to the detected antibodies, as well as the previously reported diagnostic results of this cluster. CONCLUSIONS The UCP-LF CAA serum assay has been demonstrated as the most sensitive method for the diagnosis of early Schistosoma infections and post-treatment monitoring in travellers.
Collapse
Affiliation(s)
- Pytsje T Hoekstra
- Department of Parasitology, Leiden University Medical Center, Leiden, the Netherlands.
| | - Marjan van Esbroeck
- Department of Clinical Sciences, Institute of Tropical Medicine Antwerp, Belgium
| | - Claudia J de Dood
- Department of Cell and Chemical Biology, Leiden University Medical Center, Leiden, the Netherlands
| | - Paul Lam Corstjens
- Department of Cell and Chemical Biology, Leiden University Medical Center, Leiden, the Netherlands
| | - Lieselotte Cnops
- Department of Clinical Sciences, Institute of Tropical Medicine Antwerp, Belgium
| | | | - Linda J Wammes
- Department of Medical Microbiology, Leiden University Medical Center, Leiden, the Netherlands
| | - Govert J van Dam
- Department of Parasitology, Leiden University Medical Center, Leiden, the Netherlands
| | - Jan Clerinx
- Department of Clinical Sciences, Institute of Tropical Medicine Antwerp, Belgium
| | - Lisette van Lieshout
- Department of Parasitology, Leiden University Medical Center, Leiden, the Netherlands
| |
Collapse
|
3
|
Beechler BR, Jolles AE, Budischak SA, Corstjens PLAM, Ezenwa VO, Smith M, Spaan RS, van Dam GJ, Steinauer ML. Host immunity, nutrition and coinfection alter longitudinal infection patterns of schistosomes in a free ranging African buffalo population. PLoS Negl Trop Dis 2017; 11:e0006122. [PMID: 29253882 PMCID: PMC5755937 DOI: 10.1371/journal.pntd.0006122] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2017] [Revised: 01/05/2018] [Accepted: 11/20/2017] [Indexed: 11/19/2022] Open
Abstract
Schistosomes are trematode parasites of global importance, causing infections in millions of people, livestock, and wildlife. Most studies on schistosomiasis, involve human subjects; as such, there is a paucity of longitudinal studies investigating parasite dynamics in the absence of intervention. As a consequence, despite decades of research on schistosomiasis, our understanding of its ecology in natural host populations is centered around how environmental exposure and acquired immunity influence acquisition of parasites, while very little is known about the influence of host physiology, coinfection and clearance in the absence of drug treatment. We used a 4-year study in free-ranging African buffalo to investigate natural schistosome dynamics. We asked (i) what are the spatial and temporal patterns of schistosome infections; (ii) how do parasite burdens vary over time within individual hosts; and (iii) what host factors (immunological, physiological, co-infection) and environmental factors (season, location) explain patterns of schistosome acquisition and loss in buffalo? Schistosome infections were common among buffalo. Microgeographic structure explained some variation in parasite burdens among hosts, indicating transmission hotspots. Overall, parasite burdens ratcheted up over time; however, gains in schistosome abundance in the dry season were partially offset by losses in the wet season, with some hosts demonstrating complete clearance of infection. Variation among buffalo in schistosome loss was associated with immunologic and nutritional factors, as well as co-infection by the gastrointestinal helminth Cooperia fuelleborni. Our results demonstrate that schistosome infections are surprisingly dynamic in a free-living mammalian host population, and point to a role for host factors in driving variation in parasite clearance, but not parasite acquisition which is driven by seasonal changes and spatial habitat utilization. Our study illustrates the power of longitudinal studies for discovering mechanisms underlying parasite dynamics in individual animals and populations.
Collapse
Affiliation(s)
- Brianna R. Beechler
- College of Veterinary Medicine, Oregon State University, Corvallis, OR, United States of America
| | - Anna E. Jolles
- College of Veterinary Medicine, Oregon State University, Corvallis, OR, United States of America
- Department of Integrative Biology, Oregon State University, Corvallis, OR, United States of America
| | - Sarah A. Budischak
- Odum School of Ecology, University of Georgia, Athens, GA, United States of America
| | - Paul L. A. M. Corstjens
- Department of Molecular Cell Biology, Leiden University Medical Center, Leiden, The Netherlands
| | - Vanessa O. Ezenwa
- Odum School of Ecology, University of Georgia, Athens, GA, United States of America
- College of Veterinary Medicine, University of Georgia, Athens, GA, United States of America
| | - Mireya Smith
- College of Veterinary Medicine, University of Georgia, Athens, GA, United States of America
| | - Robert S. Spaan
- Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR, United States of America
| | - Govert J. van Dam
- Department of Parasitology, Leiden University Medical Center, Leiden, The Netherlands
| | - Michelle L. Steinauer
- College of Osteopathic Medicine of the PNW, Western University of Health Sciences, Lebanon, OR, United States of America
| |
Collapse
|
4
|
Henrichs B, Oosthuizen MC, Troskie M, Gorsich E, Gondhalekar C, Beechler BR, Ezenwa VO, Jolles AE. Within guild co-infections influence parasite community membership: a longitudinal study in African Buffalo. J Anim Ecol 2016; 85:1025-34. [PMID: 27084785 DOI: 10.1111/1365-2656.12535] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2015] [Accepted: 04/04/2016] [Indexed: 11/26/2022]
Abstract
Experimental studies in laboratory settings have demonstrated a critical role of parasite interactions in shaping parasite communities. The sum of these interactions can produce diverse effects on individual hosts as well as influence disease emergence and persistence at the population level. A predictive framework for the effects of parasite interactions in the wild remains elusive, largely because of limited longitudinal or experimental data on parasite communities of free-ranging hosts. This 4-year study followed a community of haemoparasites in free-ranging African buffalo (Syncerus caffer). We detected infection by 11 haemoparasite species using PCR-based diagnostic techniques, and analyzed drivers of infection patterns using generalized linear mixed models to understand the role of host characteristics and season on infection likelihood. We tested for (i) effects of co-infection by other haemoparasites (within guild) and (ii) effects of parasites infecting different tissue types (across guild). We found that within guild co-infections were the strongest predictors of haemoparasite infections in the buffalo; but that seasonal and host characteristics also had important effects. In contrast, the evidence for across-guild effects of parasites utilizing different tissue on haemoparasite infection was weak. These results provide a nuanced view of the role of co-infections in determining haemoparasite infection patterns in free living mammalian hosts. Our findings suggest a role for interactions among parasites infecting a single tissue type in determining infection patterns.
Collapse
Affiliation(s)
- Brian Henrichs
- Department of Integrative Biology, Oregon State University, Corvallis, OR, 97331, USA
| | - Marinda C Oosthuizen
- Department of Veterinary Tropical Diseases, Faculty of Veterinary Sciences, University of Pretoria, Onderstepoort, 0110, South Africa
| | - Milana Troskie
- Department of Veterinary Tropical Diseases, Faculty of Veterinary Sciences, University of Pretoria, Onderstepoort, 0110, South Africa
| | - Erin Gorsich
- Department of Biology, Colorado State University, Fort Collins, CO, 80523, USA
| | - Carmen Gondhalekar
- Department of Integrative Biology, Oregon State University, Corvallis, OR, 97331, USA
| | - Brianna R Beechler
- Department of Biomedical Sciences, Oregon State University, Corvallis, OR, 97331, USA
| | - Vanessa O Ezenwa
- Department of Infectious Diseases, University of Georgia, Athens, GA, 30602, USA.,Odum School of Ecology, University of Georgia, Athens, GA, 30602, USA
| | - Anna E Jolles
- Department of Integrative Biology, Oregon State University, Corvallis, OR, 97331, USA.,Department of Biomedical Sciences, Oregon State University, Corvallis, OR, 97331, USA
| |
Collapse
|
5
|
CORSTJENS PAULLAM, DE DOOD CLAUDIAJ, KORNELIS DIEUWKE, FAT ELISAMTJONKON, WILSON RALAN, KARIUKI THOMASM, NYAKUNDI RUTHK, LOVERDE PHILIPT, ABRAMS WILLIAMR, TANKE HANSJ, VAN LIESHOUT LISETTE, DEELDER ANDRÉM, VAN DAM GOVERTJ. Tools for diagnosis, monitoring and screening of Schistosoma infections utilizing lateral-flow based assays and upconverting phosphor labels. Parasitology 2014; 141:1841-55. [PMID: 24932595 PMCID: PMC4265670 DOI: 10.1017/s0031182014000626] [Citation(s) in RCA: 149] [Impact Index Per Article: 14.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
The potential of various quantitative lateral flow (LF) based assays utilizing up-converting phosphor (UCP) reporters for the diagnosis of schistosomiasis is reviewed including recent developments. Active infections are demonstrated by screening for the presence of regurgitated worm antigens (genus specific polysaccharides), whereas anti-Schistosoma antibodies may indicate ongoing as well as past infections. The circulating anodic antigen (CAA) in serum or urine (and potentially also saliva) is identified as the marker that may allow detection of single-worm infections. Quantitation of antigen levels is a reliable method to study effects of drug administration, worm burden and anti-fecundity mechanisms. Moreover, the ratio of CAA and circulating cathodic antigen (CCA) is postulated to facilitate identification of either Schistosoma mansoni or Schistosoma haematobium infections. The UCP-LF assays allow simultaneous detection of multiple targets on a single strip, a valuable feature for antibody detection assays. Although antibody detection in endemic regions is not a useful tool to diagnose active infections, it gains potential when the ratio of different classes of antibody specific for the parasite/disease can be determined. The UCP-LF antibody assay format allows this type of multiplexing, including testing a linear array of up to 20 different targets. Multiple test spots would allow detection of specific antibodies, e.g. against different Schistosoma species or other pathogens as soil-transmitted helminths. Concluding, the different UCP-LF based assays for diagnosis of schistosomiasis provide a collection of tests with relatively low complexity and high sensitivity, covering the full range of diagnostics needed in control programmes for mapping, screening and monitoring.
Collapse
Affiliation(s)
- PAUL L. A. M. CORSTJENS
- Department of Molecular Cell Biology, Leiden University Medical Center, Leiden, the Netherlands
| | - CLAUDIA J. DE DOOD
- Department of Molecular Cell Biology, Leiden University Medical Center, Leiden, the Netherlands
| | - DIEUWKE KORNELIS
- Department of Parasitology, Leiden University Medical Center, Leiden, the Netherlands
| | - ELISA M. TJON KON FAT
- Department of Molecular Cell Biology, Leiden University Medical Center, Leiden, the Netherlands
| | | | - THOMAS M. KARIUKI
- Institute of Primate Research, National Museums of Kenya, Nairobi, Kenya
| | - RUTH K. NYAKUNDI
- Institute of Primate Research, National Museums of Kenya, Nairobi, Kenya
| | - PHILIP T. LOVERDE
- Departments of Biochemistry and Pathology, University of Texas Health Science Center, San Antonio, TX, USA
| | - WILLIAM R. ABRAMS
- Department of Basic Science, NYU College of Dentistry, New York, NY, USA
| | - HANS J. TANKE
- Department of Molecular Cell Biology, Leiden University Medical Center, Leiden, the Netherlands
| | - LISETTE VAN LIESHOUT
- Department of Parasitology, Leiden University Medical Center, Leiden, the Netherlands
| | - ANDRÉ M. DEELDER
- Department of Parasitology, Leiden University Medical Center, Leiden, the Netherlands
| | - GOVERT J. VAN DAM
- Department of Parasitology, Leiden University Medical Center, Leiden, the Netherlands
| |
Collapse
|
6
|
van Dam GJ, de Dood CJ, Lewis M, Deelder AM, van Lieshout L, Tanke HJ, van Rooyen LH, Corstjens PL. A robust dry reagent lateral flow assay for diagnosis of active schistosomiasis by detection of Schistosoma circulating anodic antigen. Exp Parasitol 2013; 135:274-82. [PMID: 23850995 PMCID: PMC3797160 DOI: 10.1016/j.exppara.2013.06.017] [Citation(s) in RCA: 102] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2013] [Revised: 06/18/2013] [Accepted: 06/30/2013] [Indexed: 01/05/2023]
Abstract
UNLABELLED An earlier reported laboratory assay, performed in The Netherlands, to diagnose Schistosoma infections by detection of the parasite antigen CAA in serum was converted to a more user-friendly format with dry reagents. The improved assay requires less equipment and allows storage and worldwide shipping at ambient temperature. Evaluation of the new assay format was carried out by local staff at Ampath Laboratories, South Africa. The lateral flow (LF) based assay utilized fluorescent ultrasensitive up-converting phosphor (UCP) reporter particles, to be read by a portable reader (UPlink) that was also provided to the laboratory. Over a period of 18 months, about 2000 clinical samples were analyzed prospectively in parallel with a routinely carried out CAA-ELISA. LF test results and ELISA data correlated very well at CAA concentrations above 300 pg/mL serum. At lower concentrations the UCP-LF test indicates a better performance than the ELISA. The UCP-LF strips can be stored as a permanent record as the UCP label does not fade. At the end of the 18 months testing period, LF strips were shipped back to The Netherlands where scan results obtained in South Africa were validated with different UCP scanning equipment including a novel, custom developed, small lightweight UCP strip reader (UCP-Quant), well suited for testing in low resource settings. CONCLUSION The dry format UCP-LF assay was shown to provide a robust and easy to use format for rapid testing of CAA antigen in serum. It performed at least as good as the ELISA with respect to sensitivity and specificity, and was found to be superior with respect to speed and simplicity of use. Worldwide shipping at ambient temperature of the assay reagents, and the availability of small scanners to analyze the CAA UCP-LF strip, are two major steps towards point-of-care (POC) applications in remote and resource poor environments to accurately identify low (30 pg CAA/mL serum; equivalent to about 10 worm pairs) to heavy Schistosoma infections.
Collapse
Affiliation(s)
- Govert J. van Dam
- Department of Parasitology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands
| | - Claudia J. de Dood
- Department of Molecular Cell Biology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands
| | - Melanie Lewis
- Ampath Laboratories, Department of Serology and Immunology, Private Bag X9, Highveld Techno Park, Centurion, Republic of South Africa
| | - André M. Deelder
- Department of Parasitology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands
| | - Lisette van Lieshout
- Department of Parasitology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands
| | - Hans J. Tanke
- Department of Molecular Cell Biology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands
| | - Louis H. van Rooyen
- Ampath Laboratories, Department of Serology and Immunology, Private Bag X9, Highveld Techno Park, Centurion, Republic of South Africa
| | - Paul L.A.M. Corstjens
- Department of Molecular Cell Biology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands
| |
Collapse
|
7
|
Abstract
Schistosomiasis remains a major health problem in much of the developing world. Despite decades of research, many fundamental questions on the dynamics of infection and immunity development remain unanswered. Schistosomiasis is also a common parasitic infection in cattle, and studies on livestock exposed to their own species of schistosome may help in understanding some aspects of the host-parasite relationship. Here, Jan De Bont and Jozef Vercruysse review the current knowledge on the epidemiology and control of cattle schistosomiasis.
Collapse
Affiliation(s)
- J De Bont
- Department of Parasitology, Faculty of Veterinary Medicine, University of Gen. Salisburylaan 133, 9820 Merelbeke, Belgium
| | | |
Collapse
|
8
|
De Bont J, Shaw DJ, Vercruysse J. The relationship between faecal egg counts, worm burden and tissue egg counts in early Schistosoma mattheei infections in cattle. Acta Trop 2002; 81:63-76. [PMID: 11755433 DOI: 10.1016/s0001-706x(01)00198-x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The relationships between schistosome faecal egg counts (EPG), total tissue egg counts (TEC) and adult female worm burdens (FW) recorded at post-mortem examination of 30 Friesian calves from three different field trials were analysed. The calves in study 1 (n=14) had been exposed to natural Schistosoma mattheei infections for 2 months, those of study 2 (n=9) for between 4 and 12 months, and those in study 3 (n=7) for 8 months. No clinical schistosomiasis was observed in any of the groups, and at perfusion, EPG's varied from 5 to 210, TEC's from 28,800 to 2,439,400 and FW's from 11 to 1218. There was as much variation in EPG, TEC and FW between calves with the same duration of exposure as between calves with different duration of exposure. There were very similar significant positive relationships between log transformed FW and log transformed EPG in all three groups (P<0.007, R2>0.46, slopes 0.957-1.015). There were also significant positive relationships between log transformed FW and log transformed TEC in all three groups (P<0.034, R2>0.45) and between log transformed TEC and log transformed EPG in all three groups (P<0.025, R2>0.48). All three studies had a linear relationship between log transformed FW and log transformed EPG with a slope value close to 1 (P>0.845 for different from one). This indicates that there was no evidence of density dependence in the three studies for the relationship between FW and EPG. In contrast, there was no consistent relationship (in terms of slope) between either log transformed TEC and log transformed FW in the three studies (after correcting for differential duration of exposure), or log transformed TEC and log transformed EPG. For all three sets of comparisons the predictability of one parameter based on another was poor for a single sample.
Collapse
Affiliation(s)
- J De Bont
- Centre d'Immunologie et de Biologie Parasitaire, Inserm-IPL U547, Institut Pasteur de Lille, 1, rue du Professeur Calmette, BP 245, 59019 Cedex, Lille, France.
| | | | | |
Collapse
|
9
|
Polman K, Stelma FF, De Vlas SJ, Sow S, Fathers L, Le Cessie S, Talla I, Deelder AM, Gryseels B. Dynamics of egg counts and circulating antigen levels in a recent Schistosoma mansoni focus in northern Senegal. Trop Med Int Health 2001; 6:538-44. [PMID: 11469948 DOI: 10.1046/j.1365-3156.2001.00742.x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Serum circulating anodic antigen (CAA) levels were compared with faecal egg counts in four subsequent population samples, randomly selected at 8-month intervals, in a recent Schistosoma mansoni focus in northern Senegal. In all four samples, antigen levels showed the same age-intensity profiles as egg counts, with a strong decline in adults. Also across population samples, a consistent relationship was found between egg counts and antigen levels. Assuming the level of CAA to be a direct reflection of worm burden, these findings support the idea that the observed egg count patterns and levels indeed reflect dynamics of worm burdens, and not of egg excretion or worm fecundity. Remarkably similar levels of both egg counts and CAA were observed in the first and last sample, collected in the same season (August--September), but 2 years apart. This suggests that a steady state of S. mansoni infection had already been reached shortly after the onset of the epidemic in this focus (3 years). Significantly lower infection levels were found in the intermediate population samples collected in January and April. The differences in infection levels across the four population samples may be because of seasonal transmission patterns. They would indicate a substantial turnover of worm populations, with an estimated average life span of only 7 months, probably less, in this recently emerged, intense S. mansoni focus.
Collapse
Affiliation(s)
- K Polman
- Department of Parasitology, Leiden University Medical Centre, Leiden, The Netherlands.
| | | | | | | | | | | | | | | | | |
Collapse
|
10
|
Affiliation(s)
- J De Bont
- Department of Parasitology, Faculty of Veterinary Medicine, University of Gent, Merelbeke, Belgium
| | | |
Collapse
|