Posted by: glamielle | September 10, 2012

West Nile Virus in California – Wild birds as a sentinel for the disease

In my opinion, vector-borne diseases represent the most classical example of how One Health integrates human and animal medicine. These diseases usually affect both animals and people and are transmitted by insects, which are very dependent on the environment for their survival and spread.

Mosquitoes such as this Culex sp. act as vectors for West Nile Virus infection in animals and people (Image source)

One great example of this is the disease West Nile Virus (WNV) and its accidental introduction in the Americas. WNV is a present worldwide and is transmitted by mosquitoes such as Culex spp.

West Nile virus was first seen in the USA in 1999 in New York City, where health officers noticed a new type of encephalitis spreading through the city. While they were trying to diagnose the disease, wild birds on the premises of the Bronx Zoo were dying in large numbers. This sounded the alarm for the veterinary pathologist at the zoo who collected samples from the dead birds and tried to have them analyzed. The veterinarian was turned down several times as most authorities thought that she was dealing with an “animal-only” problem which was unrelated to the outbreak in the human population. However, her samples were tested by the Army and West Nile Virus was found to be the culprit of both the animal and human outbreak. It took the virus about 5 years to spread from the East Coast to the West Coast of the USA, disseminated by mosquitoes and bird reservoirs. It was later speculated that the virus entered the country by boat through introduction of mosquitoes that laid eggs in old tires that accumulated some standing water.

Life cycle of West Nile Virus (Image source)

In people, the syndrome is usually mild and some individuals will not even notice they’ve been exposed, others will have a mild flu-like disease and recover on their own. In about 1% of cases, however, the virus can invade the brain and cause severe neurologic symptoms which can be life-threatening. This disease is called Neuroinvasive West Nile and affects more commonly those who are immune-suppressed such as the elderly or people suffering from chronic diseases.

A wide range of animal species can suffer from WNV infection as well and it is commonly seen in birds. Some birds do not suffer from the disease and act as reservoir, in which the virus multiplies and spreads through the bite of mosquitoes. Other birds, especially crows, are very susceptible to illness and will often suffer from sudden death (as seen in the Bronx Zoo in the original outbreak). These birds are the ones that people use the most to monitor WNV in the environment, and an increase in crow deaths in summer months is often associated with West Nile activity. Other animals that can be affected by the disease include dogs, where the infection is somewhat mild and horses, in which the disease can be much more severe. In equids, infection often leads to severe neurologic signs such as ataxia and can lead to death. Because of this, a vaccine against WNV in horses has been developed and is widely used. There is also some evidence that, at least in birds, the virus may be transmitted from one animal to another by direct contact, since it is present in oral secretions; however mosquitoes remain the most common mode of transmission.

Dead crows in Los Angeles County can be reported to the County’s Veterinary Public Health Program. The mouth of the bird is then swabbed and tested for WNV antibodies.

West Nile Virus has now a pretty strong hold in the Americas and eradication methods would be difficult to put in place. Some insist that the presence of the disease has become normal and that monitoring for the disease has become useless, as we already know the virus has implanted itself. However, as the current outbreak in Texas shows, WNV has made a comeback and several people have been affected, including 87 deaths (as of this post’s publication date). This shows that there should not be any laxity in zoonotic disease surveillance. One of the aspects that make WNV an interesting model for One Health is the relationship between human and animal infections. Experimentally, a zoonosis will affect animals first and then spread to people. West Nile Virus follows this pattern almost exclusively. As we enter hot summer months such as August, we usually see a sharp increase in cases in animals such as crows as well as horses, followed by an increase in human cases about a month after. Monitoring the impact of the disease in wildlife will surely give us an idea of how it will affect human populations later in the season so that health professionals can be ready for it.

I am taking an active participation to this disease surveillance working in public health in Los Angeles County and one of my duties is to routinely test for West Nile in dead wildlife such as crows. The test used is an antigen test, which enables us to determine if the animal was infected at the time of death. As far as this season, I have seen a sharp increase in positive birds since the beginning of August but overall numbers of positive birds is less than it was at the same time in 2011. Other parts of California, however, have reported increase in both animal and human cases.

The Vectest is an antigens test used to detect WNV infection in crows. Two of the three birds tested on this picture were positive – A dark red line represents the control and a faint red line below it (red arrow) shows the positive result.

In order to reduce likelihood of contracting West Nile virus, these guidelines should be followed:

  • Wearing long-sleeves shirts during times of increased mosquito activities (dusk & dawn)
  • Making sure there is no standing water around the house where mosquitoes can breed
  • Horse owners should also vaccinate their animals for protection during peak WNV activity, under the guidance of a veterinarian

Global West Nile virus distribution in 2011. Since its introduction into the Western Hemisphere, West Nile Virus is considered to be present worldwide, unfortunately, not every country has the ability or resources to monitor and report the disease to public health organizations (Image from the OIE’s World Animal Health Information Database – WAHID)

Finally, there is no arguing that there are changes in our weather pattern. This may lead to an increase in rainfall or more extreme temperatures. Mosquito distribution depends greatly on these factors and some studies have shown that a small increase in average temperatures can lead to a wide spread of diseases classically thought to be tropical in nature, such as Dengue Fever or even Malaria (study).

More information on West Nile Virus:

CDC – West Nile Virus

Center for Food Security and Public Health – West Nile Virus Technical Sheet

Microbe World – One Health and the lessons learned from the 1999 West Nile Virus Outbreak

Rabinowitz PM & Conti LA. 2010. Human-Animal Medicine. Saunders Elseviers ed.

World Organisation for Animal Health (OIE) – West Nile Fever factsheet


  1. […] West Nile Virus in California – Wild Birds as a sentinel for the disease – Global Health Vet […]

  2. […] I presented a poster on West Nile Virus (WNV) surveillance efforts in Southern California (click here to read my previous post on WNV monitoring in wild […]

  3. […] to an increase in the number of vectors, but also their range. I have touched on that in a previous post, specifically regarding mosquitoes. However, ticks also benefit from these weather changes. A […]

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