Vector borne diseases are spread by insects (and ticks). They are not contagious directly from host to host, but in contrast,
they are running in an eternal cycle between vectors and hosts. To get to a new host they must always go through a vector first.
Because vectors are cold blooded it is the air temperature which determines whether the parasites or viruses have time to develop during the short
lifespan of a mosquito. When a mosquito is infected with malaria, for example, the malaria parasites use several days to pass from the stomach
and into the salivary glands. Only when the parasite or virus reaches the salivary glands, the mosquito can infect a new host.
It is also the temperature that determines how often the mosquitoes bite and thus how many new people the mosquito can manage to infect after
the infection has reached the salivary glands. Therefore, the number of these vector-borne diseases increases the farther south we go.
But if the warmth moves northwards the diseases will naturally be able to keep up.
In the last 10-15 years there is evidence that vector-borne diseases in fact moves from Africa and Asia into southern Europe and further north.
In recent years we have had outbreaks of dengue fever and Chikungunya in the south. And we've seen one outbreak of bluetongue virus after another
in cattle and sheep in Italy, Spain and Greece and later also in Scandinavia.
But is climate to be blamed or are there other factors driving the spread of vector-borne diseases northward?
We know that globalization of trade in goods dramatically increases the risk of introducing a vector with a dangerous disease.
Flight takes off from Africa with fruits and flowers and lands a few hours later in Europe. And tourists and migrants are traveling
around the world on a scale and at a speed never seen before.
If the outbreaks of vector-borne diseases we have seen in Europe really are caused by these modest climate changes, there is reason for concern.
And it is often brought forward in the debate on climate change that we will have more vector-borne diseases in a future warmer climate.
Nobody denies that rising temperatures will make it easier for vector-borne diseases to spread. On the other hand we do not know if the trends
we see now are actually attributable to climate change. Nor can we know with certainty what the effect is in the long term. Hence, we have
calculated on this at the Veterinary Institute. We have done this by building mathematical models of disease spreading for four infections:
Malaria is a blood parasite in humans. The disease is spread by specific species of mosquitoes, which are very common in Scandinavia, although it is almost a hundred years ago since they last spread the parasite in our latitudes.
Bluetongue is a virus in ruminant animals (cows, sheep and goats). The disease is spread by midges, which are a kind bloodsucking very
small mosquitoes (in Sweden they are called "knot"). They are found in large quantities almost everywhere in the North.
A single trap in a cowshed can catch 20,000 midges in one night.
Dirofilaria is a parasitic worm in dogs, but it can also infect human beings. The parasitic worms are spread by mosquitoes.
African horse sickness is a fatal disease in horses. It is spread by the same midges that are spreading bluetongue virus. The two
viruses are closely related.
We have used a statistical model for the tiger mosquito. Not only can diseases spread northwards. New disease-spreading insects
are also expanding northwards in Europe.
Our models of disease spread are mechanical process models. This means that we try to develop models that mimic the biological processes in nature.
Every day, the models calculate how many vectors that bite an infected host. Then the models follow these vectors until the last vector of the day
is dead. Next we use temperature to figure out how long it took before the infection reached the salivary glands of the vectors and then counting
how many times the vectors have bitten new hosts in the period in which they could pass on the infection. In this way we can estimate how many new
hosts are infected in each cycle of the spread of infection. Nature is very complex and it is difficult to accurately calculate the true spread of
infection both for the present and the future. But models are probably quite good to calculate how much infection rates change in relation to today.
Therefore, we calculate the infection rates both today and tomorrow. The model calculations, for example, permit the spread malaria today.
It is not surprising since malaria has previously been quite common in Scandinavia. It is best to compare the calculated spread today with the
expected spread in the future. This gives a better impression of the effect of climate change than if you only look at the calculated spread of
infection in the future. If you only look at the calculated spread of infection in the future you can get an overly negative impression of the effect
of climate change.
The models do not show whether these diseases will be able to settle permanently in Scandinavia. The models show what could happen if, for example,
a tourist are returning from Thailand with an untreated malaria infection, or if a horse with African horse sickness was imported.
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