Pests and diseases have threatened farmers since farming began. The damage they cause can be economic (through lost output, income and investment) as well as psychological (manifested in shock and panic). Combating pests and diseases is a necessity for farmers and, as a rule, decisions regarding control are made by the individual farmer. However, the presence of a pest or disease on one farm poses a threat to adjacent farms and sometimes even to distant localities. As such, pests and diseases imply negative impacts on third parties and call for an additional response, either from affected parties or a public agency.
Infrastructure and services to prevent and combat pests and diseases are a public good that can be provided more efficiently by governments than by individual farmers. Yet, the most effective form of government intervention depends on the pest or disease in question. Experience has often shown that government provision of pest and disease control services can create a dependency among farmers and discourage their adoption of integrated pest management approaches that enable them to address the problems themselves. In such circumstances, government provision of knowledge, science and information may be the best and most sustainable way of serving the farming community in the long term.
The justification for government control intervention is stronger for transboundary pests and diseases than for those that only occur locally. Furthermore, in some countries the loss of food as a result of pests and diseases may threaten food security or rural livelihoods, making intervention politically unavoidable.
Plant pests and animal diseases pose the greatest immediate threat when they move as plagues or when they are introduced for the first time into ecologically favourable conditions where there are few natural factors to limit their spread and people do not have experience in managing them. Such occurrences often have the most evident economic impact and, in many cases, affect marginalized people most severely.
The spread of emergent diseases and invasive species has increased dramatically in recent years. At the same time, numerous developments - such as the rapidly increasing transboundary movements of goods and people, trade liberalization, increasing concerns about food safety and the environment - have heightened the need for international cooperation in controlling and managing transboundary pests and diseases.
The transboundary plant pests and animal diseases reviewed here include:
"Those that are of significant economic, trade and/or food security importance for a considerable number of countries; which can easily spread to other countries and reach epidemic proportions; and where control/management, including exclusion, requires cooperation between several countries."
Definition recommended by FAO
EMPRES Expert Consultation, 24-26 July 1996
(www.fao.org/WAICENT/FAOINFO/AGRICULT/AGA/AGAH/EMPRES)
This definition1 covers many pests and diseases that can cause damage or destruction to farmers' property, threaten food security, upset rural economies and disrupt trade relations. Box 5 lists some of the most important transboundary pests and diseases.2 This review discusses their economic impacts and the costs of control. It explains why regulatory measures are justified in restricting transboundary plant pests and animal diseases, why the issue is of growing concern and what the main measures are for combating the establishment of unwanted and economically significant pests and diseases.
Box 5 SIGNIFICANT TRANSBOUNDARY PLANT PESTS AND ANIMAL DISEASES MIGRATORY PLANT PESTS Transboundary migratory pests move in search of food and suitable breeding places. They include locusts, armyworms and the quelea bird. Such migrations can extend over thousands of kilometres, across seas and political borders. The pests usually concentrate as swarms (locusts), infestations (armyworms) or flocks (quelea birds). Armyworm Armyworms are caterpillars that develop into nocturnal moths, capable of long-distance migration (covering more than 100 km per night). The caterpillars cause extensive damage to grazing land, cereals and sugar cane. Compared with locust outbreaks, armyworm infestations usually occur on a smaller scale but may extend over several hundred square kilometres. Outbreaks and movements are usually related to the rainy seasons. Locusts Locusts are the most damaging of the migratory pests. They have adapted to semi-arid or desert environments where rainfall is scarce and irregular but often torrential when it occurs. The locusts fly to areas of recent rain, where moist and sandy conditions, developing vegetation and the absence of natural enemies offer ideal breeding conditions. Quelea The red-billed quelea is a common and destructive bird pest of ripening grain in many semi-arid parts of sub-Saharan Africa. Millet, sorghum, wheat and rice are the most frequently attacked crops. Migration is influenced by rainfall patterns that affect the availability of certain annual grass seeds, which are the staple food of this species. They migrate over distances of more than 1 000 km, consequently crossing political borders. Affected areas may lose most or all of their cereal crops. QUARANTINE PLANT PESTS Unlike migratory pests, quarantine pests can be introduced to a country in a multitude of ways and be relatively difficult to identify. There is no "worst" pest because impact depends more on local agricultural conditions than on particular biological features of pests. Fruit flies, aphids and other pests of leafy vegetables and cut flowers are increasing in importance as transboundary pests. Estimates of global losses from pests were made by Oerke et al.1 for eight crops by region. These estimates were made for all pests, not only transboundary types. The authors found that Traditional transboundary pests (for example plant seeds or long-lived timber pests) had long dormant stages and were found in dry commodities, such as grain, or in vessels, such as rats, aquatic species in bilge water, mosquitoes and ANIMAL DISEASES The introduction of animal diseases occurs in many ways. The most common occurrence is through live diseased animals and contaminated animal products entering a country either as imports or as food waste from international aircraft or ships. Other introductions result from the importation of contaminated biological products (for example vaccines) or germplasm (semen or ova); the entry of infected people (in the case of diseases that are transmittable from humans to animals); the migration of animals and birds; or even from natural spreading by insect vectors or wind currents. African swine fever African swine fever is the most lethal transboundary disease affecting pigs. It is also a virus disease that has shown a great propensity for sudden and unexpected international spread over great distances. This is often associated with the transportation of virus-contaminated pig meat products, including food scraps in waste from ships and aircraft. There are no vaccines against African swine fever. It is endemic over much of eastern and southern Africa, where eradication is not feasible at present because of wildlife cycles of infection between warthogs and other wild pigs and ticks, and now also because of endemicity in uncontrolled village pigs. The only practical disease control measure for commercial piggeries is the denial of access to wild and village pigs through fencing and other sanitary precautions. There is, however, a long-term prospect of controlling African swine fever in endemic areas through the development and breeding of genetically resistant pigs. Bovine spongiform encephalopathy Bovine spongiform encephalopathy (BSE), caused by novel infectious agents - prions - was first detected in the United Kingdom in 1986. It has since spread to a number of other European countries, although the majority of cases have been in the United Kingdom (see Box 9 on the spread of BSE). In cattle, it has been transmitted through meat and bone meal (MBM) feed supplements containing infected particles from affected animals. It can spread to humans through the consumption of infected tissues. In humans, it causes a fatal neurological disease known as variant Creutzfeldt-Jakob disease. Classical swine fever Classical swine fever, or hog cholera, is a generalized viral disease that only affects pigs. The disease is endemic in much of South and Southeast Asia, where it is a constraint to the development of the pig industry. In Europe, it caused major outbreaks in the EC in 1997 and 1999. Recent outbreaks have also occurred in Latin America and the Caribbean. Contagious bovine pleuropneumonia Contagious bovine pleuropneumonia (CBPP) is often regarded as an insidious, low-mortality disease of cattle, but this assessment is based on experiences in endemic areas. In susceptible cattle populations, the disease can spread surprisingly rapidly and cause high mortality rates. The disease is spread with the movement of infected animals, including acute cases and chronic carriers. Major CBPP epidemics have been experienced in eastern, southern and western Africa over the last few years. It currently affects 27countries in Africa at an estimated annual cost of $2 billion. Foot-and-mouth disease Foot-and-mouth disease is highly contagious and can spread extremely rapidly in cloven-hoofed livestock populations through movement of infected animals and animal products, contaminated objects (for example livestock trucks) and even wind currents. Vaccination is complicated by a multiplicity of antigenic types and subtypes. Substantial progress has been made towards the control and eradication of foot-and-mouth disease in several regions, notably Europe and parts of South America and Asia. However, outbreaks occurred in Argentina, Brazil, Greece, Japan and the Republic of Korea in 2000, and in the United Kingdom early in 2001. A serious outbreak in Taiwan Province of China in 1997 forced the slaughter of 3.8 million pigs. Eradication can only be viewed as a long-term objective in parts of Africa because of the existence of wildlife reservoirs for the virus. Newcastle disease Newcastle disease is a virus spread primarily through bird-to-bird contact among chickens, but it can also spread through contaminated feed, water or clothing. Outbreaks occur in most parts of the world, and there have been two major pandemics over the last century. It is a major constraint to the development of village chicken industries, particularly in Asia and Africa. A large number of wild bird species can harbour Newcastle disease virus and, occasionally, the disease affects large-scale commercial poultry units in developed countries, despite tight biosecurity measures. Mexico experienced a major outbreak in 2000, in which 13.6 million birds were destroyed. Peste des petits ruminants Peste des petits ruminants affects sheep and goats. The spread of this disease has been partly due to the inadequate international availability of an effective vaccine until recently, and also because small ruminants have perhaps not received adequate attention in disease surveillance and quarantine programmes in some regions. The Americas, Europe and Oceania are free from peste des petits ruminants. Rift Valley fever Rift Valley fever is a mosquito-borne viral zoonotic disease. The first recorded outbreak of the disease, in Egypt in 1977, caused an estimated 200 000 human cases of the disease and approximately 600 deaths, as well as large numbers of deaths and abortions in sheep, cattle and other livestock species. In 1997, 1998 and 2000, outbreaks of the disease in eastern Africa not only caused livestock losses and human deaths but also seriously disrupted the region's valuable livestock export trade to the Near East. Rinderpest Rinderpest is the most serious cattle disease known. The Americas, Europe and Oceania are free from rinderpest, and it was eradicated from southern Africa during the first half of the twentieth century by the strict enforcement of cattle movement controls, quarantining of infected areas and selective "stamping out" of infected herds as well as vaccination in areas at risk. However, by 1962, rinderpest remained endemic over a large swathe of the pastoral regions of eastern, central and western Africa. Considerable progress has been made towards the eradication of the disease in India; however, it is endemic in Pakistan. 1 E.C. Oerke, H.W. Dehne, F. Schonbeck and A. Weber. 1994. Crop production and protection: estimated losses in major food and cash crops. Amsterdam, the Netherlands, Elsevier. |
The review begins with a brief history of international control efforts and the incidence of selected transboundary pests and diseases by region. The factors behind a country's need to combat these pests and diseases are outlined, together with the effectiveness of interventions. It also explains the economic rationale for controlling transboundary pests and diseases. Some guiding concepts for determining the most effective level of control are to be found in theories concerning public goods and externalities. They indicate when there should be government involvement in control as well as addressing the equity issues involved in financing interventions.
The review then presents empirical evidence on the economic impacts of transboundary pests and diseases, including impacts on agricultural production, food security, trade and the environment. The impacts of control measures, such as the use of pesticides and stockpiling, are also discussed.
Following this is a summary of the primary tools used for pest and disease eradication and control as well as the range of possible responses - from exclusion to tolerance - on the part of a given organism. The same section also discusses options for managing and addressing the economic impacts of pests and diseases. Next, emerging and evolving issues affecting countries in their efforts to combat transboundary pests and diseases are presented and, finally, the institutions and policies governing international responses are discussed, along with the question of how to finance transboundary control.
Reports of desert locusts (Schistocerca gregaria) causing destruction and famine date back as far as 3 500 years when, according to the Bible, God sent locust swarms to Egypt as one of the ten plagues. Ancient locust plagues were also reported in the Koran. In a contemporary account of the 1747-1748 locust plague in Ethiopia, it is reported that the locusts "covered the land like a fog", and "devoured all the grain". A "great famine" ensued in which "all the inhabitants" of the daga (highlands) and all of the qolla (lowlands) are reported to have perished. The dead were said to have been so numerous that there were too few survivors to bury them.
The frightening attacks of locusts that occurred in seventeenth-century Europe compounded factors such as wars, diseases and droughts, which had already created considerable insecurity in people's lives. Systems of government intervention, including food distribution, compensation for damage and later control campaigns, became important at this time. During locust campaigns, a significant part of society was mobilized in attempts to prevent damage and to stop further spread of the insects. Initially, methods such as the digging of ditches to bury the marching, immature locusts (hoppers) were used. Later, arsenic bait was found to be effective. Within the past 50 years, aerial application of pesticides has become the main method of combating the pest.
The human-induced movement of plants and associated organisms has been important since the exploration of the New World, at the end of the sixteenth century, and of Australasia, from the eighteenth century. Non-native species were transported during voyages of exploration and via early trade in luxuries and spices - later supplanted by trade in food, beverage and fibre commodities.
In addition to cultivated agriculture, the introduction of crops and animals brought pests and diseases of significant social and economic impact. The early diseases moved quickly and were difficult to trace. Major potato crop failures from as far back as the early 1700s were associated with diseases imported from the Americas. Over time, the impact of introduced pests and diseases lessened as resistance and control measures were developed. The Colorado potato beetle, the next major potato pest to enter Europe (in the 1870s), was easier to see, slower to cause damage and more amenable to control. Possibly the first phytosanitary legislation was the Destructive Insects Act of 1877, passed in the United Kingdom to prevent entry of the Colorado potato beetle.
A global marketplace for major grains was solidified with the advent of the Industrial Revolution and the need to supply growing cities with raw materials for manufacturing. Before the Second World War, an estimated 30 million tonnes of grain crossed borders every year. By the 1970s, increases in animal consumption raised the global demand for feed, and most grain pests became endemic. Today, about 250 million tonnes of grain are transported internationally every year. Growth in the trade of fresh fruit and vegetables is responsible for many of the quarantine pest problems today.
Many important infectious diseases of animals, such as rabies and anthrax, have been known from antiquity. One of the plagues of Egypt, described in Genesis, could have been an epidemic of Rift Valley fever. Cultural and religious taboos against eating some livestock species may have originated as hygiene protection against zoonotic diseases (i.e. diseases transmitted from animals to humans).
Little is known about the economic and social consequences of epidemic livestock diseases in early times. One exception is rinderpest. From a probable source in Central Asia, the disease swept into and through Europe, often during periods of war and social upheaval, causing countless cattle deaths and much human misery. The rinderpest crisis in Europe in the eighteenth century, and later in Africa, was probably the main stimulus for the development of effective veterinary services. It was in this period that the first modern veterinary schools in Europe were established, starting with Lyons in 1762, followed some time later by the first state veterinary services. Although rinderpest was eradicated from Europe by the end of the nineteenth century, it was re-introduced into Belgium in 1922 with imported Zebu cattle. This incident was directly responsible for the establishment of the International Office of Epizootics (OIE).
In the mid-nineteenth century, there was an explosion in the incidence and economic cost of epidemic livestock diseases which persisted well into the twentieth century. Diseases that advanced included foot-and-mouth disease, contagious bovine pleuropneumonia (CBPP) and classical swine fever. There were three main causes: first was the rapid intensification of livestock production to feed the population explosion of the industrial age; second, improved transportation, ushered in by the steam age, enhanced international spread of both human and animal diseases; and, third, European colonization of other regions brought livestock into contact with new diseases that had previously only circulated in wildlife. Human encroachment into wild areas continues to be a source of disease spread.
Certain basic conditions affect the likelihood of transboundary pests and diseases establishing and spreading in regions or countries.4 These include:
Maps 3 through 6 show the areas of impact of the major locusts in the world today. The recession area is the distribution of non-swarming (solitary) desert locusts. The gregarization area is where the transformation from the solitary to the swarming form frequently occurs. Preventive control is most important around the gregarization areas. Table 43 shows regions that experience damage from migratory pests and the most recent outbreaks of each.
Table 43
IMPORTANT TRANSBOUNDARY MIGRATORY PESTS
Species |
Region |
Generations per year |
Last plague |
Desert locust |
Africa, Near East, Asia |
3-4 |
1986-1989 |
Red locust |
Southern Africa |
1 |
1930-1944 |
Migratory locust |
Africa, Europe, Asia |
Up to 6 |
1998-1999 |
Brown locust |
Southern Africa |
Up to 3 |
1985-1998 |
South American locust |
South America |
2 |
1946-1951 |
Central American locust |
Central America |
2 |
1939-1954 |
Moroccan locust |
Africa, Europe, Asia |
1 |
- |
Italian locust |
Europe, Near East, Asia |
1 |
- |
Senegalese grasshopper |
Africa, Near East, Asia |
1-3 |
- |
Australian plague locust |
Australia |
2 |
1984 |
African armyworm |
Africa, Asia, Pacific |
4-8 |
- |
Red-billed quelea |
Sub-Saharan Africa |
1 |
- |
Sources: Centre for Overseas Pest Research. 1982. The locust and grasshopper agricultural manual. London; and FAO (EMPRES). | |||
In recent years, outbreaks of migratory pests have become less frequent for some species and more frequent for others, but the underlying reasons have not been fully understood. There has been a conspicuous decline in desert locust upsurges and plagues during the last 30 years, which might indicate successful preventive control operations or might have been caused by changes in the rainfall patterns in key breeding areas (or by a combination of both). The threats from quelea birds and armyworms do not reach such peaks of severity, but they cause more frequent problems for farmers. The distribution of these two pests is shown in Maps 7 and 8.
Table 44
PROPORTION OF REPORTED CASES OF SELECTED TRANSBOUNDARY ANIMAL DISEASES, BY REGION, IN 1997
Region |
Foot-and |
CBPP |
Rinderpest |
Pest des |
Classical |
African |
Newcastle |
(Percentage) | |||||||
Africa |
1.8 |
99.6 |
...4 |
97.2 |
0 |
100.05 |
26.3 |
Asia |
97.5 |
0.1 |
... |
2.8 |
45.3 |
0 |
62.8 |
North America |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
Central America |
0 |
0 |
0 |
0 |
15.7 |
0 |
0 |
South America |
0.6 |
0 |
0 |
0 |
1.6 |
0 |
1.4 |
Caribbean |
0 |
0 |
0 |
0 |
0.03 |
0 |
0.24 |
Oceania |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
Europe |
0 |
0.26 |
0 |
0 |
37.57 |
0 |
9.8 |
(Thousands) | |||||||
Total number of cases |
1 177 |
24 |
... |
344 |
108 |
392 |
2 504 |
1 Cattle, buffaloes, small ruminants and pigs combined. | |||||||
Table 44 shows the distribution of selected transboundary animal diseases. The data provide a rough overview of disease incidence by region.5
