NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
THE CONTROL OF ANIMAL DISEASES
315
The owners of the animals in these understocked highland areas are entirely
unaware of the true causes why their stock do so badly, and they are extremely
reluctant to accept any advice as to management which may be offered by a veteri-
nary officer. Throughout the whole of this type of country there is an established
equilibrium of low stock-concentrations which, when upset and readjusted, tends
more frequently to lower than to higher concentrations. In other words, stock
are dying out in some of these areas, and the already large proportion of the
Territory which is without cattle tends to become yet larger.
Most of the one-ninth part of the Territory that is stocked to saturation car-
ries a vegetation indicative of arid or sub-arid conditions. Some of it is naturally
arid, and comprises open grass land, dry savannah, deciduous thicket, and culti-
vated land. In this kind of country surface-water is scarce, and stock popula-
tions are limited to the numbers which can find a bare livelihood within 15 or 16
miles of water—for to this great distance from water do stock graze in the dry
season, drinking every second or even third day. Some of the land that is now
sub-arid was not always thus ; it has been reduced to this condition by the corro-
sive action of man's activities.
This condition of artificial aridity reveals an apparent biological anomaly
which has been overlooked by most people, although it is of supreme impor-
tance in native animal husbandry.
It has been stated earlier that, for the European, overstocking is altogether
undesirable ; from the native's point of view, however, there is much to be said
in favour of overstocking. Knowing nothing of the true cause of disease he yet
takes advantage of the important fact that, under East African conditions, the
denser a stock population is, the freer is it from disease, so that the healthiest
flocks and herds are the ones living on the border line of starvation in arid and
sub-arid districts.
I refer to this as apparently a biological anomaly, because, the generally accept-
ed view is that overcrowding favours diseas . In animal ecology we think of popu-
lations of rodents or insects going in cycles; so long as the concentration of a species
is low the average individual is healthy, but as numbers increase so disease be-
comes more rife until it culminates in an epizootic which brings the numbers
down to a low level again. So with human beings ; when considering problems
of public health one accepts as almost axiomatic the view that overcrowding-
favours disease. Therefore, it is at first sight surprising that a concentration of
domestic animals high enough to constitute overstocking should be healthy. And
very healthy they often are. They almost starve during some months of every
year and they rarely attain their full potential weight, but so well adapted are they
to these conditions that, like the dwarf xerophytes which form their sustenance
they react rapidly to the better conditions of the months when food is more avail-
able, and during this time they put on flesh and attain breeding condition.
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THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VII, IV
The explanation of the healthiness is to be found by studying the nature of
the most devastating stock-diseases of these parts. They are due to parasites which
pass part of their existence away from their mammalian host. Such parasites
include piroplasms and rickettsia, which are transmitted by ticks, trypanosomes,
transmitted by tsetse and nematodes. Of these transmitters, ticks generally
leave their hosts for moulting and egg-laying, and tsetse are attached to their
hosts only for brief minutes of feeding. Most parasitic nematodes, too, pass
critical stages of their larval life on the ground. The conditions which are most
unfavourable to these disease agents are furnished by bare land exposed to the hot
sun ; conditions which, in Africa, are always associated with aridity. This is
explained why the parts of the Territory which are agriculturally the richest are
frequently extremely unhealthy for stock, but may be made comparatively healthy
by measures which favour erosion and bring about soil aridity even in the presence
of good rainfall.
As the environment gets more and more arid from such a cause as overstock-
ing, the animals appear to become smaller rather than fewer. This point is being
investigated, but already there is evidence to indicate that the seasonal under-
nourishment associated with overstocking tends, through survival of the fittest,
to produce a dwarf race of full fertility, but low size-potential, rather than mere
stunted individuals of low fertility and retaining a comparatively high size-
potential.
This is probably an important point, as combined with the effect on disease pre-
valence, it means that this artificial aridity does not readily provide the seeds of its
own remedy ; the trend is towards a lower and lower equilibrium in which depleted
soil is balanced by a more xerophytic flora and a more dwarf population of domestic
animals. In this way we can reconcile the statement of modern economists that
the great country of the Wasukuma lying south of Lake Victoria is being turned
into desert, with the known fact that Stanley described a scene of obvious over-
stocking in these parts when he passed through them with Emin Pasha fifty years
ago for there is little doubt that in the areas which were then overstocked the equili-
brium between soil, vegetation, and domestic animals was higher than at present.
The country is on its way to desert, as it was then, but only carefully recorded data,
which are non-existent, could show clearly that it is a step nearer to desert now
than it was then.
Naturally the native understands nothing of this. He does know, however,
that he is continually faced with the alternative of exposing his cattle to the risks
of starvation in dry areas, or of exposing them to the risks of such diseases as East
Coast Fever in areas where grass is abundant. One might almost say that this is
the chief problem in the lives of nomadic pastoral people, though with them the
healthy regions are the naturally arid ones of low rainfall.
THE CONTROL OF ANIMAL DISEASES
317
The real problem of overstocking in relation to soil erosion is furnished by the
agriculturalists who are also stock-owners, such as the Wasukuma, to whom re-
ference has been made. These settled in the areas of greater soil fertility and
originally, no doubt, lost a good many stock from enzootic diseases. As the direct
result of their erosive methods of agriculture and animal husbandry they rendered
these areas less fertile but healthier for stock, which consequently increased to,
and are now maintained at, saturation point.
This type of abused country is commonly known as cultivation steppe, and it
exists in patches of varying size in every province. It is associated with a dense
concentration of both natives and stock and with comparatively good water-sup-
plies. Its measure is largely the measure of the badly eroded land of the Territory.
Not only are conditions within areas of this type getting worse, but the areas them-
selves are spreading, like ulcers, wherever surface water permits their peripheries
to be extended to meet the growing need of an increasing population driven out-
wards from centres of ruined fertility.
This analysis of stock-distribution seems to indicate clearly that unaided
native husbandry tends to create the two extremes of under-and over-stocking.
Prior to the European invasion of Tanganyika Territory there were large areas
of overstocked land, and enormously larger areas of cattle-free country. Since
the arrival of Europeans some formerly cattle-free country has become stocked,
but there has been no decrease of overstocking ; rather has this evil considerably
increased.
As the veterinary department is the branch of government which has most
to do with the interests of the stock-owner, and since on our own finding these
interests have in some respects deteriorated through increased over-stocking, it is
natural that the department should be considered as in some measure responsible
for the deterioration. This is a reasonable view which does not, however, justify
the conclusion that disease-control is the main cause of accelerated overstocking.
The most important diseases of stock in Tanganyika Territory are rinderpest,
contagious bovine pleuro-pneumonia, East Coast Fever, trypanosomiases, an-
thrax, contagious caprine pleuro-pneumonia, diseases due to worms, and skin
diseases. There are about five million cattle and about the same number of sheep
and goats, besides donkeys, pigs, dogs, poultry, etc. At the end of 1934 there
were only fourteen veterinary officers in the department, including the Director
and two research officers. Owing to furlough and sickness, there are never more
than three quarters of this staff on duty at one time ; that is never more than ten
veterinarians to deal with the diseases of ten million stock. They would be
rather wonderful men if by their methods of disease-control they were able to
cause overstocking.
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THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VII, IV
It is true that the first aim of the veterinary department is the application of
measures of disease-control to the extent that these are justified economically.
I consider that an effort should be made to suppress completely both rinderpest
and pleuro-pneumonia, and that staff, knowledge, and funds should be available
to allow any other disease to be controlled locally at any time this becomes eco-
nomically desirable.
But up to now the disease-control measures or the department have perforce
been confined to a struggle to keep rinderpest and. contagious bovine pleuro-pneu-
monia within bounds, and to deal with the other diseases when outbreaks assumed
excessive local importance. The only significant way in which these attempts
have influenced mortality returns is in connexion with rinderpest, so for the pur-
pose of this article we can substitute the term rinderpest-control for the wider
term of disease-control, and consider very briefly any effect which such control
has had on overstocking.
Rinderpest is a contagious disease capable of infecting most species of rumi-
nants and a few non-ruminants. In East Africa it affects particularly cattle and,
among game animals, buffalo, eland, giraffe, kudu and wart-hog.
Within historic times the disease made its first appearance in Africa south
of Egypt about 1889, when infected cattle were brought across the Red Sea from
Asia. Once introduced it spread rapidly in the form of the greatest epizootic on
record. As it swept through Somaliland (1889), Masailand (1890), Nyasaland
(1893,) Rhodesia (1896), Transvaal (1896), and Cape Colony (1897), it almost ex-
terminated the cattle and buffaloes in its path. Other game animals also suffered
heavily.
So thorough was the destruction, that the epizootic soon died down for
want of further large groups of animals to infect, and energetic measures by
the governments of all the countries of South Africa succeeded in stamping out the
embers. Thus by about 1905, the disease was completely suppressed in all coun-
tries south of what is now Tanganyika Territory (then German East Africa), nor,
except for a brief incursion into Northern Rhodesia in 1919, has it been allowed
to spread south again since. It is one of the main duties of the veterinary depart-
ment of Tanganyika Territory so to control rinderpest within its bounderies that
there is no danger of the disease spreading to its southern neighbours. Total
suppression is aimed at but is unachievable with a small staff, and each annual
report of the department records the number of outbreaks dealt with and the
number of cattle inoculated. In 1934, there were 44 outbreaks and 130,000
cattle were inoculated.
It must be borne in mind that one attack of rinderpest confers almost lifelong
immunity, and, for reasons which need not be discussed here, the form in which
the disease occurs to-day in East Africa is much milder than the raging epizootic
which worked such havoc in the final decade of last century. It is necessary only
THE CONTROL OF ANIMAL DISEASES
319
to describe what occurs to-day in that part of Tanganyika Territory where the
disease is hardly interfered with by the veterinary department. This part is the
large and important cattle district of Musoma, lying east of Lake Victoria. Here
the disease is always smouldering and sometimes flaring up into outbreaks of
considerable size. In the herds where rinderpest is actually active, every suscep-
tible beast, that is every beast which is not immune from a previous attack, may
become infected. Some of these die, and some recover, and the disease passes
on to other herds, to return again after a period of anything between one and ten
years, usually after four or five years.
The most important aspect of this uncontrolled rinderpest is the mortality.
Whereas the original epizootic on its way through Africa killed about 90 per cent
of the cattle attacked, to-day in Musoma it kills a mere 5 to 40 per cent, depend-
ing on many factors. This means that when rinderpest is uncontrolled in a district
like Musoma, every beast is likely to get it sooner or later, with an expected mor-
tality of 20 per cent.
The normal composition of a Musoma herd of 100 head is 40 cows and 10
adult males, all of which may be immune to rinderpest, and 50 young stock of
both sexes, which may be susceptible to the disease. If rinderpest comes along,
these 50 become infected and 10 may die. But the 40 cows are capable of rearing
20 calves in a year, so that in the absence of mortality from any other cause the
herd could increase even during the year of infection.
Musoma district adjoins Usukuma and resembles it in many ways. Many
parts of Usukuma are stocked to saturation, which means that no further increase
in numbers can take place—if the herd of 100 is not kept down to 100 by disease
and slaughter, starvation will effect the adjustment. But whereas rinderpest
takes toll of young vigorous animals, starvation destroys only the weak and aged.
Both methods of culling are cruel and deplorable, but starvation does the job
better.
Having demonstrated that rinderpest-control has no effect on overstocking,
I must add that the main reasons why such, control is attempted in most over-
stocked areas is not for the local effects, but to prevent the disease spreading from
these areas, both to other countries and to the understocked parts of the same
Territory. In these understocked parts the cattle are so debilitated by enzootic
diseases (vide supra) that an outbreak of rinderpest may almost exterminate the
susceptible animals. Thus a policy of laissez-faire with regard to rinderpest
merely encourages the inevitable trend of native animal husbandry towards its
two extremes of overstocking and complete absence of stock.
I do not say that disease-control never causes or increases overstocking. What
I do say most emphatically is that in Tanganyika Territory it does little or nothing
towards accentuating the bad conditions in the already overstocked areas. In
this Territory, and doubtless in many others, the desideratum of disease-control
is not being purchased with the indispensable soil.
I
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THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [VII, IV
Pax Europa is the main reason why overstocking has increased so much in
East Africa during recent years. Before the settlement of Europeans the history
of much of Africa was ' a tangled skein of secessions, wars, migrations, and exter-
minations ' amid which even the strongest tribes had little chance to accumulate
stock unmolested for long periods. To-day the weakest tribes tend their animals
without fear. Coincident with the first decades of this era of peace was natural
recovery of stock populations from the effects of the greatest epizootic of cattle
in history.
SUMMARY
Soil erosion is due to the disturbance by man or his domestic animals of the
vegetative cover which alone can prevent the shifting of soil by rain and wind.
In many countries with a long annual dry season, overstocking is an import-
ant cause of erosion.
Since adequate disease-control tends to increase stock populations, many
people have suggested that these measures favour overstocking.
These suggestions naturally raise a doubt concerning the economic value of
some phases of veterinary activity in countries where overstocking is rife, and con-
vey a challenge to all veterinary departments.
It would be presumptuous for an individual to reply to this general challenge,
but the author uses it as a basis for a review of the situation in Tanganyika Terri-
tory where overstocking has been prevalent from time immemorial.
The primary cause of overstocking in Tanganyika Territory is the ordinary
desire for wealth acting in communities where wealth is reckoned in terms of stock
and where cattle are held in almost religious esteem. Subsidiary causes are
ignorance and indifference to the future.
A brief review of the place which stock occupy in native agriculture is given,
and the conclusion drawn that it is desirable for natives to possess stock, but de-
plorable that ignorance and indifference should be permitted to turn a laudable
industry into an economic disaster.
With a view to finding some way of improving the stock industry in the face
of many difficulties, the veterinary department of Tanganyika Territory made
a survey of the distribution of stock. One most interesting result was the dis-
covery that, generally speaking, the denser a stock population is, the freer it is
from disease.
The explanation of this is given, and it is shown that the parts of the Territory
which are agriculturally the richest are frequently extremely unhealthy for stock,
but may be made comparatively healthy by measures which favour erosion and
bring about artificial soil aridity. Unaided native husbandry tends to create
the two extremes of under and over-stocking, so that the latter may be considered
an essential feature of unaided native husbandry.
THE CONTROL OF ANIMAL DISEASES
321
In Tanganyika Territory the only form of effective disease-control which can
be carried out by a small veterinary staff is rinderpest-control, and the evidence
is clear that this does little or nothing towards accentuating the bad conditions
of the overstocked areas, since mortality from uncontrolled rinderpest in these
areas is less than the usual average wastage from starvation.
In conclusion it is pointed out that the reason for the increase of overstocking
in Tanganyika Territory during recent years is pax Europa, especially when this
had effect at the time when native herds were recovering naturally from, losses
sustained during the greatest epizootic of cattle in history.
REFERENCES
1. T. Eden, Soil Erosion. Bur. Soil Sci. 1933, Tech. Comm. No. 28.
2. Report of the Committee on Soil Erosion, Sessional Paper III, 1931, Ceylon
Govt. Press.
3. Final Report of the Drought Investigation Commission, Un. South Africa,
1923, Govt. Printers.
4. H. H. Bennett and W. R. Chapline, Soil Erosion a National Menace. U. S.
Dept. Agric. 1928, Circ. No. 33.
5. H. E. Hornby, Overstocking in Tanganyika Territory. East Afr. Agric. J.,
1936, 1,353.
6. Report of the Native Economic Commission, Un. South Africa, 1932, Govt.
Printer, Pretoria.
7. Report of the Kenya Land Commission, 1933, Govt. Printer, Nairobi.
8. R. R. Staples, Usukuma Overstocking Problem. Ann. Rept. (1931) of the
Department of Vet. Sci. and Anim. Husbandry, Tanganyika Territory.
9. H. M. Stanley, In Darkest Africa, Vol. 2 (1890) London : Sampson Low, Marston
& Co. Ltd.
10. C. G. Seligman, Races of Africa. Home Univ. Lib., 1930, Thornton & Butter-
worth, Ltd.
PARASITE REACTIONS TO HOST MODIFICATIONS *
BY
ROBERT HEGNER
Department of Protozoology, The Johns Hopkins School of Hygiene and Public Health
(Reprinted from the Journal of Parasitology, Vol. 23, No. 1, February 1937, pp 1-12)
IT is customary for the president of this society to make his address a dis-
cussion of some problem in which he is particularly interested. Continuing this
custom, I wish to bring to your attention one method of attacking the great pro-
blem of host-parasite specificity. The term specificity as used by us does not refer
to the specific identity of either host or parasite but to the specific or definite in-
fection-relations existing between species of hosts and species of parasites. The
use of the term may best be illustrated by several examples. We have found
that in most cases each species of Giardia flagellate inhabits only one species of
host and does not ordinarily live in other species of hosts in a state of nature.
This we refer to as a rigid host-parasite specificity. On the other hand, the large
ciliate, Balantidium coli, lives as a normal parasite in three species of hosts, namely,
chimpanzees, pigs and man. A condition of this sort we consider to represent
a loose host-parasite specificity in which a species of parasite lives in nature
in several species of host. Or these same relationships may be described from
the standpoint of the host as follows : almost every species of host is inhabited
by its own peculiar species of Giardia; whereas three species of hosts, pigs,
chimpanzees and man, are all inhabited by the same species of Balantidium.
Why one species of host is in general inhabited by a group of parasites that
do not occur in other species of hosts, or why one species of parasite lives in one
or only a few species of hosts instead of in others that seem equally satisfactory
to us, are very intriguing problems. One method of determining the factors in-
volved is to modify hosts by subjecting them to various conditions and to note
the reactions of their natural parasites or of foreign parasites introduced into
them. During the past fifteen years my colleagues, students and myself have car-
ried on various investigations that throw light on these problems. Our investi-
gations have included the use of modifications of a purely laboratory type as
well as alterations that might more or less easily occur in nature. The latter
are of particular interest because we wish especially to know what factors ac-
count for the host-parasite specificity observed under natural conditions.
*Address of the Retiring President, American Society of Parasitologists, December 29,
1936, Atlantic City, New Jersey. Those who took part in the investigations on which this
discussion is based are as follows : Justin Andrews, G. H. Boyd, L. R. Cleveland, M. S. Mac¬
Dougall, Lydia Eskridge, Arnoldo Gabaldon, Robert Hegner, E. C. Nelson, H. L. Ratcliffe,
Eugene Schumaker, K. S. Shah, L. G. Taliaferro, and W. H. Taliaferro.
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