How does the high level of global traffic contribute significantly to the spread of influenza?
Influenza virus is transmitted via direct skin-to-skin contact, indirect contact with environment, or inhalation of droplets from coughs or sneezes. Coughing/sneezing can produce a large sized droplet (>10 micrometers) or a small sized one (≤ 5 micrometers). Large droplets can coat exposed surfaces for hours. Small droplets are more infectious and can remain suspended in the air for extended periods of time. The droplets can be disseminated via air currents, especially in closed environments. Therefore, an enclosed environment that brings people in close proximity to others for a length of time, such as the inside of an aircraft, can transmit the virus to many hosts. Influenza virus has a very short incubation period of 2-3 days. Because of the short incubation period, a person can become ill during travel which can cause significant problems to high-risk individuals like the elderly and those with underlying health problems. Most people are infectious before showing symptoms. As a direct result of the presymptomatic transmission, the rate of secondary influenza infections would be expected to occur on air flights, further spreading the virus. Children shed more virus than adults. They can shed the virus 6 days before they are symptomatic and up to more than 10 days after their symptoms disappear. Also, people who are severely immunocompromised can shed the virus for weeks or even months. The affordability and increasing necessity of air travel worldwide has made it an efficient method for transmitting influenza between people. Air travel has a significant impact on the spread of influenza and the rate of infected travelers into the U.S. from overseas will affect the rate of spread within the U.S. Air travel is of a major concern for influenza spread, as influenza follows a seasonal pattern demonstrating low summer and high winter incidences, which is consistent with the air travel pattern. Influenza virus also can be transmitted from wild birds or pigs, the natural hosts of the H5N1 virus, to humans. Pandemics are usually caused by a genetic shift in avian influenza A viruses that enables the virus to then infect humans and be transmitted from human to human. Usually transmission occurs among humans who own small poultry flocks; however, transmission can occur by contacting birds or even their feces. Therefore, the importation of poultry products from infected countries can facilitate the spread of avian influenza. The only way to prevent an influenza pandemic is to prevent human to human contact with infected individuals. The decision to close businesses, schools, mass gatherings and public transportation in affected areas for 10 days after an outbreak may be the best way to limit contacts, but could also lead to massive economic losses. The World Health Organization (WHO) has issued several travel advisories in the past to slow the international spread of disease, however reactions to these advisories were mixed because of the adverse economic impact. A solution to this problem may be a partial restriction of air travel, which could include closing highly connected hub airports in large cities to slow viral spread. This may have a large impact if the source epidemic is controlled before there are thousands of cases of influenza. If the initial spread is confined to a region, then those individuals in that region should be vaccinated. Quarantine could also limit contacts. It could be done in homes or in public buildings (i.e. hospitals), but law prevents children from being quarantined from their parents so that issue would have to be addressed. Studies have shown that quarantine usually just delays the inevitable epidemic by a few weeks. This may be beneficial if a winter outbreak is delayed until spring when conditions are not as favorable towards transmissibility. For air travel, using respiratory masks helps to cut the transmission cycle along with routinely sanitizing the air planes. Screening travelers, especially in the cases of a severe epidemic is another public health action to prevent the spread of the virus. Travelers should have their influenza vaccination history reviewed before boarding an aircraft. If they were not vaccinated the previous fall or winter, then they should be encouraged to receive the most current vaccine. Finally, basic public health education via television or radio commercials along with hanging posters in vulnerable public places, such as train or bus stations is also strongly recommended.
A 68-year old women is evaluated in your office for mid and low back pain. Her pain has been present for several months, but last week she experienced sharp pain after picking up her grocery bag. She takes no medications. On physical examination, she is thin. She has tenderness over the middle and lower thoracic regions. An x-ray of the back shows compression fractures at T10, L1, and L2. Her blood work shows calcium, phosphorus, and alkaline phosphatase levels are normal. Her bone mineral density shows spine T-score: -2.6 and total hip T-score of 1.9. What is the most likely diagnosis?
The patient has osteoporosis. She had recent vertebral compression fractures in response to minimal trauma (fragility fracture). Osteoporosis is diagnosed by the presence of fragility fractures (fracture secondary to minor trauma, such as falling from standing position), or by a bone mineral density (BMD) T-score less than -2.5 in patients who have not experienced a fragility fracture. Bone density scan results are reported in terms of T-scores (the standard deviation from mean BMD of a young healthy population) and Z-scores (the standard deviation from the BMD of an age- and sex-matched group). At the spine, a T-score of -1 represents approximately 10% bone loss. The T-score is used to diagnose osteoporosis. Osteoporosis is diagnosed by the presence of fragility fractures or by a bone mineral density T-score less than -2.5 in patients who have not experienced a fragility fracture.
What factors besides heat affect the melting temperature (Tm) of DNA? Explain how.
The G:C content of DNA and the ionic strength of the solution affect the melting temperature (Tm) of DNA. The higher the G:C content of DNA, the higher the temperature must be to denature the DNA strand. The base-pair G:C has three hydrogen bonds, compared to A:T which only has two. The extra hydrogen bond between G:C contributes to DNA stability. More importantly, the G:C base pair stabilizes DNA by its stacking interactions between adjacent base pairs. The stacking interactions are additive along the length of a DNA molecule. The stacking interactions between neighboring G:C base pairs are stronger than those of A:T base pairs. Consequently, DNA rich in G and C is harder to disrupt than DNA with a high proportion of A and T. The Tm of DNA increases with a higher salt concentration. This occurs because the positive charges from the salt solution shields the negative charges on the phosphoryl groups which comprise the backbone of DNA. If these charges are in close proximity and are not shielded, then the electrostatic repulsion of the negatively charged phosphoryl groups will help to drive the strand separation. The charges are shielded at a high ionic concentration, and therefore the DNA is stabilized. A higher temperature would be needed to change the Tm. When the ionic concentration is low, this shielding is decreased, which increases the repulsive forces between the strands and reduces the Tm. Lastly, formamide and urea, agents that destabilize hydrogen bonds, also lower the Tm. Extremes of pH denature DNA at low temperature.