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Key Diagnostic Tests.

Although often a relatively mild fever that is difficult to differentiate from other fevers. In more serious cases, it presents as a hemorrhagic hepatonephritis (inflammation of the liver and kidneys leading to hemorrhage). In advanced stages, there is vomiting of blood from buccal and intestinal hemorrhaging.

• Isolation of the virus, it is fairly easy to recover from blood and liver biopsies;
• ELISA is available for the virus;
• hemorrhagic lesions on multiple organs;
• jaundice;
• necrosis of the median region of the liver;
• raised serum aspartate aminotransferase (AST).

Symptoms and effects.

In many cases, infection is relatively mild and difficult to differentiate from other fevers.

In more serious cases, symptoms occur 3-6 days after infection, beginning with the sharp onset of fever, chills, pain in the lower back. A rash is sometimes seen on the chest. This phase lasts 3-4 days followed by a remission and a second febrile phase

The second phase involves the liver and kidneys. The pulse slows and jaundice develops with hemorrhages in the digestive tract leading to black (or coffee ground) vomiting.

Medical and Physical Countermeasures.

Vaccination (Immunoprophylaxis)

An attenuated live vaccine of virus cultured in eggs is available for those at risk of infection. Vaccination is effective for about 10 years.

 Specific Therapy

No specific therapy is available.

Supportive care

Minimize intrusive care to protect a weakened vascular bed.

• Rehydration to maintain electrolyte balance
• use of hepatoprotectants
• use of anti-emetics
• transfusion with whole blood or platelets
• vitamin K1

The virus is killed by many disinfectants, including 1% sodium hypochlorite, 2% glutaraldehyde, formaldehyde, and 70% ethanol. It is killed by heating at 60°C for 10 minutes.

Agent Properties and Potential Uses

The virus is relatively fragile and does not survive outside its vector - a mosquito of the genus Aedes. However, in the case of urban yellow fever, which occurs in regions of high population density, it is known to be transmitted between humans. In the laboratory the disease has been spread by contaminated body fluids, including blood and urine and there is some risk of aerosol spread. Unexpected appearances of the mosquito vector would be a warning sign.

Terrorist Acquisition and Attempted Use.

• None confirmed.

The History and Natural History of Yellow Fever.

The home of the yellow fever virus is western Africa where it lives without apparent adverse effects in the monkeys of the high jungle canopy. It is transmitted to man by bites from the mosquito Aedes aegypti. The mosquito takes a blood meal from an infected monkey, and then bites a human. The mosquito injects saliva, containing the virus, into the bite to prevent blood clotting and infects the human. The yellow fever virus is known as an arbovirus or arthropod-borne virus because of the means of transmission.

When Europeans began to colonize western Africa the virus killed the colonists by the hundred and the area became known as the White Man's Grave because of it. The virus was also killing the native people about as efficiently.

Mosquito larvae carried in ships water barrels spread the virus very efficiently and it became established in the Americas where it ranged as far north as Boston and claimed up to 75,000 lives a year in the United States in the last quarter of the 19th Century.

The mode of transmission was a mystery until Walter Reed demonstrated the role of the mosquito. Once this was known, the disease was brought under control in the Western Hemisphere as the 20th Century dawned. The Cuban capital of Havana benefitted from an extremely aggressive effort by US forces stationed there. Stagnant puddles and ponds where the mosquito larvae mature were drained or treated with oil to kill the larvae. Even without the use of insecticides the disease was brought under control and it has been essentially unknown in the city since. The last major outbreak in the United States was in New Orleans in 1905.

In one of the peculiarities of history, Cuba was invaded by the United States during the Spanish-American War in part because it was seen as a source of yellow fever that its Spanish governors chose to do nothing about. Although Cuba was freed of the disease, this did not prevent it from becoming established in the southern United States. Although Central American countries respond well to outbreaks of yellow fever, the southern United States is now the major reservoir for the disease in the Western Hemisphere. Environmental concerns about the use of pesticides make it difficult to conduct the wide-scale aerial spraying that would bring the mosquito under control.

A century before the disease was defeated in Cuba, it had defeated Napoleon. When slaves led by Toussaint L'Ouverture revolted on the island of Santo Domingo, Napoleon sent a force of 33,000 to put it down. Within a few months, yellow fever had killed 30,000 French soldiers, laid the foundation for the independence of Haiti and forced Napoleon to give up on his ambitions for the Americas.

Even though deforestation brought the disease under control, it was adaptive enough to find another home as it began to appear in towns and cities and was termed urban yellow fever in contrast to the sylvatic yellow fever of the jungle. The virus could be transmitted by other members of the genus Aedes and this included a number that used small patches of water in discarded tyres and drink cans to breed.

The mosquito can still be controlled with insecticides and the vaccine against the virus gives lifelong protection, but neither mosquito control nor vaccination in areas at risk are complete and there are still occasional outbreaks. Even without its use as a weapon, there is a concern that the disease may break out again.