Viruses

• What is West Nile virus?

• How do people get WNV?

• What are the symptoms of WNV?

• What is the District doing to prevent the transmission of West Nile virus?

St. Louis encephalitis (SLE) is one of the most common mosquito-borne viruses transmitted to humans in the United States. Culex tarsalis is the most common vector of SLE in Butte County. SLE virus is a flavivirus that was first detected in St. Louis, Missouri in 1933. SLE virus is diagnosed based on symptoms, physical findings, laboratory testing, and the possibility of exposure to infected mosquitoes. There is no specific treatment for SLE. Treatment is based on symptoms, not the virus. SLE was detected in Butte County most recently in the mid 1990’s with the detection of positive infected mosquitoes and sentinel chickens. The District still actively tests for this virus in mosquitoes and sentinel chickens.

Western equine encephalitis (WEE) is a virus that can be transmitted to humans and horses by mosquitoes. Mosquitoes can transmit several strains of the encephalitis virus, but WEE is the most prevalent type in California. Culex tarsalis, is the mosquito considered to be the primary vector of this disease. It is found in all California counties, and likes to breed in relatively clean, standing water sources such as wetlands, rice fields, cemetery urns, and backyard sources. Aedes melanimon, which breeds in wetlands and flooded pastures, has also been found to be a vector of the disease. Historically, both the San Joaquin and Sacramento valleys have been the regions where most WEE has occurred. Encephalitis viruses are believed to be brought into the Central Valley by migrating wild birds, which are the natural reservoirs for the disease. When an uninfected mosquito bites an infected bird, the virus is transferred to the mosquito. The insect can subsequently transmit the virus to horses and humans through its bites. Encephalitis cannot be transmitted directly from person to person, or from birds to people.

Malaria was introduced in the central valley of California in the 1800's. Many mosquito control districts were formed primarily to combat the disease thru controlling the Anopheles freeborni mosquito. Butte County Mosquito and Vector Control District was formed in 1948 to combat malaria. The combination of case recognition, treatment and successful mosquito abatement essentially eliminated malaria as a major health concern in the early 1900's. However persistent Anopheline abundance and repeated importation of malaria in infected immigrants and travelers poses a risk of transmission and re-establishment in our agricultural based, mosquito-friendly central valley.
Malaria transmission begins when parasites present in the bloodstream of a malaria infected human are ingested with the bloodmeal of a female Anopheles mosquito. These parasites then burrow through the mosquito's gut wall. Outside of the gut, they multiply for 8 - 14 days, after which they migrate to the mosquito's salivary gland. They remain there until the mosquito takes her next bloodmeal. The parasites are transmitted to the next human in the small amount of salivary fluid secreted by the mosquito when feeding. After being transmitted into the bloodstream, the parasites travel to the liver, multiply, and then move out to attack red blood cells. Later during the infection the parasites become abundant enough to be picked up by another Anopheles mosquito during a bloodmeal. There are 4 different parasites that cause disease. The most serious is called Plasmodium falciparium, which can cause severe anemia, kidney failure, brain damage and death.

For northern California , the responsible vector is mainly the Western Treehole mosquito (Aedes sierrensis). Other mosquitoes capable of heartworm transmission include Anopheles freeborni, and Aedes vexans. Aedes sierrensis breeds in cavities that develop in trees able to retain rain or irrigation water usually known as treeholes. Often, older trees in city parks and residential yards and dense groupings of oaks in the foothills and mountains provide annual breeding grounds for springtime hatches of these mosquitoes.

Heartworm is a serious and potentially fatal disease affecting dogs. Infections have also been reported in cats, ferrets and other animals. Mosquitoes often pick up the parasite from foxes, coyotes or wolves where the disease can run rampant. Heartworm parasites (Dirofilaria immitis) are then transmitted to an animal through the bite of an infected mosquito. After the heartworm larvae have been transmitted to a dog or other animal, the larvae develop into immature adults and travel to the arteries surrounding the heart. This process takes up to 4 months. The worms reach maturity in about 6 months. Adult worms may reach lengths of 6 - 14 inches and are usually found in the pulmonary artery near the right side of the heart and also in the lungs. Infections of several hundred worms have been reported, but this is very unusual. Infected cats have fewer and smaller worms than dogs. At maturity, the worms may reproduce, releasing offspring called microfilaria. These pathogens can be picked up by another mosquito during a blood meal. Inside the mosquito, the microfilaria develop into larva and can then be transmitted to another animal. This is how the infection cycle is repeated again and again.

Infections with just a few worms can have serious consequences. So treating an infection through your veterinarian is important for re-establishing your pet's good health. Since most pets do not exhibit symptoms in the early stages of infection, annual testing at your vet's office is the best method for insuring early detection. Adult heartworms can be destroyed through a series of injections or in an emergency, removed through surgery. There is no vaccine to prevent infections, but there are methods of prevention that are nearly 100% effective. Only your veterinarian can help you choose the right medication and prevention schedule for your pet.

Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans by the bite of infected blacklegged ticks. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system. Lyme disease is diagnosed based on symptoms, physical findings (e.g., rash), and the possibility of exposure to infected ticks; laboratory testing is helpful in the later stages of disease. Most cases of Lyme disease can be treated successfully with a few weeks of antibiotics. Steps to prevent Lyme disease include using insect repellent, removing ticks promptly, landscaping, and integrated pest management. The ticks that transmit Lyme disease can occasionally transmit other tick-borne diseases as well.

Plague is an infectious disease of animals and humans caused by a bacterium named Yersinia pestis. People usually get plague from being bitten by a rodent flea that is carrying the plague bacterium or by handling an infected animal. Millions of people in Europe died from plague in the Middle Ages, when human homes and places of work were inhabited by flea-infested rats. Today, modern antibiotics are effective against plague, but if an infected person is not treated promptly, the disease is likely to cause illness or death.

Wild rodents in certain areas around the world are infected with plague. Outbreaks in people still occur in rural communities or in cities. They are usually associated with infected rats and rat fleas that live in the home. In the United States, the last urban plague epidemic occurred in Los Angeles in 1924-25. Since then, human plague in the United States has occurred as mostly scattered cases in rural areas (an average of 10 to 15 persons each year). Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year. In North America, plague is found in certain animals and their fleas from the Pacific Coast to the Great Plains, and from southwestern Canada to Mexico. Most human cases in the United States occur in two regions: 1) northern New Mexico, northern Arizona, and southern Colorado; and 2) California, southern Oregon, and far western Nevada. Plague also exists in Africa, Asia, and South America (see map).

Hantavirus pulmonary syndrome (HPS) is a deadly disease caused from rodents. Humans can contract the disease when they come into contact with infected rodents or their urine and droppings. HPS was first recognized in 1993 and has since been identified throughout the United States. Although rare, HPS is potentially deadly. Rodent control in and around the home remains the primary strategy for preventing hantavirus infection. Avoid contact with fresh rodent urine, droppings, saliva, and nesting material. The deer mouse (Peromyscus maniculatus) is the primary reservoir of the hantavirus that causes HPS in the United States.