Читать книгу Cases in Medical Microbiology and Infectious Diseases - Melissa B. Miller - Страница 69

1. Although a number of respiratory viruses could explain this patient’s symptoms, influenza is the most common febrile respiratory illness in adults, particularly during the winter months, when influenza activity normally peaks. In pediatric patients (particularly <1 year old), respiratory syncytial virus should also be considered. The clinical clues of her influenza infection are the abrupt onset of fever and sore throat with nonproductive cough seen at her initial presentation to the ED. Clinically it is difficult to distinguish infections due to influenza A and influenza B, though influenza A tends to be associated with more severe disease, is generally the cause of annual epidemics, and has been responsible for all described pandemics.

Influenza virus has two major envelope proteins that contribute to its pathogenesis: neuraminidase and hemagglutinin. Neuraminidase likely has at least two functions. Its major function seems to be the cleavage of sialic acid from the cell surface and progeny virions, which facilitates the spread of new virions from infected respiratory cells. There is also evidence supporting the role of neuraminidase in viral entry to the cell. One mechanism that has been proposed is that neuraminidase cleaves decoy receptors on mucins, cilia, and cellular glycocalix so that the virus can have greater access to the functional receptors on the cell membrane. Once the virus penetrates to the cell surface, binding to specific sialic acid-rich receptors is mediated by hemagglutinin. Proteolytic cleavage of hemagglutinin by lung serine proteases is required for hemagglutinin activity. After the virus is endocytosed into the cell, hemagglutinin plays a role in the formation of channels through which viral RNA can enter the cytoplasm and initiate the viral replicative cycle.

2. In recent years only two hemagglutinin types (H1 and H3) and two neuraminidase types (N1 and N2) of influenza A virus have been circulating in humans (H1N1 and H3N2). However, due to antigenic variation, there are annual influenza epidemics and, in 2009, a pandemic. Why does this happen? There are two major evolutionary concepts related to influenza virus—antigenic drift and antigenic shift.

A unique property of influenza viruses is that they have single-stranded RNA genomes made of eight segments. Each influenza gene is found on a separate viral RNA segment. Since the mutation rate for RNA is higher than that of DNA (10⁻³ to 10⁻⁵ versus 10⁻⁶ to 10⁻⁸ per base per generation), point mutations readily accumulate in influenza viruses. Although mutations occur throughout the influenza genome, the accumulation of mutations (and corresponding amino acid changes) in surface antigens, such as hemagglutinin and neuraminidase, have the greatest impact. For influenza A virus, these changes will not necessarily result in the change of the classification of a viral strain (which is based on the subtypes of the H and N antigens), but they may be sufficient to render patients with antibodies to the parent strain susceptible to the new mutant strain. This is the basis for the decision to reevaluate and potentially change the formulation of the influenza vaccine each year to include recent isolates, so that protective antibodies to the most recent isolates will be made in response to the vaccine. Both influenza A and influenza B are constantly changing by antigenic drift.

The more dramatic, and less common, antigenic shift is due to genetic reassortment of genes to form a novel human influenza virus, which typically has different hemagglutinin and/or neuraminidase proteins. Antigenic shift occurs during coinfection of a cell with two different influenza A viruses. Since the packaging of viral RNA segments occurs randomly, a coinfected cell could form a variety of different virions. The result could be a virus with a different classification (e.g., a shift from H1N1 to H5N1) or a virus of the same type but with divergent genomic sequences from nonhuman sources such as pigs or birds. The end result is a new virus that differs dramatically from parent strains.

The influenza A H1N1 pandemic of 2009 was a result of antigenic shift. Although an H1N1 influenza virus had circulated globally for years, a reassortant H1N1 virus was introduced and spread worldwide. The 2009 H1N1 virus was a result of the introduction of Eurasian swine segments (neuraminidase and matrix) into the classical swine influenza strain that previously had only caused swine-to-swine transmission and rare swine-to-human transmission. When an antigenic shift occurs, most of the world’s population has little or no protection against the new virus, resulting in large epidemics or pandemics.

3. There are a variety of ways of diagnosing influenza in the laboratory, including rapid antigen tests, direct fluorescent-antibody assay (DFA), viral culture, and molecular detection. Rapid antigen tests are immunochromatographic assays that have been used for decades and have been favored due to their fast time to result (~15 minutes). However, as diagnostic methods have improved and circulating strains have changed, studies have shown that these tests suffer from lack of sensitivity. Sensitivities down to 10% were reported during the 2009 pandemic. Typical ranges of sensitivity reported are 20 to 90% depending on the strain circulating and the method used as the reference method. A further concern is the positive predictive value of rapid antigen tests when used outside of peak influenza season. Since positive predictive value is dependent on the prevalence of disease, using a test with imperfect specificities (90 to 95%) during times of low prevalence increases the chance that a positive result may actually be false positive rather than true positive. However, the times when laboratory testing for influenza is the most helpful clinically are at the beginning and end of the epidemic season, when the differential diagnosis is much broader. Another rapid method (~2 hours) is DFA testing. DFA uses a pool of monoclonal antibodies to influenza and other common respiratory viruses to directly detect infected cells obtained from the nasopharynx of patients. Although it is more sensitive than rapid antigen tests, DFA also had decreased sensitivity (~47%) for detecting the 2009 H1N1 pandemic strain. DFA sensitivity and specificity are also dependent on the skill of the personnel performing the test. Therefore, if rapid antigen tests or DFA must be used, alternative methods should be available to confirm the results, as needed.

Viral culture sensitivity is virus specific and ranges from 80 to 95%. Specificity approaches 100%. The disadvantage to culture is its longer turnaround time (up to 7 days). Rapid shell vial cultures have decreased the time to result to 24 to 48 hours, but this is still not adequate to aid in treatment decisions for influenza, which must occur in the first 48 hours of illness for the greatest benefit. Nonetheless, it is important for public health laboratories to maintain the capability of culturing influenza so that epidemiologic typing and resistance testing can be performed to inform next year’s vaccine components and antiviral recommendations.

The increase in molecular testing for influenza has been largely due to the limitations outlined above for other methods. Several FDA-cleared assays exist for the molecular detection of influenza with turnaround times ranging from 20 minutes to 8 hours. Sensitivities of these tests are 90 to 99%, with specificities of 98 to 99%. Some of the tests can also type influenza (i.e., H1, H3, or 2009 H1N1), and others can detect other respiratory viruses simultaneously. However, the majority of these tests require significant laboratory expertise and are more expensive than the other diagnostic methods listed. Since influenza genomic sequences change rapidly, it is important to monitor the accuracy of molecular tests on an annual basis. The curves shown in Fig. 10.1 represent the increase in fluorescence during real-time detection of PCR amplification.

A fluorescent probe is incorporated into the PCR reaction to measure on a per-cycle basis the presence of amplicons. Once the level of fluorescence is higher than the background level, the sample is positive. A lower cycle number of positivity (the point at which the curve crosses the horizontal threshold line) indicates a greater amount of virus in the sample. The positive result for the patient is shown by the gray line. The cycle threshold (Ct value) for the positive result is displayed by the red vertical line (27.3) and represents the cycle at which the fluorescence from the real-time PCR detection exceeds background. An example of a negative result is depicted by the purple line. The horizontal red line represents the threshold required for positivity in the PCR.

4. Most cases of influenza in this age group are self-limited and do not require hospitalization. Influenza is a much greater threat to individuals >65 years of age and children <5 years old. During most epidemics the highest numbers of hospitalizations and deaths are in these age groups. Other individuals at risk for complications of influenza infection are those with underlying chronic pulmonary diseases, such as asthma, cystic fibrosis, and chronic obstructive pulmonary disease; immunocompromised individuals; pregnant women, particularly in the second and third trimesters; and those with a variety of other chronic conditions such as cardiovascular disease and diabetes. This patient was a smoker and had diabetes, both of which put her at increased risk for severe influenza disease. In 2009, obesity was shown to be an independent risk factor for increased mortality due to H1N1. During the pandemic there was still significant disease and mortality in pediatric patients, with more than double the number of pediatric patients dying than in the previous three influenza seasons. Interestingly, the death rate for those 25 to 49 years of age was greatly increased as well, but little disease was seen in those >55 years of age. This suggests that influenza strains circulating prior to 1955 provided some protection against the pandemic strain, which was confirmed by serologic surveillance studies.

5. The most common complication leading to increased morbidity and mortality is pneumonia. This could be primary influenza virus pneumonia, secondary bacterial pneumonia, or a combination of the two. The majority of reported influenza-associated deaths appear to be due to influenza with accompanying bacterial pneumonia, especially pneumonia caused by Streptococcus pneumoniae and Staphylococcus aureus. For this patient, we cannot determine whether she has influenza pneumonia or bacterial pneumonia. To differentiate these, we would need a lower respiratory specimen (preferably a bronchoalveolar lavage) obtained prior to antibiotic administration to culture for bacteria and test for influenza. The sputum specimen obtained from this patient was rejected as inadequate for culture because there were no neutrophils present, suggesting a poor specimen collection. Thus, she was treated empirically for bacterial pneumonia.

6. There are currently two classes of anti-influenza drugs. The first class of agents, M2 inhibitors, blocks formation of influenza-derived ion channels. The reason these virally derived ion channels are important is that they play an important role in the “uncoating” of the virus. This is a step in viral replication in which viral RNA is released from the viral particle and enters the cytoplasm of the cell. The two drugs in this class are the oral agents amantadine and rimantadine. The drugs must be administered in the first 2 days of illness to be effective. They have been shown to reduce the disease course by 1 day. In addition, these agents prevent influenza illness in approximately 70 to 90% of individuals who take these agents prophylactically. Unfortunately, resistance to these drugs increased rapidly in influenza A H3 and 2009 H1N1. They do not work on influenza B. Therefore, in practice, these drugs are no longer used.

The second group of agents is the neuraminidase inhibitors. Two agents belong to this class of drugs—zanamivir, which is an inhaled agent, and oseltamivir, which is an oral agent. These agents are most effective if given in the first 2 days of illness and, like the ion channel-blocking agents, reduce the disease course by 1 day. However, data suggest that giving neuraminidase inhibitors at any time to a seriously ill patient may have benefits. The advantage of the neuraminidase inhibitors is that they are active against both influenza A and B viruses. However, influenza A H1 (pre-pandemic strain) is resistant to oseltamivir, and sporadic cases of H3 and 2009 H1N1 resistance have been described. To date, the majority of circulating influenza strains maintain susceptibility to both neuraminidase inhibitors.

7. Both vaccines are trivalent vaccines containing the same three influenza strains. The strains present in the 2012 vaccine included two subtypes of influenza A, 2009 H1N1 and H3N2, and influenza B. For the first time, in 2013 the vaccine contained two antigenically distinct influenza B viruses. It is important to remember that the composition of the vaccine changes annually. This is determined by the types of viruses that circulated during the previous season in the Southern Hemisphere. Due to waning immunity and antigenic drift of the viruses, the vaccine must be given annually. The efficacy of the vaccine is dependent on the level of change that may occur from year to year in the circulating virus, but it is generally 60 to 70% effective. One vaccine is an inactivated vaccine and can be administered intramuscularly (to those 6 months or older) or intradermally (to those 18 to 64 years old). There is also a high-dose inactivated vaccine that is given to people older than 65. The other vaccine is a live attenuated vaccine given intranasally to individuals aged 2 to 49 years. The live attenuated vaccine should not be given to pregnant women, immunocompromised individuals, or those caring for immunocompromised individuals.

The Centers for Disease Control and Prevention recommends that influenza vaccines be given to at-risk populations (see the answer to question 4 for a listing of at-risk populations). This includes children aged 6 months to 4 years, people 50 years and older, and health care personnel who could transmit the virus to at-risk patients. The vaccine is not recommended for children <6 months of age, a population that would most likely benefit from influenza virus vaccination. Numerous studies have proven the efficacy of this vaccine strategy. Recent studies also show that immunocompetent children benefit from vaccination through reduction in hospitalizations, doctor office visits, antibiotic use, serious secondary bacterial infections, and spread to at-risk family members.