The widespread unusual rainfall during spring and summer 2024 have created ideal conditions for explosive mosquito population growth, experts shared in a recent webinar hosted by the Swine Health Information Center (SHIC) and the American Association of Swine Veterinarians (AASV). Mosquitos aren’t just an annoying threat for humans, they can also transmit many pathogens and cause problems for pigs.
How Much Do You Know About Mosquito Biology?
Mosquitos have four stages starting with eggs laid in, on, or near stagnant water which then develop into an aquatic larval feeding stage. Next is an aquatic transitional pupal stage followed by the terrestrial adult stage, explained Dustin Swanson with USDA Agricultural Research Service.
“Mosquitos breed in stagnant water lacking an abundance of predators such as fish. This can include ponds and some streams; however, standing water resulting from human activity such as water pots, wheelbarrows, and drainage runoff are common breeding sites,” SHIC reported in its September newsletter.
Mosquitos mate in swarms and males fly near specific landmarks waiting for females to breed. The females fly into the swarm and will typically mate with one male. Females are the only sex which consume blood meals and during the blood feeding stage, females use blood proteins to develop and hatch eggs. Each clutch of eggs typically requires a blood meal, he added.
A Danger to Pigs
The reason why mosquitos can cause harm to pigs is through physical bites and blood loss but also through the transmission of pathogens during blood feeding. Blood feeding can result in stress or allergic reactions in the host, anemia, loss of productivity, and reduced weight gain in livestock.
“For virus transmission, a mosquito must feed from a virus-infected host, then internally replicate the virus, seek a susceptible host for a blood meal, and transmit the virus through saliva to a susceptible host,” SHIC reports. “Mosquitos can carry and replicate pathogens that affect multiple species including pigs. Understanding the lifecycle and biology of mosquitos assists in the targeted development of effective control strategies to reduce the negative impacts associated with mosquito bites.”
Lessons Learned in Australia
Bernie Gleeson, a veterinarian at SunPork in Australia, opened up about Australia’s 2022 outbreak of Japanese Encephalitis Virus (JEV) in humans and pigs. Acknowledging water habitat for mosquitos will be present and wild bird populations cannot be controlled, management strategies were developed in response. Control efforts focused on issuing emergency chemical use permits for control of mosquitos in standing water, in and around piggeries, and on pigs.
Australian stakeholders collaborated to publish a guide during the outbreak, “Integrated Mosquito Management Practices for Piggeries” to help the industry. Gleeson encouraged producers and veterinarians to seek professional assistance when dealing with mosquito control needs. Australian efforts included medical entomologists as well as urban and peri-urban professional pest control experts.
Surveillance is a key element of integrated pest management. State government programs and on-farm surveillance (trapping mosquitos) were initiated using CO2 and light traps, he explained. The resulting data helped direct management actions.
Applied and recommended management strategies include environmental controls of vegetation management and removal of standing water sources. Larval stage mosquito control was found to be very productive with S-methoprene and Bacillus products used in standing water, he added. Chemical controls require careful consideration of potential residues. In Australia, the use of direct topical application to pigs for repellant options required emergency permits and Gleeson said these were a last resort, labor intensive, and had withdrawal requirements which limited their use to only breeding stock.
What’s Happening in the U.S. Now?
Natalee Judson, DVM, of Pipestone shared her experiences within their production system from late June to early July in the Upper Midwest where mosquito season typically runs from April to October annually.
“Challenges were created by extensive rainfall in the spring and early summer of 2024 in Iowa and Minnesota across areas with large concentrations of pigs. The excess rainfall created an ideal environment for mosquito breeding grounds including excess fresh, waste, and stagnant standing water,” SHIC reports.
She said the negative impact for swine due to increased mosquito populations included the increased threat of viral infections from bites as well as physical blemishes on animals’ hides on-farm and at harvest sites. At harvest, 20% to 80% of loads delivered during this time period had blemishes. Additional impacts included carcass condemnation, Judson said.
Multiple approaches targeting all stages of the mosquito life cycle are critical to reducing mosquito populations, she believes. Some environmental management efforts to consider include proper site drainage, reducing standing water, maintaining well-kept premises mowed and free of weeds, ensuring effluent flows freely, turning off outdoor lights at night, and increasing airflow (cfms) to prevent mosquitos from settling.
For chemical management, she noted pyrethrin and pyrethroid insecticides, larvicides, and insect growth regulators are potential interventions targeting different lifecycle stages. IGR products are available in liquid, wettable granules, and feed-through formulations. Options for adult mosquitos include spraying and fogging products.
Judson encouraged producers to contact their veterinarians to assess farm-specific needs related to mosquito control and management to mitigate risks of disease transmission and production losses.
“Changes in environmental conditions, specifically during high rainfall seasons, may result in increased mosquito populations that can lead to subsequent emergence of diseases and other challenges for swine health and production,” SHIC reports. “Understanding the biology and lifecycle of mosquitos can aid in the development of mitigation and control strategies to reduce the risk of transmission of pathogens and improve swine health and welfare.”
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