Chapter 2
How Is West Nile Virus Spread?

There are two main types of West Nile virus capable of causing disease. Scientists have identified these two major types, or lineages, of the virus by using sophisticated gene sequencing techniques. These methods involve chemically separating the virus's DNA or RNA strands, the structures which house genes. Once these strands are separated, the genes on them can be mapped and sequenced with the aid of a computer. This identifies where different genes are on the DNA or RNA molecules. Different gene sequences characterize different lineages of a particular virus.

Investigators have discovered that only one of the two known lineages of West Nile virus causes human West Nile fever and West Nile encephalitis, meningitis, and meningoencephalitis. This group is known as Lineage One. Lineage One virus has been conclusively identified as causing West Nile virus outbreaks in Africa, Europe, Asia, the Middle East, and the United States. The virus grouping can be further broken down into subtypes, or different strains, whose genetic sequence may differ slightly, making these subtypes more or less aggressive or resistant to an immune-system response. One subtype of Lineage One West Nile virus known as Kunjin virus has been identified in cases of disease in Australia.

Lineage Two West Nile viruses are not associated with human disease. This viral group has been found in animals in Africa but does not appear to play a role in causing infections in humans.

Going Places

Although experts are not entirely sure how Lineage One West Nile virus suddenly arrived in the United States in 1999, they do know that birds and mosquitoes are involved in the transmission of this virus to people and other animals. This leads public health authorities to believe that either imported birds or infected mosquito larvae in internationally shipped cargo were responsible for the New York City–area outbreak. They have further narrowed down the strain of the virus that invaded the United States to one that is identical to a strain seen in outbreaks in Romania in 1996 and in Israel in 1998, so it is likely that the causative virus in the United States came from one of these two locations. Scientists at the National Veterinary Sciences Laboratories performed this comparison by sending tissue taken from dead birds found in New York City to the CDC laboratory in Fort Collins, Colorado. There, biologists matched the genetic structure of the West Nile virus found in the bird tissue with samples sent from Romania and Israel.

Birds to Mosquitoes

In addition to matching the genetic structure of these samples to determine the strain of the virus, scientists were able to track the manner in which the virus was then spread from birds to humans and other animals. This has led to an understanding of how West Nile virus causes disease as well as to information about which strain of the virus is responsible for a particular outbreak.

The infection begins in birds, so they are known as the primary host. Scientists performing studies in Egypt between 1952 and 1954 first associated birds with the virus when they found antibodies to West Nile virus in several common species of birds. Later studies showed that human epidemics of West Nile virus followed the illness or death of large numbers of birds, and other research proved that birds are indeed the starting place for the spread of the virus. More than one hundred species of birds have been found to carry West Nile virus, which sometimes kills the bird but often just makes the animal temporarily sick. Studies in the United States since the 1999 outbreak of West Nile virus have shown that the birds that survive the infection are mainly responsible for spreading the virus throughout the nation. "The virus has spread in the United States along the migratory patterns of birds,"⁶ say experts at the Mayo Clinic.

There have not been any documented instances of people or other animals contracting West Nile virus directly from an infected bird, however. Although public health officials caution that it is unwise to touch a dead or ill bird for other health reasons, they say it is very unlikely that such an act would result in the transmission of West Nile virus.

While researchers are not sure how birds contract the virus, they do know how the virus is transmitted to other animals and people. Once a bird is infected with the virus, the pathogen can be spread by mosquitoes that bite the infected bird, become infected themselves, and then pass the infection on to another animal or person they later bite. This transmission cycle, like the evidence that birds are the primary hosts for the virus, was first documented in studies in Egypt between 1952 and 1954. At that time, scientists showed that four different species of mosquitoes were capable of transmitting West Nile virus after biting infected birds. Subsequent research has shown that many other types of mosquitoes as well can pass the virus to people and other animals.

Birds: The Primary Host

West Nile virus begins in infected birds and spreads when mosquitoes bite these birds and then bite people or other animals. Experts believe migrating birds are primarily responsible for spreading the disease to new areas. This is why the disease seems to have spread so rapidly throughout the United States. Evidence that migrating birds are indeed the cause of this transmission includes the following facts:

• Outbreaks tend to occur in late summer or early fall, when large numbers of birds begin their migration patterns.
• Outbreaks tend to center in wetland areas where large numbers of migrating birds and mosquitoes are present.
• Antibodies to West Nile virus are found in many species of migrating birds throughout the world.
• Migrating birds are known to transmit related viruses in the United States and other areas.

Recent evidence has shown that once a mosquito is infected, the virus does not seem to affect the insect. West Nile virus can survive inside so-called overwintering, or hibernating, mosquitoes that are inactive during the winter months. This is what experts believe happened during the winter of 2000 in New York, following the late summer 1999 outbreak in that area. The infected mosquitoes harbored the virus through that winter and then started infecting people and animals during the summer of 2000 when the mosquitoes became active again.

Once a person or animal other than a bird is infected with West Nile virus by a mosquito, there is no evidence that an uninfected mosquito that bites that person or animal can acquire the virus and pass it along to others. Because infected people and animals other than birds and mosquitoes do not usually play a role in spreading West Nile virus, they are known by epidemiologists as incidental hosts. Mosquitoes, which are necessary for transmitting the virus from the primary host to the incidental host, are known as the vector. Thus, the usual cycle of transmission is primary host to vector to incidental host.

How Does the Virus Spread?

After a mosquito bites an infected bird to feed on its blood, it carries West Nile virus in its salivary glands. Then, when it bites a human or another animal to obtain another blood meal, it injects the virus into the person or animal's bloodstream, where it can multiply and cause disease. There is no evidence that parent mosquitoes pass the virus on to their offspring when they lay eggs; experts believe newly hatched mosquitoes must bite an infected bird to become infected themselves.

Once the virus has multiplied in the bloodstream of a person or animal, it does its major damage by crossing the blood-brain barrier into the central nervous system. The blood-brain barrier is a mechanism present in people and many animals to prevent most toxins and pathogens from moving from the blood into the brain. Biologists believe the blood-brain barrier evolved to protect the body's center of thought, behavior, and automatic activities from being damaged by most poisons and germs in the environment.

However, the blood-brain barrier is selectively permeable, meaning that it is designed to allow some substances to enter the brain. This is necessary so that certain nutrients needed by the brain are permitted to enter. This selective permeability is achieved by means of the tightly packed cells that line the small blood vessels, or capillaries, in the central nervous system. In the rest of the body, these lining cells have a small space between each other to allow substances to move freely between the inside and outside of the capillary. But the tightly packed cells in the central-nervous-system capillaries allow only certain substances of a small enough size to get through.

Even with the blood-brain barrier, unfortunately, some dangerous pathogens and chemicals are able to get into the central nervous system. West Nile virus is one infectious agent that can get through the barrier. Once inside the brain and spinal cord, it can cause the inflammation and other characteristics typically found in cases of West Nile virus infection. The severity of disease in a particular person or animal is at least partly determined by how much of the virus gets into the central nervous system to reproduce.

More Mosquitoes, More Disease

The chance of being bitten by a mosquito so that West Nile virus gains a foothold in the nervous system is, in turn, determined by the number of infected mosquitoes in a given area. This factor is influenced by several variables. One variable is, of course, the number of infected birds that the mosquitoes can bite to begin the transmission cycle. The more infected birds there are, the more likely it is that many mosquitoes will become infected.

Large numbers of infected birds are commonly found in areas near large rivers. Experts believe that rivers played a role in several recent West Nile virus epidemics. These epidemics all occurred in the late 1990s, one in Romania in 1996, one in Russia in 1999, and the one in New York in 1999. "All three sites were located adjacent to large rivers, presumably providing a favorable wetland habitat for attracting both resident and migratory species of wild birds,"⁷ points out an article in the book West Nile Virus .

Other environmental factors can increase the number of mosquitoes and therefore increase the probability of being bitten by an infected mosquito. Many experts believe human population growth and its accompanying changes in ecology are largely responsible for the recent heightened incidence of West Nile virus throughout the world. With more people come changes in agriculture and irrigation practices, and this can increase the amount of wet breeding areas and hospitable habitats for mosquitoes. Destruction of forests can lead to further disruptions in the natural balance of animal and insect life.

Another environmental factor that many public health authorities believe has increased the mosquito population is global warming, the phenomenon in which the entire planet is growing warmer due to pollution and a host of other contributing conditions. Because mosquitoes thrive best in warm, moist climates, increases in the temperature or humidity can play a role in increasing the number of mosquitoes.

Along with global warming, annual fluctuations in temperature and rainfall can influence how many mosquitoes appear in a given location. Warmer winters in particular allow more mosquitoes in an area to survive, meaning that during the following summer and fall, there will be more mosquitoes around to breed and bite. Heavy rains that leave many areas of standing water can also mean greater than normal mosquito populations. This is what authorities believe happened during a 1974 epidemic in a normally dry region of the Republic of South Africa. Unusually heavy rains that year created fertile breeding grounds for mosquitoes that became infected with West Nile virus. These mosquitoes then passed the virus to more than three thousand people during the largest West Nile virus epidemic that continent has ever reported.

Public health experts point out that rainfall is not the only source of water that can provide mosquito breeding grounds. Any source of standing water can have the same effect, as shown by the 1996 West Nile virus outbreak in Bucharest, Romania. Authorities have linked this outbreak to run-down conditions in urban areas where the disease struck. These conditions gave mosquitoes a place to breed, and the urban setting allowed these mosquitoes access to plenty of people. As epidemiologists explained in the book West Nile Virus:

During the epidemic investigation in Bucharest, blockhouses were commonly found to be in poor general condition, often with basements flooded with drinking water or raw sewage. Public corridors commonly harbored large resting populations of adult Cx. Pipiens [a species of mosquito known to transmit West Nile virus].… Individual apartments and single family homes often lacked window and door screens, and those screens that were present were often in disrepair.⁸

Species of Mosquitoes That Transmit West Nile Virus

Experts say that it is not merely the number of mosquitoes in an area that determines the likelihood of someone being bitten and infected with West Nile virus. It also depends on the species of the mosquitoes present. Some species do not seem to carry West Nile virus, while others do carry it and are capable of transmitting it to people and animals. Scientists assess which species are

Mosquito Traps

There are several types of mosquito traps used to collect mosquitoes for identification and prevalence counts. Light traps are employed most often. These are compact, lightweight, and contain a small motor, fan, and light with a photocell powered by batteries. The photocell turns the light on at dusk and off at daybreak. The light attracts mosquitoes, which are then blown into a collection net or jar by the fan. Related to the light trap is the carbon dioxide–baited light trap, which uses dry ice, or carbon dioxide, placed in an insulated container, to attract the mosquitoes. Carbon dioxide is the gas emitted from the mouth when people and animals exhale; it is this gas that attracts mosquitoes to creatures from which they seek blood.

Gravid or oviposition traps are designed to capture gravid female mosquitoes—those that are ready to deposit their eggs. These traps are made of a base reservoir filled with hay or manure, known to attract these mosquitoes. The reservoir is connected to a suction apparatus that pulls the mosquitoes into a collection carton.

Fay Prince traps are used in the daytime to trap mosquitoes active during these hours. They contain contrasting glossy black and white panels that attract the mosquitoes. These traps also have a container for carbon dioxide and a suction motor to draw the bugs into a collection bag or jar.

Propane-generated carbon dioxide traps use propane gas for power and also convert the propane to carbon dioxide to attract mosquitoes. Some of these traps also emit heat and moisture so mosquitoes think they are landing on a warm-blooded animal. A suction device then draws the bugs into a collection tray.

or are not capable of carrying the virus by trapping mosquitoes in special mosquito traps and identifying the species. Then, the investigators perform laboratory tests to check for the presence of West Nile virus in the mosquitoes' salivary glands. If the virus is found, the scientists then must determine whether or not the particular mosquito is capable of transmitting the virus to animals or people it bites. Generally, researchers allow these mosquitoes to bite a newborn chicken or other laboratory animal to see whether or not the animal then contracts West Nile virus.

Experts at the University of California, Davis, point out that identifying the species that carry West Nile virus is critical for tracking, predicting, and controlling the spread of the virus throughout the nation: "As WNV expands its range westward across North America, examining the transmission potential of the different mosquito species will help to anticipate patterns of transmission."⁹

Since the 1999 outbreak of West Nile virus in the New York City area, scientists have found more than twenty-five mosquito species that carry and transmit the virus. The vector responsible for causing most cases of the disease is Culex pipiens . Other common vectors tors include Culex salinarius, Culex restuans, Ochlerotatus canadensis, Ochlerotatus japonicus, Aedes vexans, and Culiseta melanura .

Other Modes of Transmission

Until very recently, experts believed that the mosquito species capable of spreading West Nile virus was the only mode of transmission from birds to other animals and people. Doctors thought that the virus could not be passed from humans to other humans or from animals to other animals. However, recent occurrences have shown that this is not true; indeed, the virus has been transmitted to people through blood transfusions and organ transplants, through an infected mother's milk to her nursing baby, from a pregnant mother to her unborn child, and to laboratory workers stabbed by needles containing infected tissue.

One instance in which the virus was transmitted via a blood transfusion involved a forty-seven-year-old man in Michigan. Ten days after he received a liver transplant and a blood transfusion, he developed a fever and encephalitis. Tests on his cerebrospinal fluid revealed West Nile virus antibodies. The man recovered, and the infection was traced to the individual who donated the blood to him; the same donor also infected three other blood recipients.

In another recent case, medical investigators identified transplanted organs as the source of West Nile virus infections in four people. All four received organs from an infected donor who died in an automobile accident in Georgia. One patient, who received a kidney transplant, developed a fever, backache, diarrhea, rash, and breathing difficulties two weeks after the procedure. Over the next few days, she experienced a deterioration in her thought processes and required a mechanical ventilator to breathe. She eventually recovered, but laboratory tests showed evidence of West Nile virus infection. A male kidney transplant recipient who developed similar symptoms a few weeks after his transplant was not so fortunate and died from the West Nile virus infection. The other two patients who received organs from the infected donor—a woman who got the liver and a man who received a new heart—also developed symptoms. Laboratory evidence revealed they had West Nile virus, but they recovered.

Health authorities are taking steps to ensure that this type of transmission does not occur again; however, at this time, donated organs and blood are not being screened for West Nile virus. Experts say the risk of contracting the virus via these routes is extremely small, but they urge caution nonetheless.

Transmission from Mother to Baby

Recent evidence shows that West Nile virus can also be spread from a pregnant woman to her fetus and from a nursing mother to her baby. The pregnant woman, a resident of New York, was hospitalized during the summer of 2002 with fever, headache, vomiting, and back and abdominal pain. Blood tests revealed the presence of West Nile virus. When the baby was born three months later, it had extensive birth defects and its blood had antibodies to West Nile virus, indicating that infection had occurred and was possibly to blame for the birth defects. Doctors say this case illustrates the fact that it is critical for pregnant women to protect themselves against mosquito bites.

A nursing mother in Michigan proved that West Nile virus can be transmitted through breast milk. Shortly after giving birth, the mother contracted West Nile virus from a blood transfusion. Laboratory tests showed evidence of the virus in her breast milk, and her baby's blood tested positive for the virus three weeks later. The baby, however, showed no symptoms of the infection. The American Academy of Pediatrics and the American Academy of Family Physicians issued a statement saying that women who have symptoms of West Nile virus should not stop breast-feeding their infants since the risk of transmission is low.

Danger in the Laboratory

Recent incidents in which laboratory workers contracted West Nile virus after being lacerated with contaminated instruments underscored the importance of caution on the laboratory front, too. In one instance, a microbiologist using a scalpel to remove a dead blue jay's brain cut his thumb with the scalpel. Despite the fact that the wound was thoroughly cleaned and bandaged, four days later the microbiologist experienced symptoms of West Nile virus infection, and blood tests revealed the presence of West Nile virus antibodies. In another case, a microbiologist working on mouse brains infected with West Nile virus punctured a finger with a contaminated needle. Again, despite the fact that the wound was cleaned well, the man developed symptoms, and laboratory evidence confirmed that he had West Nile virus. Both microbiologists recovered from the illness, but the CDC issued alerts and new regulations for all laboratories involved in working with the pathogen.

Experts say that these instances of infection through contaminated laboratory instruments, blood transfusions, organ donations, breast milk, and from pregnant mothers to their unborn babies illustrate the fact that under certain conditions, West Nile virus can be transmitted by means other than the usual mosquito-borne route. They emphasize, though, that there is no evidence that the virus can be passed through casual contact or just by being near a person or animal that is infected. All modes of transmission seem to involve direct inoculation into the body, so there does not appear to be any reason for concern over "catching" West Nile virus in the same manner in which colds or flu are caught.