February 09, 2008

Influenza Marches On

Its winter here in Calcutta, India. The sombre wintry wind and the overcast skies bring a pall of melancholy in their wake. To add to it, the bird flu wave this part of India is experiencing, leaves us all sad in the stomach.

While this epidemic of avian influenza (bird flu) is affecting large parts of Asia, Europe and extending its grip towards other continents, the earliest known documentation of human influenza was given by Hippocrates (the famous Greek physician, known as the Father of Medicine) about 2,400 years ago.

It is known that the disease caused by orthomyxovirus ( an RNA virus) occurs more commonly during the cold winter season. In fact, the word influenza comes from the Italian word Influenza di freddo meaning 'influence of the cold'. But why this season is specially chosen by the virus is not known for sure. Many different explanations have been put forward. Some say that in winter people remain indoors, they huddle together thus facilitating the spread of infection. People also go on holidays during this period and may thus help in disseminating the infection. Others argue that in winter the immunity is low due to fall in vitamin D level (staying indoors will mean less exposure in the sun, causing decline in vitamin D levels.) Some researchers even postulated that air currents in the upper atmosphere during winter was to blame.

Peter Palese, a flu researcher of the microbiology dept. of Mount Sinai school of medicine in New York, now suggests that it is the winter's influence on the virus that may be responsible and not the other way around. During winter, the humidity in the air is less. The droplets containing the virus get dried up faster and thus suspend in the air longer. Moreover, the cold air is more conducive for the virus. The virus is more 'stable' during winter.

Compared to common cold (caused mostly by rhinovirus and spread mainly via droplet infection or direct touch or with fomites such as door-knobs, handkerchiefs etc.)droplets eject as man sneezes, influenza is a much more severe disease. It spreads via droplet infection. When we sneeze, for example, droplets of mucus are thrown out at high speeds of about 150 kph (as is shown in the figure). Thus influenza differs from colds. It should also be differentiated from Haemophilus influenzae, which is a bacteria that produces pneumonia, meningitis and other diseases.

In influenza, the patient suffers from cough and cold, myalgia or muscle weakness, headache and other symptoms. Apart from giving symptomatic relief and antiviral drugs [Tamiflu (oseltamivir) Relenza (Zanamivir): both are neuraminidase inhibitors and interfere with the budding out off the host cells], we must also vaccinate persons by giving 'flu shots' for prevention. Typically, a trivalent vaccine is used to offer immunity. The virus contains two important antigens- hemagglutinin and neuraminidase. The genes that are responsible for the antigens, H and N, mutate rapidly thus evolve into new strains easily. The avian flu virus (H5N1) is usually restricted to the birds. But they are changing patterns and are out to get us. This will be fatal!

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