Parasites and poverty: The case of schistosomiasis

Objectives: To summarize new knowledge on the range of disease due to chronic schistosomiasis and examine the debilitating burden of both light and heavy infection; to outline goals of disease prevention, including current age-targeted strategies and more extended programmes aimed at preventing transmission.Methods: A systematic search of 2004—2007 papers via PUBMED and related databases using `schistosom' and disability- or treatment-related subject headings. Reports were independently reviewed for inclusion.Results: Sixty-eight papers met review objectives. These suggest new evidence for a causative link between schistosome infection, antiparasite inflammation, and risk for anaemia, growth stunting and undernutrition in affected populations, as well as exacerbation of co-infections and impairment of cognitive development and work capacity. Formal quality-of-life assessment defines a significant 9.5—24% disability with the most aggressive schistosome species, Schistosoma japonicum.Discussion: Schistosomi...

The unacknowledged impact of chronic schistosomiasis

Anthelmintic resistance in nematode parasites of cattle: a global issue?

This paper summarizes and concludes in-depth field investigations on suspected resistance of Schistosoma mansoni to praziquantel in northern Senegal. Praziquantel at 40 mg/kg usually cures 70-90% of S. mansoni infections. In an initial trial in an epidemic S. mansoni focus in northern Senegal, only 18% of the cases became parasitologically negative 12 weeks after treatment, although the reduction in mean egg counts was within normal ranges (86%). Among other hypotheses to explain the observed low cure rate in this focus, the possibility of drug resistance or tolerance had to be considered. Subsequent field trials with a shorter follow-up period (6-8 weeks) yielded cure rates of 31-36%. Increasing the dose to 2 x 30 mg/kg did not significantly improve cure rates, whereas treatment with oxamniquine at 20 mg/kg resulted in a normal cure rate of 79%. The efficacy of praziquantel in this focus could be related to age and pre-treatment intensity but not to other host factors, including immune profiles and water contact patterns. Treatment with praziquantel of individuals from the area residing temporarily in an urban region with no transmission, and re-treatment after 3 weeks of non-cured individuals within the area resulted in normal cure rates (78-88%). The application of an epidemiological model taking into account the relation between egg counts and actual worm numbers indicated that the low cure rates in this Senegalese focus could be explained by assuming a 90% worm reduction after treatment with praziquantel; in average endemic situations, such a drug efficacy would result in normal cure rates. Laboratory studies by others on the presence or absence of praziquantel resistance in Senegalese schistosome strains have so far been inconclusive. We conclude that there is no convincing evidence for praziquantel-resistant S. mansoni in Senegal, and that the low cure rates can be attributed to high initial worm loads and intense transmission in this area.

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Are poor responses to praziquantel for the treatment of Schistosoma mansoni infections in Senegal due to resistance? An overview of the evidence

Schistosoma japonicum, Fasciola hepatica and F. gigantica are digenetic trematodes and, therefore, possess similar life cycles. While schistosomiasis japonica has for a long time been recognised as a major disease of both humans and animals, infection with fasciolids has only been considered of relevance to animals. However, a number of recent reports indicate that fasciolosis is becoming a serious public health problem, especially in South America, Egypt and Iran (sporadic cases are also on the increase throughout Europe). Vaccines targeted at animals could play an important role in controlling these three diseases in animals and, by blocking transmission of infection, have a concurrent beneficial effect on disease in humans. Approaches towards identifying and producing vaccines against these parasites are similar and are discussed in this reveiw.

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Vaccines against the zoonotic trematodes Schistosoma japonicum, Fasciola hepatica and Fasciola gigantica.

Publisher Summary This chapter provides an updated and comprehensive review on the main features of cattle schistosomiasis. The adult worms are obligate parasites of the blood vascular system of vertebrates. They live in the perivesical, nasal or mesenteric, and hepatic veins of the host where they feed on blood and produce nonoperculated eggs with a characteristic terminal or lateral spine. As many as 10 different species of schistosomes have been reported to naturally infect cattle. The geographical distribution of schistosome species infecting cattle is mainly determined by the distribution of their respective intermediate host snails. It is noted that most infections in endemic areas occur at a subclinical level. However, it has been established that high rates of prevalence of subclinical infections cause significant losses due to long-term effects on animal growth and productivity and increased susceptibility to other parasitic or bacterial disease. Despite this, schistosomes of veterinary concern have received relatively little attention. In addition, suitable drugs are not available for mass treatment in domestic stock and are unlikely to be developed in the near future. However, recent progress in identifying potentially protective parasite antigens has opened new perspectives in the control strategy against schistosomiasis.

Schistosomiasis in cattle.

The transmission and interaction of schistosomes in the Senegal River Basin in Mali and Zambia are reviewed and some preliminary field data are presented. In the Senegal River Basin four species of schistosomes are prevalent: Schistosoma mansoni, S. haematobium, S. bovis and S. curassoni as well as the following potential intermediate hosts: Biomphalaria pfeifferi, Bulinus globosus, B. umbilicatus, B. truncatus, B. senegalensis and B. forskalii. The role of each of these species in the transmission of schistosomes in man and domestic stock is discussed. Recent ecological changes caused by the construction of dams at Diama and Mananatali on the Senegal River, such as reduction in salinity, more stable water flow, creation of irrigation canals and development and extension of rice culture, have contributed towards the occurrence of new outbreaks of both intestinal and urinary schistosomiasis in the Senegal River Basin. In Mali, the four main areas of high prevalence of S. haematobium are Office du Niger (irrigation areas), Bandiagara (small dams), Selingue (dam areas) and Baguineda (irrigation areas). Apart from the Office du Niger, S. mansoni infections are rare. Surveys were carried out in the Dogon Country (Bandiagara District) in an attempt to confirm the recent independent reports of the presence of S. intercalatum. Data based on egg morphology and Ziehl Neelsen staining of egg shells suggested the possible occurrence of S. haematobium x S. intercalatum hybrids. Potential factors affecting the focal endemicity of S. haematobium in Mali are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on transmission and schistosome interactions in Senegal, Mali and Zambia.

During 1988 and 1989, the mesenteric veins of 901 cattle were examined for the presence of schistosomes at the Kandy slaughterhouse (Sri Lanka). The overall prevalence of infection was 31.2%. Animals younger than 2 years were less infected (21.3%) than those older than 5 years (47.9%). Based on the number of paired worms counted, three intensities of infection were recognized: low (1-20 pairs), moderate (21-100 pairs) and heavy (greater than 100 pairs). Intensities increased with the age of the animals but remained low (average 10 worm pairs). The worm burden increased by approximately 20% for each step in age group. The number of miracidia/100 g faeces was measured in 85 animals of all age groups and intensities of infection; 77% of the samples contained less than 100 miracidia. Miracidia counts decreased with age; moderately and heavily infected animals in each age group had the highest and the lowest counts, respectively. This may be due to a host immune response. The results raise questions on the sensitivity of faecal egg counts as a diagnostic method for visceral schistosomiasis in cattle.

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

During 1987 a total of 1393 cattle was examined for Schistosoma nasale infection at the Kandy slaughterhouse, Sri Lanka. The overall prevalence was 12.6% (monthly range 3-17%). Based on the appearance of macroscopic lesions, 6 types (0-5) were recognized; type 5 being the most severe, with cauliflower-like growths obstructing the nasal cavity. Older bovines with 8 permanent incisors were more heavily infected (29.1%) than younger ones with no permanent incisors (6.0%). The severity of the lesions increased also with the age of the animals. Observations on the localization of the lesions showed that the first sessile nodular areas appear on the medial septum, on the dorsal edge of the ventral nasal concha and on the lateral wall of the middle meatus of the nasal cavity. Later, they gradually spread over the whole mucosal surface of the anterior part of the cavity but were rarely found further than 10 cm posterior to the nasal opening. The histopathology showed that granuloma formation due to the presence of eggs was the most common feature of the respiratory mucosa.

The prevalence and pathology of Schistosoma nasale Rao, 1933 in cattle in Sri Lanka.

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 Fl S. mattheei × S. haematobium hybrid, but could also represent a S. mattheei × S. leiperi hybrid. Further studies are required to elucidate the origins of pattern B.

J. De Bont

Papers

Schistosomiasis in cattle.

Studies on transmission and schistosome interactions in Senegal, Mali and Zambia.

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

The prevalence and pathology of Schistosoma nasale Rao, 1933 in cattle in Sri Lanka.

Cattle schistosomiasis in Zambia.