Early warning as the weakest link in disease surveillance systems

Transboundary animal diseases are not only a threat to household food security but also represent a barrier to international trade, and hence an impediment to economic and livestock development

Early, accurate warning of new outbreaks of epidemic livestock diseases, and particularly the spread of such diseases to new areas, is an essential prerequisite for the effective containment and control of these diseases. As observed recently, the blame for the global spread of diseases such as foot-and-mouth disease has been put on the weaknesses of disease surveillance systems and the inability to control major diseases at their source, along with the globalization of trade.

In this publication, early warning is identified as all disease initiatives, based predominantly on epidemiological surveillance, targeted to improve the awareness and knowledge of disease or infection distribution, which might also lead to early and accurate forecasting of the evolution of an outbreak.

There have been many instances where outbreaks of serious epidemic livestock disease in new areas eluded the attention of central veterinary authorities for several weeks or months. The diseases were thus allowed to spread unchecked during this period. The consequences have been unnecessary production losses and difficult and more expensive control and disease eradication measures - or, outright, the impossibility of both.

Rinderpest epidemic in South Africa that caused acute and peracute signs in cattle
PHOTO COURTESY OF G.R. THOMSON

Failure to report new disease occurrences to neighbouring countries and trading partners either directly or through international organizations such as the Office international des épizooties (OIE) and FAO has meant that other countries have been unable to take the necessary steps to prevent the introduction of the disease. Furthermore, as observed during the Rift Valley fever epizootic in the Horn of Africa and the Arabian Peninsula, the negative impact of transboundary animal diseases (TADs) on trade is also of critical importance to food security matters and human livelihoods. Indeed, many countries are removed from the international livestock market - or were simply never able to enter it - because diseases such as those mentioned above occurred on their soil. This meant that a potential lucrative trade never developed because it was pre-empted from the start by disease occurrence - a quite negative impact, often not fully realized. It then follows that TADs are, therefore, not only a threat to household food security, but also represent a barrier to international trade, and hence an impediment to economic and livestock development. It is widely recognized that to ensure sustained livestock development, the financial return to farmers from livestock trade is necessary.

THE IMPACT OF TRANSBOUNDARY ANIMAL DISEASES IN THE ABSENCE OF AN EARLY WARNING SYSTEM

Rinderpest virus disease. Rinderpest is perhaps the most serious cattle plague because of its high morbidity and high mortality rates. When this virus disease was first introduced into Africa in the late nineteenth century, it spread over almost the whole continent within ten years, killing an estimated 10 million cattle and untold numbers of wildlife - irrevocably changing livestock husbandry and wildlife ecology. Another rinderpest pandemic, in the early 1980s, which saw a resurgence of the disease throughout much of sub-Saharan Africa, was estimated in Nigeria alone to have caused losses to livestock production in the order of US$2 billion. Wildlife populations, including endangered species in Africa, have suffered considerably from rinderpest epidemics passed on to them from cattle. The relatively small and fragmented populations are increasingly at risk from a future resurgence of the disease, as illustrated by the loss of 60 percent of the African buffalo population of the Tsavo National Park in Kenya in 1994/95.

In 1994, rinderpest spread to previously long-time free, mountainous areas of northern Pakistan, killing an estimated 40 000 cattle and yaks and devastating local agriculture. The continuing presence of rinderpest in Pakistan has resulted in trade bans that for many years have denied access of countries to high-quality meat and, perhaps more important, breeding stock of high genetic value.

Classical swine fever. Outbreaks in the Netherlands in 1997-98 led to the death or slaughter of some 12 million pigs as part of the eradication campaign. The cost of these outbreaks was estimated to be US$2.5-3 billion, half of which was public money while the other half was about equally shared among farmers and other participants in the livestock production chain. The effects of the epidemic were so severe that the Netherlands Government approved a national pig-restructuring plan, which foresaw a reduction in the national pig herd of about 25 percent within two years.

African swine fever. African swine fever (ASF) has no vaccine and no treatment, and the mortality rate is usually close to 100 percent. Pig production has become increasingly important and, owing to the changing dietary patterns and growing demand for animal protein, subsistence peri-urban production systems have turned to short-cycle species, namely poultry and pigs, to meet the increasing market demand. This trend has given rise to many peri-urban commercial pig-producing units, rearing improved and often pure exotic breeds. Recent outbreaks have caused significant losses and have threatened entire pig populations in some countries.

ASF is endemic in many parts of East, Central and southern Africa. It occurred for the first time in Côte d'Ivoire in 1996, where it killed 25 percent of the pig population and cost the country, according to various estimates, between US$13 and 32 million in direct and indirect losses and eradication costs. There has since been a serious spread of ASF to Benin, Cameroon, the Gambia, Nigeria and Togo within West Africa. Estimates indicate that in the past five years, the disease has killed almost half of the total standing pig population of the West African region. The disease also seriously constrains swine production development in a number of other countries including Angola, Malawi, Mozambique and Uganda.

FAO's contribution to various ASF intervention measures in the past five years has amounted to over US$3 500 000.

In view of the nature of the disease, control and eradication rely on early detection and, where possible, immediate and rapid stamping out should be implemented. This can only be done if there is a system in place to detect any outbreak at its earliest appearance.

With the increasing speed of global transportation and communication, ASF poses a serious threat to pig-producing areas throughout the world. Preventing and mitigating the serious effects any outbreak might have will strongly rely on an early detection mechanism.

Some of the common problems that are manifest in early warning systems for serious livestock diseases include:

lack of farmer awareness programmes on high-threat epidemic livestock diseases and generally inadequate contact between field veterinary staff and farmers;
disease reporting systems that are based primarily on passive reporting of outbreaks rather than active disease surveillance;
inadequate training of veterinary and paraveterinary staff in the clinical and gross pathological recognition of epidemic diseases that may be either unusual or exotic for the country, in the implications of delayed action, and the collection and transportation of appropriate diagnostic specimens;
poor coordination of field and laboratory veterinary services;
lengthy and overcomplicated routine disease reporting chains and failure to institute an emergency reporting system for serious disease outbreaks;
failure to establish confirmatory diagnostic capabilities for the target diseases within national laboratories;
inadequate liaison with international reference laboratories, coupled with the failure to send in new virus strains, sourced from outbreaks, to these laboratories on a regular basis for specialized antigenic, genetic and epidemiological analysis;
lack of an epidemiology unit and expertise to analyse new disease outbreaks, including traceback and traceforward activities;
failure to report new disease occurrences to the appropriate international organizations, e.g. OIE, within an acceptable time;
lack of central contingency planning and other emergency preparedness for epidemic disease.

There is therefore no doubt that early warning, which is a key component of disease surveillance, needs to be strengthened at the national, regional and international levels. The following section illustrates, through concrete examples taken from previous epizootics, the lessons that can be learned from the past.