Human medical view on zoonotic parasites

From medical point of view, a zoonosis is any infectious disease that is naturally transmissible from vertebrate animals to humans [1]. A stricter definition is a disease that normally exists in other vertebrate animals, but can be accidentally transmitted to humans [2]. In Nordic countries, parasites are rare (and zoonotic parasites even more unusual) causative agents of human infections probably due to good hygiene and climatic conditions. In most cases, parasitic infections are of foreign origin, except for some relatively common indigenous infestations such as enterobiasis (caused by the human pinworm, Enterobius vermicularis) and pediculosis (caused by the human head louse, Pediculus humanus). Worldwide, the most significant genus of human parasites is Plasmodium. It is the causative agent of malaria, a severe tropical protozoan disease, which kills globally more than one million people every year [3]. In Finland, about twenty cases of malaria are diagnosed annually [4]. In 2007, P. knowlesii infection was diagnosed in Finland in a tourist who had traveled in Malay Peninsula [4]. P. knowlesii is a Plasmodium of monkeys. This was second reported case of P. knowlesii malaria in a tourist. During the 19 century, malaria was an indoors transmitted disease in Finland, as Anopheles mosquitoes hibernated in peoples’ households [5]. Intestinal parasitoses are the most common parasitic infections. Among Finnish asymptomatic population, pathogenic intestinal parasites (mostly Giardia lamblia) can be found from 1.5 % of people [6]. However, only 300 cases of clinical giardiasis are diagnosed in Finland annually [7], and reported numbers of diagnosed amebiasis cases (caused by Entamoeba histolytica) range from 30 to more than 100 [7,8]. These protozoans are human parasites, and infections caused by them can occur through contaminated food, water or by faecal-oral route. According to the statistics of the Parasitological unit of HUSLAB (Laboratory of Hospital District of Helsinki and Uusimaa county, Finland) from 2005 to 2007, the most important intestinal helminthes were pinworms, the human whipworm (Trichuris trichiura) and intestinal roundworms (Ascaris spp.). The swine roundworm (Ascaris suum), is a zoonotic parasite, but it was not routinely differentiated from human roundworm (A. lumbricoides). Formerly, the broad fish tapeworm (Diphyllobothrium latum) was a major health problem in Finland, and it has been called “the national parasite of Finland” [9,10]. Although it has been diminished drastically, it has not been totally eradicated. Around twenty human cases are still diagnosed annually in Finland, and the situation is similar in Sweden [11]. In contrast to diphyllobothriasis, which is mostly an indigenous disease, human intestinal taeniases are imported cases. About a handful of taeniasis cases are diagnosted in HUSLAB yearly, and the beef tapeworm (Taenia saginata) is more common finding than the pork tapeworm (T. solium). In the strict sense (see the definition above), diphyllobothriasis and taeniases should not be called zoonoses, since humans are important definitive hosts of D. latum and essential for T. saginata and T. solium, although vertebrate animals (fishes, cattle and swine, respectively) act as sources of human infections. Echinococcus spp. are the most important zoonotic cestodes worldwide. Their larvae are causative agents of serious diseases called echinococcoses. Until 1960’s, human cystic echinococcosis was a significant public health problem among reindeer herding Sámi population in Swedish and Norwegian Lapland [12]. Human cases were found also in Finnish Lapland, but only few reports have been published. Later, the parasite was eradicated from the reindeer-dog cycle, and endemic human cases have not been diagnosed for several decades. In the Parasitological unit of HUSLAB, eight echinococcosis cases were diagnosed between 2002 and Department of Bacteriology and Immunology, Haartman Institute, University of Helsinki, Finland Lavikainen Acta Veterinaria Scandinavica 2010, 52(Suppl 1):S4 http://www.actavetscand.com/content/52/S1/S4