CASE NOTES


Classical swine fever virus in Eastern Indonesia and Timor-Leste

Kate Sawford, Greater Sydney Local Land Services

Posted Flock & Herd March 2015

INTRODUCTION

Pork production in Australia has increased gradually over the last five years from around 300,000 tonnes in 2009 to around 350,000 tonnes in 2014. Ninety percent of pork produced in Australia is consumed domestically. In 2010-2011 Australia's pork products had a gross value of production in excess of 882 million AUD1.

Australian pork exports remained relatively flat throughout 2013, however they increased throughout 20142. Australia has the potential to take advantage of key export markets as it remains free of a number of exotic diseases that infect pigs including foot and mouth disease, classical swine fever virus (CSFV), Aujeszky's disease, porcine reproductive and respiratory syndrome, and porcine epidemic diarrhoea virus.

Australia imports around 14,000 tonnes of pork annually. The volume of pork imported into Australia increased in both 2013 and 20142. The Australian pig population is believed to be particularly vulnerable to an emergency animal disease event due to the illegal feeding of meat and meat products, or 'swill', to pigs. Many of Australia's key import countries have pig diseases that are presently exotic to Australia2,3.

In addition, many of the pig diseases exotic to Australia are spreading in neighbouring countries3. This phenomenon represents a further threat to Australia's pork industry as pig products from these regions could be imported illegally and fed to pigs.

This report details the occurrence of CSFV in eastern Indonesia and Timor-Leste and regional efforts to better understand the disease.

GLOBAL DISTRIBUTION

CSFV is a pestivirus that causes significant morbidity and mortality events in pigs. It has been eradicated from several countries including the United States, New Zealand, and a number of countries in Central and Western Europe, however it remains endemic in parts of Asia, Central America, and South America3. There have been recent outbreaks in the domestic pig populations of countries previously free of CSFV and these events have had significant economic and animal health consequences3.

CSFV SPREAD ACROSS INDONESIA AND TIMOR-LESTE

CSFV spread from Malaysia into Sumatra in 1995 causing substantial losses and has spread eastward since that time, largely through uncontrolled pig movements. The first suspect cases of CSFV in eastern Indonesia occurred in East Nusa Tenggara (NTT) in 19974,5. Most recently, CSFV spread to the eastern-most province of Papua with cases confirmed in 2007 (Figure 1).

Figure 1: The movement of CSFV into eastern Indonesia and Timor-Leste

TRANSMISSION AND CLINICAL PRESENTATION

CSFV is a highly contagious transboundary disease. Pigs are generally infected oronasally, and spread is both direct via horizontal and vertical transmission, and indirect via contaminated fomites and pork products. Clinical disease caused by CSFV is classified as acute, subacute, or chronic, and is determined by CSFV strain, as well as host factors, including pig age, breed, stage of pregnancy, previous CSFV exposure status, and CSFV vaccination status. There are no pathognomonic signs for CSFV, and therefore laboratory diagnostics are required to make a diagnosis6.

CSFV CONTROL IN INDONESIA AND TIMOR-LESTE

In NTT, districts are classified by CSFV infection status, which is based on clinical case reports to the NTT Livestock Office and limited government-led serological surveys. Vaccination campaigns are conducted in districts with the highest pig densities and annual reports of cases in an attempt to control disease. However, fluctuations in the size of the pig population have continued, and the NTT Livestock Office has documented an increase in the number of annual reported cases7.

In Timor-Leste, CSFV is believed to be endemic and one of the main constraints to pig population growth. The Ministry of Agriculture and Fisheries (MAF) has conducted CSFV vaccination campaigns since 2003 in an attempt to reduce the impact of disease8. However, as of 2013 there was no national system for reporting of clinical cases of CSFV and no serological surveys had been done. As a result very little is known about the burden of disease or the effectiveness of control strategies. While the demand for pork and live pigs in Timor-Leste is increasing, the size of the pig population in Timor-Leste has remained virtually unchanged over the last decade and morbidity and mortality events attributed to CSFV still occur8.

One of the major challenges to disease control in both contexts is maintaining cold chain requirements and ensuring vaccine delivery.

RECENT RESEARCH EFFORTS

As part of Australia's pre-border biosecurity program, from 2006 to 2014 the Australian Centre for International Agricultural Research (ACIAR) funded a research project in the region that included three CSFV surveys in NTT and Timor-Leste, in addition to investigation into suspect cases of the disease.

Seroprevalence levels were found to vary widely within and between islands, districts, subdistricts, and villages, with village-level true seroprevalence estimates ranging from around 5% to 85%. This finding is significant because it shows that the level of herd immunity required to control the disease is not being achieved across the region.

High levels of seroprevalence were found in Manggarai Barat, a district in Indonesia on the west end of Flores Island where clinical cases have never been reported and vaccination campaigns have never been carried out. This result could be due to one or a combination of several factors. Underreporting of clinical CSFV has been reported in NTT9. In addition, the virulence of the strain or strains of CSFV circulating in NTT is unknown, and therefore infected pigs may show few clinical signs and recover. Finally, while movement of pigs from CSFV infected to suspect or uninfected areas is not permitted in Indonesia4, illegal movements from central districts to western districts on Flores are known to occur10. While such movements could result in the introduction of CSFV into Manggarai Barat, it could also result in the presence of vaccinated pigs in the region.

Two years following this surprising result, a survey was conducted only in Manggarai Barat. Village-level seroprevalence ranged from 13% to 85%, confirming the previous result. During this survey a pig was necropsied that had clinical signs consistent with CSFV. Samples of kidney, lung, liver, spleen, and mesenteric lymph node from this pig tested positive on PCR for CSFV antigen. However, CSFV could not be isolated from any of the samples. Neighbour-joining phylogenetic analysis revealed that the CSFV detected is classified in the 2.2 sub-genotype group of CSFV. Strains in this sub-genotype group are typically moderately virulent.

In both NTT and Timor-Leste, only around 45% percent of reportedly vaccinated pigs tested positive for CSFV antibody. There are a number of factors that could be contributing to this low seroconversion rate. Maternally derived antibody is the most common cause of CSFV vaccination failure, particularly in highly endemic areas11, and therefore piglets that have circulating maternal antibody may not seroconvert when vaccinated. This may be particularly important in low input systems where farmers may not actively wean piglets. In addition, vaccine storage and delivery may not be adequate for achieving the high levels of efficacy reported for CSFV modified live vaccines. Infection with other pathogens, including pseudorabies virus, porcine reproductive and respiratory syndrome virus, and porcine circovirus 2, is known to cause immunosuppression in pigs12, and as there have been no published studies on any of these pathogens in this region it is impossible to assess their potential impact on CSFV vaccination. Finally, many of the pigs sampled were in poor body condition and pigs kept in low input systems are known to be protein deficient13, which results in immune deficiency14.

On a positive note, risk factor analysis for the different surveys showed that vaccinated pigs were more likely to test positive for antibody to CSFV, suggesting that vaccination campaigns are contributing to CSFV seroprevalence in the region. Additionally, risk factor analysis showed that older pigs were more likely to test positive for antibody to CSFV. This result is expected as older animals are both more likely to have been exposed to CSFV and more likely to have been vaccinated.

LIMITATIONS

Modified live vaccines against CSFV like those used in Indonesia and Timor-Leste have a number of advantages, including early onset of CSFV immunity and full protection against vertical transmission (Suradhat et al., 2007; Schroeder et al., 2012). However, one of their disadvantages is that the antibody response they induce cannot be differentiated from that caused by CSFV infection. While marker vaccines have been developed in the hope of enabling differentiation of infected from vaccinated animals, these vaccines are less protective and the immune response is delayed when compared to modified live vaccines11,15. Additionally, the antibody ELISAs developed as accompanying marker tests have been shown to lack sensitivity15. These characteristics of CSFV vaccines and ELISAs impact significantly our ability to understand the epidemiology of CSFV in contexts where the disease is endemic and vaccination is used as a control strategy.

FUTURE RESEARCH

One of the challenges in this context is determining the impact of CSFV on the pig population. While abnormal piglets, including piglets born with congenital tremors or birth defects, and mummified piglets, and piglet deaths before three months of age were reported, it is impossible to know if these losses should be attributed to CSFV. Much of published research on CSFV over the last 10 years has focused on the development of vaccines and diagnostic tools, the 1997-1998 outbreak in the Netherlands, and the status and control of CSFV in European wild boar16. In addition, the clinical presentation of CSFV in naive groups of pigs has been extensively studied17. In contrast, our understanding of the clinical manifestations of CSFV at the population level in settings where the disease is endemic, pigs are raised with minimal inputs, and vaccination is used in an attempt to minimize losses is extremely limited. Further research is needed to both justify the expense of disease control efforts in such settings and inform how to best allocate resources. Future CSFV research in such regions may benefit from a combination of serology, antigen detection and in-depth investigation of suspect cases over a period of time to better understand the epidemiology of the disease.

REFERENCES

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  3. World Organisation for Animal Health. World Animal Health Information Database (WAHID) Interface. www.oie.int 2015. Accessed October 2014
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