December 2016
From the State Vet's Office

All Maggots Are Not Created Equal

Screwworms Found in Florida Keys

Note from author: Some of the material in this article may be a little bit gory and we do not recommend you read it just before, during or after eating. However, we do recommend you read the article any other time.

It is a little ironic that I am writing this article about flesh-eating maggots returning to the United States as Halloween approaches. The return of screwworms to the United States could be scarier than anything to do with Halloween. The biggest majority of you reading this article may not be familiar with screwworms. After an intense eradication program, the last reported case of screwworms, other than very sporadic cases, was in Florida in 1959.

It took quite a bit longer in the Southwest. Due to their proximity to Mexico, they continued to have significant problems until 1979. In 1979, southwestern United States had over 7,000 cases. By 1982, there were only six cases reported. After that, screwworms were considered to be eradicated from the United States.

Since then, on five occasions, screwworm larvae have been found on animals and at least one person and were quickly eliminated …… until now.

Before I fill you in on the details of the outbreak in the Florida Keys, I want to spend a little time discussing the parasite and its ability to have a serious negative economic impact on animal agriculture.

Most of us have experienced maggots in some form or fashion, even if we just left some garbage out and the green blow flies spent some quality time there eating the garbage and laying their eggs. In more extreme cases, I can remember, when I was in private veterinary practice, occasionally a client would bring in a shaggy dog and tell us there was some disgusting odor coming from the dog. We would usually lift up some of the long hair and find the dog had been infested with maggots. By the way, maggots from blow flies have a very distinct odor. Anyway, we would usually remove the maggots, clip the hair around the infested area, clean it really well and life would be good again.

While the maggots we are familiar with are at the very least disgusting, they do seem to have a place in the food chain or at least were put on Earth to help get rid of garbage and rotting carcasses. Yes, they have a dirty job, possibly one of the worst there is, but somebody has got to do it.

The Cochliomyia hominivorax, the scientific name for the screwworm, on the other hand, eats the flesh of living animals and sometimes humans. Although hominivorax means human-eating, we are far from the top of the menu for this parasite. According to some entomologists, we are their last choice. However, as uncommon as it is, there are instances when humans have been infested with the parasite. In fact, in late July or early August 1998, a Huntsville man travelled to Brazil and brought back a few screwworm larvae in a wound on the back of his neck. That is an interesting story, but I won’t go into it now.

All you have to do to find out what kind of problems these flies and their offspring can cause is to talk to some of our older farmers who remember fighting the maggots known to kill a 600-pound steer if left untreated. It seems that the flies would lay their eggs in wounds left from dehorning, castration and navel cords that were still wet on newborn calves, pigs, sheep, goats and other livestock. These flesh eaters would often do enough damage to kill their prey or at least cause enough to considerably devalue the animals.

Screwworms were eradicated by releasing sterile male flies into the environment. They would mate with the female flies, but the eggs laid would not produce offspring. The males were sterilized using radiation and massive quantities of them were released. Little by little, the screwworm population gradually went away.

I have talked to some people who ranched out in Texas when screwworms were in their heyday. They said it was awful. But it was a good day when screwworm maggots were no longer formidable adversaries.

Now fast forward to this past Sept. 29. A biologist from a wildlife refuge on Big Pine Key, Florida, contacted the Florida Department of Agriculture concerning the increased number of maggot infestations in deer. Larvae were submitted to the National Veterinary Services Laboratory in Ames, Iowa. The next day, NVSL confirmed the larvae were Cochliomyia hominivorax. Oct. 11, sterile male releases began and over 3 million flies have been released at 22 sites on nine different Keys.

These are the results of 22 releases since Oct. 11: During the past about four weeks, reports of close to 100 deer in the Florida Keys have died. Also a pet pig and two dogs have been reported to have screwworm larvae affect them.

As the crow flies, Big Pine Key is around 600 miles from Dothan. So, it is not likely that a fly will make that pilgrimage on its own. But that is not what we are concerned about. Notice I said, "On its own." I was at a meeting last spring and a veterinarian told us he knew for sure a fly could travel over 300 miles in one day. He knew that because the fly was in his car.

Not only that, but, if Hurricane Matthew had hit the Keys and carried a few pregnant female screwworm flies a few hundred miles, this could have gotten pretty ugly pretty fast. We just have to be very vigilant to watch for any signs of screwworms making the trip from the Florida Keys to Alabama.

You should always be on the lookout for maggot infestations on livestock and pets. Your local veterinarian can not only help with the control and treatment of these maggots, but can also help identify suspicious maggots. If they are not sure about screwworms, some of our field people can come by and work with your local veterinarian to collect maggots and get them officially identified. I would tell you to do this until further notice, but with screwworms always being just a ride on an airplane away, I do not think there will be a time when we can let our guard down completely. But right now, for sure, be on the lookout.

If you have questions, do not hesitate to call me.



Dr. Tony Frazier is the State Veterinarian for Alabama. You can contact him at 334-240-7253.