The United States has reported its first human case of New World screwworm, according to its Department of Health and Human Services (HHS).
The flesh-eating parasite usually eats cattle and other warm-blooded animals, and is fatal if left untreated.
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The case, investigated by the US Centers for Disease Control and Prevention (CDC), was confirmed on August 4 and involved a patient who returned from El Salvador to the US state of Maryland, HHS spokesman Andrew Nixon said in an email to the Reuters news agency.
Nixon did not address an earlier report that said a case had been confirmed in Maryland in a person who had travelled from Guatemala. The US government has not confirmed any cases in animals so far this year.
What’s a screwworm?
It’s the larva of a screwworm fly.
Female screwworm flies lay their eggs in the wounds of warm-blooded animals, and those eggs hatch into hundreds of screwworm larvae.
The larvae then use their sharp mouths to burrow through the living flesh of their hosts for about a week.
At the end of that week, they drop to the ground, form a pupa and develop in the ground for another week to two months before emerging as an adult fly.
Screwworm can be devastating in cattle and wildlife and has been known to infect humans.
What are the symptoms?
Screwworm infestations in wildlife and humans cause painful, progressive wounds as larvae burrow into living tissue. Symptoms include open, foul-smelling sores that enlarge rapidly, with visible maggots inside.
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Affected animals may show weakness, reduced mobility, weight loss, and abnormal behaviour from distress. In humans, symptoms include severe pain, swelling, fever, and secondary infections.
Screwworm is diagnosed primarily by visual identification of larvae in wounds. The maggots have distinctive features, such as dark tracheal tubes visible through their bodies with bands of spines around each segment.
If untreated, infestations can lead to tissue destruction and even death.
How is it treated?
Painfully.
The hundreds of larvae have to be manually removed, and the wounds disinfected.
Antibiotics are often prescribed to control secondary bacterial infections, and pain management may be necessary in severe cases.
In animals, topical or systemic insecticides are also used to kill remaining larvae and prevent re-infestation.
With thorough care, recovery is possible, but untreated cases can lead to extensive tissue damage and death.
As adult screwworm flies are capable of travelling many kilometres in search of hosts, infestations can spread quickly across wildlife populations, livestock herds and even to humans. The patient in Maryland represented the first-ever recorded case of human screwworm in the US.
In addition to the potential human health toll, the spread of the parasite could also rattle the US cattle industry. In particular, newborn calves are at high risk because the post-partum navel has yet to scar.
Beef producers and livestock traders in the US are already on alert for potential infestations, as screwworms have recently been found moving north from Central America into southern Mexico.
The US government’s confirmation of a screwworm case comes just more than a week after US Department of Agriculture (USDA) Secretary Brooke Rollins travelled to Texas to announce plans to build a sterile fly facility there as part of efforts to combat the pest.
A sterile fly facility produces a large number of male flies and sterilises them. These are then released to mate with females in the wild, which reduces the wild population over time. This method eradicated screwworm from the US in the 1960s.
The USDA has estimated that a screwworm outbreak could cost the economy in Texas, the largest cattle-producing state, about $1.8bn in livestock deaths, labour costs and medication expenses.
Earlier this year, Mexico also began efforts to limit the spread of the pest by kicking off construction of its own $51m sterile fly production facility.
Why is the US cattle industry nervous?
Screwworms have been travelling north through Mexico from Central America in recent years.
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They are endemic in Cuba, Haiti, the Dominican Republic and countries in South America, according to the USDA.
When Mexico reported a new case about 595km (370 miles) south of the US border in July, the USDA ordered the closure of livestock trade through southern ports of entry, after previously halting imports in November and May.
The US legally imports more than a million heads of cattle from Mexico a year, used to fatten feedlots and for slaughtering.
But illegal cattle smuggling, long considered one of the most efficient money-laundering means for Central American drug cartels, is now regarded as the main potential threat for the advance of the parasite in the US.
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