Ranchers are asking lots of questions as New World Screwworm spreads across southeastern Arizona.

What are the restrictions on moving healthy animals? Is there a quarantine? Do producers really need a Premises Identification Number?

The federal response playbook, which Arizona is closely following, contains important nuances.

We asked Devon Kartchner, Arizona Department of Agriculture’s assistant state veterinarian, to help explain them.

What are infested premises?

Infested premises only include the location of a confirmed detection, not the 12.4-mile area around it (that’s called the infested zone).

Only infested animals must be quarantined on infested premises — and by quarantined, that means they cannot leave the property for 21 days or until their wounds have fully healed, he said.

Healthy animals can still move off infested premises but must be inspected first.

There is no requirement to isolate infested animals from others on infested premises because New World Screwworm is not an infectious disease. 

It’s a fly that targets cuts, wounds and orifices to lay its eggs.

Those eggs quickly grow into maggots that burrow deep into flesh, causing painful but highly treatable wounds if caught early.

Infested animals should be treated, cleaned and closely monitored to ensure no larvae fall onto the ground.

“We don’t want those larvae to burrow into the soil and complete their life cycle, becoming a new generation of flies,” Kartchner said.

What are infested zones?

Livestock with no signs of screwworm can move freely within infested zones.

But they can’t leave the zone without first obtaining a movement permit and an inspection from a state livestock inspector or licensed veterinarian.

Healthy livestock typically receive preventive screwworm treatment before they move, unless they are heading for slaughter.

In that case, a special 72-hour permit is issued and an inspection is conducted, but no preventive treatment is administered so that no medication enters the meat.

Like livestock, healthy pets must complete an inspection and follow their destination’s treatment requirements before leaving Arizona.

An inspection is recommended but not required if dogs and cats leave an infested zone but remain in state.

The distinction exists partly because more preventive treatments are available for pets, Kartchner said.

While also not required, he urged all pet owners in infested zones to begin a monthly, over the counter or prescription treatment to prevent screwworms.

Do ranchers need a PIN?

A Premises Identification Number (PIN) is preferred but not required to move livestock within Arizona.

The answer gets trickier if producers want to sell or transport their animals beyond state borders.

Some ranchers have voiced concerns about how PINs might be used.

But a growing number of states won’t accept livestock from those with screwworm outbreaks if animals lack official identification, as spelled out in the federal playbook.

“If we want to protect our ability to move Arizona livestock out of state, it’s very important that we are playing by the same rules as everyone else,” Kartchner said.

“If we decide to make our own rules, there’s a potential for other states to put added restrictions on Arizona livestock — which is exactly what we don’t want.”

The federal PIN system has been in place for two decades to help trace animal origins during disease outbreaks, minimizing the spread.

Each location where livestock is kept is assigned a unique number. That number is then matched with official ear tags distributed to that premises.

How long can infested zones last?

According to the federal playbook, an infested zone should remain in place for at least 84 days — about four screwworm life cycles — after the last detection.

The clock resets with each new case.

But environmental factors also can extend that duration. Cold weather slows fly development, for example, lengthening its life cycle.

Boundaries can change over time if no additional cases in part of a zone are detected. That has happened in Texas.

Meanwhile, New Mexico recently designated the southwestern part of the state as a single infested zone, so livestock could move more easily within its boundaries.

Arizona has established three infested zones in Cochise and Greenlee counties as of Nov. 6, following the confirmation of four screwworm cases in horses, a dog and a sheep.

What’s a surveillance zone?

Surveillance zones provide an additional 12.4-mile buffer around infested zones.

There are no movement restrictions for animals in this zone.

They exist to heighten public awareness about where the screwworm could move next.

Officials increase outreach efforts to livestock producers, pet owners and hunters in these areas.

They also bolster surveillance, including the use of trail cameras and drone flights to keep tabs on wildlife.


Joanna Allhands writes about water, land use and other issues important to the Arizona Farm Bureau. Reach her at joannaallhands@azfb.org.