[COAGDU] FW: Update: dangerous new parasite in the US

Mon Jun 22 14:57:46 UTC 2026

From: cmcg216 at gmail.com <cmcg216 at gmail.com> 
Sent: Sunday, June 21, 2026 1:57 PM
To: barbara.bski at gmail.com; 'Maryann Migliorelli' <maryannmigs at gmail.com>;
'Susan Emmerling' <emmerlingsusan at yahoo.com>
Subject: FW: Update: dangerous new parasite in the US

I'm passing this along FYI.  

From: Southwest Veterinary Hospital PC
<SouthwestVeterinaryHospitalPC at vetmatrix.com
<mailto:SouthwestVeterinaryHospitalPC at vetmatrix.com> > 
Sent: Wednesday, June 17, 2026 5:02 PM
To: cmcg216 at gmail.com <mailto:cmcg216 at gmail.com> 
Subject: Update: dangerous new parasite in the US

Hi friends -

As USDA Accredited Veterinarians, we take seriously our duty to promote the
One Health of all animals on the planet - human and otherwise. Today I'm
writing with a serious topic impacting animals in the US and asking each of
you to be vigilant for yourselves and your pets. Apologies in advance for
this long letter!

Please take a few minutes to read this information about a serious parasitic
disease threat that has breached the US border. You may have been seeing
information in the media about this - and it will get worse before it gets
better.

New World Screwworm (NWS) is an aggressive, invasive fly. It had been
eradicated from the continental US since 1966 due to intense management
efforts which had pushed these flies south to the Panama Canal. Regrettably,
DOGE decided that funding these efforts was something which could be
stopped. As a result, over the last 15 months we have seen the continual
march of these insects north through Mexico - and now they are back in the
US. As of June 3 rd , in the area north of the Panama Canal (and now into
the US) over 171,700 animals have been infested, along with over 2,070
humans!

So why is this big deal?

Many of you are likely familiar with the "bot flies" - Cuterebra - which our
pets and other animals can develop here in Colorado during the spring and
summer. With Cuterebra, a single fly larva appears under the skin of the
animal, where it creates a single hole in the skin. It is using the host as
a warm, safe place to grow, and uses energy from the body fluids to survive.
It is not consuming the host animal. We must then extract the living worm
from under the skin. These are gross, admittedly, but once that single larva
is removed it heals uneventfully in most cases. Similarly disgusting are
maggots from the common house fly - but they do not invade tissue and only
consume dead tissue.

NWS is not so simple.

The NWS fly is attracted to the tiniest of wounds on an animal or human.
That wound can be as small as a tick bite - far too small for us to see.
Once it lands, a female fly deposits 300 to 3000 eggs. Within 12 to 24
hours, those eggs hatch and those larvae penetrate through that wound. They
then begin eating the living tissue of the animal they are on. They
proliferate rapidly and cause extensive, devastating damage to the animal or
human they have invaded. In livestock, these infestations have high economic
impact.

While this is primarily a concern for our livestock industry, each of us
needs to be worried and vigilant. This threat is serious enough that the
USDA is updating numbers daily. The US - Mexico border has been closed to
all animal movement. Regulation of interstate animal movement is tightening.
As the cattle industry is impacted by this infestation, beef and dairy
prices will go even higher.

The facilities that DOGE stopped funding are finally being returned to their
original purpose - but there is a significant lag time before those efforts
will be back up to effective levels. (These facilities produce sterile male
flies. The female flies only mate once - so mating with the sterile fly
leads them to lay eggs which cannot hatch. An elegantly simple means of
control - but it requires repeated release of approximately 500 million
sterile flies in each aerial release to gain control in any given area.)

So, how can we pet owners help?

1. Use effective fly/flea/tick control on your pets continuously. These
insects cause pets to scratch and damage the surface of their skin, creating
the kinds of wounds these flies love. Credelio Quattro for dogs and Credelio
for cats, which we recommend, has rapid flea/tick kill. Any appropriately
labeled permethrin spray can also be used in limited amounts to prevent fly
strike in outdoor dogs. Permethrin is not safe for cats! If you choose to
use topical sprays, be sure to read and follow label directions carefully!

2. Check your pets and domestic livestock daily for any evidence of wounds.
Also monitor any body openings (eyes, nose, ears, mouth, genitals) for
evidence of larva. Any wound which looks like it might have a larva in it
needs to be examined immediately, as at this time ALL larva infestations are
reportable to the Colorado Department of Health and Environment. (That means
we collect the larva(e) and submit that to the state for tracking purposes.
They do not seize your pet!)

3. Be aware of any fly that "looks different". Many fly species look alike,
and in Colorado we are most likely to see common house flies and some stable
flies or "horse flies". The NWS fly is slightly larger than a house fly. It
has characteristic orange-red eyes. They only fly in 13 second long spurts,
but have a travel radius of 2 to 15 miles.

4. Avoid travel with your pets to affected areas - currently southeastern
New Mexico and the entire state of Texas. Under an emergency rule issued by
the Colorado Department of Agriculture, all animals entering the state of
Colorado from a state where NWS is present must (1) have a Certificate of
Veterinary Inspection issued within 5 days of entry, and (2) must be
inspected upon arrival in Colorado by an accredited veterinarian.

5. Infested animals must be treated immediately. They will need to be given
one of the handful of medications that FDA has approved for emergency use,
then will need surgical intervention to remove all affected tissue and
larvae. Currently, Credelio Quattro - the parasiticide we recommend - has
conditional approval which is being reflected on their packaging. The next
lower level of approval includes plain Credelio, Nexgard, Nexgard Combo, and
Capstar which have emergency use approval for treatment once infested.

Our entire staff here at Southwest Vet has been educated on this issue. Our
policy is that any suspected larval wound will be seen same day. That may
mean that we have to ask you to drop off your pet or wait patiently until we
have someone available to make an assessment - but in these times, a larval
wound cannot wait. The odds of seeing NWS are not high - but we are not
willing to take that chance with your pets.

There are many available resources to learn more about NWS. The Colorado
Department of Agriculture has a YouTube channel where they have posted a
webinar about it. There is the USDA-APHIS (United States Department of
Agriculture - Animal and Plant Health Inspection Service) New World
Screwworm landing page at screwworm.gov

We appreciate each of you and your dedication to keeping your pets and
yourselves safe - pleased reach out to us if you have additional questions
about the medication plan to protect your pets.

Sincerely,

Dr Sara, Dr Jensen, Dr Kim, and the entire team at Southwest Veterinary
Hospital

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