What Is The Sindbis Virus?

Introduction 

The Sindbis virus is a type of zoonotic virus, meaning that it is passed from animals to humans. It is mainly passed from birds and mosquitoes to humans, with previous outbreaks occurring in areas of Africa and northern Europe.¹

This article will cover two main sections: the history and biology of the Sindbis virus as well as the clinical aspects of the virus. The first section will cover how the Sindbis virus was discovered, historical outbreaks, Sindbis classification and how the virus passes between animals and humans.

The second section will cover health aspects of the Sindbis virus, such as what the symptoms of Sindbis virus infection are, where it can be found, how it's diagnosed as well as current treatments used to treat the virus.

History of the Sindbis virus

First discovered in the Sindbis region of Egypt in 1952, the Sindbis virus was found in mosquitos near the river Nile.² It wasn’t until 1961 in Uganda that the first cases of Sindbis virus were found in humans.³ 

Although there have been human cases since 1961, the first outbreak occurred in South Africa in 1974 with roughly 4,000 cases. Since then, there have been outbreaks in South Africa, Sweden, Finland and the Soviet Union.³ Outbreaks in Sweden and Finland were first recorded in the early 1980s with minor outbreaks occurring, on average, every 7 years.⁴ Large outbreaks in recent years include ones in Finland in 2002 and 2021.⁴

Historically, outbreaks may have only been limited to these countries but Sindbis virus has been found in other countries as well. The virus has also been found in countries in Oceania, Asia as well as central and western Europe. However, these cases have been mostly in animals such as birds and mosquitoes, with very few human cases outside Africa and Northern Europe.⁴ 

Structure and classification 

Viruses can be classified in a number of different ways, including classification based on the structure of the virus, the organisms that they infect, how they reproduce and what kind of condition they cause during infection.⁵

The virus taxonomy system allows for viruses to be classified according to Order, Family, Genus and Species. The Sindbis virus is classified into the following:

Using this system, certain characteristics of the Sindbis virus can be learnt. Family groups viruses on the structure and shape of virus cells. Viruses of the Togaviridae family have an outer layer with the inside of the cell containing a single strand of RNA. Genus gives an idea of the type of organisms that the virus is present in. Alphaviruses are spread by insects (arthropods), including mosquitos and ticks.⁶

Transmission and hosts 

Viruses require other living organisms to survive. By themselves, viruses cannot replicate and need the cells of other organisms to produce new viruses. Viruses also need to spread to other organisms to continue existing. This includes two things; vectors and disease reservoirs.⁷

Disease reservoirs are organisms in which the disease reproduces. They can include humans, animals, food or water. Viruses usually only cause limited effects on the disease reservoir organisms, meaning that they can reproduce without killing the host before they get a chance to spread.⁷ Birds belonging to the Grouse and Passerine families are believed to act as a reservoir for the Sindbis virus in northern Europe, with other migratory birds also thought to act as a reservoir in areas of Africa.²

Vectors are also another important part of a virus's life cycle. Vectors are organisms responsible for carrying the virus from an infected organism to a new, uninfected organism. In the case of the Sindbis virus, Culex and Culiseta mosquitoes act as vectors. By biting an infected animal such as a bird, they draw in blood containing Sindbis viruses. 

The virus then develops in the mosquito’s salivary gland until it bites another organism such as a human. When biting the person, viruses within the mosquito saliva can spread into the bloodstream, infecting that person.² Thankfully, because the Sindbis virus needs mosquitoes to spread, the virus can’t spread from one person directly to another.¹

Symptoms of Sindbis virus infection 

Once bitten by an infected mosquito, symptoms begin to be seen within seven days. The condition caused by Sindbis virus infection has several names, depending on the country. These include Pogosta disease in Finland, Ockelbo disease in Sweden and Karelian fever in Russia. It may also simply be called Sindbis fever.⁶ 

Symptoms can include:

• Arthritis 
• Rash
• Mild fever 
• Fatigue
• Joint pain (arthralgia) 
• Muscle pain (myalgia)

Most people with Sindbis fever tend to recover within three weeks after symptoms begin to show but symptoms linked to the joints, such as arthritis and arthralgia, can last for several years. 

One study found that approximately 39% of people diagnosed with Sindbis fever had arthralgia and myalgia of various severity 6-8 months after symptoms first began to show.¹ There is very limited evidence to suggest that the Sindbis virus has been directly responsible for causing any deaths but infection can considerably affect the quality of life of those affected with long-lasting joint problems.

Epidemiology 

Currently, human Sindbis virus infection is seen in only a select number of countries. These include Sweden, Finland, Russia and certain areas of Southern Africa. The number of cases in these countries is linked to the populations of disease reservoir birds and mosquito vectors. The number of these animals changes depending on seasons and population cycles. Because of this, the number of Sindbis virus infections rises and falls with the number of birds and mosquitoes present.

Sindbis virus infections are mostly seen at the end of summer, usually in August and September when mosquito numbers are at their highest. Similarly, the Grouse and Passerine birds that act as disease reservoirs for the Sindbis virus have a 6- or 7-year population cycle. The peaks during this 7-year cycle allow for more birds to act as a disease reservoir. Because of this, there are larger outbreaks of human Sindbis virus infections every 7 years. Because of this cycle, outbreaks have occurred in 1995, 2002 and 2021.⁴

With these cycles, the prevalence of the Sindbis virus can vary greatly. During outbreak years, the prevalence is 11.5 per 100,000 people. However, during non-outbreak years, the prevalence is much lower, at 2.4 per 100,000 people. 

These figures are only based on the number of people that are actually diagnosed with Sindbis virus infection. Because of difficulties in diagnosing infection as well as people that don’t have symptoms not being diagnosed; the actual number of people infected is thought to be higher.²

Diagnosis and detection

People who have symptoms of Sindbis fever and have a recent history of coming into contact with mosquitoes can have tests done to confirm Sindbis virus infection. These tests include:

• Enzyme-linked immunosorbent assay (ELISA) testing 
• Polymerase chain reaction (PCR) testing

ELISA testing is the most common method used and works by finding antibodies in the blood. There are two Sindbis virus antibodies produced by the body that can be used to prove infection; IgM and IgG. Both are produced by the body within days of infection and can be found within the blood once symptoms have cleared too.⁸

These antibodies can also be used to find out if someone has previously been infected. Sindbis virus IgG antibodies will be present in the blood for the rest of a person's life, while IgM will only be present for a couple of months. So, if someone has been found to have IgG antibodies but not IgM antibodies, it means that they are not currently infected but have been in the past.⁸

PCR testing can also be done to find out if there is any Sindbis virus RNA present in the blood. PCR testing for Sindbis virus infection is limited due to there only being a very short period of time in which it can detect the virus, so it isn’t used as much as ELISA testing.⁸

Sindbis virus infection can be difficult to diagnose because special laboratories are needed to carry out these tests. With the short period in which Sindbis virus infection can be detected and the facilities needed to do so, many cases are not diagnosed.⁸

Treatment and prevention

Currently, there are no vaccines for the Sindbis virus, and treatment of its infection focuses on treating its symptoms. Treatment of symptoms includes:

• Antihistamines - to treat rash symptoms
• Painkillers - to treat arthritis, myalgia and arthralgia symptoms

Prevention of Sindbis virus infection revolves around mosquitoes. Raising public awareness of the virus can help limit the number of people infected. Avoiding areas with large numbers of mosquitoes as well as stopping them from biting can prevent contracting the virus.⁸ 

Conclusion 

To summarise, the Sindbis virus is spread by birds and mosquitoes and causes fever, rash, muscle and joint pain as well as arthritis. The symptoms usually last for 1-3 weeks but joint problems can last for months or years in a small number of people. The virus is mainly found in northern Europe and southern Africa with the condition caused by the virus having different names depending on the country it is diagnosed.

Sindbis virus infection is diagnosed by finding antibodies in the blood, however, the period in which Sindbis infection can be accurately diagnosed is slim, which can lead to many cases not being diagnosed. There is no vaccine for the virus, but treatment includes managing the symptoms of rash and joint/muscle pain via antihistamines and painkillers.