Transmission of encephalomyocarditis virus in pigs estimated from field data in Belgium by means of R₀

Abstract

-Transmission of encephalomyocarditis-virus (EMCV) has been estimated in experiments, but never using field data. In this field study, a farm in Belgium was selected where the presence of EMCV was confirmed by necropsy and virus isolation. Serology was used to estimate the transmission parameter R 0 . In one compartment with 630 pigs, 6 pens were fully sampled, in the remaining 38 pens, 2 randomly selected pigs were bled. The 151 pigs were bled twice and their serum was tested in a virus neutralisation test. Seroprevalence at the first and second sampling was 41 and 43% respectively, with a cut off value of 1:40. R 0 was estimated for 2 scenarios, in-and excluding mortality based on the final sizes from the serological results of the second sampling. The R 0 for the fully sampled pens was estimated between 0.6 and 1.7, the combined estimated R 0 of these 6 pens was 1.36 (95%-CI 0.93-2.23). The median of the estimated R 0 of the partially sampled pens was 1.3 and 1.4. Sampling two pigs per pen provided insight into the spread of the virus in the compartment, while the fully sampled pens provided an accurate estimation of R 0 . The low R 0 strongly suggests that EMCV is not very effectively transmitted between pigs. The number of seropositive pigs in a pen and the spread in the compartment suggests that other routes of infection are more important, in this case most likely rodents. Preventing viral spread should therefore be focussed on rodent control instead of reduction of contact between pigs. encephalomyocarditis virus / pigs / transmission / R 0 / field data

The encephalomyocarditis virus

“…Indeed rat infections are usually asymptomatic and the virus can replicate and be excreted up to 29 d post-infection 90 . The presence of infected rodents in proximity to farms with infected swine also suggests they play an important role in virus spread, 91 , 92 even more so because horizontal transmission between pigs seems limited 93 . However, a study performed in Greece points out that the possibility of a reservoir existing in wild boar should not be neglected 88 …”

Vaccination of influenza a virus decreases transmission rates in pigs

“…Transmission experiments and mathematical models have been used to quantify vaccine-induced reduction in the spread of Mycoplasma hyopneumoniae, pseudorabies virus, classical swine fever, Actinobacillus pleuropneumoniae, encephalomyocarditis virus (EMCV), foot and mouth disease (FMDV), porcine reproductive and respiratory syndrome virus (PRRSV), hepatitis E virus, and porcine circovirus type 2 (PCV-2) in pigs [15][16][17][18][19][20][21][22][23]. In order to quantify transmission of a pathogen, a key parameter is the reproduction ratio (R) of the infection which is defined as the average number of secondary cases caused by an infectious individual in a population during its entire infectious period [24,25].…”

Clinical problems due to encephalomyocarditis virus infections in two pig herds

“…The infection pattern on these herds (i.e. clustering) however, could be explained by multiple introductions because of the many infected pens and the low basic reproduction ratio (R 0 ) between pigs within pens [22] described by Kluivers et al (2006). The most convenient cause of multiple introductions in this case would be rodents.…”