Mosquitoes and Mosquito-Borne Diseases

Mosquitoes

Mosquito Species in Pennsylvania

Over 60 species of mosquitoes can be found in Pennsylvania. The most common species are the common house mosquito (Culex pipiens), white-dotted mosquito (Culex restuans), inland floodwater mosquito (Aedes vexans), rock pool mosquito (Aedes japonicus), eastern tree hole mosquito (Aedes triseriatus). Two invasive mosquito species, the yellow fever mosquito (Aedes aegypti) and the Asian tiger mosquito (Aedes albopictus) have been found in southern Pennsylvania; however, their range may change and move further north.

Mosquito Biology and Life Cycle

The mosquito life cycle has four distinct life stages: egg, larva, pupa, and adult. Some mosquitoes can complete their life cycle (egg to adult) in as little as eight to 10 days under optimal conditions, but the overall amount of time to complete their life cycle will depend on the mosquito species, availability of resources, and weather conditions.

The egg, larval, and pupal stages are found in water habitats. To lay their eggs, adult female mosquitoes identify water habitats that contain organic material (decomposing grass, leaves, dirt, etc.) in order to provide immediate nutrition for larval mosquitoes. Females will lay their eggs in "rafts" (clusters of eggs) or singly on the water surface or areas prone to flooding. Eggs may hatch in two to three days under favorable conditions. Some eggs can survive for up to two years.

Once mosquito eggs hatch, larvae will feed on the organic material found in the water. Larvae, also known as "wigglers," have a small head, expanded thorax, and a cylinder-shaped abdomen. The larval stage is a stage of feeding, during which larvae filter through their mouthparts the organic materials and microscopic plants and animals found in the water habitat. Some mosquito larvae are predatory and feed on other mosquito larval species. Within four to 10 days, most mosquito larvae pass through four larval stages (instars) before pupating.

The pupal stage is a transitional stage before the adult stage. Pupae, also known as "tumblers," are apostrophe shaped and will remain near the surface of the water unless they are disturbed. This is a nonfeeding stage. Within one to 10 days, the mosquito pupa is ready to emerge as the adult form.

Adult mosquitoes have one pair of true wings, long legs, elongated abdomens, and variable colorations and patterns. Male mosquitoes have full (plumose) antennae, while female mosquitoes have more sparse (pilose) antennae. Males will typically emerge first and wait near the emergence site for females. Males feed on nectar to get the required carbohydrates for energy, but they do not feed on blood. In addition to nectar, most females will also feed on blood as a source of protein and fat to produce eggs. Males will soon die after mating, while females can live for another month or longer depending on environmental conditions and the presence of hibernacula (protected areas where females can hide or rest).

During the blood-feeding process, female mosquitoes use their mouthparts to pierce the host's skin, simultaneously taking blood from the host and injecting saliva into the host. Saliva from an infected mosquito can contain pathogens, which can enter the host as the mosquito feeds on blood. After blood-feeding, female mosquitoes will go in search of nutrient-rich water or areas prone to flooding to lay their eggs, thereby restarting the cycle.

Mosquito-borne Diseases

Mosquitoes are known to transmit many kinds of pathogens that cause diseases in humans and animals, but the types of pathogens that are transmitted depend on the mosquito species. For a mosquito-borne disease to establish itself in a geographic location, the pathogen, susceptible hosts, and competent vectors must all be present.

Mosquitoes can transmit pathogens such as:

• West Nile Virus
• Eastern Equine Encephalitis Virus
• Zika Virus
• Heartworm

Mosquito Control and Prevention

Because it is difficult to prevent mosquitoes from biting us or our animals, mosquito management is a high priority in preventing mosquito-borne diseases. Effective long-term mosquito management may require using more than one management technique such as:

• Mechanical/physical control
• Biological control
• Chemical control
• Personal protection

Understanding mosquito biology will provide further guidance for the appropriate measures to take when controlling mosquito populations. For example, removing standing water will remove immature stages (egg, larva, or pupa), while insecticides applied in trees and bushes will target adult mosquitoes.

Mechanical/Physical Control

Mechanical/physical control of mosquito populations will kill the mosquito directly, block mosquitoes, or make the environment unsuitable for mosquito survival. Examples of these controls include:

• Dumping standing water sources or regularly draining water
• Removing containers that can potentially hold water
• Maintaining pools (draining regularly and chlorinating water)
• Maintaining window/door screens
• Removing vegetation or debris from water sources
• Checking and removing water from tree holes
• Using traps and bug zappers

Biological Control

Biological control refers to the use of natural enemies (predators, parasites, pathogens, and competitors) to control mosquitoes. To control mosquito populations at home, the following biological controls can be used:

• Mosquito fish (Gambusia affinis)
• Bacteria, such as Bacillus thuringiensis israeliensis or Bacillus sphaericus
• Mermithid nematodes
• Cyclopoid copepods

Chemical Control

Chemical control is the use of pesticides against mosquito populations. Chemical controls should be used sparingly and in combination with other control methods for effective, long-lasting control. Chemical controls can be used against adults or immature stages, but applying these controls will require background knowledge of where the adults and larvae are likely to be found. It is also important to follow the EPA guidelines and manufacturer instructions on how and when to apply chemical controls to maximize efficiency, reduce insecticide resistance, and to prevent negative effects on nontarget insects and animals.

Personal Protection

Personal protection measures can protect you when the other controls are not available or feasible during outdoor activities such as hiking, camping, going to a park, fishing, or gardening.

• Protective clothing (long sleeves, long pants, loose-fitting, and light colors)
• Repellents registered by the EPA (DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undecanon)
• Permethrin-treated clothes (0.5%)

For More Information

Centers for Disease Control and Prevention

U.S. Environmental Protection Agency

Penn State Extension

Mosquito-Borne Diseases

Mosquitoes are known to transmit many kinds of pathogens that cause diseases in humans and animals, but the types of pathogens that are transmitted depend on the mosquito species. For a mosquito-borne disease to establish itself in a geographic location, the pathogen, susceptible hosts, and competent vectors must all be present.

West Nile Virus

Mosquitoes are known to transmit pathogens that cause disease in humans and animals and different species can transmit different pathogens. The most well-known disease in Pennsylvania is caused by West Nile virus, which is mainly transmitted by the common house mosquito (Cx. pipiens) but can also be transmitted by other mosquitoes such as the white-dotted mosquito (Cx. restuans). WNV can affect humans, birds, and horses. Birds serve as the primary reservoir for the virus and allow the virus to move long distances. Horses and humans are considered dead-end hosts and are not able to infect mosquitoes with WNV that subsequently feed on them. Furthermore, WNV cannot be transmitted through direct contact with infect individuals. Two forms of WNV illness can occur in humans: West Nile fever (WNF) and West Nile neuroinvasive disease (WNND).

West Nile fever symptoms include flu-like symptoms, headache, body/muscle aches, and a mild rash that develops on the arms, chest, or back. Approximately 80% of cases will not exhibit symptoms of WNF and 20-50% will not develop the rash. West Nile neuroinvasive disease is more severe and affects the nervous system. Those with WNND can develop encephalitis, meningitis, and muscle weakness. Permanent neurological complications may result from WNND, such as paralysis, memory loss, and sensitivity to light. There is no cure or specific treatment for WNF/WNND in humans or animals other than management of symptoms and supportive treatment. A vaccine against WNV is available for horses, but not for humans, which emphasizes the importance of mosquito bite prevention.

Eastern Equine Encephalitis (EEE)

Photo adapted from the U.S. Centers for Disease Control and Prevention.

Like WNV, the cycle for eastern equine encephalitis virus (EEEV) involves an avian reservoir and affects horses and humans. The main vector for EEE among birds is the black-tailed mosquito (Culiseta melanura), which feeds almost exclusively on birds. Because this mosquito rarely bites humans or horses, another mosquito may play a role as a "bridge vector," transmitting the virus from birds to humans and horses.

Infection with EEEV can cause systemic or encephalitic illnesses. Systemic infection is characterized by flu-like symptoms, joint pain, and muscle pain. The encephalitic form includes abrupt onset of flu-like symptoms, headache, swelling of the brain, drowsiness, convulsions, and coma. EEEV has a 30% mortality rate in humans and horses, with death usually occurring 2-10 days after the onset of symptoms. Those who recover may suffer from permanent mental and physical effects of infection. There is no human vaccine or specific treatment for EEEV other than supportive treatment for the patient.

Zika Virus

Zika virus is primarily transmitted to humans by the bite of the yellow fever mosquito (Aedes aegypti) or the Asian tiger mosquito (Aedes albopictus). In addition, the virus can be spread through sexual transmission, blood transfusion, and during childbirth (mother to child). Humans and wild primates are the primary reservoirs for Zika virus, meaning that if a mosquito feeds on a human with Zika, the mosquito will then become infected and pass the virus during future bloodmeals.

Image from Brar_j- Flickr

Symptoms can last approximately 2-7 days and are often mild. The most common symptoms reported for Zika virus include fever, skin rash, conjunctivitis (red eyes), and joint pain. More severe symptoms include muscle pain, headaches, and vomiting. If pregnant women are infected with Zika virus during the first trimester, the fetus may develop a birth defect called microcephaly that results in a small head due to an underdeveloped brain, however, the connection between Zika virus infection and microcephaly in the fetus has not been firmly established. There is no vaccine or specific treatment for Zika virus, but supportive treatment may be administered.

Heartworm Disease

Heartworm disease is caused by a filarial worm called Dirofilaria immitis and can be transmitted by many species of mosquitoes including those in the Culex, Aedes, and Anopheles genera. Dirofilaria immitis can be transmitted from dog/cat to mosquito to dog/cat, however, mosquitoes can also transmit the worm to humans albeit rarely. Once in the host, the worm migrates to the heart and causes heart damage, blockages, and eventually death if it is not treated. Symptoms in dogs include coughing, difficulty breathing, fatigue, weight loss, and loss of appetite. Once detected and diagnosed, heartworm can be treated under the advisement of a veterinarian. Since mosquito bites are inevitable for pets, prophylactic drugs prescribed by veterinarians can prevent worms from reaching the adult stage.