I I 0 Parasitology Today, vol. 5, no. 4, 1989

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Dermatobia Revisited or Tickling Cows with Feather Dusters

Due to illness I have only just read the article on Dermatobia hominis i which refers to our work on cow colour influencing larval density in the hide 2. In sunlit pasture the black cows heat up quicker and seek the shade sooner where the paratenic egg-carrying hosts of D. hominis 3 await. The role of skin temperature gradient as the key factor promoting egg occlusion demanded investigation. A farmer, a Cambridge medical student, a black and a white cow and sensitive skin thermocouples gave us data which have never been published. I would like to comment briefly on other unpublished work. In spite of its economic importance ~ we were not encouraged to continue our studies on Dermatobia.

Our abattoir-derived maps of cow hides show that the greatest concentration of fly maggots in the skin is found on the hump of the zebu cow. Tickling cows with feather dusters (in Brazil you can get a very nice feather duster of rhea feathers) shows that the hump skin cannot be trembled on stimulation like other skin areas. Anatomical dissection of humps shows fat-laced muscle (delicious eating when roasted) but no muscle insertion into the skin such as is seen in platysma muscle in man and almost generally in the cow and horse. The hump is out of reach of tongue licking and tail whisking. It is postulated then that phoretic insect carriers (mosquitoes, flies) have a better chance of discarding first-stage infective larvae on the cow hump since they are left undisturbed in comparison with the rest of the cow surface.

Due to the frequency ofD. hominis in cattle and its relative rarity in man there is an argument for calling it D. boris but in reality its true name must depend on its original host since man and cow are recent intruders into its geographical domain. The original host is still unknown. I have shot a few deer without encountering larvae but no one has made a systematic search, to my knowledge. The people of Mambai, Goi~s, Brazil say that parrots have Dermatobia but this is unconfirmed and an original rodent host is more likely. The domestic host distribution is curious. Cows and dogs are frequently infected, chickens and man rarely, but cats and horses never, lain still tryingto interest a doctorate student in the immunology of D. hominis. I cannot agree with Sancho i that because the maggots are in the superficial layers of the skin this accounts for the total absence of immunity that cows exhibit. The skin is a sensitive, competent immunological organ even in the cow.

Finally, for me, D. hominis raises doubts as to whether Darwin's theory of natural

selection can be universally applied. How can it be biologically advantageous to a mouthless female fly, with limited energy reserves and a short life, to catch other fly species in flight to deposit her eggs? She has 400 to lay in batches of 30-40 on each captured insect and her desperation can be sensed by the haphazard way she lays them on the walls of her restraining glass container after capture in the field. An easier method would be for the female to deposit the egg directly on the cow hide. What would Darwin have thought?

The Scourge on the Land

I read with interest the article byJ.E. Senghor and E.M. Samba,'Onchocerciasis Control Programme- The Human Perspective '~. I would like to comment on two issues that are of personal interest to me.

Samandeni is the first village where longitudinal studies on the disease and its vector were initiated in the late 1950s 2. Although onchocerciasis was highly endemic (causing more than 15% blindness in the total population)the villagers did not wish to move away because there was a bridge and a ranch nearby that provided additional resources. When they moved, in 1964/1966, the villagers did not settle away from the river, as they usually do. They simply crossed the bridge and built a new village at a site as close as their previous village was to one of the scientifically best known Simulium breeding sites in Africa 3. Experimental control of onchocerciasis by vector control started in October 1962 in this area of the black-Volta basin 4. After a long period of interruption, weekly treatments resumed in 1972, before the beginning of the OCP. Since this time (and not since 1979) the village and surrounding area have been entirely protected, as can be seen from entomological and epidemiological data collected by ORSTOM s and OCP.

The authors state that 'the West African rural communities have long recognized an association between the disease, the vectors, and the areas around riverine breeding sites'.

Gertrude Elion and George Hitchings

There seems to be something missing in the article about Nobel prizewinners George Hitchings and Gertrude Elion in the February issue of Parasitology Today I . Five fulsome paragraphs on George and one sentence near the end on Gertrude. What did she do then- make the tea? I'm glad to see that the Nobel committee, if not Len Goodwin, valued her contribution to science sufficiently to award her a prize.

W. Gibson Comparative Pathology Laboratory School of Veterinary Science Langford Bristol BS 18 7DU, UK

P.D. Marsden N~cleo de Medicina Tropical Universidade de Brasilia 70910 Brasilia, DF, Brasil

References I Sancho, E. (1988) Parasitology Today 4,

242-246 2 Marsden, P.D., Shelley, A.J. and Armitage, P.

(1979) Trans. R 5oc. Trap. Med. Hyg. 73, 458-459

3 De Belder, M.A. (I 977) Report of a Cambridge Medical Expedition to Brazil Mim. Doc. pp 121-126

The only evidence we have of rural communities having associated rivers with blindness, comes from some populations of the 'Mossi' ethnic group. They rightly believe that 'the river eats eyes'. It has inspired the title of a film which is available in French, 'Les mangots mangent les yeux', and in More, the language of the Mossi.

As usual in Africa, onchocerciasis and other diseases are attributed to divine curses but also to other causes. In Kankela at the end of the last century, river blindness was connected with the arrival of mangoes in the market place; a story that I hope will soon be written by M. Pangalet, a pioneer of studies of onchocerciasis in West Africa.

Otherwise, I enjoyed reading this journalistic article which relates the great success of the OCP in protecting the poorest of the poor from a real 'scourge on the land'.

R. Le I ~ r r e Special Programme on Research

and Training for Tropical Diseases World Health Organization 121 I Geneva 27 Switzerland

References I Senghor, J.E. and Samba, E.M. (1988) Parasit-

ology T0day 4, 332-333 2 Grenier, P. and Feraud, L. (1960) Bull. Sac.

Pathol. EXot. 53,563-58 I 3 Le Berre, R. (I 966)MEre. Orstom 17, 204 4 Le Berre, R., Ovazza, M. and Juge, E. (1964)

Proceedings of the 12th International Congress on Entomology

5 Philippon, B. (1976) Tray. Doc. Orstom 63,308

Reference I Goodwin, L. (I 989) Parasitology Today 5, 33

Apology

Oh dear. Poor Trudy. I thought that it was clear that she had worked with George from the beginning- planning the synthesis of new compounds, arguing, fighting sometimes- a real partnership. rm very sorry.

L C . ~ l w i n Shepperlands Farm Park Lane, Finchampstead Berkshire RG I I 4QF, UK