ERADICATION OF CLASICAL SWINE FEVER IN DEVELOPING COUNTRIES – EXAMPLE OF THE REPUBLIC OF SERBIA

Slavoljub Stanojevic, DVM, Ph.D.

Directorate of the National Reference Laboratories

Abstract

Classical Swine Fever is very contagious disease of domestic pigs and wild boars, a viral etiology that can pass in acute, sub-acute and chronicle form and also as inapparent infection. Wild pigs are the natural reservoirs of the virus. An important role in the maintenance and spread of disease occupy low immune, chronically infected pigs with low virulent strains if CSFV. The spread of the virus through susceptible population and emergence of the disease depend of virulence of the viruses, age and condition of animals, herd immunity, trade and transport of the animals, environmental factors, and technology applied, education of the farmers, population density, geographic and other epidemiological factors. CSF occurs worldwide and is present on all continents. Economically speaking CSF is the biggest threat to pig production of a country. CSF caused huge economic damage and has affected pig production to the countries of the European Union (EU) over the past few decades. By applying effective measures of eradication, including protective vaccination and maintaining the immunity of pigs, in most EU countries CSF has been eradicated. However, the risk of recurrence of the CSF is still present. In some EU countries the virus is endemic in the population of wild boars. EU countries apply measures that are based on control policy without vaccination. In the last two decades there has been an intensification of pig production and significant growth in demand for meat and animal protein. Swine production meat is an important source of food and protein for mankind. For these reasons as well and because of the widespread awareness of animal welfare, control strategies that are based on the mass destruction of animals are not optimal and economic justified, especially for developing countries. 

Control of CSF in developing countries

In areas where the CSF is endemic, active immunization of pigs is important disease control measure. If the vaccinations carried out systematically on the whole territory of the country, a significant reduction of clinical cases and interruption of CSFV circulation will be achieved. Successfully braking of the circulation of the virus requires a high level of immunization of pigs. Herd immunity and proportions of vaccinated animals, which are sufficient to stop the chain of infection and the circulation of the virus depends on herd size, the type of pig production and Basic reproductive number of infection (Ro). In conditions when 75-80% of pig are immune, in case of extensive production, can be consider as critical threshold of herd immunity. In case of intensive pig production on industrial farm, the coverage should be higher. Taking into account the stated reasons, in the final stage before the eradication of CSF, it is essential to ensure a high level of immunization of pigs. By this way we interrupt the chain of infection, decrease Basic reproduction number (Ro), and virus finally fades out from the population. The spread of CSF virus could be influenced by many factors like production technology, a way of keeping pigs, level of introduced biosecurity measures, population density, trade of animals, geographical and other epidemiological factors. These factors may vary from country to country and significantly influence the selection of appropriate strategies of control and eradication of CSF. EU countries apply a strategy based on control of disease without vaccination. This strategy is also based on the depopulation of diseased pigs in the infected areas, usually in combination with large-scale preventive killing of clinically healthy pigs. However, despite the fact that the application of these measures provides long-term absence of disease, that strategy has often been criticized by the professional community and society, both for economical and for ethical reasons. For the stand point of developing countries, thus conceived measures are appropriate to the conditions of production and rearing of pigs in the developed countries of the EU but are not adjusted to the extensive pig production, which are found in underdeveloped countries. Due to the significant impact which way of keeping pigs, technology, environment and education of farmers could have on the emerging and spread of the CSF virus, there is no universal pattern of control and eradication of CSF. The choice of alternative strategies can lead to a reduction in the number of animals killed during the epidemic and avoiding the occurrence of unnecessary economic losses. On the other hand, selection of the wrong strategy could cause huge economic damage and cause large-scale spread of the virus. In conditions when the disease is already present in the field, adapting and changing strategies is not easy to perform, and often impossible. Since the material resources are prepared and veterinary service are practiced during the absence of an epizootic, incorrect assessment of needs may lead to inadequate capacity building and neglecting key segments strategy against CSF.

Control of CSF in The Republic of Serbia

Production technology of pigs in the Republic of Serbia is significantly different from the conditions that exist in most EU countries. There are obvious differences in terms and ways of trade and transport of pigs. In addition to the intensive farm production, a significant number of pigs are reared in the extensive manner. In some areas, pigs are kept as free range, where the possibility of contact with the natural reservoir of the virus is very high. In the Republic of Serbia, control and eradication of CSF is conducted for several decades. However, despite the fact that the eradication of CSF in the Republic of Serbia lasts for several decades, the country is still not free from this disease. Implementation of measures is often limited with resources, the economic possibilities of animal owners, the capacity of veterinary services and other factors. In addition to general and specific measures to control CSF, different levels of applied biosecurity measures on farms and households has been implemented. Based on experience from past epidemics and on results of Monte Carlo simulation done by experts, conclusions were drawn that vaccination, safe disposal of infected animals and early diagnosis proved to be the most effective measures to control the disease. During the simulation of CSF in articles published by Stanojevic S. et al. (2015), has shown that risk of introduction of CSF virus and the probability of occurrence of CSF in rural households is high. However, in the event of a disease outbreak on rural farms, spreading of CSF virus usually happens in the immediate vicinity of the outbreak and, usually, there is no spreading of the virus over long distances, which is not the case when it comes to the appearance of the disease in commercial farm. This conclusion seems that limited preventive depopulation strategy, for the extensive pig production, economically and epidemiologically speaking, is significantly more acceptable compared with those applied in the EU. During the Monte Carlo simulation it was shown that in case of extensive pig production, the radius of preventive depopulation of pigs should not be higher than 50 meters around backyard and family farms type A and B, and 100 around infected commercial farm, in combination with protective vaccination. Measures to control and eradicate CSF must be appropriate to the conditions of pig production and technology, to situation and possibilities of veterinary services to implement them. During the simulation is shown that planned and systematic rising of biosecurity measures and improvement in technology in all strata of pig production, are base for long-term CSF control strategy. The efficient control and eradication of CSF by applying vaccination of pigs require a high degree of vaccination coverage in all categories of pigs. Experience shows that CSF can be eradicated in endemic areas with a strict vaccination regime within a year. To ensure satisfactory level of heard immunity and to avoid perverse effects of vaccination like complications due to low coverage, such as the presence of infected undetected pigs, i.e. the presence of "masked" clusters of diseased pigs with a mild clinical picture difficult to differentiate, it is necessary to ensure constant vaccination. Vaccination of pigs should be constantly monitored. Monitoring of vaccination is based on sampling plan of priory estimated required number of samples which have to be tested for the presence of seroconversion, filed investigation and detection of risk factors that may affect the quality and coverage of immunization.

In The Republic of Serbia last CSF outbreaks were reported in 2010 and no new outbreaks were reported since then. However the situation is not clear enough. Even though the wider monitoring program for CSF vaccination has not been implemented since 2010, according to the reports the critical level of herd immunity has not reached, which is crucial for stopping of circulation of CSF virus in pig population. Appropriate critical level of herd immunity is necessary to ensure a decrease of Basic reproductive rate (Ro) and disappearance of the virus from the pig population. It is common standard in eradication scheme to vaccinate entire population and maintain the high proportion of vaccinated animals for at list one year, until the end of the virus circulation. In many countries, Classical Swine Fever has been successfully eradicated with the application of wide control measures such as monitoring of CSFV in natural reservoirs, biosecurity measures, depopulation of diseased pigs and pigs which were in contact with diseased pigs, movement restriction, together with the vaccination of pigs. The efficient control and eradication of CSF by applying vaccination of pigs require a high degree of vaccination coverage in all categories of pigs.

In The Republic of Serbia, there is surveillance of CSF in wild boars in place based on virological and serological surveillance. However should be improved due to the fact that problem with appropriate number of samples taken has been noticed, also should be improved with samples taken from so called “fallen” animals e.g. wick or dead wild boars (surveillance should cover both shot and fallen animals). According to the results, there is evidence of possible circulation of CSF virus in wild boars. That opinion is based on positive serological findings in shot wild boars. Based on available data and findings, it is necessary to improve surveillance CSF of wild boar and implement active surveillance e.g. syndromic surveillance of CSF in domestic pigs and to insure regular frequent visits of veterinary practitioners on back year holdings. Also to improve passive surveillance and pay attention on possible disorders which could make suspicions on CSF etc. Prior implementation of the plans for cessation of vaccination of pigs it is necessary to stop the circulation of the virus in the population of domestic pigs and it is under control in wild reservoirs. It is of paramount importance to keep clear distance between wild reservoirs and domestic pigs, by implementation of good bio sanitary measures. High biosecurity measures and technology have to be implemented on pig farms and also on pig holdings in villages, which is quit demanding for veterinary services. Only good biosecurity measures implemented on the farms and pig holding in combination with efficient sensitive surveillance system of CSF in domestic pigs and wild boars could replace the vaccination as a measure for CSF control. Without this prerequisite cessation of vaccination is not justifiable. To make this possible, competent authority should provide appropriate financial support to veterinary service, ensure technical resources and for implementation of animal health measures. Also public and all interesting parties should be informed and educated about the risk, about the benefits and challenges regarding CSF control without vaccination. Special attention should be dedicated to the farmers and their rule in CSF control. The producers of pigs should be educated and actively involved in the process of CSF control by active implementation of biosecurity measures on their farms. As a conclusion, competent authority should do the assessment of all interesting parties, benefits and risk of CSF and work on developing the plans for implementation of measures, based on graduate or phases approach. Prohibition of natural matting and limitation of use only within the framework of one's own backyard farm, significantly reduce the risk of occurrence of CSF and transmission of the virus. In case of cessation of vaccination, the veterinary service of the Republic of Serbia, despite the technical equipment, it seems would not be able to stop the spread of the virus without the use of protective vaccine in the event of the occurrence of CSF, and in this sense this measure is necessary. When deciding on the cessation of vaccination, the economic justification of such measure as well as the goal to be achieved should be considered beforehand. The decision to stop vaccination depends on the ability of the industry to place pigs, meat and meat products on markets where the condition of sale is a cessation of vaccination. After examining the economic feasibility and sustainability of such a program, the next step is the adoption of a strategy on phasing out vaccinations, where it would be worked on establishing good practice, introducing high biosecurity measures on farms and farms, and cessation of vaccination in these areas (vaccine-free zones). In the conditions currently in the field, it is not justified to stop a vaccination program, especially in rural farms and family farms without implementation of general hygiene and biosafety measures, ensuring enough financial resources for cost compensation to the farmers, implemented syndromic surveillance of CSF virus in pigs and wild boars etc.

Discussion and conclusions

Mathematical models and simulation of occurrence of infectious diseases cannot predict the occurrence of the diseases or give an estimate where it will occur. On the other hand, modeling is a tool for decision making and planning. A model is a representation of the natural environment in which the disease can occurs. With the help of well-developed and meaningful model it is possible to assess in advance the optimal control strategies, quantify the possible outcomes of the disease based on a "worst-case scenario", adjust plans, carry out assessment of veterinary service and assess the necessary resources. In case of occurrence of CSF or any other disease control strategy couldn’t be simply changed or adjusted to the reality. If it turns out to be applied measures which are not sufficiently effective, huge losses would be expected. Through simulation and mathematical modeling prior the crisis we are checking the existing strategy and according to the findings and economic power of countries adjust or determine the most optimal measures (strategy) before the occurrence of the disease. In this way we can plan in advance funds, material resources, measures and priorities. In case of Serbia, simulation has shown that special place in the spread CSFV at the level of the villages takes the uncontrolled natural mating of pigs, or the practice of using so-called "walker" boars for breeding sows. Occurrence of the CSF on the small family farms and backyard holding is most commonly associated with the procurement of piglets for the purpose of fattening from the category of type B, rural farms marketplaces, from illegal trade etc. When it comes to commercial farms there are typically two risks associated with the emergence of CSF: the introduction of infected animals in herds and contact with the backyard holdings or family farms over workers who work on the farm and have their own pigs at home. Stamping out and removal of diseased animals from infected holding in all cases proved effective measures and also measures of a urgent vaccination. On the other hand in scenario where preventive vaccination has been applied, there is the least economic damage. However, these measures cannot be applied indefinitely, and also carries certain economic consequences when it comes to intensive production and the ability to export animals and pork meat, especially on the markets of countries that control the CSF without vaccination. The simulation also showed that the extensive production is very susceptible to the occurrences of CSF. However, the potential to spread of CSFV over longer distances from these smaller farms is not huge. The spread of the disease on longer distances more often associated with objects of type A and type B, or commercial farms and in most cases with direct transport of live pigs. Rural farms which are engaged in production of pig to fulfill its own needs for fresh pig meat are not potentially high risk for the spread of the disease but are important for maintaining chronicle CSF unnoticed. The model also showed that, when it comes to rural farms, the most common way of transmitting CSFV is local spread and indirect contacts. Simulation has been shown that raise of hygiene and biosecurity measures on farms and rural holding significantly reduces the intensity of the spread of the disease, the number of diseased animals and the height of the economic damages in the event of outbreaks. In combination with other measures, such as good manufacturing practices on farms, controlled access of people and vehicles, eliminating unnecessary contact with other owners of pigs, avoid contact with wild pigs, contributes to the risk of occurrences of the CSF to be significantly smaller. Investing in technology education and raising the level of biological protection on family farms and rural holdings type B, early diagnostics and raising the level of sensitivity of surveillance system of CSF in domestic pigs and wild boars have a significant effect on eradication of disease. Troubleshooting the natural mating and avoiding of unnecessary indirect contacts between commercial farms and rural holdings, contributing to a significant reduction in the risk of occurrence of CSF. In case of the limitation of natural mating just for reproduction of pigs on the farm without “sharing” the boars, the risk of introduction of CSFV on the farm is lower by 43%-59%, and the economic the damages are less than 34%-36%. Reducing the indirect contacts by 25% in combination with controlled natural mating reduces the number of pigs infected during the simulated epidemics about 65%-68%, while the economic damage in less than 47%-50%. Epidemics lasted on average 66 days. Reducing indirect contacts for 50% in combination with controlled natural mating, reduces the number of pigs infected during the simulated epidemics for about 73%-90%, while the economic damage in less than 72%-77%. Epidemic lasted on average 64 days.

Active participation of owners of pigs in measures of control of CSF, education of farmers and veterinarians about the method of recognizing symptoms of CSF and importance of biosecurity measures, significantly reduces the risk of occurrence of diseases. Early detection of disease, clinical examination of animals by the veterinarians and reporting of suspect cases by the owner of the pigs, represent a major challenge for veterinary services. Stopping the spread of the virus through a susceptible population at the beginning of the disease is a key element of a good control strategy. Despite the resources and available equipment to combat major epidemics, delay in detecting the disease leads to uncontrolled spread of the disease and the impossibility of controlling the CSF without vaccination. The economic damage could be enormous and containment measures are insufficient to halt the spread of the disease on larger production systems such as large commercial farms, nucleus farms and reproduction farms. When making decision of cessation of vaccination an economic justification should be consider for such measures and the objective wants to be achieved. The decision of cessation of the vaccination depends on the ability of industry to sell pigs, meat and meat products in the markets where it is a requirement to stop sales of vaccination, the available animal health funds and the preparedness of veterinary service to control CSF without vaccination, based on conventional measures.

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