Setaria tundra, an emerging parasite of reindeer, and an outbreak it caused in Finland in 2003-2006
Year: 2010
Type: dissertation
Abstract: Recent Finnish studies have revealed an array of filarioid nematodes and associated diseases that appear to be emerging in northern ungulates. All filarioid species produce microfilariae that are present in the host blood, and known vectors are haematophagous arthropods. Infections attributable to a species of the genus Setaria appear to have emerged in Scandinavian reindeer in 1973. The infections were associated with an outbreak of peritonitis. In the same year, tens of thousands of reindeer died in the northern part of the reindeer herding area of Finland. Severe peritonitis and large numbers of Setaria sp. worms were common findings. However, the prevalence of Setaria sp. in Scandinavian reindeer subsequently diminished. In Finland, the latest outbreak of peritonitis in reindeer started in 2003 in the southern and middle parts of the reindeer herding area. The proportion of reindeer viscera condemned due to parasitic lesions identified during meat inspections increased dramatically. These increases caused substantial economic losses and increased the workload associated with meat processing. The focus of the outbreak moved northward by approximately 100 km/yr, and by 2005 only the reindeer in Upper Lapland were free of lesions. During the same period, the peritonitis outbreak was apparently fading away in the southern area. The causative, agent based on morphological and molecular data, was identified as Setaria tundra. Reindeer calves with heavy infections of S. tundra expressed decreased thriftiness, poor body condition, and an undeveloped winter coat. Meat/post mortem inspection of diseased reindeer carcasses revealed ascites fluid, green fibrin deposits, adhesions, and live and dead S. tundra nematodes. Histopathology indicated granulomatous peritonitis with lymphoplasmacytic and eosinophilic infiltration. No specific bacterial growth was found. No significant impact on pH values of meat or on the organoleptic evaluation of meat was found. There was a significant positive correlation between worm counts and the degree of peritonitis, and a negative correlation between the degree of peritonitis and the back-fat layer. Based on the evidence in both ante and post mortem inspections and histological examinations, present studies and historical data indicate that S. tundra can act as a significant pathogen in reindeer. The prevalence and density of Setaria microfilariae (smf)in blood circulation were higher in reindeer calves than in adults; the overall prevalence was 42%. In order to monitor the dynamics of S. tundra in nature, wild cervids also were sampled. The overall smf prevalences for moose, wild forest reindeer and roe deer were 1.4-1.8%, 23% and 44%, respectively. The focus of microfilaremia in reindeer moved north while simultaneously declining in the south as the observed peritonitis outbreak decreased. Experimentally, in reindeer calves infected in their first summer of life the peak microfilaremia was recorded in their second summer. Captive reindeer were smf positive throughout the year, but smf disappeared from the blood after 2 years. The prepatent period of S. tundra was estimated to be about 4 months, with a life span of at least 14 months. Moose are apparently not a suitable reservoir host for the S. tundra haplotype occurring in reindeer. The previous report of a peritonitis outbreak in moose associated with Setaria sp. nematodes in Finnish Lapland in 1989 was caused by another S. tundra haplotype. It may well be that among other factors, the high percentage of wild forest reindeer with signs of peritonitis caused by S. tundra may also have contributed to a substantial population decline for this herd in Kainuu (1700 to 1000 in 2001-2005). Although S. tundra is at present maintained primarily in the reindeer population, roe deer seem to be a suitable host and asymptomatic carrier. Mosquitoes, particularly Aedes spp. and to a lesser extent Anopheles spp., play an important role in the transmission of S. tundra in reindeer herding areas in Finland. The prevalence of S. tundra larvae in naturally infected Finnish mosquitoes varied from 0.5-2.5%. The rate of development in mosquitoes is temperature-dependent. The life cycle of Setaria tundra; Adult nematodes inhabit the peritoneal cavity of reindeer and produce microfilariae in to the host’s blood circulation, especially on summer months. The life span of the adult S. tundra female is at least 14 months.Microfilariae get with the blood meal into the intermediate mosquito (Culicidae) host. Microfilariae penetrate the gut of the mosquito and develop through two moults into infective third-stage larvae. The development is temperature dependent and takes about two weeks at 21°C (mean). When the mosquito is feeding again, the third-stage larvae break out and penetrate the skin of the host through mosquito’s puncture wound. Then they develop to the adult stage through two moults in the host and find their way to the abdominal cavity. The prepatent time is approximately 4 months. Ivermectin has good efficacy against adult S. tundra nematodes and circulating smf, and therefore there is an obligation to treat heavily infected reindeer calves with ivermectin by injection for animal welfare reasons. At the population level, massive antiparasitic treatment with ivermectin can reduce the number of carriers among reindeer population. The fact that this could not prevent the emergence of the S. tundra outbreak in new areas in the North indicates that the transmission dynamics of S. tundra are efficient. The 1973 outbreak of S. tundra in Sweden was associated with unusually warm weather and abnormally high numbers of mosquitoes and gnats. The summers of 1972 and 1973 in Finland were also very warm, as were those in 2002 and 2003. Warm summers apparently promote transmission and the genesis of disease outbreaks by favouring the development of S. tundra in its mosquito vectors, by improving the rate of mosquito development and reducing their mortality from frost, and finally, by forcing reindeer to stay in herds on mosquito-rich wetlands. Mosquito-borne diseases are among those most sensitive to weather and obviously will be influenced by climate change. Thus, I predict that global climate change will promote the further emergence of filarioid nematodes and diseases caused by them in the subarctic ecosystem. Moreover, I believe that future outbreaks can be predicted based on the mean temperatures of two consecutive summers. This study indicated that S. tundra probably has an important impact on boreal ecosystems. It also revealed the absence of baseline knowledge concerning temporal parasitic biodiversity in cervids at high latitudes. Therefore, it is important to gain knowledge about these parasites, their ecology, transmission dynamics, and their impact on human and animal health. The putative relationship between climate change and a vector-borne disease identified in this thesis indicates the potential and obvious threats to the individual and population health of arctic ungulates.
Source: Työväentutkimus Vuosikirja
Author: Sauli Laaksonen
Language: English
Subfield: Infectious Diseases
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W1510091464