The epidemiology of trichinellosis is characterized by two main cycles: a synanthropic-domestic cycle (Trichinella spiralis) and a sylvatic cycle (all species and phenotypes). Trichinella spiralis, T. britovi, and T. nativa are found in the north.

Over the years, several outbreaks have occurred in Sweden (Table 3.1) with the last outbreak occurring in 1984. Since then only a few cases have been diagnosed, and these

Table 3.1 Trichinella outbreaks in Sweden during the twenty-first century


Year No. of cases

Source of infection


Kiruna, Stockholm, and Karlskrona 1917 >18

Bear meat

Berglund (1917)




Lundmark (1937)




Hallen (1937, 1938)



Home slaughter

Bergwall (1941)



Home slaughter

Bergwall (1943)




Norup and Roth (1944)



Uninspected pork

Wird (1946)




Arvidsson (1954)




Ringertz et al. (1962)




Odelram (1973); Ljungström (1974)

Vastergôtland 19699 Uncooked sausage Ljungstrôm (1974)


Kristianstad 19844 Imported pork Personal communication

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patients have contracted the infection outside the Nordic countries. However, Trichinella infection occurs in wildlife such as foxes and wild boar.

There are few documented outbreaks in Norway. The first reported outbreak was in 1881, with four cases (Bull 1882), and the last ones in 1953-1954. These last outbreaks were in a farming family who all became infected at five different occasions (Hauge 1969). The largest outbreak occurred in Halden in 1940, when 681 German soldiers and at least eight civilian Norwegians became infected. The cause of infection was a cold smoked sausage 'Braunschweiger Wurst' or 'Mettwurst' containing 50% pork and 50% beef (Sannum 1980). The same year an outbreak occurred in Akerhus County with seven reported cases. In addition, other people from the same area showed symptoms suggestive of trichinella infection, but the diagnosis was never verified (Kobro and Owren 1941).

At the compulsory slaughterhouse control of pigs in Norway, trichinellosis has been found only occasionally. The last two cases were in 1994. The sylvatic trichinellosis (T. nativa and T. britovi) are found in wildlife and T. spiralis seems very rare (Tharaldsen 1997). The compulsory control of pigs in Sweden has shown that very few pigs are infected with trichinellosis, and the same holds true for Finland.

Both domestic and sylvatic trichinellosis have increased dramatically during the last decade in Finland. The raccoon dog population has increased over the years, and they might be an important reservoir of sylvatic trichinellosis in Finland (Oksanen et al. 1998). The raccoon dog has also been suspected as one of the links in transferring Trichinella between wildlife and farm animals. Fortunately, very few human cases have been reported, mainly in bear hunters (Nurmi 1983).

Denmark is regarded as Trichinella free with respect to domestic trichinellosis and sylvatic trichinellosis is very rare (Kapel 1997).

Protozoal infections Toxoplasma

Toxoplasma gondii is a globally widespread parasite that is common in the North. The seroprevalence in sheep is high both in Sweden and in Norway (10-45%; Lunden et al. 1994) while it is much lower in swines (»3%; Hirvela-Koski 1992; Nielsen and Wegener 1997). The prevalence is also high in many other groups of animals: 42% of cats are sero-positive, 31% of red foxes, 23% of dogs, 21% of wild rabbits, 12% of red deer, 1% of horses, and 3 out of 732 small rodents (0.4%) were found sero-positive for toxoplasmosis while reindeer were always sero-negative (Kapperud 1978; Uggla et al. 1990). The first case reports occurred during the 1950s when »50% of pregnant women were sero-positive (Forsgren et al. 1991). In one study it was reported that antibodies were more common in females than in males. Even small girls were more often infected than boys of the corresponding age. The difference became marked during puberty and was significant in adults (Huldt et al. 1979). The main risk factors of contracting toxoplasmosis during pregnancy according to a Norwegian study was found to be the consumption of undercooked minced meat products, unwashed raw vegetables or fruits, and raw or undercooked mutton or pork. Cleaning cat litter was also associated with the higher incidence of toxoplasmosis (Kapperud et al. 1996). The sero-prevalence in pregnant women is today 25-30% in the southernmost parts of Sweden (Evengärd, personal communication) and in Denmark (Lebech et al. 1999). The sero-prevalence in pregnant Finns in the Helsinki area is about 20% (Lappalainen et al. 1992). It is lower in the northern parts of Sweden and Norway

(7.5 versus 6.7%) while an intermediate sero-prevalence level is found in Stockholm (14%; Evengard et al. 1999) and in Oslo (13.2%; Jenum et al. 1998).

Some recent incidence figures suggest that congenital spread of the parasite to the foetus occurs in 1 in 10000 pregnancies in Sweden (Evengard, personal communication) while it is higher in other countries (0.9-5/1000 sero-negative women; Jenum et al. 1998; Lebech et al. 1999; see also Chapter 14). A remarkably high prevalence of anti-toxoplasma antibodies has been found among slaughterhouse and greenhouse workers in Denmark (Lings et al. 1994). Other risk groups are the immunosuppressed who have been transplanted or those who have AIDS. A few cases of toxoplasmosis have been described where patients acquire the infection from heart transplants (Andersson et al. 1992), and the sero-positivity of Swedish homosexual men was found to be higher than the rest of the population (37%; Bergqvist et al. 1984). A longitudinal comparison of the sero-prevalence over the last four decades in Stockholm, Sweden, has shown the prevalence to be on a steady decrease from 47.7% in 1957 to about 10-15% in 1997 (Forsgren et al. 1991; Evengard et al. 1999). Thus, the incidence of toxoplasmosis has decreased in the Nordic countries over the last few decades.


Malaria does not occur in northern Europe today, but for around 500 cases imported each year; 463 cases were, for example, imported in 1998 whereof 181 were to Denmark, 55 to Finland, 74 to Norway, and 153 to Sweden.

Malaria is known to have occurred for several 100 years in the north; 'ague' and 'quartana' were already well known during the Middle Ages. At this time, malaria was caused by a Plasmodium vivax parasite, which had a prolonged incubation time, so that the individual was sometimes infected the year before the clinical symptoms appeared. Transmission was also secured by winter-hibernating female Anopheles mosquitoes infected during the previous year, as well as by relapses of malaria in individuals infected previously. Although the parasite present in the north has always been P. vivax the death rates were quite high. This provokes the thought that P. vivax previously caused malignant infection. An early record of malaria is in the thesis of Carolus Linnaeus presented to the University of Haderwijk, Holland, in the year 1735 (Linnaeus 1735) where he describes the epidemiology of malaria and his thoughts on current treatment of the disease. He suggests that malaria is frequent around lakes and water holes and argues that malaria is caused by the clay in the water. It should also be remembered that the malarial parasite was used world-wide to treat cerebral syphilis until the 1940s when penicillin became generally available. For example, P. vivax was used in the treatment of syphilis until 1945 at dermatology and venerology clinics, such as St Gorans Sjukhus in Stockholm. Five large epidemics of malaria occurred in Europe, including the Nordic countries, during the nineteenth century (1812-1816, 1819-1821, 1830-1832, 1846-1848, and 1853-1862). The most extensive epidemic in Denmark occurred during 1830-1831 with its centre at Lolland and Falster (Andersen 1976; Bengtsson 1992). Aland was severely affected by malaria during the 1850s when half of the population was infected and 149 died. There were about 500 cases of malaria per year during the First World War in Finland that had spread from the Cossacks in southern Russia. The incidence decreased between the wars; but malaria was re-introduced in Finland during the Second World War with 1252 patients who had contracted the disease on Carelian isthmus during 1944 (Figure 3.1).

Figure 3.1 Malaria cases in Finland during 1945. Each point represents five cases of malaria (from Hernberg and Tuomela 1948).

A few cases of malaria remained in Finland into 1948 suggesting that the incubation time for the disease could be very long (Hernberg 1947; Hernberg and Tuomela 1948; Bengtsson 1992). Some small epidemics are known to have hit Norway around Oslo and Fredriksstad during the early nineteenth century. A malaria epidemic also appeared later during the century with 143 cases recorded in Oslo (previously Christiania) during 1870. This was due to a receding water level at the seashore, which created novel marshlands where mosquitoes could breed (Grelland 1965; Bengtsson 1992). Malaria was endemic in Sweden during the nineteenth century along the southeast coast and around Lake Malaren, with an epidemic spread throughout southern Sweden (Figure 3.2). The last record from Sweden dates back to 1930 in Oxelosund.

Figure 3.2 The endemic and the epidemic spread of malaria in Sweden during the 1850s (from Bengtsson 1992).
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