6.3 Vector-Borne Diseases & Pest Control

Vectors of Public Health Significance

A vector is a living organism, typically an arthropod or mammal, that transmits an infectious agent from an infected animal or human to a susceptible host. Understanding the biology and behavior of vectors is critical for disease prevention.

Mosquitoes

Mosquitoes are the deadliest animals on earth, responsible for millions of deaths annually through the transmission of various pathogens.

• Anopheles: The primary vector for malaria. They typically breed in clean, unpolluted, standing water and are most active between dusk and dawn.
• Culex: The primary vector for West Nile Virus (WNV) and various forms of encephalitis (e.g., St. Louis Encephalitis). They often breed in stagnant, organically rich water such as storm drains and catch basins.
• Aedes: Specifically Aedes aegypti and Aedes albopictus, are vectors for Zika virus, Dengue fever, Yellow Fever, and Chikungunya. They are aggressive daytime biters and breed in artificial containers like tires, flowerpots, and buckets.

Mosquito Control Strategies:

• Source Reduction: The most effective method is eliminating standing water to prevent larval development.
• Larval Control: Using larvicides such as Bacillus thuringiensis israelensis (Bti), a naturally occurring soil bacterium that produces toxins specifically lethal to mosquito larvae, without harming other wildlife.
• Adult Control: When adult populations pose an immediate disease threat, Ultra-Low Volume (ULV) spraying of adulticides (e.g., pyrethroids) is conducted during peak mosquito activity times.

Rodents

Commensal rodents live in close association with humans, relying on human habitats for food, water, and shelter.

• Norway Rat (Rattus norvegicus): Also known as the brown or sewer rat. They are burrowers, prefer ground levels, and are excellent swimmers.
• Roof Rat (Rattus rattus): Also known as the black rat. They are excellent climbers, preferring to nest in elevated areas like attics and trees.
• House Mouse (Mus musculus): Small rodents that nest in concealed spaces. They are curious but have a small home range (10-30 feet).

Rodent-Borne Diseases:

• Plague: Caused by the bacterium Yersinia pestis, transmitted primarily by the bite of infected fleas that have fed on infected rodents.
• Hantavirus Pulmonary Syndrome (HPS): Caused by the Sin Nombre virus, transmitted through the inhalation of aerosolized urine, droppings, or saliva from infected Deer Mice (Peromyscus maniculatus).
• Murine Typhus: Transmitted by infected rat fleas.

Rodent Control: Control relies on exclusion (sealing entry points as small as 1/4 inch for mice and 1/2 inch for rats), sanitation (removing food sources), and trapping. Rodenticides (e.g., anticoagulants) should be used cautiously in tamper-resistant bait stations to prevent accidental poisoning of non-target species.

Ticks

Ticks are obligate blood-feeders that can transmit bacterial, viral, and parasitic diseases.

• Blacklegged Tick (Ixodes scapularis): Commonly known as the deer tick, it is the primary vector for Lyme disease (caused by the bacterium Borrelia burgdorferi). Ticks must typically be attached for 36-48 hours to transmit the Lyme disease bacterium.
• Dog Tick (Dermacentor species): Primary vectors for Rocky Mountain Spotted Fever (RMSF), a severe bacterial infection.

Rabies

Rabies is a fatal viral zoonotic disease that causes progressive inflammation of the brain and spinal cord.

• Transmission: The rabies virus is transmitted primarily through the saliva of an infected animal via a bite or scratch.
• High-Risk Vectors: In the United States, the primary reservoirs are wild mammals, including raccoons, bats, skunks, and foxes. While any mammal can contract rabies, small rodents (e.g., mice, squirrels) are rarely infected and have not been known to transmit rabies to humans.
• Post-Exposure Prophylaxis (PEP): If exposed to a potentially rabid animal, the wound should be immediately washed with soap and water. Medical evaluation is critical. Rabies PEP involves a dose of human rabies immune globulin (HRIG) and a series of four rabies vaccine doses over 14 days. PEP is nearly 100% effective if administered promptly before the onset of symptoms.

Integrated Pest Management (IPM)

Integrated Pest Management (IPM) is a comprehensive, science-based approach to pest control that focuses on long-term prevention and minimizes risks to human health and the environment.

• Principles of IPM:
  1. Inspection and Monitoring: Identifying the pest and determining population levels.
  2. Sanitation and Habitat Modification: Removing food, water, and shelter to make the environment inhospitable to pests.
  3. Exclusion: Using physical barriers (e.g., screens, caulking) to prevent pest entry.
  4. Targeted Interventions: Using physical/mechanical controls (traps), biological controls (natural predators), and, as a last resort, chemical controls.

Pesticide Safety and Regulation

In the United States, pesticides are regulated by the EPA under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA).

• Registration and Labeling: FIFRA mandates that all pesticides be registered with the EPA. The pesticide label is a legal document; using a pesticide in a manner inconsistent with its labeling is a violation of federal law.
• Restricted Use Pesticides (RUP): Pesticides that pose a higher risk to human health or the environment are classified as RUPs and may only be purchased and applied by certified applicators.