Agricultural Diseases & Pests

An outbreak of serious disease on crops or livestock can set off a chain reaction that would include:

Direct economic losses (herd destruction, containment measures, disposal of dead animals).
Rise in consumer prices for meat.
Agricultural industry layoffs and unemployment.
Impact on retail food business and restaurant industries (both from increased prices and from loss of business because of consumer fear).
Trade restrictions, loss of exports, and drop in international trade. (A very small outbreak can prompt international export restrictions.)

The largest recent animal disease outbreak in the United States occurred in 1983-84, when avian influenza swept through Pennsylvania and neighboring states. Poultry prices for consumers jumped by $350 million. A 6-month eradication plan cost the federal government $63 million.

American officials say that a food contamination scare similar to the one that hit the Belgium poultry industry in the late 1990s could jeopardize $140 billion in annual U.S. agricultural exports. Soybean rust could wipe out an $8 billion/year industry. Asian longhorn beetles could be used to kill maple trees and cripple syrup production in New England. Any targeted agricultural industry could suffer catastrophic losses.

In 1970 leaf blight destroyed about $1 billion worth of corn in the United States. Between 1993 and 1998, fusarium head blight affected successive wheat harvests in the Dakotas, Minnesota, and Manitoba. The disease spread over 10 million acres, probably with the help of abnormally wet weather and cost an estimated $1 billion in lost production.

Diseases that can be passed to humans would have aneven greater impact. In 1988, the value of British beef and beef products was estimated at US $880 million. After bovine spongiform encephalopathy (BSE, or "mad cow disease") emerged, its value dropped considerably. After a 1996 announcement of a probable link between consumption of BSE-affected meat and a new variant of Creutzfeld-Jakob disease in humans, the value fell to zero.

Disease Transmission among Animals

Animal diseases can be spread in three primary ways.

Airborne transmission - Some diseases (e.g., foot-and-mouth (FAM) disease, avian influenza, Newcastle disease) can travel in aerosol form very long distances in the air. (In 1981, FAM broke out in France and traveled 175 miles to Great Britain in 3 days.) Airborne diseases are extremely difficult to contain and would present an enormous challenge to emergency responders in the event of an outbreak. These diseases can also be transmitted by direct contact.
Direct contact - Some diseases (e.g., FAM, rinderpest, vesicular stomatitis, hog cholera, African swine fever) can be spread by direct contact among animals, contact with contaminated objects such as feed and water troughs, milking machines and other equipment, and people's clothes and shoes. This makes biosecurity measures, keeping animal facilities clean and restricting human and vehicle traffic around animals absolutely critical.
Vectors - Some diseases (e.g., vesicular stomatitis, lumpy skin disease, Rift Valley fever, bluetongue, African swine fever) can be spread by other organisms, such as mosquitoes and ticks. In these cases, disease control depends on insect control.

Transmission of Animal Diseases to Humans

Some animal viruses are zoonotics; they can be transferred to another species including humans. Zoonotics usually do not affect humans in the same way they do animals. For example, FAM, vesicular stomatitis and Newcastle disease can be transmitted to humans, but the resulting illnessis mild and not considered dangerous to human health. However, a few pathogens have been known to seriously harm humans. For example, people have died from avian influenza, and 74 cases of a new variant Creutzfeldt-Jakob disease (a fatal neurological disorder) have been linked to ingestion of BSE-infected beef products.

Crop Diseases

Most crop diseases produce failed harvests rather than killing the plants outright. They do so by drastically reducing crop quality and quantity. Fungi present the biggest threat to crops. Crop diseases are caused by fungi, viruses and bacteria. These plant pathogens are transmitted by wind, water, or vectors. Because they depend heavily on environmental factors (e.g., temperature, humidity, rainfall, sunlight),the introduction of a pathogen does not necessarily result in widespread infection.

There are three primary transmission modes of crop diseases.

Airborne (Fungal Diseases) - Fungi produce dry spores, which are dispersed on the wind and can travel great distances. After a fungus has infected an area, it is very difficult to eliminate all of the spores. Although fungicides are helpful, fungi can persist in other hosts, allowing the disease to continue infecting plants for a long time.
Vectors (Viruses and Bacteria) - Insects such as aphids are often virus carriers. When an aphid feeds on a leaf, it pierces cell walls and transmits the virus. Although viruses can be extremely damaging to crops, their ability to spread is limited by insect movement. Crop viruses are currently untreatable. Virus control depends on insect control and the use of virus-resistant crop strains. Insects also can transmit bacteria.
Waterborne (Bacteria) - Bacteria require moisture for transmission. Although they cannot be transmitted on the wind, they can travel via wind-driven rain. Splashing rainwater can spread bacteria among individual plants, and irrigation runoff can spread bacteria over entire fields. Although bacteria can cause serious plant diseases, they generally cannot spread over vast areas.

Crop Pests

Insects can damage crops directly. Infestations of particular insects can prompt export restrictions. (The Mediterranean fruit fly, or "Medfly," lays its eggs on many types of fruit on which the larvae later feed. If the Medfly became established in the United States, the USDA estimates that it would cost $1.5 billion per year in lost production and export restrictions.The agriculture industry must be particularly vigilant in preparedness for natural disasters or intentional actions:

Need for Surveillance
Surveillance is the first line of defense against a disease outbreak. U.S. agriculture relies upon ground surveillance, plant pathologist and field veterinarians for disease reporting. The greater the number of human monitors and the better trained they are to recognize diseases, the better the chance that serious diseases do not become widespread outbreaks. Disease outbreaks in wildlife also should be monitored because they can serve as early warning signs of agricultural outbreaks.
Need for Quick Diagnosis
A fast diagnosis is critical in the case of a disease such as FAM, which can spread hundreds of miles during the time lag between when the disease is noticed and when a national lab confirms it. Currently there are no rapid screening tests for Foreign Animal Diseases (FADs). State labs do not routinely check for FADs because these diseases are so rare, and in some cases they do not have the resources to diagnose particular FADs. The samples have to be sent to a national lab. As a result, it can take several days for a FAD to be diagnosed.

Protecting Against Animal Diseases

Biosecurity is an important means of preventing the introduction of disease to farms, feedlots, and other livestock and poultry facilities. Biosecurity should include the following.

Keeping vehicles and people (e.g., non-business visitors) away from livestock and poultry buildings to prevent them from introducing or transmitting diseases.
Isolating new animals from the rest of the herd for several days to let potential symptoms appear.

Protecting Against Plant Diseases

Bio-security measures are unrealistic for crops. The primary protections against crop diseases include:

Use of disease-resistant strains
Herbicides and pesticides
Crop Diversity

Review Steps to Protect Your Farm from Pests and Disease (Checklist 15 in the Appendix of the Guidebook or the checklists page of this site) to protect against a deliberate attempt to infect your crops or livestock.