Abstract
BackgroundIn northern Finland (Lapland), reindeer are reared as semi-domesticated animals. The region has a short summer season of 2–3 months, yet reindeer are infected with the mosquito-borne filarioid parasite Setaria tundra. The infection causes peritonitis and perihepatitis, which cause significant economic losses due to reduced body weight of infected animals. The objective of this study was to: (i) describe the spatial and temporal pattern of outbreaks in three different areas across Finnish Lapland; and (ii) construct a temperature-driven mechanistic transmission model to quantify the potential role of temperature on intensity of S. tundra transmission in reindeer.MethodsWe developed a temperature-driven transmission model able to predict the number of S. tundra potentially transmitted from an infectious reindeer. We applied the model to the years 2004–2015, and compared the predictions to the proportion of reindeer whose livers were condemned due to S. tundra infection at the time of slaughter.ResultsThe mean proportion of liver condemnation increased in reindeer slaughtered in late autumn/winter compared to earlier dates. The outbreaks were geographically clustered each year but there were no fixed foci where outbreaks occurred. Larger outbreaks were recorded in the southern regions of reindeer-herding areas compared to the central or northern parts of Lapland. Our model showed that temperatures never allowed for transmission of more than a single generation of S. tundra each season. In southern (Kuusamo) and central (Sodankylä) Lapland, our model predicted an increasing trend from 1979 to 2015 for both the duration of the effective transmission period of S. tundra (P < 0.001) and for the potential number of L3 S. tundra larvae being transmitted from an infectious reindeer (P < 0.001).ConclusionsThe effective transmission period for S. tundra in reindeer is very short in Lapland, but it increased over the period studied. Only one generation of S. tundra can be transmitted in one season among reindeer in Lapland. Increasing temperatures may facilitate a range expansion and increasing duration of effective transmission period for S. tundra.
Highlights
In northern Finland (Lapland), reindeer are reared as semi-domesticated animals
We expected them to correlate, though not perfectly, because: (i) the data were not equivalent; (ii) the data could not be expected to be linearly correlated; (iii) we do not know how many worms must be present before condemnation will occur; (iv) there is a potential human bias when identifying the pathology caused by S. tundra in different slaughterhouses; and (v) we do not know how many reindeer are infected at the start of the season, as this depends on last year’s transmission, the deworming treatment and slaughter rates during winter
There was a geographical variation in the proportion of liver condemnation due to S. tundra infection across different reindeer-herding areas and in different years with more outbreaks being recorded in southern region
Summary
In northern Finland (Lapland), reindeer are reared as semi-domesticated animals. The region has a short summer season of 2–3 months, yet reindeer are infected with the mosquito-borne filarioid parasite Setaria tundra. An increase in outbreaks of mosquito-borne filarial Setaria tundra infection has been documented in Finnish reindeer (Rangifer tarandus tarandus) husbandry in recent years [1]. At least three large outbreaks of S. tundra in ungulates have been documented, in 1973, 1989 and 2003 [1] These outbreaks were all associated with relatively warm summers, and a relationship between climate change and increasing S. tundra outbreaks in Finnish reindeer has been suggested [3]. Studies showed a correlation between higher mean temperatures of two successive summers and S. tundra outbreaks in Finland [3]. Many other studies on vector-borne diseases have suggested a similar correlation between increasing temperatures and disease outbreaks [6,7,8,9]. The exact mechanism is not well described, but may include: (i) an increase in the duration of the annual transmission periods, allowing more generations of the pathogens; (ii) shortened pathogen development time in vectors; and (iii) and increased vector abundance
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