Remedies for Treating Screwworm Larvae

by Douglas Fodge



We had chased the bull calf all over the pasture until we panted like one of Daddy’s hounds on a hot day, but still the calf couldn’t be cornered and caught. I suppose he suspected that he wouldn’t like what we had in store for him that afternoon! We had discovered that the calf had a bad wound on his belly, probably on his naval, and the wound was teeming with the maggots (insect larvae). A frothy, bloody discharge appeared on the surface surrounding the wound, and up close the stench emanating from the open site was pretty strong. If left to the designs of the screwworm larvae, the calf would soon be a part of history. We had to do something, and fairly rapidly at that, and there was no time to wait several days for the arrival of a Vet, moreover we had little or no money to spend on his services.

Screwworm larvae represented the parasitic stage in the lifecycle of a fly (now named Cochliomyia hominivorax), and prior to about 1960 they, and close kin, caused severe damage to domestic and wild animals, and infrequently people (most frequently the nasopharynx tissues), across Texas and much of the south. These flies were nearly twice the size of the common housefly. More than 100,000 species of two-winged fly species had been identified by the 1950s, and several attacked animals. Myiasis was the veterinary term for a fly strike, and each species of fly had a slightly different mode of attack, a different life cycle and a different range of occurrence. Some flies attacked rabbits (Cuterebra sp or Dermatobia hominis), others deer etc., but if any of the troublesome species laid eggs near open wounds on animals, the eggs had plenty of rich nutrients for their needs. Of all of the species, the screwworm fly was the worst one because each individual female fly could lay perhaps 500 eggs, and the eggs hatched shortly thereafter.

The eggs quickly changed (metamorphed) into larvae and migrated into a wound. These beasts had strong mandibles for chewing that were complimented by an enzyme-laden digestive system, and they were capable of destroying an animal, inside out. Wounds did not need to be elaborate and could be caused by anything from a scratched insect bite or a thorn’s penetration of an animal’s skin, no matter how slight it may have seemed. An animal’s navel or a thorn penetration (holes) usually concealed the extent of the problem, but surface wounds could easily be enlarged from simple scratches to large holes by the larvae. It was rumored that screwworm flies carried infectious bacteria and viruses, and perhaps other things as well, but the big problem with them were the larvae. Left unattended, screwworm maggots could kill an animal in less than 2 weeks, and during that time thousands of the larvae would mature into pupae, leave the wound and burrow into the topsoil where the fly’s lifecycle was completed. This species was an obligate parasite (to complete their life cycle the larvae could only develop in wounds of animals). Fortunately, by the mid 1960s, screwworm flies were eradicated as far south as Central America when the USDA (U.S. Dept. of Ag.) released X-irradiated, sterile, male flies. When these males mated with females, their offspring did not lay fertile eggs and the life cycle was stopped in its tracks.

Navels were particularly vulnerable sites for screwworm flies to lay their eggs, and the bull calf was a prime example of this since his navel was teeming with the maggots. After about 15-30 minutes of chasing the calf, we finally cornered and downed him. Daddy tied the calf’s back feet together and sat on them, and I lay across his front legs and held his head on the ground. I don’t know who was worse from the chase, the calf or me, but at least I wasn’t about to be worked on, and he was. I suppose that had a Vet attended to the calf’s miseries, he would have used sterile, stainless steel instruments to make the corrective measures that were necessary, but we didn’t have any of those. Daddy had his pocketknife, and it certainly wasn’t sterile, but neither was the wound.

With the knife Daddy cut a small hardwood limb and removed the bark and limbs from it. Then he fashioned a small hooked point on one end of the tapered stick. This foot-long stick was used to dig the maggots out of the wound, and since there were hundreds, if not thousands in the wound, the process to snare the maggots one-at-a-time and pull them out of the decaying flesh was lengthy, estimated at 30-45 minutes. Although the screwworm maggots would have eventually killed the calf, maggots of some other species have been used, even in modern times, to clean out a wound leaving it almost bacterial- and fungal-free. However, the medical community used maggots raised under aseptic conditions for this practice, rather than allowing some flies to blow the wound! After fishing maggots out of the wound, Daddy decided that he had cleared out enough of them to take the next step in the treatment process.

Any time we treated an animal for screwworm infestations, we were concerned and uncertain that all the maggots had been removed, much less any other contaminating microbial species. Therefore, the next procedure was aimed at killing any leftover maggots and preventing a second infection before the wound healed. On a farm there were plenty of other marauders (blowfly species of the genera Lucilia, Calliphora, Phormia, and Chrysomyia ) awaiting the attractiveness of an open wound, and any number of them were capable of introducing infectious agents to the site if they got the chance, although these flies were not obligate parasites and did not need to lay eggs in wounds to complete life cycles. Out of his pocket Daddy produced his omnipresent bottle of inexpensive disinfectant (coal oil, kerosene), and he doused the interior and exterior surface surrounding the wound with this solution. Coal oil was used to disinfect anything that needed to be disinfected on our farm. Then he smeared (Product 66, a Balm or Salve of unknown commercial sources) a tarry product about one-half inch deep on the inside and outside of the wound. The balm was composed of things to disinfect the wound, and it had a black, thick consistency, and I doubt that anything could penetrate it. For all I know, it may have been asphalt, but it didn’t have the consistency of asphalt and didn’t need to be heated before use. Since we didn’t have suturing materials, the calf was let loose, complete with an open wound covered thoroughly with this product. The calf recovered pretty nicely, and when we sold him that fall he brought a decent price.

Maggots did not have lungs, but they needed oxygen for survival. Since there weren’t lungs to inspire the air and oxygen, the maggots relied on a series of tubes (spiracles) located on their posterior surface to breath the air. These tubes carried air to the interior of the worm, and if any substance clogged the spiracles the maggots would be suffocated. I am confident that Product 66 killed maggots because it clogged their breathing apparatus, and they suffocated in the process. Many farmers used any oily substances that were available, ranging from axle grease, motor oil, mineral oil and even cooking oils to coat the interior of wounds that contained screwworms. According to my cousin James R. Miller, Uncle Jack often mixed some sulfur with Vaseline, and used this mixture to treat cattle for screwworm infestations. Since it contained sulfur, it probably also inhibited microbial growth in the wounds.

Cattle weren’t the only animals that suffered screwworm infestations, and on other occasions we treated various farm animals that had an infected wound, usually started as a small cut. One special animal that we treated was one of Daddy’s priced foxhounds that suffered either a cut or scratched the skin on his scrotum. We suspected that it occurred while the dog tried to clamber through a barbed wire fence, but as talented and well trained as Daddy’s hounds were, they couldn’t talk, and we never knew what happened. Sure enough in a few days after suffering this indignity, flies of some species laid eggs near the wound and within hours the foxhound, and we, had a big problem. However, catching one of Daddy’s hounds could be difficult even when they were healthy, and this one avoided us for more than a day before we finally caught him. Some of Daddy’s hounds were practically wild animals, and I recall vividly one prized Beagle, Buck, who lived at our house for 12 years, eventually dying of old age, and Mama’s hand ever laid on him during the 12 years, and she caught him only twice after he had been injured in some hunting incident and didn’t run away from her. By the time we caught Daddy’s foxhound with the infestation, the maggots had already done considerable damage. As rapidly as possible, we repeated the procedure that we had used on the calf, and the hound recovered after a few weeks. Eventually the hound was carried on hunting trips and he began to hunt at a level up to his previous standards.

Following the foxhound’s successful show of vigor in the hunts, Daddy began to wonder if the foxhound had suffered permanent sterility (castration) from the larval infestation. As soon as a female was available, Daddy penned the foxhound in a kennel with a female in estrus to test his virility. In due time, we had evidence that the foxhound was fully recovered. The female gave birth to a fine litter of healthy pups, and many of them became fine foxhounds.

While the field surgery procedures described herein were gruesome and seem primitive by modern standards, others in the region practiced them just as we did, and similar procedures are still used today in many areas of the world. They worked almost every time. Nonetheless, screwworms took a major toll on cattle ranches and farms across Texas and the region, and as mentioned previously, screwworm infestations often went undetected until it was too late to save an animal. Experts in modern Vet Schools have estimated that an outbreak of screwworm flies, similar to those of the first half of the 20th century, would cost the US cattle industry upwards of nearly $1 billion per year, not to mention the impact that such an outbreak would have on other animal production industries. While such an outbreak is unlikely, should such an occurrence happened, I take comfort in the knowledge that one procedure would be available to help save animals, at least for those of us old enough to recall the treatment.