The epidemiology of Schistosoma spindale Montgomery, 1906 in cattle in Sri Lanka

One Health Concept against Schistosomiasis: An Overview

Schistosomiasis (bilharziasis) is a parasitic disease caused by Schistosoma spp. that belongs to trematode worms. These worms are known as “blood parasites”. This disease is included in “neglected tropical diseases” and “water-borne diseases”. The main species are Schistosoma (S.) haematobium, S. japonicum, S. mansoni, S. intercalatum, S. mekongi, S. guineensis and S. intercalatum, though there are more than 20 different species. The parasite in the definitive host may affect many organs and systems. The disease may become chronic and lasts 3–8 years and even up to 20–30 years. The definitive host is primarily human; however, in endemic areas animals such as monkeys, cattle, horses, rodents, cats, dogs are reservoirs. According to World Health Organization (WHO), schistosomiasis affects 250 million people, and causes 1.9 million deaths yearly in endemic areas. Moreover, due to global warming, the spread of the disease may increase. The effective way to fight against schistosomiasis is following the “one-health system”. Indeed, to overcome or “eradicate” this disease, we have to strive against different forms at different evolutionary stages of the worm such as, forms in humans, domestic or wild animals, and freshwater snails. If we combine the knowledge of professionals, we may achieve this goal.

Spillover, hybridization, and persistence in schistosome transmission dynamics at the human-animal interface

Zoonotic spillover and hybridization of parasites are major emerging public and veterinary health concerns at the interface of infectious disease biology, evolution, and control. Schistosomiasis is a neglected tropical disease of global importance caused by parasites of the Schistosoma genus, and the Schistosoma spp. system within Africa represents a key example of a system where spillover of animal parasites into human populations has enabled formation of hybrids. Combining model-based approaches and analyses of parasitological, molecular, and epidemiological data from northern Senegal, a region with a high prevalence of schistosome hybrids, we aimed to unravel the transmission dynamics of this complex multihost, multiparasite system. Using Bayesian methods and by estimating the basic reproduction number (R₀ ), we evaluate the frequency of zoonotic spillover of Schistosoma bovis from livestock and the potential for onward transmission of hybrid S. bovis × S. haematobium offspring within human populations. We estimate R₀ of hybrid schistosomes to be greater than the critical threshold of one (1.76; 95% CI 1.59 to 1.99), demonstrating the potential for hybridization to facilitate spread and establishment of schistosomiasis beyond its original geographical boundaries. We estimate R₀ for S. bovis to be greater than one in cattle (1.43; 95% CI 1.24 to 1.85) but not in other ruminants, confirming cattle as the primary zoonotic reservoir. Through longitudinal simulations, we also show that where S. bovis and S. haematobium are coendemic (in livestock and humans respectively), the relative importance of zoonotic transmission is predicted to increase as the disease in humans nears elimination.

The phylogeography of Indoplanorbis exustus (Gastropoda: Planorbidae) in Asia

BACKGROUND

The freshwater snail Indoplanorbis exustus is found across India, Southeast Asia, central Asia (Afghanistan), Arabia and Africa. Indoplanorbis is of economic importance in that it is responsible for the transmission of several species of the genus Schistosoma which infect cattle and cause reduced livestock productivity. The snail is also of medical importance as a source of cercarial dermatitis among rural workers, particularly in India. In spite of its long history and wide geographical range, it is thought that Indoplanorbis includes only a single species. The aims of the present study were to date the radiation of Indoplanorbis across Asia so that the factors involved in its dispersal in the region could be tested, to reveal potential historical biogeographical events shaping the phylogeny of the snail, and to look for signs that I. exustus might be polyphyletic.

RESULTS

The results indicated a radiation beginning in the late Miocene with a divergence of an ancestral bulinine lineage into Assam and peninsular India clades. A Southeast Asian clade diverged from the peninsular India clade late-Pliocene; this clade then radiated at a much more rapid pace to colonize all of the sampled range of Indoplanorbis in the mid-Pleistocene.

CONCLUSIONS

The phylogenetic depth of divergences between the Indian clades and Southeast Asian clades, together with habitat and parasitological differences suggest that I. exustus may comprise more than one species. The timescale estimated for the radiation suggests that the dispersal to Arabia and to Southeast Asia was facilitated by palaeogeographical events and climate change, and did not require human involvement. Further samples from Afghanistan, Africa and western India are required to refine the phylogeographical hypothesis and to include the African Recent dispersal.

The epidemiology and control of cattle schistosomiasis

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.

Immunoepidemiology of human schistosomes: taking the theory into the field

Much is known about human immune responses to schistosome infection, but it has proved difficult to determine the impact of these responses on schistosome epidemiology in the field. In this paper, Mark Woolhouse compares epidemiological patterns from field data with the predictions o f simple mathematical models of different immunological processes. The comparison gives some indications as to which types of immune response may be important, and of their strength and duration. The results are consistent with a significant impact of a process similar to 'concomitant' immunity, with a possible role for anti-fecundity effects.

Variations in the immune response to natural Schistosoma mattheei infections in calves born to infected mothers

During previous work Schistosoma antibodies and circulating antigens were detected at birth in the serum from some calves born to Schistosoma mattheei infected mothers. The objectives of the present survey were: (1) to investigate the proportion of calves, born to cows infected with S. mattheei, which have specific antibodies and circulating schistosome antigens present in their serum at birth and (2) to investigate whether the presence or absence of these specific antibodies and/or circulating antigens at birth may affect the pattern of a natural S. mattheei infection in calves from 4 to 5 months of age, when the colostral antibodies are thought to be of negligible importance. A total of 28 calves born to infected mothers were randomly selected. Faeces, serum and colostrum samples were collected from the cows at calving, serum samples were collected from the calves at birth (day 0), after intake of colostrum (day 1) and monthly thereafter up to the age of 10 months. Both serum and colostrum samples were analysed for IgG(H+L) against SWAP mattheei and schistosome circulating anodic antigen (CAA) levels. The calves were exposed to a natural challenge from the age of 4-5 months. Faecal samples were collected from the calves monthly, starting at an age of 5 months up to 10 months, and were examined for faecal egg counts. Nine (group 1) out of the 28 calves were found to have specific antibodies in their serum at birth, in 5 of them CAA levels were also detected. In the other 19 calves (group 2) no IgG(H+L) or CAA were detected. At the end of the study faecal egg counts and CAA levels were significantly lower in calves from group 1 compared to group 2. Results confirm earlier work that specific antibodies and circulating antigens may be present in serum from calves at birth, and show that these calves have lower faecal egg counts and CAA levels after exposure to a natural challenge.

Transplacental transfer of schistosomal circulating anodic antigens in cows

The present work investigated the transplacental passage of circulating anodic schistosome antigens (CAA) and the production of foetal antibodies in response to antigenic stimulation in Schistosoma mattheei infected cows. Three groups were available: six calves born to non-infected cows received colostrum from a pool from non-infected cows (group 1), six calves born to non-infected cows (group 2) and six calves born to infected cows (group 3) received colostrum from a pool from infected cows. Schistosoma-specific IgG1 antibody and CAA levels were measured in the colostrum pools, the sera collected from the cows, and the sera collected from the calves at birth, after intake of colostrum and at day 30. The specific IgG1 antibody levels were significantly higher in the sera from cows of group 3. In four cows of group 3 high CAA levels were detected. The specific IgG1 antibody levels were 0.646 and 0.176 OD for the infected and non-infected colostrum pool, respectively, and the CAA levels were 5667 and 2557 pg CAA/mL, respectively. At birth high levels of specific IgG1 antibody and CAA were detected in 4 calves of group 3; levels in the other two calves were negligible. After intake of colostrum, specific IgG1 antibody levels of group 1 increased slightly at day 1 to become again insignificant at day 30. In group 2 specific IgG1 antibody levels increased significantly between days 0 and 1, to decrease, although not significantly, at day 30. Finally, in group 3 the delta OD values increased at day 1 and remained high until day 30. After intake of colostrum the CAA level increased very slightly for groups 1 and 2 to become again undetectable at day 30. In group 3 a nonsignificant decrease in CAA levels was observed at day 1 followed by a further significant decrease to reach low levels at day 30. The suggested intrauterine antigenic stimulation may be important not only for generating immune responses to natural early infections, but also for enhancing the immunogenicity and efficacy of vaccines administered to newborns.

The relationship between faecal egg counts, worm burden and tissue egg counts in early Schistosoma mattheei infections in cattle

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.

Variations in Schistosoma mattheei egg morphology and viability according to age of infection in cattle

Comparison of the numbers of Schistosoma mattheei eggs and miracidia per gram faeces in groups of naturally infected calves, heifers and adult cows showed that the reduction in faecal egg excretion recorded as infection progresses is associated with a decline in the ability of eggs to hatch. While 50% of the eggs from calves produced a miracidium, only 15% of those excreted from adult cows did the same. The decline in egg viability is at least partly associated with morphological changes of the eggs. About twice as many smaller and vacuolated eggs were found in the faeces of heifers and adult cows (33.8%) as compared to animals in early infection (16.1%).

Observations on worm population dynamics in calves naturally infected with Schistosoma mattheei

The evolution of faecal egg output, worm burdens and tissue egg counts in young calves was monitored during the first year of natural exposure to Schistosoma mattheei infection on a Zambian farm. According to the duration of their stay on the farm, these calves were classified into 2 groups of 14 temporary tracers (TT calves) which were introduced on a 2-monthly basis for residential periods of 2 months, and 12 permanent tracers (PT calves) introduced either at the beginning of the experiment (Group A) or 2 months later (Group B) and gradually removed after residential periods of 2, 4, 6, 8, 10 and 12 months on the farm. Worm counts in the TT calves showed that infection occurred throughout the year on the farm and that levels of infection acquired during each period of 8 weeks correlated well with the respective infected snail densities observed at the main transmission site. Marked differences in worm population dynamics were recorded between the 2 groups of PT calves. In Group B animals which apparently were initially exposed to heavy transmission, according to the results from TT calves, much higher worm counts and greater susceptibility to reinfection were observed than in Group A animals initially exposed to lighter exposure. These results suggest that the development of resistance to natural infection with S. mattheei may depend on the initial exposure to the parasite. Low initial exposures may lead to resistance whereas high initial exposures may result in decreased immune responses resulting in susceptibility to infection.

Schistosoma mattheei infections in cattle: changes associated with season and age

The Schistosoma mattheei egg output was monitored in 31 cattle over a 18-month period on a dairy farm near Lusaka (Zambia). The animals were kept on pasture with free access to two streams which were suitable for the intermediate host, Bulinus globosus. Individual faecal egg excretion reached an average peak of 130 eggs per gram, around 9 months after birth and decreased markedly before the age of 18 months. Average counts declined significantly with age, down to less than five eggs per gram in adult cows. A seasonal increase in B. globosus snails and S. mattheei transmission during the rainy season had no effect on the egg output of animals older than 18 months. Two calves and two adult cows were necropsied to compare fluke and tissue egg counts in young and old infections. There was a marked decline in tissue egg accumulation in older cows, in spite of an increase in the numbers of adult female flukes, as compared with young animals. A shift of egg accumulation from the large intestine towards the liver was also observed as infection progressed. It is concluded from the results of faecal egg counts that cattle reared under conditions of continuous challenge develop acquired resistance to S. mattheei infection within the first year following primary infection. Comparison of fluke and tissue egg counts in farm animals of different ages suggests the acquisition of an anti-fecundity effect as infection progresses.

Cattle schistosomiasis in Zambia

A total of 358 cattle was examined for schistosome infection in Zambian slaughterhouses. A total of 542 worms collected from 104 infected individuals was examined for glucose-6-phosphate dehydrogenase and phosphoglucomutase using isoelectric focusing. The overall prevalence of infection was 51%. Ninety three percent of the infected animals had less than 100 worm pairs in the mesenteric veins. Schistosoma mattheei was the predominant species (75%); S. leiperi (12%) and S. margrebowiei (2%) were also identified. The remaining 11% of the worms showed one of two distinct heterozygote patterns. Pattern A is identical to that of a laboratory-produced F1 S. mattheei x S. haematobium hybrid, but could also represent a S. mattheei x S. leiperi hybrid. Further studies are required to elucidate the origins of pattern B.