Fly strike occurs when common flies lay their eggs in a moist, warm tissue somewhere on the body of a newborn calf. It can occur in older animals, but generally that requires some sort of injury, or widespread skin inflammation before fly strike would become a serious problem in an older animal. Usually, they are capable of licking, rubbing or scratching and physically removing the fly eggs or larva from their bodies.
In the newborn calf however, flystrike becomes a potentially life threatening condition. The flies lay their eggs, usually around the anus, where there is a buildup of soft, milk-manure, or around the umbilicus that may still be moist as it hasn’t healed from birth yet.
The eggs hatch into tiny larva, or maggots, that feed on organic matter until they pupate. These larva are tiny specks when they are first hatched, but rapidly grow into inch long, fat grub-like maggots. While they do not feed on living tissue, like the screwworm, the inflammation and irritation that they cause contributes to necrosis or death of the tissue they are on. The inflamed, oozing tissue attracts more flies, which lay more eggs, and on and on. The intense dermatitis causes discomfort, infection, fever and dehydration. Untreated, this becomes a death sentence for the calf.
Early signs of flystrike usually are limited to evidence that the calf is uncomfortable. The tail may twitch and twirl when the calf is not nursing. It may kick at its stomach, if the umbilicus is infested. If you examine the calf at this point, you may find tiny white debris that almost resembles pollen, stuck in clumps in the hair over the tail, under the tail, or around the umbilicus. These are newly hatched larva that really don’t even resemble the maggots that they will become.
If you miss the early signs, later in the infestation, the larva will be obviously grub-like, although they may be hidden under clumps of hair stuck together with serum. At this point, the skin underneath the maggots is moist, red and painful. Still later on, the inflammation and infection in the skin may actually eat holes through the skin and you may see the subcutaneous tissue underneath. The skin and tissue may appear to be gray in color.
Treatment for flystrike depends on how severe the condition is when you find it. Cases noticed early, when the larva are still tiny, will respond to scrubbing and clipping hair, with the application of a fly repellent. It is important to remove the dampened hair where the larva are, so that it can completely dry. Dry tissue will not attract more flies.
More severe cases may require shaving large areas of skin, thorough scrubbing and searching out all the maggots that may be hiding, and removing them. If large areas are involved, and you are a novice at maggots and nursing care, please consult a veterinarian. At this point, the calf may require IV fluids, antibiotics and possibly forced feedings if it is too weak to nurse.
I have found that prevention is far easier that treatment. We try to calve all our cows (and sometimes we fail miserably) early enough in the spring so that the fly season hasn’t started. Depending on your part of the country, that may, or may not be practical. When I do have later calves, I wait until I know they are nursing normally, and then I apply a small amount of either Ivomec Pour- On or Ivomec Eprinex Pour-On to the calf’s back. I do weigh all my calves, so that I can put an accurate amount on the animal. Most of my calves weigh between 50 and 85 pounds, so that I apply 2 ½ to 4 c.c.s (mls) to the calf. (The dosage for both of these products is 5cc per 110 #.)
These products seem to work by killing the larva after they hatch, and it appears to last about 3 weeks in most cases, although I have reapplied it at weekly intervals on rare occasions without any ill effects. If the fly eggs hatch and die, they do not seem to cause any irritation, although they may attract yet more flies. I believe that the mother’s grooming then removes the dead larva, and the problem is solved.
Sometimes even with this prevention, you may have to intervene to remove large deposits of manure from under the tail. Occasionally new mothers just won’t clean the calf adequately and this mess attracts the flies.
Although the larva won’t live, the manure stuck to the skin may cause irritation and inflammation.
Fly strike is not limited to the newborn. Any age animal can become susceptible due to injury. Any wound that attracts flies can develop into a serious problem, particularly if the animal’s tongue, tail or horn cannot reach the area. Matted hair that remains soaked with moisture due to severe downpours can result in inflammation and dermatitis, attracting flies and resulting in a secondary fly strike. Pour-on dewormers such as Ivomec or Eprinex have worked well as preventatives in the face of open draining sores that cannot be sutured, in adults as well as juveniles.
There are several other topical medications that can be used to kill maggots and help prevent flystrike. Follow label directions carefully, as many of these are potent insecticides that deserve your respect.