2.2.1 Course of the Parasitosis
The pathogen of river blindness, Onchocerca volvulus, is a filarial worm of the class Nematoda whose larvae, the microfilariae, are transmitted by Simuliids or black flies (Diptera, Nematocera).
The adult worms live subcutaneously coiled up in a capsule of connective tissue, the onchocercoma. They are viviparous, producing microfilariae which inhabit the Iymph tissue of the host's skin, close to the epidermis, causing the condition termed: Microfilaridermia (fig. 2.14, page 35).
During transmission by Simulium, some of the microfilariae which were taken up in the blood meal, migrate from the insect's gut into the thoracic flight muscles and moult twice intracellularly. The resulting third larval stages, the metacyclic or infestive larvae, migrate to the proboscis, penetrate the vectors skin and migrate into the stab wound. In the definitive host, man, the infestive larvae moult a fourth time to become the juvenile stages, that already show sexual differentiation into males and females. During a prepatency lasting 6 to 9 months, they develop into mature adult worms, which copulate and propagate almost continuously only interrupted by short intervals for new inseminations. Their life expectancy is about seven yearsr (fig. 2.13, page 34).
The symptoms of the parasitosis, such as skin irritation and eyes lesions, are caused by the microfilariae. Onchocercomata are mostly located around the hip region.If they are located at the host's head or if microfilariae are found in the temporal region, they are particularly dangerous to the eyes. However, in most cases eye leasons appear not until several years of morefold infestations by the parasite.
The course of the parasitosis remains non-apparent for years in spite of a fully developed patency. Therefore, in endemic areas, the prevalence of the disease increases to high levels within the first two to three decennial age classes.
The parasitological diagnosis is performed by a superficial skin biopsy, the skin snip. For chemotherapy, substances are available, which act predominantly against the microfilariae. Substances active against the adult worms, - adulticides - are not well tolerated by the patient. Ivermectin, an antibiotic, lowers the microfilaridermia after a single oral administration to 10% of the original value lasting for three years. However, even by a morefold increased dosage the microfilaraemia does not disappear completely. Thus, the transmission of the parasitosis cannot be interrupted by Ivermectin only.
People living in endemic regions are classified clinically into groups with non-apparent, apparent or locally-acute onchocerciasis. Persons also exposed to transmission but giving permanently negative results in parasitologicai tests, are called "endemic normals". It is impossible to determine whether such persons are naturally completely resistent, or exhibit an occult or early postpatent onchocerciasis (see fig. 2.22, page 53).
2.2.2 Biology of Simuliids
Simuliids or blackflies are Diptera and belong to the suborder Nematocera which are characterised by a threadlike antennal flagellum. Only the females of blackflies are sucking blood, whereas the males live on nectar from flowers.The larvae need predominantly fast running fresh waters
The sexes find each other by visual orientation of the males, which have highly differentiated ommatidia in the dorsal region of their compound eyes. Females find their blood hosts over short distances by visual stimuli including colour, movement and shape (fig. 2.15, page 39).
During blood-feeding the female fly cuts the skin of the host using their scissor-like mandibles and parts the wound with hooks on the labrum and teeth on the hypopharynx. The maxillae anchor the proboscis whereas the labellae of the labium are spread out. If the insect in question is carrying metacyclic parasite larvae, these penetrate the streched ligula and migrate into the stab wound whereas they are protected against desiccation by exuding hemolymph (fig. 2.16, page 41).
Feeding activity is highest after dawn and in the early evening. The stimulus of this response is the the rapid increase and decrease in day light. Biting happens only outdoor. The resting places of adult flies during digestion of the blood meal are largely unknown.
For oviposition, the female flies land on plants material floating on the water surface, such as reeds or branches hanging down into the river. The jelly of the eggs gives off a pheromone which attracts further females to oviposit nearby and the eggs thus aggregate to thick layers.
The resulting hatched larvae fasten themselves to solid supports by means of a salivary cone. They filter detritus and bacteria from the passing water using their cranial fans. When prepareing for pupation they spin a bag-like cocoon in which they moult to produce the pupal stage. This carries breathing filaments which function under water as well as in the open air. The imago hatches inside the cocoon and is carried unwetted to the surface in a glistenintg film of air.
Cytotaxonomic types or even species can be determined according to the banding pattern on the polytene chromosomes of the larval salivary glands. The different types require special hydrochemical conditions in the waters in which they breed (fig. 2.17, page 44). The types are important for the vectoring characteristics of the imagoes
Several Simulium species live phoretically as larvae and pupae on decapod crustaceans, which stay under rocks in rapid flowing surface and underground rivers, the latter sometimes occurring in areas appearing otherwise completely dry. Some phoretic species are vectors of onchocerciasis in Western and Eastern Africa.
Eye lesions caused by onchocercosis are more frequent in the savannah regions of Africa than in the bordering rainy forest regions.
According to the tendency of the parasite to cause eyelesions and the development rate of the microfilariae in the vector flies, there exist corresponding pathogen-vector-complexes in the forest and the savannah regions.
The daily biting rates (DBR) relate to the density of the vector population capable of transmission. The proportion of parous (flies coming to their second blood meal) and nonparous flies allows calculation of the mean life expectancy of the vectors. The number of infestive larvae carried by flies attacking man can be extrapolated over one year to give the annual transmission potential (ATP) (fig. 2.20, page 47).
The degree of endemicity of onchocerciasis reflects the community microfilarial load (CMFL) and is expressed as the geometric mean of the microfilarial density in the skin of all persons more than 20 years old.
Loose clothing covering especially the legs as well as the wearing of sockings and shoes, considerablv reduce the biting rate.
In spite of the strongly changing population densities of the definitive host, (man), and the seasonal and locally extreme changing densities of the vectors, the annual transmission potential is sufficient in all cases to guarantee the maintenance of the parasitosis (fig. 2.19, page 46).
For the control of onchocerciasis, insecticides or biologically derived toxins are used in microincapsulated formulations against the larvae in running waters. The dosage is related to the actual running volume in such a way that 0.05 ppm (parts per million) are reached at the application point during 10 minutes.
The orientation behaviour of the female flies during oviposition is too poorly known to be used effectively for purposes of control.
The visual orientation of the fly to the blood host, man, and the attraction effected by CO2 used in traps equipped with colored clothes, exert an over-optimal success in terms of attraction and may thus be applayed for supervision during the maintenance phase of an control-campaign.
The success of entomological control during such a campaign assessed by the daily biting rate (DBR) is highly sensitive: it is delayed by only one or two days at the breeding sites. From haching experiments, the density of pupae can be estimated. In this way, the recolonisation of the breeding site can be followed exactly and repeated control applications timed. For validation of the success of control, it is necessary to extensively differentiate the Simulium species and of the infestive larvae they carry within any given region.
Assessing success in terms of parasitological control during a campaign by the age grading of adult worms (tegument, excretions as concrements) is less sensitive; only after several years are significant changes noted. Even so, it is highly valid approach and enables one to substantiate long-lasting trends, in order to calculate the timetable for implementing positive action.
Measures for control should lower the rate of blindness in humans equal to other regional causes of blindness.
The seasonal migrations of the Simulium vectors between savannah and the bordering forest regions and their blood-feeding on wild animals and cattle deserve particular attention (fig. 2.18, page 45).