An outbreak of viral encephalomyocarditis in pigs

Encephalomyocarditis virus infection in a splenectomised calf

BACKGROUND

Encephalomyocarditis (EMC) caused by EMC virus (EMCV) was diagnosed in a 5-month-old splenectomised calf, which died suddenly on an experimental farm that had a high infestation of rodents.

RESULTS

At postmortem examination, the lungs were dark purple and diffusely congested. On histological examination, the calf had severe necrotising myocarditis. EMCV was isolated from the heart. The polyprotein gene of the EMCV isolate was amplified by PCR and had 85-91% identity with published EMCV sequences, including 89% identity with isolates from Queensland. On phylogenetic analysis, the polyprotein gene had highest sequence identity with South Korean EMCV strain, CBNU.

CONCLUSION

This is the first report of naturally occurring EMC in cattle in Australia and the first report of naturally occurring bovine EMC from which EMCV has been isolated.

Transmission of encephalomyocarditis virus (EMCV) among pigs experimentally quantified

Two types of transmission experiments were performed to estimate the basic reproduction ratio R(0), indicating the level of encephalomyocarditis virus (EMCV) transmission among pigs. In a first experimental set-up with nine separate pairs, one randomly chosen piglet per pair was inoculated with a Belgian (myocardial) EMCV strain (B279/95, 10(3)TCID(50)/ml oronasally) and placed back into the pen. In the second experiment with two separate groups of five piglets, two piglets in each group were inoculated at the start. During the experiments, viraemia in blood and excretions was measured as well as the serological response against EMCV antigen. After death or euthanasia, the piglets were checked for heart lesions and virus isolation was done on various tissues. In both the experiments, the majority of the inoculated piglets either died with typical heart lesions (five out of nine and three out of four resp.), or produced high levels of neutralising antibody. EMC virus was isolated from the hearts of all piglets that died during either one of the experiments. The pairwise experiment revealed a point estimate for R(0) of 2.0 (95% confidence interval (CI)=0.37-10.74), while the group experiment resulted in a R(0)-value of 0.71 (95% CI=0.08-4.93). Combining the information from both experiments results in an estimate for R(0) of 1.24 (95% CI=0.39-4.35). Since R(0) has values around the threshold value of 1, the spread of EMCV due to contacts between pigs will in most cases be limited, but due to chance processes may lead to large outbreaks as well.

Detection of encephalomyocarditis virus in clinical samples by immunomagnetic separation and one-step RT-PCR

A method of immunomagnetic separation and one-step reverse transcription-polymerase chain reaction (RT-PCR) was developed for the detection of Encephalomyocarditis virus (EMCV). EMCV was captured from sample on magnetic beads with homologous monoclonal antibody and then heat denatured. The heated beads were used directly in one-step RT-PCR reaction to amplify a 285-bp PCR fragment at the 3' end of the genomic region that encodes the viral polymerase. This method detected as little as 3.5 TCID(50) of EMCV from infected cell culture. It was shown with this method that the sensitivity of RT-PCR increased when applied for the detection of EMCV added to fecal extract. Using this protocol EMCV was detected from heart homogenate samples containing less than 100 TCID(50)/ml. The amplified product was sequenced to ensure specificity. The immunomagnetic-RT/PCR procedure described here should be useful for the rapid, specific and sensitive detection of EMCV in clinical samples. This technique is rapid, reliable and can be readily adapted to detect EMCV from other clinical samples.

Nucleotide sequence and construction of an infectious cDNA clone of an EMCV strain isolated from aborted swine fetus

A full-length cDNA clone of an Encephalomyocarditis virus (EMCV) strain (2887A) isolated from aborted swine fetus was constructed and sequenced. Sequence comparison showed more than 99% nucleotide and amino acid sequence identity with two other EMCV strains, EMCV-PV21 and -R. However, the 2887A genomic sequence showed only about 84% nucleotide identity and 96% amino acid identity with EMCV-B, -D and -PV2 variants. RNA synthesized by in vitro transcription of this cDNA clone was infectious upon transfection of BHK21 cells, as shown by cytopathic effects and identification by neutralization test, and by propagation of the virus released into the culture media. The transcript RNA led to the production of infectious particles despite the presence of two nongenomic nucleotide residues at the 5' end, the short poly(C) tract (C(10)TCTC(3)TC(10)), the short poly(A) tail (7A), and the presence of six nongenomic nucleotides at the 3' end. The rescued virus was also found to be highly pathogenic for mice by intra-peritoneal inoculation producing a fatal disease indistinguishable from that of wild-type virus. An important finding concerning the molecular basis of infectivity was that the in vitro synthesized EMCV RNA transcript is infectious, although it contains a very short poly(A). The availability of the infectious cDNA clone of the reproductive failure strain of EMCV should prove to be useful for studying the molecular basis of the pathogenicity of EMCV in pig.

Experimental encephalomyocarditis virus infection in pigs

A field isolate of Encephalomyocarditis (EMC) virus was inoculated intravenously into 8 pigs. Four animals died at post inoculation day (PID) 2, the remaining being sacrificed at PID 5, 7, 11 and 15. Two control, in-contact pigs were sacrificed at PID 19. Virus was isolated from leucocytes and nasal swabs until PID 4, from rectal swabs until PID 2 and, in the pigs found dead at PID 2, from several organs. EMC virus was further isolated from brain and spleen of the pig sacrificed at PID 7. One of the 2 control pigs became infected: virus was isolated from nasal swabs at days 6 and 7 and from leucocytes at day 4 of the experiment. Serum-neutralizing (SN) antibody was detected in the injected pigs starting from PID 4; two days later, it was also revealed in the infected, in-contact control. To our knowledge, this is the first report of an experimental transmission of EMC virus infection in pigs by contact exposure.