Salmonella control programs in Denmark - Synopses

Emerging Infectious Diseases, July, 2003 by Henrik C. Wegener, Tine Hald, Danilo Lo Fo Wong, Mogens Madsen, Helle Korsgaard, Flemming Bager, Peter Gerner-Smidt, Kare Molbak

We describe Salmonella control programs of broiler chickens, layer hens, and pigs in Denmark. Major reductions in the incidence of foodborne human salmonellosis have occurred by integrated control of farms and food processing plants. Disease control has been achieved by monitoring the herds and flocks, eliminating infected animals, and diversifying animals (animals and products are processed differently depending on Salmonella status) and animal food products according to the determined risk. In 2001, the Danish society saved U.S.$25.5 million by controlling Salmonella. The total annual Salmonella control costs in year 2001 were U.S.$14.1 million (U.S.$0.075/kg of pork and U.S.$0.02/kg of broiler or egg). These costs are paid almost exclusively by the industry. The control principles described are applicable to most industrialized countries with modern intensive farming systems.

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Salmonellosis is one of the most common causes of foodborne diarrheal disease worldwide. Most of these infections are zoonotic and are transmitted from healthy carrier animals to humans through contaminated food. The main reservoir of zoonotic Salmonella is food animals, and the main sources of infections in industrialized countries are animal-derived products, notably fresh meat products and eggs. In developing countries, contaminated vegetables, water, and human-to-human transmission are believed to contribute to a comparatively larger proportion of the human cases than those in industrialized countries (1). However, the incidence of human salmonellosis increased in most industrialized countries in the 1980s and 1990s. Rapid spread of a limited number of successful Salmonella clones in different sectors of food animal production (swine, broiler chickens, and particularly layer hens) has been suggested as the most important cause of this increase (2).

Despite much research and many national and international attempts to implement control strategies, the incidence of human salmonellosis in most countries remains high. One notable exception is Sweden, which remains essentially free from the Salmonella problems typical for most other industrialized countries. The background for the Swedish success has been described (3). Unfortunately, other countries cannot apply the Swedish model of Salmonella control, which requires near freedom from Salmonella in domestic food animal production from the onset. In the European Union, the Zoonosis Directive (4) was an attempt to initiate a European Union-wide control effort against foodborne zoonoses, particularly Salmonella in broiler chickens and layer hens. Most European Union countries found that they either could not or would not implement the directive, which did not permit use of vaccines, antimicrobial drugs, or both as elements in the control program of Salmonella in broiler chickens or layer hens. This constraint was seen as an obstacle by some countries. Recently a new directive has been formulated, which is awaiting final approval by the European Union Parliament.

In Denmark, the incidence in human salmonellosis increased rapidly in the second half of the 1980s because of the spread of Salmonella in broiler chickens. This increase led to the initiation of a targeted national control program (5). Subsequent spread of Salmonella in swine and layer hens has also led to increases in human disease incidence and subsequently to the development and implementation of targeted control efforts (6-8). We review Denmark's Salmonella control programs and the effect on Salmonella in food animals, food, and humans. We also evaluate and discuss control costs and public health economy aspects.

Control of Salmonella in Broiler Chickens

Objectives, Program, and Effects

The initial aim of the program was that <5% of broiler flocks would be infected with Salmonella. The program was successful and was gradually revised towards assurance of complete freedom from Salmonella in broiler production.

The program is based on the principle of top-down eradication, ensuring freedom from Salmonella from the top of the broiler-breeding pyramid down. Infected flocks of breeding animals are destroyed, and infected birds are processed for slaughter. The testing program has developed gradually to adjust to higher food safety objectives. As progress stalled, more intensive serologic and bacteriologic testing was developed and applied (5,9-11). The current testing scheme is shown in Table 1.

Birds from infected flocks are slaughtered on separate slaughter lines or late in the day to avoid cross-contamination. Farmers get a better price for birds from Salmonella-free flocks, and slaughterhouses can use the label "Salmonella-free" for birds that meet criteria determined by the authorities. No decontaminants, such as organic acids or chlorine, are used during carcass processing.

The proportion of Salmonella-infected broiler flocks has been markedly reduced since the initiation of the control program. Figure 1 shows that >65% of broiler flocks tested positive for Salmonella during the first year of the program, 1988-89, versus <5% in 2000. This decrease in Salmonella has led to a concomitant reduction in the proportion of infected broiler carcasses after slaughter and at retail.