West Nile virus is predominantly an infection of birds and mosquitoes.
Virus circulates in the blood of birds, then when mosquitoes take a blood meal from an infected bird they take up the virus as well.
The mosquito then transmits the virus to the next bird from which it feeds.
Other infected species, such as horses or humans, are incidental victims.
West Nile virus is not transmitted from person-to-person, except accidentally after blood transfusion.
The incubation period in man is typically 3 to 15 days.
Eighty percent of infected humans are asymptomatic while almost 20% have mild flu-like illness.
A small proportion (less than 1%) develop more severe disease such as encephalitis, meningitis, or meningoencephalitis, which is occasionally fatal.
Most deaths have been reported in those over 50 years of age.
In temperate zones, WNV occurs in late summer or early autumn. Most mosquitoes that carry WNV are likely to bite around dusk and dawn.
When outdoors, particularly during this time of day, wear loose-fitting, light-weight clothing that covers as much skin as possible and use an effective insect repellent on exposed skin and clothing.
Reduce mosquito bites indoors with use of air conditioning, insect-proof screens, and insecticides.
Bed nets can be used if necessary.
Avian influenza in birds
Avian influenza (AI) naturally circulates in wild waterfowl causing little or no symptoms, however AI may cause severe disease affecting the respiratory, digestive and/or nervous system in many other bird species.
Clinical signs may include depression, anorexia, ruffled feathers, diarrhea, ataxia, respiratory distress, and petechia.
Outbreaks associated with high bird mortality are called highly pathogenic avian influenza (HPAI).
Large AI outbreaks in poultry have been described in Pennsylvania in 1982 (H5N2), Mexico in 1993 (H5N2), Hong Kong in 1997 (H5N1), and The Netherlands in 2003 (H7N7).
In January 2004, AI in poultry was confirmed in Vietnam.
Since then, there have been HPAI outbreaks in a number of countries caused by the H5N1 subtype of influenza A virus.
Given the scale of the outbreaks in poultry, the virus does not appear to easily infect humans.
Humans are usually infected through close contact with live, infected birds.
Birds shed virus in their feces, so contact with feces is also a possible transmission route.
There is evidence to support limited human-to-human transmission (which presents the major concern for the future).
In the 1997 Hong Kong outbreak there were 18 confirmed human cases with 6 deaths, and in the Netherlands in 2003 there were 83 confirmed mild human cases and 1 death.
In the current outbreak of AI, human cases and deaths due to H5N1 have been reported from a number of countries.
Influenza outbreaks with high mortality affecting thousands and sometimes millions of people have occurred in 1918, 1957, 1968, and 1977.
These pandemics were caused by new influenza subtypes that were probably formed by combination of avian and human influenza viruses.