Incidence of Classical Swine Fever (CSF) in Wild Boar in a Densely Populated Area Indicating CSF. Virus Persistence as a Mechanism for Virus Perpetuation

Abstract

A virological survey was carried out to establish the distribution of classical swine fever (CSF) virus among wild boar in the Federal State of Brandenburg, Germany. Organ materials and blood samples were collected from 11,670 wild boar shot or found dead during the period March 1995 to December 1997. In total 211 (1.8%) wild boar were positive for CSF virus or antigen. The incidence of CSF-positive animals decreased continuously from 4.6% at the beginning of the epidemic in 1995 to 0.7% in 1997. The highest incidence of positive animals (22%) was found in wild-boar piglets younger than 3 months of age in 1995. The findings were indicative for the decisive role which young wild boar play in the epidemiology of CSF. Following intrauterine transfer some of the wild-boar piglets were probably persistently infected with CSF virus as experienced experimentally. Such piglets can be held responsible for CSF virus perpetuation within the wild-boar population. No CSF virus was isolated from adult wild boar weighing more than 75 kg. During 3 years of monitoring a sufficient number of susceptible wild boar, in particular young animals, was available to maintain the infection chain in that area. It was concluded that persistently infected piglets and the high population density of wild boar in the Brandenburg region offered optimal conditions for the establishment of an CSF epidemic.

Long-term monitoring of classical swine fever in wild boar (Sus scrofa sp.) using serological data

“…CSF monitoring can be performed on harvested wild boars by direct diagnosis, i.e., viral isolation [21] or indirect diagnosis, i.e., antibody counts [25]. Viroprevalence (or the proportion of viral positive animals) is generally used as an estimator of incidence [10,21] and sanitary authorities generally declare the extinction of epizootics from the absence of virus isolation [24].…”

“…Viroprevalence (or the proportion of viral positive animals) is generally used as an estimator of incidence [10,21] and sanitary authorities generally declare the extinction of epizootics from the absence of virus isolation [24]. However, infected hosts may either die rapidly (within 10 to 30 days post infection in acute cases) or develop antibodies that neutralise the virus and offer lifetime protection [39].…”

“…However, infected hosts may either die rapidly (within 10 to 30 days post infection in acute cases) or develop antibodies that neutralise the virus and offer lifetime protection [39]. Generally, the virus is not detected a few weeks post infection [39], so viroprevalence underestimates incidence at the level of the year [21]. Moreover, very young piglets that likely act as the infection reservoir [13,21] are poorly sampled by hunting.…”

“…Generally, the virus is not detected a few weeks post infection [39], so viroprevalence underestimates incidence at the level of the year [21]. Moreover, very young piglets that likely act as the infection reservoir [13,21] are poorly sampled by hunting. Thus the viroprevalence we observe on hunted animals is only useful during the very early stages of epizootics, when instantaneous incidence peaks.…”

“…Thus the viroprevalence we observe on hunted animals is only useful during the very early stages of epizootics, when instantaneous incidence peaks. Viroprevalence is not powerful enough to describe the evolution of epizootics over the long term [10,21], even when observed in large samples, which can be both time-consuming and expensive.…”