A medical director of a blood center wonders about a US-born blood donor who is in good health, but who has a history, many years ago, of being treated for filariasis that was apparently acquired overseas. Details are not available documenting the species of the infecting filaria or the treatment that was administered to the donor to 'cure' the infection. The inquiring colleague wonders if other blood centers would accept this prospective blood donor?

Editor's note: The following information is extracted from chapter 53 authored by the Webmaster and Dr. Marcia Haimowitz, and published in ROSSI’S PRINCIPLES OF TRANSFUSION MEDICINE (THIRD EDITION 2002, published by LIPPINCOTT WILLIAMS & WILKINS; Editors: Drs. TOBY L. SIMON, SUNNY DZIK, ED. SNYDER, CHRIS STOWELL & RONALD STRAUSS).

"Filarial worms can affect the lymphatic, subcutaneous, or cutaneous tissue in humans: Wuchereria bancrofti (found in tropical and subtropical areas); Brugia malayi (found in Asia and the Indian subcontinent); Onchocerca volvulus (found in Central Africa, Latin American, and the Middle East); Loa Loa and Mansonella streptocerca (found in Africa); Mansonella perstans (found in tropical Africa and South America); Mansonella ozzardi (found in the Western hemisphere); and Brugia timori (limited to some Indonesian islands). Filarial worms share the characteristic that the adult female produces microfilariae (primitive larvae) which may circulate in the blood stream. This creates the possibility of a transfusion-transmitted infection, and microfilariae have been observed to be transmitted by blood. The microfilariae of W. bancrofti have been reported to survive in stored blood products at 4-6 C for at least 12 days, however at least one report suggests that the larvae may be viable for up to 21 days after collection. In a study of Loa Loa, the concentration of microfilariae in banked blood steadily decreased to 66% of the initial concentration during the first 18 days of storage, with counts dropping off sharply thereafter; filtration of stored blood removed a large proportion of Loa Loa microfilariae. Even though the larvae can survive in stored blood products, filarial infections are only transmitted by arthropod vectors. Thus, a transfusion-acquired microfilaremia is self-limited because transfused microfilariae do not develop into adult filarial worms. Microfilaria develop to the infective stage only in the arthropod vector and then must be passed back to man in order for an adult worm to develop. There have been isolated reports of microfilaremic blood transfused to humans, none of which have been reported recently. In one report, microfilariae of W. bancrofti transfused into a healthy subject survived for 14 days, but in other human transfusion experiments, there usually has been an immediate disappearance of most of the transfused microfilariae, without any evidence of anaphylaxis. In another study, Gönnert injected himself with blood containing microfilariae of both Loa Loa and M. perstans; Loa Loa disappeared by the fourth day, but M. perstans persisted for 3 years. Human transfusion experiments demonstrated that M. ozzardi microfilariae may persist for over 2 years. Acute inflammatory reactions to destroyed microfilariae often seen in true filarial infections have not been seen in transfusion recipients, although there is a theoretical concern that allergic reactions to breakdown products of the dead microfilariae could occur. Fever, headache, and rash have occurred in individuals who were intentionally transfused with microfilariae, but these reactions may have been caused by other factors. Only one US blood donor has been documented to be microfilaremic (in this case, with M. ozzardi); transfusion-transmitted microfilariasis was never proven to occur in any of the recipients of that donor's blood. Since the clinical consequences following transfusion of microfilariae appear limited, routine testing of donors does not appear warranted. However, it seems prudent NOT to transfuse blood if it is known to contain microfilariae since an allergic reaction is a theoretical possibility."

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