West Nile virus in blood: stability, distribution, and susceptibility to PEN110 inactivation

Natural Host-Environmental Media-Human: A New Potential Pathway of COVID-19 Outbreak

Identifying the first infected case (patient zero) is key in tracing the origin of a virus; however, doing so is extremely challenging. Patient zero for coronavirus disease 2019 (COVID-19) is likely to be permanently unknown. Here, we propose a new viral transmission route by focusing on the environmental media containing viruses of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or RaTG3-related bat-borne coronavirus (Bat-CoV), which we term the "environmental quasi-host." We reason that the environmental quasi-host is likely to be a key node in helping recognize the origin of SARS-CoV-2; thus, SARS-CoV-2 might be transmitted along the route of natural host-environmental media-human. Reflecting upon viral outbreaks in the history of humanity, we realize that many epidemic events are caused by direct contact between humans and environmental media containing infectious viruses. Indeed, contacts between humans and environmental quasi-hosts are greatly increasing as the space of human activity incrementally overlaps with animals' living spaces, due to the rapid development and population growth of human society. Moreover, viruses can survive for a long time in environmental media. Therefore, we propose a new potential mechanism to trace the origin of the COVID-19 outbreak.

Improving the safety of whole blood-derived transfusion products with a riboflavin-based pathogen reduction technology

Worldwide safety of blood has been positively impacted by technological, economic and social improvements; nevertheless, growing socio-political changes of contemporary society together with environmental changes challenge the practice of blood transfusion with a continuous source of unforeseeable threats with the emergence and re-emergence of blood-borne pathogens. Pathogen reduction (PR) is a proactive strategy to mitigate the risk of transfusion-transmitted infections. PR technologies for the treatment of single plasma units and platelet concentrates are commercially available and have been successfully implemented in more than 2 dozen countries worldwide. Ideally, all labile blood components should be PR treated to ensure a safe and sustainable blood supply in accordance with regional transfusion best practices. Recently, a device (Mirasol® Pathogen Reduction Technology System) for PR treatment of whole blood using riboflavin and UV light has received CE marking, a significant step forward in realising blood safety where WB transfusion is the norm, such as in sub-Saharan Africa and in far-forward combat situations. There is also keen interest in the ability to derive components from Mirasol®-treated whole blood, as it is seen as a more efficient and economical means to implement universal PR in the blood centre environment than treatment of components with different PR systems.

A West Nile virus NS4B-P38G mutant strain induces cell intrinsic innate cytokine responses in human monocytic and macrophage cells

Previous studies have shown that an attenuated West Nile virus (WNV) nonstructural (NS) 4B-P38G mutant induces stronger innate and adaptive immune responses than wild-type WNV in mice, which has important applications to vaccine development. To investigate the mechanism of immunogenicity, we characterized WNV NS4B-P38G mutant infection in two human cell lines-THP-1 cells and THP-1 macrophages. Although the NS4B-P38G mutant produced more viral RNA than the parental WNV NY99 in both cell types, there was no detectable infectious virus in the supernatant of either cell type. Nonetheless, the attenuated mutant boosted higher innate cytokine responses than virulent parental WNV NY99 in these cells. The NS4B-P38G mutant infection of THP-1 cells led to more diverse and robust innate cytokine responses than that seen in THP-1 macrophages, which were mediated by toll-like receptor (TLR)7 and retinoic acid-inducible gene 1(RIG-I) signaling pathways. Overall, these results suggest that a defective viral life cycle during NS4B-P38G mutant infection in human monocytic and macrophage cells leads to more potent cell intrinsic innate cytokine responses.

West Nile virus infection in blood donors in the New York City area during the 2010 seasonal epidemic

BACKGROUND

A uniform threshold strategy for converting from minipool (MP)-nucleic acid testing (NAT) to individual donation (ID)-NAT screening for acute West Nile virus (WNV) infection among blood donors is lacking. We report on WNV screening at the New York Blood Center during the 2010 seasonal WNV epidemic, the most severe epidemic in that state since the original outbreak in 1999.

STUDY DESIGN AND METHODS

Between July 1 and October 31, 2010, blood donations were screened by MP-NAT or ID-NAT and the presence of anti-WNV immunoglobulin (Ig)M and IgG was evaluated among NAT-positive donations.

RESULTS

Twenty presumed viremic donations were identified for a frequency of 0.0129% (1 in 7752 donations). Nine donations that could have been missed by MP-NAT were identified. Two of these donations were both IgM and IgG negative, one of which would have been missed if more than one positive donation was required for initiating ID-NAT. Retrospective ID-NAT revealed two positive donations. The majority of the NAT-positive donations in New York (16/19) were from donors who lived in counties that had the highest incidence of human WNV cases in the state.

CONCLUSION

Our data details the identification of WNV NAT-positive blood donations during a severe seasonal epidemic in the New York area. By initiating ID-NAT after one positive donation, using retrospective testing, and triggering ID-NAT regionally, we were able to prevent the release of presumably infectious donations. The detection of NAT-positive donations with retrospective testing, however, may indicate the need for changes in our trigger criteria.

Developing pathogen reduction technologies for RBC suspensions

Numerous studies have evaluated a wide variety of photosensitizers and alkylating agents as candidates for a pathogen reduction process to be used in RBC suspensions. The methodologies that produce robust inactivation of pathogens with maintenance of RBC properties during storage involve those that specifically target nucleic acids. This has been demonstrated through in vitro studies by flexible photosensitizers, which specifically target nucleic acid but do not engage in photochemistry when free in solution and nucleic acid alkylating agents in conjunction with extracellular quencher(s) to protect against RBC membrane alkylation. The flexible photosensitizer method must be scaled up to entire units, and toxicology studies would need to be performed for further development. Clinical trials will ultimately be necessary to further develop either flexible photosensitizers or nucleic acid alkylating methods with quenchers for use in Transfusion Medicine.

Rapid and accurate in vitro assays for detection of West Nile virus in blood and tissues

West Nile virus (WNV) is a mosquito-borne single-stranded RNA virus, which has relatively recently emerged as a blood transfusion and organ transplantation transmissible pathogen. Low levels of WNV (viremia) are found in asymptomatic blood transfusion or cell/tissue donors with an infection, which poses a health threat to recipients. Since the introduction of nucleic acid testing (NAT) in 2003, many changes have occurred in the field of WNV detection and diagnosis. This review will focus on the recent progress in the in vitro assays for rapid and accurate detection of WNV in blood and tissues.

Transmission dynamics and changing epidemiology of West Nile virus

West Nile virus (WNV) is a flavivirus that is maintained in a bird-mosquito transmission cycle. Humans, horses and other non-avian vertebrates are usually incidental hosts, but evidence is accumulating that this might not always be the case. Historically, WNV has been associated with asymptomatic infections and sporadic disease outbreaks in humans and horses in Africa, Europe, Asia and Australia. However, since 1994, the virus has caused frequent outbreaks of severe neuroinvasive disease in humans and horses in Europe and the Mediterranean Basin. In 1999, WNV underwent a dramatic expansion of its geographic range, and was reported for the first time in the Western Hemisphere during an outbreak of human and equine encephalitis in New York City. The outbreak was accompanied by extensive and unprecedented avian mortality. Since then, WNV has dispersed across the Western Hemisphere and is now found throughout the USA, Canada, Mexico and the Caribbean, and parts of Central and South America. WNV has been responsible for >27,000 human cases, >25,000 equine cases and hundreds of thousands of avian deaths in the USA but, surprisingly, there have been only sparse reports of WNV disease in vertebrates in the Caribbean and Latin America. This review summarizes our current understanding of WNV with particular emphasis on its transmission dynamics and changing epidemiology.

Pathogen inactivation: the definitive safeguard for the blood supply

CONTEXT

Pathogen inactivation provides a proactive approach to cleansing the blood supply. In the plasma fractionation and manufacturing industry, pathogen inactivation technologies have been successfully implemented resulting in no transmission of human immunodeficiency, hepatitis C, or hepatitis B viruses by US-licensed plasma derivatives since 1985. However, these technologies cannot be used to pathogen inactivate cellular blood components. Although current blood donor screening and disease testing has drastically reduced the incidence of transfusion-transmitted diseases, there still looms the threat to the blood supply of a new or reemerging pathogen. Of particular concern is the silent emergence of a new agent with a prolonged latent period in which asymptomatic infected carriers would donate and spread infection.

OBJECTIVE

To review and summarize the principles, challenges, achievements, prospective technologies, and future goals of pathogen inactivation of the blood supply.

DATA SOURCES

The current published English-language literature from 1968 through 2006 and a historical landmark article from 1943 are integrated into a review of this subject.

CONCLUSIONS

The ultimate goal of pathogen inactivation is to maximally reduce the transmission of potential pathogens without significantly compromising the therapeutic efficacy of the cellular and protein constituents of blood. This must be accomplished without introducing toxicities into the blood supply and without causing neoantigen formation and subsequent antibody production. Several promising pathogen inactivation technologies are being developed and clinically tested, and others are currently in use. Pathogen inactivation offers additional layers of protection from infectious agents that threaten the blood supply and has the potential to impact the safety of blood transfusions worldwide.

Transfusion-transmitted infections

Although the risk of transfusion-transmitted infections today is lower than ever, the supply of safe blood products remains subject to contamination with known and yet to be identified human pathogens. Only continuous improvement and implementation of donor selection, sensitive screening tests and effective inactivation procedures can ensure the elimination, or at least reduction, of the risk of acquiring transfusion transmitted infections. In addition, ongoing education and up-to-date information regarding infectious agents that are potentially transmitted via blood components is necessary to promote the reporting of adverse events, an important component of transfusion transmitted disease surveillance. Thus, the collaboration of all parties involved in transfusion medicine, including national haemovigilance systems, is crucial for protecting a secure blood product supply from known and emerging blood-borne pathogens.

Pathogen-reduction systems for blood components: The current position and future trends

The current multi-layered interventional approaches to blood safety have dramatically reduced the risk of viral contamination of blood components. Nowadays most of the residual transfusion transmitted infections (TTI) occur as the result of the interval between the time the donor is infected and the moment at which tests are capable of detecting the agent, the so called "window period" which has been considerably reduced by the increased sensitivity of nucleic acid testing (NAT). However, the residual risk of bacterial contamination and the unexpected appearance of some other emerging pathogens, almost every five years, are still of major concern to the public, politicians, regulatory agencies and place immense pressures on the organisations responsible for the provision of safe blood and its components. In view of these bleak scenarios, the use of human blood as a raw biological source is inherently unsafe, and screening/testing alone cannot exclude all the potential human pathogens; hence we need to put in place some sort of safer alternatives and/or additional preventative safety measures. Recently, several substitutes (alternatives) to virtual blood components have been developed and tried. Moreover, various mechanical methods such as cell washing and leukofiltration have been implemented as additional preventative safety measures but with limited success in abrogating the risk of transfusion transmitted cell-associated agents. The most promising approaches, so far, are methods that target pathogen nucleic acids (Methylene blue; Psolaren and Riboflavin UV light treatment). These procedures have undergone considerable in vitro studies to ensure their extremely high safety margins in terms of toxicity to the cells or to the recipients. In essence, while the technology of targeting nucleic acid to stop viral proliferation is common to the above three strategies, in practice these procedures differ in terms of operational, physicochemical and biological characteristics; including the potential impacts of their metabolites and photo-adducts; their effects on the spectrum of pathogens affected and the log reductions in culture infective studies. Accordingly, any strategy that involves addition of an extraneous agent or physicochemical manipulation of blood must balance the benefits of pathogen reduction against the loss or alteration to the cells and plasma functional integrity, short and long term toxicity to the cells and to the recipients, as well as the risk to the personnel involved and the community at large. Moreover, it must be noted that each method will have a different profile of adverse reactions and may differ in terms of the risk to particularly vulnerable groups of patients, requiring in depth clinical trials, while taking into consideration the cost benefit of the final process. Newer diagnostic procedures must be in place to establish the storage stability of products that have undergone pathogen inactivation, in particular tests reflecting the release of platelet-derived cytokines, cellular apoptosis or microvesiculation and their role in immunosupressiveness. This overview aims to provide an update on the continual improvements in blood component safety, in particular using methods that target pathogen nucleic acid. Emphasis is placed on methylene blue light treatment (MBLT) and Intercept or Mirasol PRT systems for platelets and plasma. The status of pathogen reduction of whole blood and red cells is also highlighted, though the progress in this area has been virtually stopped after the finding of antibody development in the clinical trial.

Inactivation of a European strain of West Nile virus in single- donor platelet concentrate using the INTERCEPT blood system

BACKGROUND AND OBJECTIVE

In order to prevent West Nile virus (WNV) contaminations by transfusion, the French National Blood Service decided to evaluate the INTERCEPT Blood System's efficiency on a European strain.

MATERIALS AND METHODS

Culture supernatant of WNV was used to infect six platelets concentrates. Viral titre was determined by plaque reduction neutralization test before and after viral inactivation using the INTERCEPT Blood System.

RESULTS

In all assays, the absence of plaque forming unit was observed after viral inactivation. The log reduction observed ranged between > 5.1 logs to > 5.2 logs.

CONCLUSION

INTERCEPT Blood System is a commercially viral inactivation method potentially useful in order to prevent WNV transmission by blood products in France during re-emerging outbreaks.

Pathogen inactivation techniques

The desire to rid the blood supply of pathogens of all types has led to the development of many technologies aimed at the same goal--eradication of the pathogen(s) without harming the blood cells or generating toxic chemical agents. This is a very ambitious goal, and one that has yet to be achieved. One approach is to shun the 'one size fits all' concept and to target pathogen-reduction agents at the Individual component types. This permits the development of technologies that might be compatible with, for example, plasma products but that would be cytocidal and thus incompatible with platelet concentrates or red blood cell units. The technologies to be discussed include solvent detergent and methylene blue treatments--designed to inactivate plasma components and derivatives; psoralens (S-59--amotosalen) designed to pathogen-reduce units of platelets; and two products aimed at red blood cells, S-303 (a Frale--frangible anchor-linker effector compound) and Inactine (a binary ethyleneimine). A final pathogen-reduction material that might actually allow one material to inactivate all three blood components--riboflavin (vitamin B2)--is also under development. The sites of action of the amotosalen (S-59), the S-303 Frale, Inactine, and riboflavin are all localized in the nucleic acid part of the pathogen. Solvent detergent materials act by dissolving the plasma envelope, thus compromising the integrity of the pathogen membrane and rendering it non-infectious. By disrupting the pathogen's ability to replicate or survive, its infectivity is removed. The degree to which bacteria and viruses are affected by a particular pathogen-reducing technology relates to its Gram-positive or Gram-negative status, to the sporulation characteristics for bacteria, and the presence of lipid or protein envelopes for viruses. Concerns related to photoproducts and other breakdown products of these technologies remain, and the toxicology of pathogen-reduction treatments is a major ongoing area of investigation. Clearly, regulatory agencies have a major role to play in the evaluation of these new technologies. This chapter will cover the several types of pathogen-reduction systems, mechanisms of action, the inactivation efficacy for specific types of pathogens, toxicology of the various systems and the published research and clinical trial data supporting their potential usefulness. Due to the nature of the field, pathogen reduction is a work in progress and this review should be considered as a snapshot in time rather than a clear picture of what the future will bring.

Replication of West Nile virus in equine peripheral blood mononuclear cells

A cell model of primary monocytes and other mononuclear cells isolated from equine blood was used to study the kinetics of West Nile virus (WNV) replication in a natural host. West Nile virus has emerged on the North American continent as a significant cause of morbidity and mortality in a wide range of avian and mammalian species. While other flaviviruses are known to infect monocytes and lymphocytes, the ability of WNV to productively replicate in specific immune cells of peripheral blood has not been assessed. In this study, enriched populations of monocytes and lymphocytes as well as purified monocytes, CD4+, CD8+ and B lymphocytes were obtained from equine blood. Productive WNV replication was demonstrated by viral growth curves, quantitative RT-PCR for WNV RNA, and indirect immunofluorescence detection of a non-structural WNV protein. Enriched and purified monocytes consistently supported productive viral replication in blood from nine of nine horses tested while a minor subset of CD4+ lymphocytes supported productive replication in cells from three of the nine horses tested. Peak viral titers of 3.2-6.6 log10 PFU/ml were reached at 6 days post-inoculation (p.i.) and titers were maintained through 10-15 days p.i. Activation of monocytes with bacterial lipopolysaccharide, which resulted in activation of nuclear transcription factor kappaB (NF-kappaB) plus elevation of nitric oxide and type I interferon levels, reduced or eliminated WNV replication. These results suggest that immune cells of the peripheral blood may serve as target cells for initial replication of WNV and may play a role in subsequent viral dissemination. Furthermore, primary equine immune cell cultures represent a potentially useful model of a natural WNV host when testing compounds such as antivirals for use in WNV treatment.

Protecting the blood supply from emerging pathogens: the role of pathogen inactivation

Although the risk of infection by blood transfusion is relatively low, breakthrough infections still occur, Transfusion-related fatalities caused by infections continue to be reported, and blood is not tested for many potentially dangerous pathogens. The current paradigm for increasing the safety of the blood supply is the development and implementation of laboratory screening methods and restrictive donor criteria. When considering the large number of known pathogens and the fact that pathogens continue to emerge, it is clear that the utility of new tests and donor restrictions will continue to be a challenge when considering the cost of developing and implementing new screening assays, the loss of potential donors, and the risk of testing errors. Despite improving the safety of blood components, testing remains a reactive approach to blood safety. The contaminating organisms must be identified before sensitive tests can be developed. In contrast, pathogen inactivation is a proactive strategy designed to inactivate a pathogen before it enters the blood supply. Almost all pathogen inactivation technologies target nucleic acids, allowing for the inactivation of a variety of nucleic acid-containing pathogens within plasma, platelets, or red blood cells thus providing the potential to reduce transfusion-transmitted diseases. However, widespread use of a pathogen inactivation technology can only be realized when proven safe and efficacious and not cost-prohibitive.

Pathogen inactivation technology: cleansing the blood supply

The calculated residual infectious risk of HIV, hepatitis B virus (HBV) and hepatitis C virus (HCV) from blood transfusion is extremely low. However, the risk of bacterial contamination remains and a variety of other agents including emerging viruses, protozoa and tick-borne agents threaten blood supplies and undermine public confidence in blood safety. Traditional methods of donor screening and testing have limited ability to further reduce disease transmission and cannot prevent an emerging infectious agent from entering the blood supply. Pathogen inactivation technologies have all but eliminated the infectious risks of plasma-derived protein fractions, but as yet no technique has proved sufficiently safe and effective for traditional blood components. Half-way technologies can reduce the risk of pathogen transmission from fresh frozen plasma and cryoprecipitate. Traditional methods of mechanical removal such as washing and filtration have limited success in reducing the risk of cell-associated agents, but methods aimed at sterilizing blood have either proved toxic to the cells or to the recipients of blood components. Several promising methods that target pathogen nucleic acid have recently entered clinical testing.

West Nile virus—an old virus learning new tricks?

West Nile virus (WNV) has spread across the United States causing annual outbreaks since its emergence in 1999. Although severe disease develops only in about 1% of infections, WNV has claimed a total of 564 lives in the 5 years from 1999 to 2003. Observation of flaccid paralysis due to WNV infection at a higher incidence than previously documented and the devastating mortality recorded in infected American bird species triggered concerns about a potentially enhanced virulence of this virus. Here we summarize recent observations made during the American outbreaks regarding host range and transmission modes of WNV, and discuss epidemiological aspects of the emergence of this pathogen in the new habitat.

Infectious Risks Associated with the Transfusion of Blood Components and Pathogen Inactivation in Japan

Even after the implementation of the nucleic acid amplification testing (NAT) system, there remains a residual risk of viral transmission through blood transfusion because of the limited sensitivity of the reagents used and the pooling strategy of the current NAT system. From the calculation using NAT yield and the length of the window period, we presume that we will obtain 0.75 donations for human immunodeficiency virus and 0.58 donations for hepatitis C virus annually that are individual donation-NAT positive but 50-individual pool-NAT negative, figures that are comparable with those in other developed countries. The number of donations potentially positive for the hepatitis B virus genome is, however, considerably high in Japan and is estimated to be more than 100 annually, which is the sum of the donors in the minipool-NAT window period and the chronic carriers with a low viral load. The incidence of bacterial sepsis after transfusion is relatively low in Japan. This incidence is possibly attributable to the short shelf lives of platelet concentrate and red blood cell component, which are 3 and 21 days, respectively. In Japan, the implementation of a new technology to screen out or abrogate infectious agents in blood components is necessary while considering the balance between benefits and possible new risks or costs.

Induction of latent human cytomegalovirus by conventional gamma irradiation and prevention by treatment with INACTINE PEN110

BACKGROUND AND OBJECTIVES

Two different leucocyte-inactivation technologies--gamma irradiation and INACTINE PEN110--were evaluated for their effects on cell-associated human cytomegalovirus (CMV).

MATERIALS AND METHODS

In vitro CMV-infected cells were spiked into leucoreduced red blood cell concentrates (RCC) or medium at a final concentration of 0.5 - 1 x 10(7) cells/ml to mimic non-leucoreduced levels of leucocytes. The spiked RCC/medium was divided into three equal units and treated with gamma irradiation at the US Food and Drug Administration (FDA)-approved dose of 25 Gy, with 0.1% v/v PEN110 at 22 degrees C for 24 h, or stored at 4 degrees C as a control. The treated and control cells were recovered and tested using infectivity, viability and polymerase chain reaction (PCR) assays.

RESULTS

Gamma-irradiated CMV-infected cells produced active virus, as shown by both infectivity assays and PCR quantification of viral DNA. PCR analysis demonstrated higher CMV DNA levels in gamma-irradiated, latently infected monocytic THP-1 cells than untreated control cells. The increased virus production in gamma-irradiated cells was paralleled by an increased metabolic rate and the development of enlarged multinuclear cells. In contrast, PEN110 treatment terminated virus replication and completely inactivated the infected cell.

CONCLUSIONS

These results demonstrate that gamma irradiation, at levels currently used to treat RCC, has the capacity to induce expression of CMV, whereas PEN110 inhibits CMV replication and efficiently inactivates the infected cells.

West Nile virus in plasma is highly sensitive to methylene blue-light treatment

BACKGROUND

The epidemic of West Nile virus (WNV) in the US resulted in cases of transfusion-transmitted WNV. Effective pathogen reduction methods could have removed this infectious agent from the blood supply We have evaluated the efficacy of photodynamic treatment of fresh frozen plasma (FFP) with methylene blue (MB), a decontamination method applied in several European countries.

STUDY DESIGN AND METHODS

FFP units (300 ml each) were spiked with WNV. MB was added, and the units were illuminated with white or monochromatic yellow light. WNV infectivity was determined by bioassay. WNV-RNA was quantitated by real-time PCR. The inactivation of WNV was investigated under standard and under suboptimal conditions, respectively. In addition, rechallenge experiments with multiple addition of WNV at maximal load (approx. 105 CFU/ml) and repeated illumination without replenishing MB were performed.

RESULTS

Complete inactivation of WNV was achieved by MB (0.8-1 mmol/l) and illumination with white light (30,000-45,000 Lux) within 2 min. White yellow light 20-40 J/cm(2) (2.5-5 min) were sufficient for inactivation by 5.75 log10-steps. The rechallenge experiments revealed the substantial reserve capacity of the procedure to inactivate WNV. Quantitative PCR indicated that the viral RNA was rapidly destroyed.

CONCLUSION

All experimental data demonstrate the enormous potency of phototreatment with MB to inactivate WNV in plasma.

West Nile virus encephalitis in a kidney transplant recipient

We describe a case of West Nile virus encephalitis in a 54-year-old kidney transplant recipient. The clinical course was rapid and fatal. Serial CSF samples showed an evolving mononuclear pleiocytosis and serial MRIs showed increasing signs of cytotoxic edema in her basal ganglia. Seroepidemiological testing indicated that the infection was most likely acquired from transfusion of fresh frozen plasma at the time of transplantation.

Longitudinal Studies of West Nile Virus Infection in Avians, Yucatán State, México

Following the introduction of West Nile virus (WNV) into North America in 1999, surveillance for evidence of infection with this virus in migratory and resident birds was established in Yucatán State, México in March 2000. Overall, 8611 birds representing 182 species and 14 orders were captured and assayed for antibodies to WNV. Of these, 5066 (59%) birds were residents and 3545 (41%) birds were migrants. Twenty-one (0.24%) birds exhibited evidence of flavivirus infection. Of these, 8 birds had antibodies to WNV by epitope-blocking enzyme-linked immunosorbent assay. Five (0.06%) birds (gray catbird, brown-crested flycatcher, rose-breasted grosbeak, blue bunting and indigo bunting) were confirmed to have WNV infections by plaque reduction neutralization test. The WNV-infected birds were sampled in December 2002 and January 2003. The brown-crested flycatcher and blue bunting presumably were resident birds; the other WNV seropositive birds were migrants. These data provide evidence of WNV transmission among birds in the Yucatán Peninsula.