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West Nile Virus (WNV) Q & As

West Nile Virus (WNV) and the Blood System

WNV and Me

  1. What is WNV?
  2. Should I donate blood even if I’ve been bitten by a mosquito?
  3. What if I need blood this summer?
  4. Is all blood tested for WNV?

Overview of WNV Precautions

  1. What are Canadian Blood Services’ plans for WNV testing?
  2. Once single unit testing has begun, will it continue for the remainder of the summer?
  3. Are there any other measures being taken?
  4. What happens if you find out a recent donor has become infected with WNV?
  5. What happens if you find out a blood recipient has become infected with WNV?

Testing – General

  1. What is the difference between mini-pool and single unit testing?
  2. What advantage does single-unit testing provide over pooled testing?
  3. How successful has the test been?

Other

  1. What role will stopping collections play in WNV plans?
  2. Can Canadian Blood Services feasibly stop collecting blood in certain areas?

WNV and Me

1. What is WNV?

West Nile Virus (WNV) is a mosquito-borne virus transmitted primarily among birds. In the past decade, WNV has caused large outbreaks of infection in humans in parts of Europe, the Middle East and Russia. In 1999, the first reports of WNV infection in humans in North America were reported from New York State.

WNV infection does not cause severe illness or symptoms in all those exposed to the virus. Typically, many people may become infected, with the vast majority (80 per cent) not developing any symptoms, with some people (20 per cent) experiencing flu-like symptoms (fever, chills, etc.), and with fewer than one per cent of infected people developing severe symptoms such as either encephalitis or meningitis (swelling of the brain lining or brain itself).

WNV is a seasonal infection, typically occurring in the summer months, associated with mosquito presence. The usual incubation period in humans is between three and 14 days from exposure to symptoms. However, in people with weakened immune systems it may take up to 28 days to develop symptoms. If someone is exposed and does not develop symptoms within a month (28 days), then they have either had an asymptomatic infection or were not infected.

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2. Should I donate blood even if I’ve been bitten by a mosquito?

Yes! If you are eligible to donate blood, the best way you can help is by rolling up your sleeves at a blood donor clinic. Each time you do, you’ll be helping save or improve up to three lives. Moreover, the more blood you donate, the more flexible we can be in protecting the blood system from WNV.

Remember:

  • donors do not become infected with WNV by giving blood;
  • most mosquitoes do not carry WNV;
  • mosquito bites do not disqualify people from giving blood.
  • every unit of donated blood is tested for WNV.

The greatest risk to the recipients of blood this summer and fall would be blood shortages.

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3. What if I need blood this summer?

It’s important to understand that the benefits of blood transfusions outweigh the risk of WNV in life-saving situations. There have been over a million blood products transfused in Canada and more in the United States during the WNV seasons over the past 3 years. In Canada, there have been no known cases of WNV infection from blood transfusions. However, in the United States, while none were identified in 2005, there have been 12 confirmed cases to date: 6 in 2003 and 6 confirmed in 2004.

No one can predict exactly how the virus will behave this year – but Canadian Blood Services is prepared. We’ve put in place a number of protective measures to once again minimize the risk to patients.

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4. Do you test all blood tested for WNV?

Yes. Since July 2003, every unit of blood donated to Canadian Blood Services has been tested for WNV using an investigational test developed by Roche Diagnostics. Blood found to contain the virus is discarded. Donors whose donations contain the virus must wait 56 days before they can donate again, at which time their blood no longer poses a risk.

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Overview of WNV Precautions

1. What are Canadian Blood Services’ plans for WNV testing?

We will continue to test every blood donation for WNV with the investigational Roche test using a testing procedure where six samples are mini-pooled prior to testing. If a mini-pool of blood samples tests positive for WNV, each unit is tested individually to identify the infected unit, which is then discarded. Any units that are found to contain WNV will be withdrawn and donors will be deferred from giving blood again for 56 days.

Single unit testing will commence when 1 positive donation is found through the mini-pool testing program or when recent human cases are identified in the population of a health region at a rate of greater than 1 in 1000 in rural areas or greater than 1 in 2500 in urban areas.

Once single unit testing is begun in an identified population, single unit testing may also be initiated in adjacent populations if it is deemed appropriate by medical authorities and if system capacity permits.

Pooled testing will continue on all donations where single unit testing is not being undertaken.

Testing will cease in the affected health region after seven days. If new positive donations are reported or if human cases in the health region reach the population trigger of I in 1,000 in that health region or 1 in 2,500 in urban areas during that timeframe, the seven day period will recommence.

We are committed to providing Canadians with a safe and secure blood system. In the event of a large or widespread WNV outbreak, the triggers for implementing or discontinuing single unit testing in a health region may have to be modified because the system’s total capacity for single unit testing is limited and Canadian Blood Services must also ensure an adequate blood supply for those who need it. In such a situation of a widespread outbreak, Canadian Blood Services will also consider additional measures, such as suspending collections in areas of intense human activity while increasing collections in areas of no or less activity.

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2. Once single unit testing has begun, will it continue for the remainder of the summer?

Single Unit Testing will cease in the affected health region after seven days. If new positive donations are reported or if human cases in the health region reaches the population trigger of I in 1,000 in that health region or 1 in 2,500 in urban areas during that timeframe, the seven day period will recommence.

We are committed to providing Canadians with a safe and secure blood system. In the event of a large or widespread WNV outbreak, the triggers for implementing or discontinuing single unit testing in a health region may have to be modified because the system’s total capacity for single unit testing is limited and Canadian Blood Services must also ensure an adequate blood supply for those who need it. In such a situation of a widespread outbreak, Canadian Blood Services will also consider additional measures, such as suspending collections in areas of intense human activity while increasing collections in areas of no or less activity.

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3. Are there any other measures being taken?

Yes, in addition to testing every donation, there are a variety of other measures being put in place by Canadian Blood Services to protect the blood system from WNV including:

  1. Deferring ill donors – people who say they feel ill on the day of their donation are deferred from donating and donors are asked to contact Canadian Blood Services if they begin to feel ill in the days following their donation;


  2. Withdrawal of infected units – Any units that are found to contain WNV will be withdrawn and donors will be deferred from giving blood again for 56 days;


  3. Surveillance – Canadian Blood Services will continue to work closely with The Canadian Public Health Agency and Héma-Québec as well as public health units and public health laboratories across the country, to ensure it is immediately aware of the presence of the virus in humans and can act accordingly; and,


  4. Cancelling blood donor clinics in severely affected areas, if required – in the event that WNV activity is higher than expected Canadian Blood Services may take the added precaution of stopping the collection of blood in severely affected areas.

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4. What happens if you find out a recent donor has become infected with WNV?

Canadian Blood Services checks the names of any donors with probable or confirmed cases of WNV that are reported to us by public health authorities. If any of those people have made recent blood donations, any of their blood remaining in the system is destroyed. It is possible to have multiple recipients of a donor’s blood because the various blood components are separated and used in different ways for different people. If any of the blood from such a donation has already been given to patients, the recipients’ doctors are informed so the appropriate follow-up work can be conducted.

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5. What happens if you find out a blood recipient has become infected with WNV?

All of the people who donated blood that was received by the recipient are stopped from making additional donations until the virus is no longer in their system. As well, other blood components remaining in the system that were donated by those donors are withdrawn and discarded. It is common for blood recipients to receive multiple units of blood and each unit would be from a different donor. If any other blood components from those donors have been given to other recipients, those recipients’ doctors are advised.

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Testing – General

1. What is the difference between mini-pool and single unit testing?

Pooled Testing – the standard method of WNV testing across North America – involves pooling, where samples from individual units of blood are “pooled” together before performing the test. Every unit of blood is tested, but the testing is done in pools in order to enable blood operators to quickly and efficiently test the high volume of blood required to meet ever-growing patient needs. If a positive test is received in any given “pool”, each sample will then be tested individually and the infected unit or units will be discarded. The other donations in the pool whose samples are not positive for WNV can be used. This process, known as mini-pool testing, enables Canadian Blood Services to rapidly test the high volumes of blood needed by hospitals each day.

Single Unit Testing – undertaken when a higher level of risk to the blood system has been identified. In single unit testing, a sample from each unit of blood is tested individually. It takes much longer to conduct the testing and thereby increases the time it takes to make blood products available to those in need. It requires additional equipment and manpower, but it is thought to improve the likelihood of detecting low levels of the virus because it does not dilute infected samples with uninfected samples. For this reason it is used when increased risk to the blood system has been identified.

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2. What advantage does single-unit testing provide over pooled testing?

People who are at the very beginning of a WNV infection may have very low levels of the virus in their blood. Single-unit testing is believed to be better at detecting these very low levels of virus because samples are not diluted in pools of six. For this reason it is used when increased risk to the blood system has been identified.

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3. How successful has the test been?

While it remains investigational, the test has added an important layer of safety to the blood supply. As with all new investigational tests, work is ongoing to find ways to continuously advance the test so that over time it will become better and better.

Since July 2003, we have tested all donations for WNV. To date, 27 infected donation have been found: 14 in 2003 and 13 in 2005. In all cased the infected donations were destroyed, preventing the virus from being spread to blood recipients.

There have been no known cases of WNV caused by blood transfusions in Canada since testing began.

However, while no such cases were identified in 2005, the Centers for Disease Control and Prevention (CDC) in the United States says that six cases of WNV infection in 2003 and six in 2004 are related to infected blood donations that were not detected through testing. This shows that a small risk from WNV still exists even with testing in place, since our test uses the same technology as the test in the United States. This is the reason that we have been taking measures, in addition to testing, to protect the blood system from WNV.

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Other

1. What role will stopping collections play in WNV plans?

Stopping collections in well-defined areas where there is a lot of WNV is something Canadian Blood Services will consider doing if necessary.

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2. Can Canadian Blood Services feasibly stop collecting blood in certain areas?

The bigger the blood inventory, the more options there are to protect the blood system – so the generosity of Canadians continues to be extremely important. If Canadian Blood Services collects a lot of blood over the summer, we will likely be able to stop the collection of blood in some areas if necessary. We will let people know about such actions before clinics are suspended.

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