Eradication of enzootic bovine leukosis from Finland

Abstract

Enzootic bovine leukosis (EBL) was recognized among Finnish cattle in 1966. Administrative decisions specifying and refining official control measures were given in 1966, 1976, 1980, and 1993. The measures’ key principle always has been ‘test and slaughter’. The EBL/bovine leukosis virus (BLV) infection situation was monitored at meat inspection, and hematologically between 1970 and 1977 and serologically between 1978 and 1989. Annual surveys including all dairy herds and samples from beef animals were conducted in 1990–2001. Bulk-tank milk samples represented the dairy herds in the surveys; the beef animals were sampled individually at slaughter. The maximum positive herd-level percentage in the surveys was 0.03%. EBL/BLV infection was evenly dispersed in the southern part of the country and nonexistent in the northern part. We conclude that herd-level prevalence of EBL/BLV infection never exceeded 5%. It nevertheless took 30 years to eradicate the disease and the infection. EBL was eradicated from mainland Finland in 1996 and from the island district of Ahvenanmaa in 1999. Annual monitoring of the EBL situation continues.

Introduction

Enzootic bovine leukosis (EBL, Office International des Epizooties (OIE) B108) is a viral disease of adult cattle, characterized either by persistent lymphocytosis (PL) or lymphosarcoma formation (LSA), or both. Animals with PL frequently show no clinical signs; those with LSA have symptoms which reflect the site of the tumor. Cattle with LSA almost invariably die either suddenly or weeks or months after the onset of clinical signs. There is a genetic predisposition to both PL and LSA which occur independently but often coincidentally (Ferrer, 1979). The epidemiological cause of the disease is bovine leukosis virus (BLV), a deltaretrovirus in the Retroviridae family, but other factors contribute to development of the condition (Burny et al., 1980).

Iatrogenic infection through blood-contaminated needles and surgical instruments is probably the major route of transmission in most settings. Field transmission of BLV requires direct prolonged contact between animals or an exchange of blood, exudates, or tissues (e.g. during parturition). Spread of BLV between herds can occur via infected animals. Vertical transmission, semen, or hematophagous insects probably account for only a small proportion of cases (Hopkins and DiGiacomo, 1997). Transmission is thought to be cell (lymphocyte)-associated because BLV-infected cattle do not produce important amounts of cell-free virus in tissues, secretions, or excretions.

EBL occurs worldwide in cattle-raising countries. In most of these countries, EBL is a notifiable disease, and official control measures include screening or monitoring, precautions at borders, control of movement inside the country, and stamping out (OIE, 2000a). No vaccine is available for BLV (OIE, 2000b)—and indeed, vaccination against it is prohibited in many countries. Although susceptible to the infection, the role of sheep and goats as reservoirs of the virus is unclear.

It is not known when BLV infection or EBL entered Finland. Berger and Henriksson (1966) were the first to acknowledge the existence of the disease and the need for control measures against it. Between 1966 and 1996, its prevalence and geographic distribution were investigated, an official control program was set up and the infection successfully was eradicated from Finnish cattle. Our purpose is to review the measures applied and the eradication process.