If you read my last post, you have probably been made aware that the most common way humans are infected by West Nile Virus is by mosquitoes. It is important to note, however, that a very small number of people have acquired West Nile Virus through other routes. These modes have included blood transfusions containing WNV, organ transplants from infected individuals, exposure in laboratory settings, and from an infected mother to her baby during pregnancy, delivery, or breastfeeding. You cannot, however, acquire West Nile Virus from casual contact with an infected human or animal. You also will not become infected from handling infected birds (dead or alive), but it is best to still avoid bare-hand contact. You also won’t acquire WNV from consuming birds or animals that carried the virus.
Mosquitoes are the main mode of transmission for West Nile Virus and although 60 species have been known to carry the virus, there are 3 main species of mosquitoes that primarily influence its spread. The primary species differ by region within the United States and include the Culex pipiens in the north, Culex quinquefaciatus in the south, and Culex Tarsalis in the west.
Centers for Disease Control, 2015
How do these mosquitoes become infected with West Nile Virus? Where does the virus come from originally? The natural host of the virus is birds. Birds are considered an amplifying host for the virus because within birds, WNV is able to easily enter the bloodstream and spread throughout the body. Consequently, birds carry a high concentration of WNV in their blood and are great for transmitting the virus. In addition, most ill birds can be infectious while still being able to fly around happily. Mosquitoes become infected when they feed on infected birds, and these infected mosquitoes proceed to go on and infect even more birds. This increased number of infected birds leads to even more mosquitoes becoming infected; this transmission cycle continues through the spring and summer months. By the middle of the summer, there are huge numbers of infected mosquitos so that humans and other animals become at risk for WNV from being bitten by these mosquitoes. Humans and horses are considered “dead-end” hosts for West Nile Virus because they become, but are not able to create huge amounts of virus in their blood. This makes it hard for us to infect other mosquitoes when they bite us.
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In the United States, West Nile Virus is most prevalent in the late summer months into early fall because by this time a considerable number of mosquitoes have been infected. In addition, warmer weather plays a role in increasing incidence of WNV because heat shortens the incubation period of the virus in mosquitoes. Therefore, in temperate regions of the world, WNV occurs primarily in late summer to early fall, while tropical climates can experience incidences of WNV all year. As of June 16th, 2015, CDC surveillance of WNV activity in the U.S. has shown that the virus has been most active in southern states, including Kansas, New MExico, Oklahoma, and Texas. In 2014 in the U.S., California, Arizona, North Dakota, South Dakota, Nebraska, and Louisiana showed the greatest incidences of West Nile neuroinvasive disease (greater than 1 per 100,000 population). In 2014, the CDC report also seemed to suggest that the East Coast region of the United States had lower rates of incidence of WNV than the Midwest and Southern regions of the country.
In the last blog post, you were also introduced to the history of West Nile Virus, as well as its many consequences on infected humans. You may now be wondering what your risk is for acquiring the disease this summer. Who is at risk for acquiring West Nile Virus? The truth is that since WNV is most commonly transmitted to humans through mosquito bites, anyone that lives in an area where the virus is present in mosquitoes is at risk for being infected. As discussed in the previous post, however, the people who have the greatest risk of being infected are older and immunocompromised individuals. These individuals are not only at greater risk for acquiring the disease, but are also at greater risk for suffering from serious complications of the disease. The main factors that put someone greater risk for developing the more serious form of WNV, West Nile neuroinvasive disease, are also age and immunosuppression; the biggest factor seems to be old age. The incidence rate for neuroinvasive disease among infected individuals increases significantly with age and are greatest for the elderly who are 70 years of age or older (Sejvar, 2003).
WNV Incidence Rate by Age Group by Centers for Disease Control for 2010
The unfortunate thing is that to date, there is no licensed vaccine for use in humans to prevent West Nile Virus infections (Fillette et al., 2012). There is also no specific treatment or antiviral medication for WNV in humans and the only available care is supportive treatment to combat or relieve symptoms. For the more mild symptoms, over-the-counter pain medications can be given for fevers and headaches associated with the virus. The more severe cases of WNV, such as the neuroinvasive diseases, may require further support in the form of hospitalization for intravenous fluids and monitoring (Fillette et al., 2012). Patients that develop WNE usually have to be hospitalized and monitored because they have the potential to develop elevated intracranial pressure or dangerous seizures. For both WNE and WNP, hospitalization and close monitoring by nurses is recommended in case the patient goes into respiratory failure and require ventilator support.
The current lack of a antiviral medicines or vaccinations for WNV make prevention an especially important endeavour for this virus. Prevention of the spread of WNV by mosquitoes is crucial at both the personal level and community level (Fillette et al., 2012). To protect yourself and your family from being infected with WNV, it is important to avoid to implement strategies to avoid your exposure to mosquitoes and risk of being bitten. You can decrease your risk of getting mosquito bites by using mosquito nets and insect repellent, in addition to wearing light coloured clothing, long sleeves, pants, and socks (Fillette et al., 2012). Preventing mosquitoes from entering your home is also helpful for reducing your exposure to infected mosquitoes; to protect your home you can use screen on windows and doors, and turning up the air conditioning to make conditions less desirable for mosquitoes. Eliminating mosquito breeding grounds is also important; mosquitoes can breed in less than 1 inch of water, so it is important to empty standing water around your home (Fillette et al., 2012).
One important aspect of preventing West Nile Virus in the U.S. involves enhanced surveillance to help us more effectively detect the presence of WNV in an area and quickly implement preventative measures that would keep the virus from spreading. Surveillance should monitor the incidence of infection in birds, mosquitoes, and humans. Recognizing the presence of disease in animals like horses and birds is crucial because it can give us vital clues that a human outbreak will occur in the future (Fillette et al., 2012). There is a vaccine available for horses, so utilizing these vaccines and preventing the transmission of WNV to horses is also important in pretending the spread of the virus. According to the Centers for Disease Control (CDC), the most effective method of preventing the transmission and spread of WNV is through reducing human exposure through mosquito control (Fillette et al., 2012). Implementation and establishment of a larger number of well-equipped state and local health departments focused on mosquito control are currently underway.
Current prevention measures rely heavily on local measures, such as community mosquito control programs (Sejvar, 2003). These programs focus on identifying local mosquito species playing roles in WNV transmission, and implementing active control measures through water management, chemicals, and biological control strategies (Fillette et al., 2012). While it is noted that surveillance of WNV incidence in many different species is helps us understanding disease spread, research has shown that the best way to recognize and halt an outbreak is by frequently monitoring the vector index for WNV. The vector index is the product of the mosquito population size, and the estimated rate of WNV infection in local mosquito populations. A study analyzing a 2012 outbreak in Dallas, has supported the theory that monitoring mosquito cases of disease and vector index, rather than human cases, was an effective means of predicting future epidemics and allowing early intervention. Through statistical models, the study showed that Dallas public health officials could have prevented 100 cases and 12 deaths if they had monitored the vector index and intervened as soon as the vector index hit the crucial threshold level indicating an epidemic was coming (Fillette et al., 2012).
In addition to frequent monitoring of infected mosquito populations, community mosquito control programs often aim to eliminate mosquito larval habitats and apply insecticides to areas with mosquito larvae or larger densities of infected mosquitoes. Control measures, such as spraying pesticides to kill infected mosquitoes, has been proven to stop further spread of outbreaks in the past. For instance, New York City, New Jersey, and Connecticut were able to successfully stop further spread of the 1999 disease outbreak in this way (Fillette et al., 2012). You can play a role in supporting your community mosquito control programs by helping to report dead birds to local authorities. Reporting dead birds to state and local health departments is crucial for effective monitoring of WNV in your area. These birds need to be tested for the virus and can serve as indicators that West Nile Virus is circulating in a specific area. Helping spread awareness of the importance of these personal preventative measures is another way you can help protect yourself and the community from West Nile Virus outbreaks.
Are we doing a good job of preventing outbreaks of West Nile Virus in the United States right now and do I think we have a reliable plan in place? Yes and no. I do believe that we are taking necessary steps to develop an effective plan of controlling mosquitoes and transmission of West Nile Virus. There is ongoing surveillance in place to measure the incidences of disease and to help us better understand patterns of how WNV spreads. We have also improved diagnostic techniques that allow us to accurately report WNV and make unequivocal diagnoses of the virus. These are all important steps in the right direction, but I still think that we have a ways to go in terms of efficiently and effectively regulating mosquito transmission and human exposure to WNV. In my opinion, one of the things that needs to be improved is public outreach that educates communities about the disease, how they are transmitted, and how to prevent and reduce risk of exposure. I also think that it would be helpful for more local health infrastructures to be put in place at the state and local levels to address prevention and control of vector-borne diseases. No only should these organizations be in place for surveillance, but I think these agencies need to have access to proper training and equipment that would render them capable of responding to West Nile outbreaks. Since birds migrate and influence the spread of disease at more than just a local level, I believe we could also improve coordination and data exchange between agencies at the local, state, and federal levels. Cooperation and teamwork would give all of these public health agencies a clearer picture of the patterns of WNV transmission around the nation as a whole. Greater cooperation among branches of surveillance, including vector control, agriculture, and wildlife departments, would also help health officials develop more efficient and effective strategies to prevent disease transmission in a variety of hosts. By confronting the issue of transmission from many different angles among many different hosts, I believe we would be most effective in cutting off modes of viral spread.
In addition to improved surveillance measures, I feel that it would be beneficial for researchers to continue in their quest for understanding the mechanisms that allow West Nile Virus to infect and spread so quickly. A better understanding of the virus persistence mechanisms, mosquito behavior, viral pathogenesis, and vector relationships, may enable us to come up with antiviral therapy specifically for WNV. Specifically, understanding the mechanisms that allow WNV to surpass the blood-brain barrier and cause neuroinvasive disease in individuals is important. If we are able to inhibit these mechanisms and decrease the more severe forms of disease from occurring, we would be able to decrease the number of WNV-related fatalities. Development of a vaccine for humans against WNV should also be a research priority, since it would prevent the possibility of outbreaks and eventually provide herd immunity.
References
Filette, M., Ulbert, S., Diamond, M., & Sanders, M. (2012) Recent progress in West Nile Virus diagnosis and vaccination. Vet Res, 43(1), 16
Sejvar, J. (2003). West Nile Virus: A historical Overview. Oschner Journal, 5(3), 6-10
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