Schistosomiasis

Organism The main parasites are Schistosoma haematobium, S. mansoni and S. japonicum. Other species, such as S. intercalatum and S. mekongi do occur, but they are only important in well-defined areas and their epidemiology and control are similar to one of the three main types.

Clinical features Infection normally starts in childhood, with often very little signs of the disease until adulthood. Passing blood in the urine is one of the first signs of S. haematobium disease, but because it is so common in the local area, it is generally ignored, with boys assuming it quite normal that they should have period bleeding like girls do. Infection and egg output increases up to about 15 years of age and then declines. Individuals vary in their response, with some persons acquiring heavy infections and developing severe pathological changes, while others have only minor symptoms. The more serious manifestations are liver fibrosis, portal hypertension and obstructive urinary problems, with the pathology depending upon the species of parasite and the number of eggs deposited in the tissues. Infections with S. mansoni and S. japonicum lead to intestinal and liver damage, while that with S. haematobium results in bladder complications, including bladder cancer.

Diagnosis is made by finding the characteristic eggs (Fig. 9.1) of S. haematobium in the urine and those of S. mansoni and S. japonicum in the faeces or from a rectal snip. Urine samples are best collected between 1100 and 1500 h when egg output is at a maximum. Leaving the urine to stand, cen-trifuging it, or passing it through a filter increases the chance of finding eggs. While the qualitative diagnosis is required in the individual case, quantitative estimates are more valuable in epidemiological investigations. In S. haematobium, the simplest method is to pass 10 ml of urine through a filter in a Millipore holder. The paper or membrane is taken out, dried and stained with ninhydrin and the eggs counted directly. Immuno-logical methods, indirect fluorescent antibody test (IFAT) and enzyme-linked immunosorbent assay (ELISA) test have also been developed for schistosomiasis, but they only indicate recent or past infection, so eggs must be looked for to confirm the diagnosis. They are useful in epidemi-ological surveys for rapidly defining the extent of the infected area.

Pathology is related to the number of worms, which can be measured by the number of eggs produced. In S. haematobium, the production of 50 eggs/ml of urine or above is regarded as the level of severe pathology and much of present day control strategy is aimed at reducing the egg count below this level.

Transmission Infection results from cer-cariae directly piercing the skin of a person when they go into the water. On penetrating the subcutaneous layer of the host, the cercaria becomes a schistosomule, migrates to the lungs and finally develops into an adult in the portal vessels of the liver. Both male and female worms are required so that pairing can take place prior to migration to the final destination in the mesenteric or vesical plexus. Adult worms can live for 20-30 years, but are active egg producers for 3-8 years, although some have produced viable eggs for over 30 years. The egg output per day in S. haematobium is some 20-250, in S. mansoni 100-300 and in S. japonicum 1500-3500. It is this massive output of eggs in S. japonicum that leads to the more rapidly developing and severe pathology.

Less than 50% of eggs manage to pass through the bladder or intestinal wall to develop further, the remainder being trapped in the tissue. On reaching water, a temperature of 10-30°C and the presence of light induce hatching, resulting in miracidia swimming out. They actively search out a snail using geotactic and phototactic behaviour, homing-in on a chemical substance 'miraxone' inadvertently liberated by the snail. The miracidium must penetrate a snail within 8-12 h, but their chance of success decreases with age. Some 40% of snails are infected at a distance of 5 m in still water, but where the water is flowing, similar infection rates can occur at a far greater distance. Normally, infection occurs in water flowing at 10 cm/s or less. Even after the rigours of the journey when miracidia have entered the correct species of snail, many are inactivated and only a small proportion develop into sporocysts. This is determined by the part of the snail entered and immunity to reinfection developed by the snail (Fig. 11.1).

Cercariae are stimulated by light to emerge from the snail when the ambient temperature is between 10°C and 30°C. Cer-carial emergence increases as daylight penetrates the watery environment producing a peak for S. mansoni at 1200 noon and for S. haematobium, mid-to-late afternoon. With S. japonicum, the stimulus produced by light is delayed and maximal cercarial liberation occurs at 2300 h. The number of cer-cariae issuing from a snail can be immense, in the order of 1000-3000/day, but this depends upon the species and relative size of the snail. Where more than one miracid-ium has penetrated a snail, there is depression of cercarial production; this may also occur if the snail is host to other trematode infections. Cercarial output is greatest in S. mansoni, less in S. haematobium and least of all in S. japonicum. Cercariae survive for 24 h, but their greatest chance of penetrating the host is when they are young. When cer-cariae enter within 2 h of release, only 30% die, but this rises to 50% at 8 h and 85% at 24 h.

The snail intermediate hosts are species specific, Bulinus spp. in S. haematobium, Biomphalaria spp. in S. mansoni and Onco-melania spp. in S. japonicum. They are illustrated in Fig. 11.1. They can adapt to a wide range of habitats from natural waterways to temporary ponds and cultivated rice fields. Whenever there is sufficient organic matter on which to feed, snails will be found. Within a body of water, distribution

S. japonicum

Fig. 11.1. Schistosomiasis species and life cycles.

may be quite irregular with dense colonies in some places and complete absence in others. Various factors, which may influence snail colonization, are:

• Electrolyte concentration. Snails demand a minimum calcium concentration, cannot tolerate high salt content or a low pH.

• Light is not required by the snail, and they can often survive in near total darkness.

• Rainfall may herald the end of the dry season and provide water in which snail populations can increase, but if the rainfall is too heavy, it may flush out the snails, resulting in a subsequent decrease. Snail populations, therefore, may follow a seasonal pattern.

• Temperature rise encourages expansion of the population up to a maximum of approximately 30°C.

• Density is a limiting factor and results in reduced growth.

• Aestivation or the ability of snails to survive out of the water for weeks or months allows populations of snails to continue from one season to another, possibly also transferring immature infections of S. haematobium and S. mansoni. The snail host of S. japonicum can survive conditions of desiccation best of all.

Snails are capable of self-fertilization, although cross-fertilization is more common. Their reproductive capacity is phenomenal and a single snail can produce a colony within 40 days and be infective in 60 days. When conditions are optimal, many species of snails will double their population in 2-3 weeks. In measuring the age of snail populations, size of snails is a useful indicator. A large number of small samples from many different areas are preferable to a few large samples in estimating the numbers and density in water courses. Infection rates in snails are generally low, with only some 1-2% of the colony being infective, but even so this level is sufficient to account for high prevalence rates in the human population.

Humans contaminate water either by urinating or defecating into or near water courses. Egg output is variable between in dividuals and their age, with a few individuals having heavy infections and egg outputs, while the majority have light infections. In areas of high endemicity, children between 5 and 14 years are responsible for over 50% of the contamination. As the infection rate declines, older age groups become more important.

People are infected by collecting water, washing (both clothes and the person), in their occupation (such as fishing) or during recreation. Children are most commonly infected when they play in water, while in adults, it is when they carry out their domestic duties or occupation. Infection is generally due to repeated water contact over a long period of time, but can occur from a single immersion if it coincides with a large number of cercariae in the water.

Animals, such as water buffalo, cattle, pigs, dogs, cats and horses, can also serve as reservoirs of S. japonicum, but they are less important than humans as sources of infection.

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