COMMENT
The migration of humans and cattle from the Old World to the New World during the nineteenth century led to the discovery of new cattle diseases. Texas fever, East Coast fever, tristeza and redwater fever, as they used to be called, became household words and the terror of cattle producers in the tropical and subtropical areas of Africa, Australia and North and South America. These tick-borne diseases have been a major constraint on efforts made to improve the cattle industries on several continents over the last 100 years. The losses that resulted from these diseases led to the ruin of many cattle owners, but they also stimulated pioneering research. For the first time in medical history, the transmission of an arthropod vector-borne disease - babesiosis - was described. This pioneering observation made at the turn of the century by the American scientists Smith and Kilbourne opened up a new field of endeavour in both veterinary and human medicine, which led to a better understanding of many other vector-borne diseases of animals and humans, such as malaria, and the introduction of therapeutic and preventive methods for their control.
Methods of tick-borne disease control that were established early in this century are still applied today, and these continue to determine the success of livestock exploitation and the economic prosperity of cattle industries on the different continents.
Tick (vector) eradication has been the method of choice in the Americas. This has been successful for Boophilus spp. in the United States and in areas of Argentina, but less so in Mexico. Eradication has been attempted in other areas and countries as well, with varying degrees of success. On other continents, success of tick eradication has been determined by the degree of isolation of the infested area and by the ease of prevention of reinfestation by the tick concerned. Amblyomma variegatum has been eradicated from islands such as Puerto Rico, Culebra, Vieques and St Croix in the Caribbean, while Boophilus microplus was eradicated successfully from Magnetic Island but not from northern New South Wales State in Australia. Eradication has led to the development of economically viable cattle industries in the areas and countries involved, free of the constraints imposed by ticks and associated diseases. However, freedom from ticks has also resulted in a situation in which cattle within an eradicated area have become tick-naive, and therefore fully susceptible to tick-borne diseases should they be introduced. The cost of maintaining the tick-free status of an area varies, depending largely on the extent of the shared zone between tick-free and tick-infested areas.
A policy for tick control and eradication has also been implemented in Africa with the objective of preventing losses resulting from tick-borne diseases, particularly East Coast fever (ECF). This policy apparently has been successful in eradicating ECF from a large area of southern Africa. The eradication of ticks, however, has not been possible because of the availability of many alternative hosts for ticks; therefore, vector ticks have survived in all countries. In those countries in East and Central Africa from which ECF has not been eradicated, tick-borne diseases have been, and continue to be, contained through the close-interval dipping of cattle in acaricide once a week or, at times, even twice a week. This method, though expensive in terms of labour input and the cost of acaricide, effectively controls all tick-borne diseases in the area, maintaining a tick-naive population of cattle fully susceptible to the five or more tick-borne diseases endemic to the area. Any break in the control programme, whether caused by a lack of water or acaricide or by natural or human failure, can result in serious cattle mortality from tick-borne diseases. To illustrate this point, the breakdown of the dipping service during the war of independence in Zimbabwe resulted in the loss of one million head of cattle mainly to tick-borne diseases. This control method provides all the advantages of tick eradication; however, it is at the expense of recurrent costs of close-interval tick control to contain tick-borne diseases. In addition to the tick-naive cattle population and the recurrent costs, other problems of potential acaricide residues, environmental contamination with acaricides and tick resistance to acaricides must be considered. Needless to say, this method has only been successful in allowing the further development of profitable cattle enterprises, where it is possible to maintain the necessary strict discipline of the veterinary services.
In Australia, initial attempts to eradicate ticks and tick-borne diseases, such as babesiosis, were unsuccessful; however, control of the diseases is currently being achieved through immunization. The tick has been recognized as a parasite in its own right and is being controlled as such. Tick control, though less intense than that practiced in East Africa, has led to serious problems with the emergence of tick populations resistant to several types of acaricides. The fuller use of natural resistance to tick infestation of certain cattle breeds and their crosses, together with a reduced strategic/threshold regimen for tick control, has allowed the development of economically sustainable cattle production in the tropics that is almost free from the threat of tick-borne diseases. This method is more or less a compromise between eradication, as practiced in the Americas, and close-interval tick control, as practiced in East Africa.
It must be appreciated that in large areas of the tropical world indigenous breeds of cattle have adapted to the ticks and tick-borne diseases existing in their environments. It is only when tick-naive cattle or cattle susceptible to the diseases are imported to these areas that ticks and tick-borne diseases become a problem.
The changing realities of the modern world, particularly those resulting from a much more demanding economic situation and a more complex environmental appreciation, demand that the options for the control of these important cattle parasites be reassessed. Except in special isolated circumstances, such as the eradication of the tropical bont tick from the Caribbean, it is probably too late for the widespread eradication of ticks using acaricides to be economically or environmentally acceptable. Similarly, close-interval tick control is increasingly difficult to justify on economic or environmental grounds.
Continuing research has resulted in:
· an appreciation of the adaptive characteristics of indigenous breeds of cattle in tropical environments, for example, resistance to ticks;· a better understanding of tick ecology and population dynamics, the economics of tick control, and the epidemiology of different tick-borne diseases, particularly the place of enzootic stability as a factor to be exploited in the control of tick-borne disease;
· the development of improved, though not yet perfect, living vaccines against tick-borne diseases.
These and other achievements have contributed to a situation in which governments and cattle owners can develop new strategies for coexisting with ticks and tick-borne diseases. These are likely to involve the control of tick-borne diseases through immunization and the control of ticks as parasites in order to reduce their economic impact. The latter should involve reduced dependence on acaricide application and full exploitation of natural resistance of cattle to ticks.
The future holds great hopes for the development of improved subunit vaccines, not only for tick-borne diseases such as babesiosis and East Coast fever, but also for specific ticks, including B. microplus. The introduction of such new products should allow a further fine-tuning of the modified strategies proposed above.
P.J. McCosker
The author is Senior Officer, Infectious and Parasitic Diseases Group, Animal Production and Health Division, FAO, Rome, Italy.
