INTRODUCTION
This chapter describes a number of factors likely to have an impact in the future on the incidence and significance of animal diseases for all livestock farmers, particularly the poor, who are most vulnerable to the ravages of these diseases, and factors likely to impinge on communal and national abilities to counter livestock diseases.
During the 1970s there was hope that the major epidemic diseases of livestock and humans were being brought under control in many countries and practically eliminated from Organisation for Economic Co-operation and Development (OECD) countries. Most predictions emphasized the increasing importance of endemic and productivity-limiting non-infectious diseases with a concomitant reduction in the relevance of epidemic diseases to livestock production. With increasing intensive livestock farming in the industrialized world, it was believed that at worst infectious diseases could be confined to the least developed parts of the world and therefore would have little impact on development, food security and trade.
During the last 15 years, however, infectious and vector-borne animal diseases have become increasingly important worldwide and disease emergencies are occurring with increasing frequency. Even industrialized nations have been affected. In 1997, the World Health Organization (WHO) observed, of human health:
“Experience has shown that reducing resources to control infectious diseases in favour of other priorities leads to the resurgence of disease and can create problems more widespread and costly than before.”
This is equally true of animal health, as is borne out by recent examples of outbreaks of old diseases, newly recognized diseases and re-emerging or evolving diseases. Some examples are summarized in Box 9.
IMPACT OF STRUCTURAL ADJUSTMENT PROGRAMMES
The collective geographical location of livestock diseases of major economic importance - FMD, rinderpest, contagious bovine pleuropneumonia (CBPP), CSF, ASF, sheep and goat pox, trypanosomosis, tick-borne diseases, ND and probably infectious bursal disease (IBD) - extends from Africa across the Near East and into Asia, encompassing many of the poorer countries of the world. Sustained control of these diseases requires:
Most countries across this sector of the globe do not have the resources to support all of those elements, so they resort to strategic approaches. The national veterinary services in developing countries have, like other departments, to compete for scarce resources; unfortunately they are often politically weak and fare badly when the cake is cut up. Furthermore, economic structural adjustment programmes have tended in several cases to weaken the administrative, legal and financial capacity for dealing with major animal diseases. Progress in the control of animal diseases in many developing countries has consequently become a tediously slow and unpredictable business.
In the immediate postcolonial period of the 1960s, the public-sector veterinary services of most developing countries were engaged in delivery of the full range of veterinary activities and services with little or no participation by the private sector. By the mid-1970s, many countries were experiencing serious economic difficulties and starting to seek financial remedial assistance. It was felt that the rescue lay in structural adjustment of their economies. Changes in fiscal, financial and pricing policy included elimination of subsidies and removal of tariffs; institutional reforms included privatization of government-owned enterprises and the introduction of cost recovery. It was argued that in seeking to move services from public to private sectors, in most domains any form of private enterprise is likely to outperform the public sector. This led to a drive for the privatization of veterinary services, with the aim of diminishing drastically the role of the state in these activities. Animal health was seen as a private good and veterinary services were seen essentially as providing an animal healthcare delivery system. The sale of veterinary medicines and vaccines, provision of clinical services or vaccinations were thus uppermost in implementing the privatization programme. Surveillance, early warning, laboratory diagnostic services, planning, regulation and management of disease-control programmes and assurance of the quality and safety of animal products became secondary considerations. The concepts of control of epidemic and trade-related diseases and the international obligation to manage and report on these diseases were lost. As a result of restructuring and decentralization, government veterinary officers were often placed under the control of regional and local authorities within a general agricultural extension system. The chain of veterinary command requiring notification of disease outbreaks to respond to disease emergencies and manage national disease-control programmes was often effectively dismantled.
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BOX 9 Examples of recent epidemics of transboundary animal diseases (TADs) Rinderpest is perhaps the most serious cattle plague. The optimism of the 1970s was shattered when during the 1980s rinderpest spread throughout south Asia, the Near East and tropical Africa, affecting cattle, buffalo and wildlife. The disease has come under control again, thanks to an international partnership through the Global Rinderpest Eradication Programme (GREP). It is currently confined to three isolated ecosystems: southern Somalia, southern Sudan and parts of southern Pakistan. The success of GREP will depend on whether rinderpest can be eliminated from these foci before the end of 2003, otherwise there remains a risk that it could flare up again as it did in the 1980s. FMD is a highly contagious virus disease of cloven-hoofed animals. There are seven distinct types of FMD virus. It is the animal disease with the greatest impact on international trade. The OECD countries are normally free from this disease, but it is endemic in less-developed countries (LDCs). The endemic distribution of the seven types of FMD is broadly as follows: Type O: Asia, Africa, the Near East and South America; Type A: Asia, Africa, the Near East and South America; Type C: Asia, Africa, South America (this type occurs rarely and tends to be sporadic); Type Asia 1: Asia; Types SAT 1, SAT 2 and SAT 3: Africa. In recent years, serious epidemics of FMD have occurred outside areas of endemicity, causing major economic losses. Examples are: Type O in Taiwan province of China in 1997 and again in 2000; Type O pan-Asian topotype, which over 10 years spread eastwards from south Asia to China, Japan, South Korea, Viet Nam, Cambodia and Taiwan Province of China and westwards to the Near East and south-east Europe; during 2000-2001, it leapt to South Africa, the United Kingdom, France, the Netherlands and Ireland. Type SAT 2 spread to Saudi Arabia in 2000, which is the first time this type has been recorded outside Africa. PPR was until recently considered to be limited to West Africa. It is now, however, the most evolving epidemic of small ruminants. It has extended throughout sub-Saharan Africa from Mauritania to Somalia and southwards to the coastal belt of the Congo Republic in the west and Sudan, Ethiopia and Somalia in the east. In the Near East there have been serious epidemics in Jordan, Saudi Arabia and Iraq and now PPR has extended as far west as Turkey, which borders Europe; it now extends as far eastwards as Bangladesh. It appears that there has been an actual extension of its range as well as increasing aetiological differentiation between PPR and other causes of pneumonic disease in sheep and goats. In India, many cases in sheep formerly ascribed to rinderpest are now known to have been caused by PPR. It has been responsible for heavy losses in small ruminants in Nepal, Pakistan, India and Bangladesh. CBPP is a serious mycoplasmal disease of cattle. There has been a catastrophic spread of CBPP over the last few years in Africa, where it now affects 27 countries and causes losses estimated at US$2 billion annually. In 1995, the disease was reintroduced to Botswana for the first time in 46 years. As part of the eradication campaign, all cattle (approximately 320 000) in an area of northern Botswana had to be slaughtered at a direct cost of US$100 million; indirect losses were over US$400 million. CSF is a generalized virus disease affecting only pigs. It is endemic throughout many of the swine-rearing areas of the world. It is a major and constant constraint to swine production in the countries of eastern and southeast Asia. It has been endemic in some Latin American countries and Cuba since the 1980s. In 1996 it was introduced into Haiti, causing major losses, and is now endemic there. It has spread to the Dominican Republic. In 1998, outbreaks were reported in Costa Rica. CSF is a disease that poses a serious threat to the swine industry of the Americas. It is practically absent from most of Northern America, so the recent epidemic in the Caribbean is seen as a serious threat to North America as well as non-infected Caribbean countries. In Europe, the most serious recent epidemics have been in Germany, the Netherlands, Spain and the United Kingdom. Molecular genetic studies indicated that the causal virus strain was more related to those isolated from southeast Asia than those circulating in wild suidae in Europe. ASF is another generalized virus disease affecting pigs. It is endemic in southern and eastern Africa, where it is maintained in an endemic cycle involving soft ticks (Ornithodorus porcinus) and wild suidae (warthogs and bushpigs). Since the mid-1990s, there have been serious outbreaks in areas which either had never experienced ASF before or had not had outbreaks for a long time. In 1994, for example, ASF moved from the endemic area in northern Mozambique to Maputo and devastated the pig population, killing 80 per cent of the estimated 4 000 pigs in the area. In 1996 it occurred for the first time in Côte d’Ivoire, where it killed 25 percent of the pig population and, according to various estimates, cost the country between US$13 million and US$32 million in direct and indirect losses and eradication costs. There has since been serious spread of ASF to Togo, Benin, Gambia and Nigeria. In 1999 the disease spread to Ghana, where it has since been eradicated. ND is one of the most important viral diseases of poultry. The history of ND is marked by at least three pandemics in domestic birds. The first began with the emergence of the disease in fowl in the middle of the 1920s and spread slowly from Asia throughout the world. The second outbreak appeared to emerge in fowl in the Near East in the late 1960s, reaching all continents by the mid-1970s. A third outbreak in the 1970s, also starting in the Near East, was associated with a mainly neurotropic and viscerotropic velogenic disease in pigeons. We are currently witnessing the fourth panzootic. Since 1991, there has been an increase in incidence with a series of related outbreaks affecting poultry in many European countries. Iran, India and southeast Asia were hit by the worst epidemic ever reported. In 1999, the panzootic reached the American continent and Australia. ND is regarded as endemic or epidemic almost all over the world. IBD/Gumboro emerged in 1957 as a clinical entity responsible for acute morbidity and mortality in broilers in the United States of America. The disease has now been reported in most parts of the world and is widespread in commercial chickens as well as scavenging chickens. IBD is caused by infectious bursal disease virus (IBDV). Recently, IBDV isolates were described in the United States of America and Europe displaying an antigenic drift. These new “hot” isolates are very virulent for chickens. The disease has an acute stage followed by immunosuppression, resulting in lowered resistance to a variety of infectious agents and poor response to commonly used vaccines. The acute stage of the disease and the immunosuppression that follows are major factors contributing to its economic significance. |
The combination of poor financial resources and an inadequately organized national veterinary service has often led to deterioration in animal-health services, with epidemic diseases frequently spreading unchecked. The resurgence and unchecked spread of CBPP in many parts of Africa can often be related to the breakdown of national veterinary services. Control of ticks and tick-borne diseases has deteriorated, in many cases along with provision of healthcare to pastoral communities. Privatization has, however, improved availability of veterinary drugs and vaccines for peri-urban farming communities who could reasonably afford the cost of private service. There are examples where involvement of the private sector has actually improved control of epidemic diseases. The most notable is the case of countries of the Mercosur of South America. Here, the private farming and trading sectors became involved in the planning and monitoring of disease-control programmes, exerting pressure on governments to the extent that the efficiency of public-sector supervision and regulation actually improved. As a result, South America has made great strides in FMD control and government services have been able to react resolutely to disease emergencies. Another example is provided by the Indian National Dairy Development Corporation’s involvement in FMD control on members’ farms.
There is an increasing realization today that structural adjustment programmes for sector reforms have not consistently resulted in adequate provision by the private sector and civil society of essential services and markets once provided by the state. The reasons are complex, but the result is that the great majority of the rural poor do not yet enjoy access to the range and quality of services and markets that they need to support a robust livestock-related livelihood. There is increasing realization that a balance needs to be struck between developing a robust private veterinary sector, providing animal healthcare services to the vulnerable poor groups and securing a responsible and effective public-sector regulatory service for aspects of animal health that affect the public.
THE IMPACT OF POLITICAL AND SOCIAL INSTABILITY ON ANIMAL HEALTH
When political upheaval leads to conflict, the consequences for disease-control programmes can be catastrophic. For example, following the Gulf War (1990) and the military offensive by the Iraq government forces against the rebellious Kurds in the north, there was mass migration into Turkey. The refugees took as many of their animals as possible with them and in doing so introduced rinderpest into Turkey’s susceptible livestock population. Turkish farmers in the southeast, rushing to dispose of their sick animals as quickly as possible, spread the disease through the marketing chain to Ankara and as far west as the Sea of Marmara. Similarly, the 1994 upheaval in Rwanda was followed by sudden and heavy migrations of people and livestock. This was followed by widespread outbreaks of FMD and CBPP in southern Uganda, Rwanda, northwestern Tanzania and eastern Congo. In Somalia and southern Sudan, the conflicts have been hindering the progress of GREP, because vaccinators have been denied access to livestock, kidnapped for ransom or robbed and killed. As a consequence, the control campaigns have been severely disrupted. The continued occurrence of rinderpest in these countries is a matter of great concern both for the countries themselves and for the region, especially disease-free neighbouring countries, which are ceasing vaccination so that they can move along the Office International des Epizooties (OIE) pathway and achieve the goal of freedom from infection.
There are strong associations between political upheaval, civil strife and increased incidence of disease. Predicting conflicts is not easy, but judged by the experience of Africa and Asia the trend seems to be in an upward direction. This does not bode well for the animal-disease situation in affected countries or in neighbouring countries. Attending to animal health through professionally guided community-based programmes will need to be an increasing component of humanitarian programmes in conflict-affected areas in order to avoid major epizootics.
EFFECT OF CLIMATIC CHANGE ON ANIMAL DISEASES
Climatic factors can have a major effect on the rate of transmission of many infectious diseases. Microbial agents and their vector organisms are sensitive to factors such as temperature, humidity, precipitation, surface water, wind and changes in vegetation. This applies particularly to vector-borne diseases (VBDs) such as RVF, transmitted by mosquitoes, African horse sickness (AHS) and BT, both transmitted by biting midges (Culicoides spp), ASF, East Coast fever, anaplasmosis, babesiosis and Nairobi sheep disease, both transmitted by ticks, and trypanosomosis, transmitted by tsetse flies. Lumpy skin disease has long been suspected of being transmitted by arthropod vectors. In general, increased temperature and moisture will enhance transmission. It is projected, therefore, that climate changes and altered weather patterns will affect the range, intensity, and seasonality of vector-borne and other infectious diseases.
Considerable progress has been made in investigating and defining the climatic and environmental factors that influence vector biology. The data has generally been obtained by a combination of field and laboratory studies. These approaches, combined with satellite remote sensing, geographical information systems and biomathematical modelling, could be used to develop models to predict when and where disease outbreaks are likely to occur and how the situation might alter with climate change. Armed with this information, control strategies, such as the use of prophylactic vaccination and vector control, could be used to protect animals in advance of the spread of a disease and thereby reduce its impact.
Successful attempts have been made, for example, to model the abundance and distribution in southern Africa of Culicoides imicola, the vector midge of AHS and BT viruses. The abundance of C. imicola and associated climate data have been analysed in combination with satellite-derived variables with the aim of developing models of C. imicola abundance to predict risks of AHS and BT. Similarly, for the 1997/98 RVF in eastern Africa, an examination of the satellite remote sensing images could readily identify areas for intensive ground surveillance for RVF and other VBDs.
Some VBDs are zoonotic diseases and cause serious illness and death in humans. Climate change is likely to increase the prevalence and incidence of such diseases, either geographically or from seasonal to year-round. Global warming and resulting rising sea levels, for example, would displace some human populations, perhaps resulting in migration into wilderness areas where zoonotic infectious agents are being transmitted in silent life cycles.
It is predicted that global warming will be characterized by more frequent storms and flooding in certain areas. Higher temperatures, increased humidity and more extensive surface water might result in increased insect populations and higher incidence of VBD. On the other hand, periods of drought will cause extensive migration of pastoral herds in search of water and grazing and favour the spread of disease by vectors and by contact between animals. These conditions would increase the likelihood of livestock mingling with wildlife populations and the transmission of pathogens. Support for these predictions is provided by the strong association shown between the major epidemics of AHS in South Africa, which occur every 10 to 15 years, and the warm El Niño phase of the El Niño southern oscillation (ENSO), which is mediated by the combination of rainfall and drought brought to South Africa by ENSO. Warm-phase ENSOs bring both rainfall and drought to southern Africa. Populations of C. imicola can increase 200-fold in years of heavy rain. Although heavy rainfall occurs for other reasons in many non-ENSO years, epidemics of AHS do not result; it appears therefore that a combination of heavy rainfall followed by drought is the critical combination that leads to epidemics. It has been proposed that this is because the high temperatures during droughts increase vector population growth rates; the coincidence of this with the congregation of horses with the virus reservoir (zebra) at the few remaining sources of water creates the conditions favourable for the vector to transmit AHS virus.
It follows that climate change, in particular global warming, will increase the incidence of animal diseases, especially VBDs, in regions where they are endemic. Since these are mainly the tropical regions, where there is the highest concentration of developing countries, this will impact on the livelihoods of many poor farmers. It is probable that climate change will extend the geographical distribution of many insects that act as vectors. It has been estimated that a 1°C rise in temperature will correspond to 90 km of latitude and 150 m of altitude. In this context it would be interesting to know what factors led to the introduction of bluetongue (BT) into Bulgaria and Greece in 1998 and into Italy, Spain and France in 2000, as these outbreaks occurred significantly north of 40° latitude in areas where C. imicola has previously been shown to be absent.
THE IMPACT OF ANIMAL MOVEMENT AND TRADE ON DISEASE INCIDENCE
In developing countries, movement of livestock is common in order to find grazing and water, move away from drought or follow natural seasonal migrations, or because of migrations precipitated by social tensions or local trade. Such movements inevitably bring livestock from different groups into contact. Where there are heavy concentrations of wildlife animals, the migrations of wildlife and livestock bring the two groups into contact. Inevitably, such contacts are a source of disease dissemination. The introduction of CBPP in the early 1990s into Botswana and Tanzania was due to movement of only a few subclinically infected animals from endemic areas. FMD, rinderpest, sheep pox, PPR, ND, IBD and others have been disseminated through such movements.
Increased road construction across Central and South America, Africa and Asia, aimed primarily at responding to expanding industrial needs, has made it easier and cheaper to transport animals over long distances on land. Similarly, the growth of sea- and air-freight systems facilitates the transport of animals around the world. The most common mechanism for the transmission of infectious organisms is contact between infected and susceptible hosts. Modern animal transport systems are ideally suited for spreading disease. The animals commonly originate from different herds or flocks and they are confined together for long periods in a poorly ventilated stressful environment, all of which will favour the transmission within the group of infectious disease should sick animals be present. If not destined for slaughter, the animals will be introduced into new herds or flocks, where they will be subject to social and dietary stress and an exchange of microorganisms with the resident population.
A spectacular intercontinental trade transfer of a pest was exemplified by the New World screwworm (Cochliomyia hominivorax) in Libya in 1988: for the first time, this pest became established outside its natural range in the Americas. Recent years have seen some spectacular examples of the consequences of extended trade links. The outbreaks of FMD type SAT 2 in dairy herds in Saudi Arabia and in sheep in Kuwait during 2000 probably resulted from the importation into the Arabian Peninsula of cattle or sheep from eastern Africa. While FMD type SAT 2 virus is endemic in many parts of Africa, this was the first occasion that the SAT 2 serotype had been recorded outside Africa. Another instance of long-distance spread of FMD occurred in 1999 in North Africa. In February 1999, the disease was reported in Algeria in cattle. It spread quickly there, then crossed the border into Tunisia and Morocco. Sequencing of the virus at the World Reference Laboratory (WRL) for FMD, Pirbright showed that it was closely related to strains of virus isolated previously in Côte d’Ivoire. The Algerian veterinary authorities reported that they had seized cattle illegally imported from Mali before the start of the epidemic and they suspected that these animals were the origin of the outbreaks. It was considered that heavy rainfall across the Sahara had created favourable circumstances for long-distance transport of cattle across Mali and the link with Côte d’Ivoire.
The movement of infected animals is the most common mechanism by which infectious diseases such as FMD and ND are transmitted. Spread can also result from feeding animals with contaminated foodstuffs such as hay and contaminated unheated waste food of animal origin. The transport of contaminated meat and fodder around the world is a mechanism by which FMD can be spread over long distances and by which exotic strains can be introduced into new territories. Advances in molecular biology, in particular reverse-transcription polymerase chain reaction (RT-PCR) sequencing, have proved to be extremely valuable for characterizing isolates of virus and identifying their probable origin. The application of such techniques to FMD is probably the most advanced. Sequence analysis of the 1996 Albania type A virus, for example, showed that it was very closely related to isolates submitted previously to the WRL for FMD from Saudi Arabia and India. A consignment of buffalo carcass meat imported from India was found by a European Union mission visiting Albania during the epidemic and that provided further evidence of a link between the two countries.
The most dramatic example, however, is the spread of FMD serotype O that is now referred to as type O Pan-Asian topotype, which over a ten-year period has spread through most of Asia and affected parts of Europe and South Africa. This virus was first identified in northern India in 1990. It spread westwards into Saudi Arabia during 1994 and throughout the Near East and into Europe (Turkish Thrace, Bulgaria and Greece) in 1996. In 1993 it was found in Nepal, in 1996 in Bangladesh and in 1998 in Bhutan. In 1999, it was reported from mainland China (Tibet, Fujian and Hainan) and then detected in Taiwan Province of China. In late 1999 and in 2000 it reached most of southeast Asia. Most recently it has been introduced into the Republic of Korea, Japan, the Primorsky Territory of the Russian Federation and Mongolia, areas free from FMD since 1934, 1908, 1964 and 1973 respectively. The total cost of the outbreaks in Japan alone has been calculated at US$77 million. The virus has been isolated from a wide variety of host species - cattle, water buffaloes, pigs, sheep, goats, camels, deer and antelope.
In September 2000, the Pan-Asian O topotype FMDV was identified on a pig farm in Kwa Zulu, South Africa. It is believed that infection was introduced there through swill collected from a ship originating from southern Asia.
In February 2001, the same strain was identified in the United Kingdom, again probably introduced through swill feeding. This virus has now (early April 2001) already resulted in over 1 000 outbreaks in the UK and a small numbers of outbreaks in Ireland, France and the Netherlands.
In Europe, the spread of the virus from its original focus in the United Kingdom to Europe was through movements associated with normal trade. It is feared that the cost to the British economy alone caused by the Pan-Asian O topotype FMDV outbreak could reach £40 billion.
EFFECTS OF CHANGES IN DISEASE-CAUSING AGENTS
There are examples for which the molecular basis for virulence of disease-causing agents has been defined.
It is often difficult, however, to determine whether the observed change in disease pattern is the result of specific genetic change in virulence, selection of more adaptive disease-causing agent strains, variation in genetic susceptibility of host animals or environmental factors such as changes in farming practices.
CONCLUSIONS
It is imperative to accept that control of animal diseases is an international public good. It has been amply demonstrated that a long period of freedom from such diseases is no protection from tomorrow’s catastrophe. In order to safeguard sustained livestock development in developing countries and permit legitimate participation of these countries and the poor communities within them in local, regional and international trade, and to diminish the risk of epizootics to livestock farming in industrialized countries, the international community must heed the call by the World Food Summit for internationally coordinated measures for prevention and progressive control of transboundary animal diseases and pests.
