parasites

parasites Like many words in the English language, ‘parasite’ has its origins in ancient Greek, where its full meaning was ‘one who eats at the table of another’. With the growth of knowledge of infectious agents, worm infestations, and general microbiology in the eighteenth and nineteenth centuries, the term became eventually identified with organisms, whether of plant or animal origins, living in or upon others (‘host’ organisms), from which they derive nutriment. Bacteria and viruses qualify for inclusion in this definition; but once bacteriology and, later, virology, acquired their own academic disciplines and terminologies from the late nineteenth century onwards, ‘parasite’ in common usage came to refer, more often than not, to the larger and more or less visible organisms preying on human and animal bodies either externally (mites and ticks, fleas and lice) or internally (the large, multicellular helminths — i.e. worms or ‘flukes’ — and the unicellular protozoa).

External parasites as disease vectors

Mites, ticks, and lice have a long history as true parasites, as well as carriers of infections, notably of typhus (the body louse identified as carrier by Charles Nicolle in 1909), of African relapsing fever (carried by a spirochete, Dutton and others in 1904), and of Rocky Mountain spotted fever (carried by the wood tick, as shown by H. T. Ricketts in 1907). The demonstration by Theobald Smith of transmission of the protozoon, Pyrosoma bigeminum, of Texas fever of cattle by the cattle tick Boöphilus bovis, in 1893, was a milestone in the history of the study of disease transmission by ticks and insects. Insects rarely parasitize mammals, having originated in the Palaeozoic with a long period of adaptive evolution long before the arrival of mammals. Far more important is their role as vectors, transmitting some of the most serious tropical diseases such as yellow fever.

Although a nuisance rather than a serious threat to general health, scabies is the best known of the diseases in man which are caused directly by mites. The itch mite acarus (Sarcoptes scabiei var. hominis, a.k.a. Acarus scabiei) is ubiquitous, but outbreaks of scabies vary in frequency, both seasonally and geographically. Historically, it may or may not have been described in Biblical texts, but Hildegard of Bingen certainly referred to itch mites (suren) in her Physika, published in the twelth century. Five centuries later, Cestoni of Leghorn and Giovanni Bonomo famously provided in 1687 the first complete evidence for the causal role played by the mite in scabies in man. Noted by Francesco Redi in the same year, the acarus mite and its biology and pathology were used from then on as model systems for smaller organisms — visible only with the help of microscopes — as responsible for development and spread of ‘contagious’ diseases. First introduced by C. F. Cogrossi in his Pensieri in 1713, this use of scabies and its pathogenesis as a favourite paradigm for the cause and spread of infections was to last well into the nineteenth century.

Anthony van Leeuwenhoek, using one of his own early microscopes, had found the ciliated protozoon Giardia lamblia in his own stools during an attack of diarrhoea in 1681. Today giardiasis is an increasing, if relatively innocuous threat to local populations and travellers in the tropics and the Western world.

Helminths

Worms are more easily recognized, even with the naked eye, in bodily fluids and waste products, and have been associated with disease in man and his domestic animals since the seventeenth century. ‘Worms’ and ‘insects’ were used as convenient synonyms for what were then unknown agents of ‘contagion’ of any kind, even when no such outward signs were observed. Scrutiny of the Ebers papyrus (1550 bc), and evidence from tissue samples of Egyptian mummies, have suggested that infections caused by the common roundworm (Ascaris lumbricoides), tapeworms, the guinea worm (Dracunculus medinensis), and the African schistosome (S. haematobium) were present in Egypt long before. More recent results also point to the presence of Trichinella spiralis, the nematode worm of global distribution whose pathology may have been responsible for the Jewish kosher taboo on the eating of pork products. In our own century, trichinosis has been prevalent in Germany and Eastern Europe — and in settlers from those countries on the North American continent — because of habits of using undercooked meat in sausages etc.

Another nematode worm, the humble hookworm, played a major role in the formation of the public health policies of disease control and prevention, which began in Europe and the US at the turn of the century, and were to grow into campaigns with worldwide concerns and consequences throughout the twentieth century. Hookworm disease in man (Ancylostoma duodenale and Necator americanus) had long been considered characteristic of tropical latitudes without any understanding of its epidemiology, until an outbreak among workers on the StGotthardt Tunnel in 1880 forced the Italian medical authorities to review the situation. In spite of work by G. B. Grassi and E. Perroncito, little progress was made until Arthur Looss's discovery, at the turn of the century, of the ability of infective hookworm larvae to penetrate intact skin. This opened the way to introduction of control measures, at first tested in the early years of the twentieth century in mines in Belgium: the use of sanitary buckets, regular testing and treatment of infected miners, and instruction in personal prophylaxis. By then it had also been noted by Perroncito that salt was toxic to hookworm larvae, and that miners working in areas with a high salt content, in Poland, France, and Cornwall, showed resistance to infection. Scattering of salt around mines was recommended, but implementation was slow, and only became standard practice in South African gold mines more than twenty years later. Meanwhile, John D. Rockefeller had been persuaded to launch his philanthropic work by giving financial support to the Sanitary Commission's Hookworm Eradication Campaign, envisaged by C. W. Stiles, in the southern US, where the debilitating effects of hookworm disease had long been a threat to poor working populations. Although full eradication was never achieved, a satisfactory measure of control was reached after four years.

After that, the Rockefeller Foundation set its sights on wider targets: it was the beginning of the Foundation's concerns with the improvement of public health on a global scale. As a by-product, it also paved the way for Rockefeller support for expansion of Patrick Manson's London School of Tropical Medicine into a full-scale school of public health concerned with temperate as well as tropical localities.

Among helminths classed as trematodes, the schistosomes occupy an important position as agents of the disease formerly called bilharzia (after Theodor Bilharz), now professionally referred to as schistosomiasis. The worms (vesical blood flukes) of the disease develop through stages in intermediate hosts via life-cycles as complex as those of many protozoa. Different species of schistosomes use different species of water snails as intermediate hosts. The cercariae emerge from their snail host into the water (river, rice field, etc.), and from there penetrate the skin of persons unlucky enough to swim or wade in contaminated water. If the disease remains undiagnosed and untreated, what initially appears in patients as merely vague feelings of being ‘below par’, leads at best to years of reasonably well tolerated infection with occasional acute episodes of decreased working capacity, at worst to liver failure or involvement of the central nervous system.

Parasitic zoonoses

Other parasitic zoonoses (parasites involving animals as primary or intermediate hosts of disease in man), rare but closer to home, include infestation with tapeworms of dogs and cats transmitted by the human flea Pulex irritans, and with Toxocara canis, sometimes transmitted via dog faeces to children, either playing in public parks or otherwise in contact with puppies. Unhappily this parasite can cause serious disease, including impairment — at worst, total loss — of sight. Such tragic consequences of local nematode infections are rare in the West; but in parts of Africa, Onchocerca volvulus, transmitted by the blackfly Simulium damnosum, continues to cause thousands of cases a year of ‘river blindness’ in Kenya's ‘Valley of the Blind’.

Among parasites causing diseases regarded as largely tropical, three have been subjects of intense research from the beginning of the twentieth century: the plasmodia of malaria; the trypanosomes of African sleeping sickness (Trypanosoma gambiense and rhodesiense); and that of Chagas' disease in South America, T. cruzi. Yet they were not described, let alone linked to the serious diseases they cause, until the late nineteenth and early twentieth centuries.

Malaria (Italian: mal aria, ‘bad air’, named at a time when miasma theories of contagion prevailed) has a long and colourful history, which, in spite of real advances in understanding of its aetiology, and past and present hopes of the therapies, and lately of effective vaccines, still shows no firm promise of nearing an imminent conclusion. Malaria is caused by infection with one of four species of Plasmodium (P. falciparum, P. vivax, P. malariae, P. ovale) and transmitted by female anopheline mosquitoes, again of different species; the life cycle of malaria parasites is complex and the disease caused by P. falciparum is the most severe — in recent years increasingly so. It is now eradicated in Europe: it disappeared from northern latitudes early in this century, and a determined campaign saw it finally eradicated in Italy after World War II. There were high hopes of eradication in Africa, India, and the Far East following the discovery of DDT and the synthesis of new anti-malarial compounds. These hopes were dashed when the mosquitoes developed resistance to DDT, and the Plasmodium parasites to new synthetic drugs, in step with their development. Additional problems include serious side-effects caused by synthetic quinine substitutes. Malaria ranks second only to the diarrhoeal and respiratory diseases in terms of global morbidity and mortality.

African sleeping sickness, like a number of other tropical diseases caused by protozoa, is almost invariably fatal unless promptly treated with drugs, which were introduced shortly after the trypanosomes were discovered in the first decade of the twentieth century. Transmitted by tsetse flies, in which the trypanosomes of African sleeping sickness develop, they enter their human host by the bite of the fly. The American form of trypanosomiasis, (Chagas' disease), is transmitted through the faeces of insects of the family Reduviidae (‘assassin bugs’) often infesting walls in dwellings in poor and remote areas of South America, making attempts at eradication difficult.

Humans and parasites have coexisted uneasily for centuries; only in the latter half of the twentieth century, following the end of hostilities after World War II, did hope burgeon for well-organized scientific control. Supported by the WHO and various private foundations, this might at last win the battle with parasites in tropical and temperate zones worldwide. The best hope lies in vaccine development. Advances in this area, especially against malaria and schistosomiasis, are not yet conclusive, but give cause for more optimism than before.

Lise Wilkinson

Bibliography

Donaldson, R. J. (ed.), (1979). Parasites and Western man. MTP Press Ltd. Lancaster.
Warren, K. S. and Bowers, J. Z. (ed.), (1983). Parasitology. A global perspective. Springer-Verlag, New York etc.
Englund, P. T. and Sher, A. (ed.), (1998). The biology of parasitism. A molecular and immunological approach. Alan R. Liss, Inc., New York.