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Cases of Babesiosis on the Rise, Testing Proves Challenging

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December 15, 2011

Babesiosis is a parasitic blood infection most often spread by the bite of an infected tick. Though relatively uncommon, incidence of the disease has been increasing in recent years. Now, the Centers for Disease Control and Prevention (CDC) has published a report in the October issue of Annals of Internal Medicine noting an increase in babesiosis related to blood transfusions. Effective and efficient donation testing is needed in those areas that have reported cases of babesiosis, but developing a test that is sufficiently sensitive and specific has presented some challenges.

In humans, infections are caused by a few Babesia species, the most common of which is Babesia microti. It is similar to malaria in that the parasite infects red blood cells. Illness ranges from asymptomatic or very mild to severe. The disease can be especially serious in people with underlying conditions as well as in newborns, the elderly, and people without a spleen or with a weakened immune system. Since the parasite infects red blood cells, one of the more serious complications is an increased destruction of red cells, resulting in hemolytic anemia. If the disease is not recognized and treated in people who are seriously ill, death can result.

Babesiosis is the most common parasitic blood disease in the U.S. Tick-borne disease occurs mainly in the Northeast and upper Midwest. Cases resulting from blood transfusions are fewer but occur throughout the U.S. as donors travel and as some blood donations are shipped outside the endemic area. According to the CDC, a total of 162 babesiosis cases have been reported from blood transfusion between 1979 and 2009. Most of these cases (77%) occurred from 2000-2009, 31 were reported from 2000 to 2004, and 91 from 2005-2009. Actual numbers may be greater because many people do not develop symptoms, or very mild ones, and don't know that they are infected. To better track the number of cases and spread of the disease, the CDC made babesiosis nationally notifiable on January 1, 2011.

Health officials acknowledge the need for an effective way to keep babesiosis out of the blood supply. An important concern is that, due to the nature of the infection, specifically, that the time between initial infection and appearance of symptoms may be as long as 6 weeks, some individuals who are unaware that they are infected may donate blood. However, testing people who may be ill with babesiosis has been an ongoing challenge and it is likewise problematic for screening blood donors who have no symptoms.

Currently, the most definitive but least sensitive test to detect infection is examining a blood smear under the microscope. This technique is time-consuming, labor-intensive, and requires expertise. With this test, babesiosis is difficult to distinguish from malaria. These issues make a blood smear impractical to use for blood donor screening.

Tests that detect antibodies produced in response to infection are more sensitive but cannot determine if the antibodies are the result of a current infection or one that has already resolved. If antibody testing were to be used for blood donation screening, it could result in the needless discarding of uninfected blood units and potentially exclude large numbers of donors who could otherwise donate. Alternatively, there are molecular tests that detect the genetic material of the parasite, but they may not be sensitive enough to detect very low numbers, when it is most likely that a donor feels healthy and may donate.

To date, there is no commercially available test that has been approved by the U.S. Food and Drug Administration (FDA) for screening donors. If one were to be developed, it would have to overcome several obstacles. It would have to be practical to perform, fast, have good sensitivity and have good specificity with a reliable way to confirm positive results. Ideally, the test would be automated, allowing a high number of screens to be completed in a short time. Plus, a good screening program would have a protocol in place that would allow previously deferred donors to begin donating again, after an infection has resolved.

For now, the CDC, with state and local public health laboratories, will continue to track cases to monitor spread of the disease. The CDC reports that future plans include working with partners to develop a test to use locally, in areas where infection rates are highest. Until a better system is in place, blood donation centers ask all potential blood donors if they have ever been diagnosed with babesiosis and, if so, the individual is deferred from donating indefinitely.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Herdwald B, et al. Transfusion-Associated Babesiosis in the United States: A Description of Cases. Annals of Internal Medicine. 2011 Oct 18;155(8):509-19. Epub 2011 Sep 5. Available online at through Accessed November 2011.

(August 2009) TRANSFUSION Volume 49, Supplement. PDF available for download at through Accessed November 2011.

Centers for Disease Control and Prevention, Babesiosis. Available online at through Accessed November 2011.

Center for Disease Control and Prevention. Babesiosis and the Blood Supply. PDF available for download at through Accessed November 2011.

Center for Disease Control and Prevention. Laboratory Identification of Parasitic Diseases of Public Health Concern, Babesiosis. Available online at through Accessed November 2011.

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