Risk Analysis for Human-Mediated Movement of Pests and Pathogens

Introduction

Humans have always played an important role in dispersing plants, animals, and other organisms—either intentionally or inadvertently. Over the last several decades, rapid developments in infrastructure and transportation have led to dramatic increases in trade, travel, and mass migration; this in turn has accelerated the human-mediated spread of exotic organisms across the globe (Hulme 2009).

In most instances, new species arrivals do not lead to the establishment of permanent populations because conditions (e.g. climate, availability of hosts or mates) are not favorable for establishment. However, under suitable conditions, introduced species may thrive and in some cases, can cause severe economic and ecological impacts in their new locations. For example, the Asian longhorned beetle, Anoplophora glabripennis, and the emerald ash borer, Agrilus planipennis, are tree pests that were introduced from Asia to the United States, presumably through wood packaging materials used in trade (Haack, Herard et al. 2010). By killing their host trees, both species have severely impacted forests and urban landscapes in the United States, causing billions of dollars in economic losses (Herms and McCullough 2014).

Citrus greening or Huanglongbing (HLB), is a bacterial disease introduced from Asia that infects citrus trees, shortening their lifespan and reducing fruit yield and quality. The first U.S. detection of citrus greening was in Florida in 2005; today, an estimated 90% of citrus acreage and 80% of citrus trees in Florida are affected by the disease (Singerman and Useche 2017). Although no one knows how citrus greening arrived in the United States, illegally imported plant material is believed to be the most likely pathway (USDA 2020). The bacterium is transmitted from tree to tree by an insect vector, the Asian citrus psyllid. Infected psyllids can be transported over large distances by prevailing winds or in shipments of citrus nursery stock or fruit (Stelinsky 2012). Over the past 5 years, citrus greening has caused economic damages of over $1 billion annually, with nearly 5,000 jobs lost per year (Li, Wu et al. 2020). Since the detection of the disease in 2005, Florida has suffered a 30% reduction in grove-bearing area and a 74% decline in production (Court 2017).

The yellow star thistle, Centaurea solstitialis, is a weed that was accidentally introduced to the United States in the 1800s, probably through importation of contaminated alfalfa seed. The thistle is toxic to horses, often killing them (Plant Conservation Alliance's Alien Plant Working Group 2005). Since its introduction, the thistle has spread over at least 15 million acres in 17 states, reducing biodiversity and rangeland values, and causing significant economic impacts (Juliá, Holland et al. 2007) (Eagle, Eiswerth et al. 2007).

Exotic Newcastle disease, one of the world’s most important poultry diseases, was introduced to the United States in infected live pet birds or game fowl on three separate occasions, and the U.S. Federal Government spent hundreds of millions of dollars to eradicate the resulting outbreaks (ARS, 2016). The movement of asymptomatic, infected birds and contact with smuggled birds at live-bird markets (USDA 2013) contributed to the silent spread of the disease. The pathogen can also be spread via contaminated clothing or equipment.

Foot-and-mouth disease is a contagious disease of cattle, swine, sheep, goats, deer, and other cloven-hoofed animals. The primary means of introductions is through imports of livestock, as well as human contact with infected animals and eating infected meat. Foot-and-mouth disease is among the most economically devastating livestock diseases in the world because of its rapid spread, the number of species affected, and the difficulty of controlling outbreaks. The United States experienced nine outbreaks of foot-and-mouth disease between 1905 and 1929. In 2001, an outbreak in Great Britain resulted in the slaughter of more than 6 million animals, costing 20 billion dollars (American Veterinary Medical Association 2007).