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Outbreak of vole fever affects platelets

Rodents in northern Sweden are having a good year, and now the number of cases of the haemorrhagic illness vole fever is increasing. New research results from Linköping and Umeå Universities show that the virus behind the illness causes patients’ blood platelets to function to a lesser degree than they normally do.

Sork.The Puumala virus, which causes the haemorrhagic fever, is similar to the notorious Ebola and Marburg viruses, and spreads to humans through inhaled air. Examples of risk groups are wood-burners and summer cottage owners who clean out the winter’s vole droppings in the spring. The incubation period takes from one to five weeks.

This winter, cases of the illness are expected to increase in Norrland owing to the large stock of bank voles (see Gert Olsson’s picture above). The last large outbreak occurred in 2007 when more than 900 cases were registered in Västerbotten alone. But the number of unknown cases is greater. The real total could be several thousand.

This gives researchers a chance to investigate more closely how the vole fever virus attacks blood platelets – or thrombocytes – and prevents them from healing damages blood vessels. The result of an initial study of eleven patients in the acute phase of the illness is now ready.
“We see that the number of thrombocytes decreases significantly in the blood tests. But above all, we see that those circulating in the blood aren’t doing well. They are not activated normally, and their ability to repair injuries is severely reduced,” says Sofia Ramström, research assistant in clinical chemistry at Linköping University.

Alongside Professor Tomas Lindahl, Ramström is working on an analysis method called flow cytometry, which is normally used in blood tests from patients with other kinds of mild haemorrhaging, inflammatory illnesses, and leukaemia. The study of vole fever patients is being carried out in collaboration with infectious disease researchers at Umeå University.

“Vole fever symptoms for most people are like a heavy flu, suffering from symptoms such as high fever, headache, and body aches. Many get nosebleeds and blood in their urine, even faeces blood and bleeding gums can occur. Isolated incidents where patients suffered cerebral haemorrhages have also been reported. Around one-fourth of those who fall ill suffer from blurred vision; the kidneys could be affected at a later stage,” says Clas Ahlm, senior lecturer and physician at the department of Clinical Microbiology in Umeå.

It is believed that the virus infects cells in the walls of blood vessels and damages surface vessels around the body. To repair them, a large number of blood platelets are used; a shortage then occurs in the bloodstream. This leads to faster new production, but not enough regenerated platelets, which can cause bleeding.

“Collaboration with the LiU researchers has really been successful. They’re among the leaders in Sweden in blood platelet analysis. We hope that, with their help, we can straighten out more questions about the illness,” Ahlm says.

Thrombocytes, cell fragments a few micrometers across, are found everywhere in the bloodstream and play an important role in healing damage to blood vessels. The very second damage occurs, they rush to the location and form a plug that prevents blood from streaming out. The next step is coagulation, and the wound is healed permanently.

If there are not enough of them – and they’re not activated as they should be – healing is impaired. Viral infections could be a cause, but side effects of medicines and autoimmune or genetic factors could also lie behind it. People with chronically reduced thrombocyte activity risk bleeding too much during operations, tooth extractions, and menstruation.

Thrombocytes themselves have no cell nuclei but are cloned from megakaryocytes in bone marrow. Apart from covering holes in blood vessel walls, they have tasks like attracting white blood cells to an infection and contributing to new blood vessel construction. If the balance in the blood is upset, they could instead become overactive and give rise to blood clots, which bring a risk of heart attack and stroke.
Sofia Ramström, research assistant, Linköping University, 010-1038937, sofia.ramstrom@liu.se
Clas Ahlm, university lecturer, Umeå University, 090-7852309, 070-3172965, clas.ahlm@climi.umu.se

Text: Åke Hjelm


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Last updated: 2019-04-15