GRIPPE AVIAIRE HUMAINE: UN TAUX DE MORTALITE PLUS IMPORTANT QUE PREDIT, selon les recherches effectuées après l'épidémie au Cambodge.

Mild H5N1 cases weren’t missed in Cambodian outbreak

Mar 27, 2006 (CIDRAP News) – Researchers who looked for mild or asymptomatic human cases of H5N1 avian influenza following an outbreak in Cambodia last year didn't find any, challenging the view that human cases have gone undetected, according to findings presented last week.

A la suite de l'éruption de l' épidémie de H5N1 l'an dernier au Cambodge, les scientifiques qui cherchaient des cas humains "doux"ou asymptomatiques n'en ont trouvé aucun, défiant quiconque de déceler des cas humains non détectés.

The research described Mar 20 at the International Conference on Emerging Infectious Diseases in Atlanta implies that surveillance for human cases might be more effective than some experts assumed, according to a story by the Canadian Press (CP). However, the findings also imply that the case-fatality rate for avian flu is higher than some experts thought.

Suite à la recherche décrite au congrès des maladies infectieuses qui s'est tenu à Atlanta le 20 mars dernier, celle-ci sous-entend que la surveillance des cas humains doit être plus efficace que ce qui était supposé par certains experts. le taux de mortalité de la grippe aviaire est bien plus élevé que ne le pensent certains experts.

Dr. Philippe Buchy and his colleagues at the Institut Pasteur in Phnom Penh last spring tested blood samples from 351 residents of a Cambodian village where poultry and one person had died of avian flu, the CP reported. No signs of antibodies to H5N1 were found in the samples, indicating the residents had not suffered even mild cases of avian flu.

Le Dr. Philippe Buchy et ses collègues de l' Institut Pasteur de Phnom Penh ont testé des échantillons de sang de 351 habitants d'un village cambodgien, où la volaille, ainsi qu'une personne, étaient décédés de la grippe aviaire. Ils n'ont trouvé aucun anticorps de H5N1, signifiant que les habitants n'avaient pas souffert de la forme faiblement pathogène de la grippe aviaire

Some of the people tested had "significant" exposure to poultry or infected people, the story said. For example, a doctor who inserted a tube down an H5N1 patient's windpipe without wearing any protective gear did not show any antibodies indicating infection. The same was true for other healthcare workers, including two veterinarians who had autopsied H5N1-infected birds. The healthcare workers did not know at the time they were dealing with avian flu cases.

Le récit dit que certaines personnes testées avaient une exposition "significative" à la volaille ou à des gens infectés: par exemple un médecin ayant intubé un patient dans la trachée, sans gants ni équipement de protection, n'a présenté aucun anticorps indiquant l'infection. Idem pour des infirmiers et 2 vétérinaires qui ont autopsié des volailles contaminées au H5N1 (à l'époque ils ignoraient qu'il s'agissait de ce virus).

"We didn't find any cases of H5N1, so nobody seems to have been asymptomatic or with mild symptoms during this outbreak in Cambodia," Buchy told the CP.

"Nous n'avons trouvé aucun cas de H5N1, ainsi personne ne semblait avoir de syptomes asyptomatiques ou à faible prévalence pendant cette éruption."

On the bright side, Buchy was quoted as saying the data indicate the virus still finds it difficult to jump from poultry to humans.

Dr. Nancy Cox, head of the influenza branch at the US Centers for Disease Control and Prevention (CDC) in Atlanta, concurred with Buchy, but added: "On the other side of the coin, it means that the case-fatality rate is still very high. And that is a negative thing."

"Le revers de la médaille, c'est que cela signifie que le taux de mortalité est très élevé. Et ceci est très négatif."

Experts have suggested that the current case fatality rate for avian flu, about 56%, could be inaccurate because milder or asymptomatic cases have not been identified.

Les experts suggéraient jusqu'à présent que le taux de mortalité pour la grippe aviaire était de 56%, du fait des cas asymptompatiques ou faiblement prévalents  non identifiés: ceci pourrait être faux.

"The work in Cambodia is extremely important because it shows that we really aren't missing that much," Cox told the CP. However, she added that it is important to conduct research on a larger scale to determine whether mild or asymptomatic cases are occurring, and said such studies are planned.

Le travail au Cambodge est extremement important, parce qu'il montre qu'en fait, nous ne manquons pas vraiment de données; le médecin a ajouté qu'il fallait absolument conduire des recherches à plus grande échelle pour déteminer si ces cas "doux"ou de faible prévalence se produisent réellement.

SUCPETIBILITE GENETIQUE AU H5N1 , UNE QUESTION DE RECEPTEURS ?
<http://www.promedmail.org>

d'apres cet article ,est cela semble coherent, les patients infectés viennent 
souvent de la meme famille
Mais il s'agit souvent d'enfant et de leur mere ou pere, mais jamais 
de mari et femme..ce qui laisse penser à un facteur genetique
Celui evoqué : presence de receptuers 2 3 ( aviaires) dasn le tractus 
respiratoire sup, permettant au virus de " s'implanter" dans l'organisme
bien que les etres humains aient des recepteurs 2 3, les gens qui ont été 
infectés devraient en avoir plus que la moyenne

des etudes vont etre faites sur les survivants pour verifier cette idée




Date: Thu 4 May 2006



People who have been infected with the H5N1 avian influenza virus 
might be especially susceptible to avian viruses because they are 
genetically predisposed to them, leading disease experts suggested on 
Thu 4 May 2006. Of the 205 [now 206 - Mod.CP] reported cases of human 
infections since late 2003, there have been many family clusters 
involving blood relatives, such as father and children, mothers and 
daughters. Of the total infections, 113 people died in 9 countries.

"There have been family clusters. So there has to be certainly a 
genetic aspect to it," Robert Webster of the St Jude Children's 
Research Hospital told a bird flu conference organized by the Lancet 
medical journal in Singapore.

Another leading expert, Hiroshi Kida, who has spent more than 3 
decades working on viruses, has long harbored the same theory. "There 
has not been a single case of infection involving husband and wife 
[i.e. family members not related by blood] ," Kida said told the 
Reuters News Agency in an interview. Kida is with the Department of 
Diseases Control at Hokkaido University in Japan. Kida explained that 
people infected with H5N1 have a carbohydrate receptor on cells 
lining their throats. The receptor -- called alpha 2,3 -- is 
predominantly found in birds and avian influenza viruses like to bind 
to this class of receptors to replicate and cause disease.

Human influenza viruses, however, prefer to bind to another receptor 
called alpha 2,6, which is dominant in humans. "I think people who 
are infected with avian strains are special. They must have alpha 2,3 
receptors," Kida said. Although humans have some amount of alpha 2,3, 
Kida said alpha 2,6 was by far more "dominant" in most people. [For a 
more precise discussion, see ProMED-mail post archived as "Avian 
influenza virus (03): virus receptor distribution 20060322.0893". - Mod.CP]

Kida is now trying to look for H5N1 survivors in Viet Nam and 
Thailand to verify his theory, and if it proves to be true, it could 
mean that most people simply cannot catch H5N1 easily -- unless the 
virus mutates. "If it changes receptor specificity, then it must be 
dangerous," Kida said.

Many experts see H5N1 [virus] as possibly triggering an influenza 
pandemic that is long overdue. But that could happen only if it 
mutates sufficiently to become easily passed among people. Most of 
its victims contracted the virus directly from sick birds. And there 
have not been any proven cases of human-to-human transmission.

Although very little is known about the virus, much work has been 
done to find out how it is transmitted, why so few people have been 
infected, and why it hasn't yet become infectious among people. A 
group of researchers recently postulated that the virus lodges itself 
deep in the lungs, and not in the upper respiratory tract where it 
could more easily dislodge itself, get out of the body and spread. 
But while Kida does not dismiss this theory, he thinks it is not the only one.

http://www.who.int/csr/disease/avian_influenza/guidelines/case_definition2006_08_29/en/index.html
trad : Anne
Background :

 Le report précis et rapide des cas de grippe H5N1 à l'OMS est la pierre angulaire pour surveiller l'évolution globale de cette maladie et le risque correspondant de l'emergence d'un virus pandémique.
En collaboration avec plusieurs associés, l'OMS a développé des définitions normalisées des cas pour faciliter :
 1. report  et classification des cas humains de l'infection H5N1 par les services d'hygiène nationaux et internationaux.
 2. Standardisation de langue pour la communication.
3. Comparabilité des données à travers le temps et les secteurs géographiques.

 Application des définitions de cas H5N1 :

1. Les définitions de cas s'appliquent à la phase actuelle de l'alerte de pandémie (phase 3) et peuvent changer pendant que de nouvelles informations sur la maladie ou son épidémiologie deviennent disponibles.

2. Les autorités nationales devraient formellement informer  les seuls cas H5N1 probables et confirmés à l'OMS. Les définitions de cas pour les personnes sous investigation et les cas suspectés ont été développées pour aider des autorités nationales en classifiant et en dépistant les cas.

 3. Les définitions de cas ne sont pas prévues pour fournir des descriptions complètes de la maladie des patients mais pour normaliser plutôt le report des cas.

4. Dans des situations cliniques exigeant des décisions au sujet de traitement, du soin ou du triage des personnes pouvant avoir l'infection H5N1, ces décisions devraient être basées sur le jugement clinique et le raisonnement épidémiologique, et non sur l'adhérence aux définitions de cas. Alors que la plupart des patients présentant l'infection H5N1 se sont présentés avec de la fièvre et les plaintes respiratoires basses, le spectre clinique est large.

Définitions des cas :

Personne sous investigation :

personne que les autorités sanitaires ont décidé d'étudier, de surveiller pour déceler ou non l'infection du virus H5N1.

1) cas H5N1 suspecté:

 personne présentant un symptôme respiratoire aiguë bas non expliqué avec une fièvre inférieure à 38°C, toux, souffle court ou difficulté respiratoire
ET
Une ou plusieurs expositions dans les 7 jours précédant le début des symptômes :

a. Contact étroit (à moins de 1 mètre, par ex en soignant, parlant, touchant )  avec une personne etant un cas probable ou suspecté ou avéré de H5n1

b. Exposition par manipulation, abattage, plumage, préparation pour la consommation de volailles ou d' oiseaux sauvages ou leurs restes ou une exposition aux environnements souillés par leurs fèces dans un secteur où des infections H5N1 chez les animaux ou les humains ont été suspectées ou confirmées au cours du mois écoulé.

c. Consommation de produits crus ou pas assez cuits de volaille dans un secteur où des infections H5N1 chez les animaux ou les humains ont été suspectées ou confirmées au cours du mois écoulé.

d. Contact étroit avec un animal infecté par le H5N1 (cas confirmé) autre que la volaille ou les oiseaux sauvages (par exemple le chat ou le porc)

e. Manipulation des échantillons (animal ou humain) suspectés de contenir le virus H5N1 dans un laboratoire ou tous autres endroits.

2 ) Cas H5N1 probable :

- definition 1 :
Ce cas regroupe tous les aspects du cas suspecté
ET
Un des critères additionnels suivants :

- Suspicion ou évidence d'une pneumonie aiguë sur la radiographie de thorax plus signes de deffaillances respiratoire (hypoxémie,  tachypnée severe)
OU
- confirmation positive  laboratoire d'une infection de la grippe de type A mais sans preuve pour le H5N1.

-  definition 2 :
Mais aussi cette définition regroupe les éléments suivants:
mort  par maladie respiratoire aiguë non expliquée,  considérée être épidémiologiquement liée par le temps, l' endroit, et l' exposition à un cas probable ou confirmé H5N1.

3) Cas H5N1 confirmé

Il s'agit d' une personne regroupant les critères pour un cas suspecté ou un cas probable
ET
- Un des résultats positifs suivant conduit par un laboratoire national, régional ou international de la grippe dont les résultats des tests sont acceptés par l'OMS comme confirmant le virus H5N1:

a. Isolement d'un virus H5N1

b. Résultats positifs d'une PCR de type H5 en utilisant deux cibles différentes au niveau de la PCR, par exemple amorces spécifiques pour hemagglutinine de la grippe A et H5 .

c .Une élévation de 4 fois ou plus des anticorps spécifiques au H5N1.,baséesur le test d'un specimen serique en phase aigue ( pris 7 jours ou moins apres le debut des symptomes ) et d'un serum convalescent
le serum convalescent doit avoir un titre d'anticorps de 1/80 ou plus

d . un titre d'ancicorps par microneutralization pour le H5N1, de 1/80 ou plus sur un serum simple pris à J14, ou plus tard apres le debut des symptomes, et un resultat positif usant un test serologique different, par exemple, un test d'inhibition de l'hemagglutination des globules rouges de cheval d'un titre de 1/60 ou plus, ou un test Western blot specifique du H5 positif

Menno D. de Jong, M.D., Ph.D., Tran Tan Thanh, M.Sc., Truong Huu Khanh, M.D., Vo Minh Hien, M.D., Gavin J.D. Smith, Ph.D., Nguyen Vinh Chau, M.D., Bach Van Cam, M.D., Phan Tu Qui, M.D., Do Quang Ha, M.D., Ph.D., Yi Guan, M.D., Ph.D., J.S. Malik Peiris, D.Phil., M.D., Tran Tinh Hien, M.D., Ph.D., and Jeremy Farrar, D.Phil., F.R.C.P.

SUMMARY

Influenza A (H5N1) virus with an amino acid substitution in neuraminidase conferring high-level resistance to oseltamivir was isolated from two of eight Vietnamese patients during oseltamivir treatment. Both patients died of influenza A (H5N1) virus infection, despite early initiation of treatment in one patient. Surviving patients had rapid declines in the viral load to undetectable levels during treatment. These observations suggest that resistance can emerge during the currently recommended regimen of oseltamivir therapy and may be associated with clinical deterioration and that the strategy for the treatment of influenza A (H5N1) virus infection should include additional antiviral agents.

---

Influenza A (H5N1) virus causes severe disease in humans and poses an unprecedented pandemic threat.^1,2,3 The neuraminidase inhibitor oseltamivir constitutes an important treatment option, and stockpiling of this drug is part of pandemic-preparedness plans.⁴ However, data on the efficacy and development of drug resistance in human influenza A (H5N1) virus are scarce. We report the isolation of oseltamivir-resistant influenza A (H5N1) variants from two patients who died of the infection, in one case despite the early initiation of treatment. Furthermore, we provide evidence suggesting that the presence of detectable virus after the completion of treatment is associated with a poor outcome. These observations have implications for the treatment of influenza A (H5N1) virus infection.

Case Report

A previously healthy 13-year-old Vietnamese girl weighing 28 kg (Patient 1 in Table 1) presented to a hospital in Dong Thap Province on January 22, 2005, with a one-day history of fever and cough. The day before, her mother (Patient 2 in Table 1) had died of influenza A (H5N1) virus infection after one day of oseltamivir treatment. Virus isolated from the mother did not reveal oseltamivir-resistance mutations. Because influenza A (H5N1) virus infection was suspected in the child at presentation, she received an initial 75-mg dose of oseltamivir and was transferred to a pediatric referral hospital. On admission, she had a temperature of 40.3°C, a pulse of 106 beats per minute, a respiratory rate of 36 breaths per minute, and normal blood pressure. Results of physical examination and routine biochemical measurements were unremarkable. Hematologic measurements showed a white-cell count of 4800 cells per cubic millimeter (normal range, 5500 to 15,500), with 12 percent lymphocytes, and a platelet count of 183,000 cells per cubic millimeter (normal range, 250,000 to 550,000). A blood culture showed no growth. A chest radiograph revealed a small focal pulmonary infiltrate in the right middle lobe (Figure 1A).

View this table: [in this window] [in a new window]   Table 1. Patients' Characteristics and Clinical and Virologic Outcome.

 

View larger version (89K): [in this window] [in a new window]   Figure 1. Serial Chest Radiographs Obtained from Patient 1. A chest radiograph obtained at admission revealed a small focal pulmonary infiltrate in the right middle zone (Panel A). A chest radiograph obtained two days later showed minimal progression of the infiltrate (Panel B). On day 3, the pneumonia had progressed to involve most of the right middle zone (Panel C), and further progression was noted on day 4 (Panel D) and day 6 (Panel E), the day the patient died.

 
The patient received a second 75-mg dose of oseltamivir within 6 hours after the first, followed by a third dose within the first 24 hours after admission. Treatment was then continued for four days at the standard dose of 75 mg twice daily. Antibiotic treatment with ceftriaxone and amikacin was also given. During the first three days after admission, the patient remained in stable condition and did not require supplemental oxygen. A chest radiograph obtained on January 24 showed minimal progression of the infiltrate (Figure 1B). At that time, her white-cell count was 3100 cells per cubic millimeter, with 26 percent lymphocytes.

On January 25 (the fourth day of oseltamivir treatment), the child's respiratory condition worsened and supplemental high-dose oxygen was given, initially by nasal cannula and later by continuous positive airway pressure. Antibiotic treatment was switched to vancomycin, ciprofloxacin, and amikacin. At this time, the pneumonia involved most of the right middle zone (Figure 1C). A chest radiograph obtained on January 26 showed further progression of the infiltrate (Figure 1D). Hematologic and biochemical measurements revealed a white-cell count of 1800 cells per cubic millimeter, with 41 percent lymphocytes; a platelet count of 97,000 cells per cubic millimeter; and increased serum alanine aminotransferase and aspartate aminotransferase levels (144 and 279 U per liter, respectively; normal range, less than 55 and less than 50, respectively). Her respiratory condition continued to worsen, and she was intubated and ventilation was begun on January 27. A chest radiograph obtained on January 28 showed pneumonia involving the entire right lung and extension to the left lung (Figure 1E). She died the same day. No autopsy was performed.

Methods

Patients and Clinical Specimens

We examined sequential pharyngeal swabs from Patient 1 and seven additional patients with influenza A (H5N1) infection from whom at least one pharyngeal swab obtained before treatment and during treatment with oseltamivir was available. The swabs were collected in viral-transport medium and stored at –80°C.

Virologic Investigations

Virus isolation was performed in Madin–Darby canine-kidney cells in biosafety level III culture facilities, and influenza viruses were identified by serotype-specific reverse-transcriptase–polymerase-chain-reaction (RT-PCR) assays and hemagglutination-inhibition assays, as described previously.^2,5,6 For the diagnosis of influenza A (H5N1) infection, nucleic acids were purified from 100 µl of pharyngeal specimens and subtype-specific RT-PCR assays were performed as described previously.^2,7,8 Quantitation of influenza A RNA was performed with the use of a real-time RT-PCR assay as described previously.⁶ The limit of detection of this assay is 1000 copies of RNA per milliliter. Quantitative analyses of serial pharyngeal specimens were performed in a batch-wise fashion. Positive and negative controls were included in all tests.

Sequence Analysis of Neuraminidase Genes

The sequence of the neuraminidase gene of initial viral isolates was analyzed as described previously,⁹ with the use of a BigDye Terminator Cycle Sequencing Kit (version 3.1, Applied Biosystems) on an ABI PRISM 3700 DNA Analyzer (Applied Biosystems). Sequence fragments were assembled and edited with the use of Lasergene software (version 6.0, DNASTAR). For direct sequencing of viral RNA from clinical specimens and follow-up isolates, a 200-bp fragment of N1 encompassing amino acid position 274 was amplified (primers available on request) and sequenced with the use of a CEQ Dye Terminator Cycle Sequencing Kit on a CEQ8000 system (Beckman Coulter). Alignment and residue analyses were performed with the use of BioEdit software (version 7). GenBank accession numbers of N1 sequences are shown in the Supplementary Appendix (available with the full text of this article at www.nejm.org).

Results

Isolation of Oseltamivir-Resistant influenza A (H5N1) Variants from Patient 1

Influenza A (H5N1) virus infection was diagnosed in another local laboratory by RT-PCR assay of a pharyngeal swab obtained at admission. This specimen was not available for further analysis. In our laboratory, influenza A (H5N1) virus was isolated from a throat swab obtained from Patient 1 on the fourth day of oseltamivir treatment (January 25, 2005). Sequence analysis of the neuraminidase gene revealed the substitution of tyrosine for histidine at amino acid position 274 (H274Y), associated with high-level resistance to oseltamivir in influenza (N1) viruses.¹⁰ Analysis of the raw sequencing traces revealed the presence of a minor subpopulation of wild-type 274H variants among predominating 274Y mutants (Figure 2). Virus was also isolated from a throat specimen obtained on January 28, 2005, two days after the completion of treatment. Sequence analyses of this strain as well as of viral RNA extracted directly from the swab also revealed the H274Y change in N1. Although sequencing traces also revealed the presence of a minor wild-type 274H population in viral RNA from the swab, only 274Y variants were observed in the isolate, possibly reflecting overgrowth of the predominant mutant population during culture. Determination of influenza A (H5N1) RNA levels showed that the viral load had increased in the second specimen (Figure 3).

View larger version (13K): [in this window] [in a new window]   Figure 2. Sequencing Trace of the Neuraminidase Gene of Influenza A (H5N1) Virus Isolated from Patient 1 on the Fourth Day of Oseltamivir Treatment. The arrow indicates the dominant T peak, with a smaller C peak at nucleotide position 763. A CT mutation at this position results in the substitution of tyrosine (encoded by TAC) for histidine (encoded by CAC) at amino acid position 274, which confers resistance to oseltamivir in N1-subtype influenza viruses.

 

View larger version (23K): [in this window] [in a new window]   Figure 3. Influenza A (H5N1) Viral RNA Load in Throat Swabs from Eight Patients. Blue lines represent patients who survived influenza A (H5N1) virus infection, and red lines represent patients who died. The dashed horizontal line denotes the limit of detection of the RT-PCR assay. The arrows indicate the specimens from which oseltamivir-resistant influenza A (H5N1) variants were isolated. No virus was isolated from any other specimen besides samples obtained at admission.

 
Virologic Responses and Isolation of a Second Oseltamivir-Resistant Strain

Of 13 patients with RT-PCR–confirmed influenza A (H5N1) infection who were admitted to the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, between January 2004 and February 2005, at least one throat swab obtained before treatment and during treatment was available from 7 patients (Patients 2 through 8 in Table 1; additional information on clinical features at admission, treatment, and outcomes is available in the Supplementary Appendix). Treatment with oseltamivir at the recommended dose and duration (75 mg twice daily for five days, with a weight-based reduction in the dose in children less than 13 years old) was begun on the day of admission in all patients. The remaining six patients, for whom only specimens obtained at admission and not during treatment were available, presented at similar days of illness (median, six days; range, four to seven) and also began to receive oseltamivir therapy on the day of admission. Five of these patients died within one to eight days after admission (median, three).

Influenza A (H5N1) virus was isolated from throat swabs obtained at admission from six of the seven patients (Patients 2, 3, 5, 6, 7, and 8). Sequence analysis of the neuraminidase genes of these viruses revealed the wild-type 274H residue alone. Measurements of the viral RNA load in sequential throat specimens showed rapid declines to undetectable levels in four patients who survived, whereas viral RNA was still detectable at the completion of oseltamivir treatment in two patients who died (Patients 3 and 4 in Figure 3). The remaining patient died during the second day after admission, at which time an increase in the viral RNA load was observed (Patient 2 in Figure 3). Direct sequencing revealed only wild-type 274H virus in the second specimen from this patient.

All subsequent throat specimens from Patients 3 through 8 were cultured. Of these specimens, influenza A (H5N1) virus was isolated only from the last specimen from Patient 4, obtained three days after the completion of treatment (Figure 3). Sequence analysis of this isolate revealed the H274Y substitution in N1. Although sequencing traces of the isolate revealed only mutant 274Y variants, direct sequencing of viral RNA from the same swab revealed evidence of a minor subpopulation of wild-type 274H viruses similar to that in specimens from Patient 1. Patient 4 died of respiratory failure six days after the isolation of resistant virus. Direct sequences of viral RNA from swabs obtained at admission and after two days of treatment showed wild-type 274H virus alone. The limited sensitivity of the method precluded direct sequencing of further samples from this patient. Likewise, no direct sequences could be obtained from the last specimen obtained from Patient 3.

Discussion

We report the isolation from two Vietnamese patients of influenza A (H5N1) viruses with a H274Y substitution in the neuraminidase gene, which confers high-level resistance to oseltamivir.^10,11 In contrast to the recent report of a partially resistant influenza A (H5N1) virus isolated during once-daily prophylactic treatment with oseltamivir,¹¹ the viruses in our patients were isolated during or shortly after a course of oseltamivir at therapeutic twice-daily doses, and mutant 274Y variants predominated. Furthermore, although the patient with partially resistant virus ultimately received oseltamivir at therapeutic doses and survived,¹¹ both of our patients died.

Patient 1 was treated with doses of oseltamivir that were relatively high for her weight, especially during the first day of treatment. Moreover, in this patient, unlike most patients with influenza A (H5N1) virus infection, treatment was started when the greatest clinical benefit could be expected: within 48 hours after the onset of symptoms. Indeed, her clinical condition remained stable during the first three days of treatment without the need for supplemental oxygen. However, on the fourth day of treatment she became progressively dependent on oxygen, her white-cell and platelet counts fell, and there was laboratory evidence of hepatitis. At the time of her death, the viral load in her throat had increased. These observations suggest that the development of drug resistance contributed to the failure of therapy and, ultimately, the death of this patient. In the second patient, the viral RNA load declined during treatment, but not to undetectable levels. Whereas only wild-type 274H virus was detectable after two days of treatment, 274Y mutant virus was isolated shortly after treatment. Although a direct relationship between the emergence of resistance and this patient's death was less clear, the presence of replicating virus after 14 days of illness suggests an effect on the outcome.

The emergence of resistant influenza A (H5N1) variants during oseltamivir treatment should not be surprising. In adults with influenza A (H1N1) or (H3N2) virus infection, the development of resistance to oseltamivir is rare, but resistant viral variants have been detected in up to 18 percent of children who receive oseltamivir.^12,13,14 The difference in resistance rates between adults and children may be explained by the occurrence of a primary infection in children associated with higher rates of viral replication owing to a lack of previous immunity. Since humans have no previous immunity to influenza A (H5N1) virus, all human infections with this virus are primary infections. In addition, studies in animals indicate particularly high levels of replication of current influenza A (H5N1) strains.^15,16

The efficacy of oseltamivir is unlikely to be optimal when treatment is instituted late in the course of illness, as has been the case in most patients with influenza A (H5N1) virus infection reported to date.^1,2 However, antiviral treatment could still be expected to be beneficial when there is evidence of ongoing viral replication. Such benefit is suggested by the rapid decline in the viral load to undetectable levels in all four survivors in the current series. In contrast, virus was still detectable at the end of treatment in three patients who died of the infection after receiving the full course of treatment, two of whom had oseltamivir-resistant virus isolated from throat specimens. Our observations suggest that at least in some patients with influenza A (H5N1) virus infection, treatment with the recommended dose of oseltamivir incompletely suppresses viral replication. Besides allowing the infection to proceed, such incomplete suppression provides opportunities for drug resistance to develop. Multiple mechanisms may account for inadequate viral suppression by oseltamivir, including overwhelming viral replication, as suggested by the results of studies in mice¹⁵ and altered pharmacokinetics in severely ill patients, who are apt to have diarrhea.^1,2,3 Strategies aimed at improving antiviral efficacy (e.g., the use of higher doses, longer durations of therapy, or combination therapy) may deserve further evaluation. In addition, antiviral agents to which oseltamivir-resistant influenza viruses remain susceptible should be included in treatment arsenals for influenza A (H5N1) virus infections.

Supported by the Wellcome Trust, United Kingdom.

No potential conflict of interest relevant to this article was reported.

Source Information

From the Oxford University Clinical Research Unit, Hospital for Tropical Diseases (M.D.J., T.T.T., D.Q.H., J.F.), Pediatric Hospital Number One (T.H.K., B.V.C., P.T.Q.), and the Hospital for Tropical Diseases (V.M.H., N.V.C., T.T.H.)— all in Ho Chi Minh City, Vietnam; and the Department of Microbiology, University of Hong Kong, Queen Mary Hospital, Hong Kong Special Administrative Region, China (G.J.D.S., Y.G., J.S.M.P.).

Address reprint requests to Dr. de Jong at the Oxford University Clinical Research Unit, Hospital for Tropical Diseases, 190 Ben Ham Tu, District 5, Ho Chi Minh City, Vietnam, or at mddejong@hcm.vnn.vn .