A 53-year-old French horticulturist felt something enter his right eye while working.
Then, the eye began to itch, and after a few hours it still didn't get better. The gardener realized that it was unlikely that the symptoms would disappear on their own, so he went to the emergency room and sought the doctor's help.
The doctor examined the gardener's vision and found that the left and right eyes were 1.0, and there was no abnormality. But after a closer look in his right eye, doctors found that he had symptoms of conjunctival congestion. In addition, the cornea, bulbous conjunctiva, upper conjunctival dome and subconjunctival dome of the gardener's right eye have translucent fly larvae "swimming", a total of more than a dozen.
▲ The larvae of the fly in the eyes of the gardener are "swimming" (Image source: NEJM case report)
So, where did these young creatures come from? After questioning the gardener himself, the doctor learned that he worked next to a farm with sheep and horses. The maggots in the gardener's eyes are the larvae of the sheep fly.
A life that begins in a sheep's nose
Many flies prefer to live in warm, moist places with a putrefactive smell. The former represents temperature and moisture suitable for reproduction; the latter may mean that there is plenty of food to escort reproductive activities.
For example, when animal remains begin to decay, lilies usually fly quickly to the scene and thrive in some open areas (such as the eyes, mouth, nose, and mouth), followed by the family. Therefore, forensic scientists often speculate on the time of death of corpses based on the reproductive cycle of insects.
However, the sheep fly (Oestrus ovis) is somewhat different from these relatives. It comes from the family Oestridae, a family of insects that, while also fond of warm, moist environments, are not known for chasing the scent of death. The ideal breeding ground for sheep flies is often a living animal.
This is because the sheep fly is an obligate parasite. That is, part of its life cycle must be done parasitically. The sheep fly spends most of its life in larval form and requires a host to continuously provide it with nutrients until the larvae are fully developed before leaving the host. Therefore, before the mother gives birth to offspring, she must choose the future growth environment for the child.
Sheep parasitized by sheep fly larvae (Credit: Jack Lloyd, University of Wyoming)
The nostrils of sheep have become a popular spawning place for sheep flies. However, the mother does not directly lay eggs, but first hatches the eggs in her own body as larvae, and then lays larvae in the nostrils area of the sheep, which is what we commonly call maggots. From then on, maggots about 1 mm long can squirm up the nasal passage and then into the sinuses. There, the larvae absorb the nutrients of the host and continue to grow (to about 20 mm), fully develop back near the nostrils, burrow into the soil as the sheep sneezes, and begin to pupate; finally pupate out, mating as adults, starting the next breeding cycle.
In fact, not only sheep, goats and deer are also common hosts of sheep flies, and they can carry such a cycle of life. Humans may not be a good host for sheep flies compared to these animals, but when humans are near those animals, they have the opportunity to be selected.
If deep into the human eyeball
Each time, the sheep fly lays several to a dozen larvae on the host, and the host's health is also affected by the presence of these parasites.
▲Black is a mouth hook like a tentacle (Image source: doi: 10.1086/588046)
For example, when larvae use small spines and hooks in the abdomen to climb in the nasal cavity of an animal, it irritates the nasal mucosa and causes the host to have a runny nose. At the same time, swelling of the nasal mucosa can cause difficulty breathing, and even if it does not cause breathing difficulties, it may cause the host animal to eat less due to discomfort or distraction, or even die of malnutrition. A 2013 study found that some sheep, parasitized by sheep flies, lost up to 4.5 kilograms in weight, 500 grams in wool production, and up to 10 percent in goat milk production.
In humans, cases of infection of the nasal cavity by sheep flies are relatively rare, and the typical symptoms are inflammation of the nasal mucosa and sneezing, but maggots can usually be excreted by the body on their own, and it is difficult to leave sequelae. In contrast, eye infections caused by sheep flies are more common, as was the case with the unfortunate gardener, who suffered from a disease called ophthalmomyiasis.
▲ The "sharp object" on the maggot can cause damage to the eyes (Source: NEJM case report)
Although the site of infection changes from the nose to the eye, the spines and hooks on maggots can also cause damage to the host. As the larval body squirms, these "sharp objects" may make the eyes itchy, red, burning, swollen or tearful, etc., and can also cause a foreign body feeling.
If there is any obvious difference between human eyes and sheep noses for sheep flies, it is probably that human eyes are not suitable for the growth of maggots, and sheep larvae usually die prematurely before they reach the moment of full development, unable to go through the entire life cycle.
Most of these maggots, which cannot escape, do not penetrate deep into the internal structure of the eyeball, but only live in some external structure. For example, the gardener's infected sheep fly larvae were active in the cornea, bulbous conjunctiva, and upper and lower domes of the conjunctiva when they were found by doctors.
This condition is external ophthalmomyiasis, and a common treatment is manual removal of foreign bodies. The doctor used forceps to help the gardener remove the maggots. In addition, due to the large number of invasive maggots, doctors are worried that their thorns and hooks will cause abrasions to the patient's cornea, so after the maggots are removed, they are also supplemented with external antibiotics as a preventive treatment.
The medical team published the horticulturist's case report in the New England Journal of Medicine. The report states that during a follow-up visit 10 days after the removal of the sheep fly larvae, the patient's symptoms had been relieved.
Of course, not all eye fly maggot patients are as lucky as he is. In a few extreme cases, sheep fly larvae will also dig holes in the eyeballs and burrow into internal structures. As a result, even if the maggots die, the hard-to-remove cadavers can cause severe inflammation of the eyeballs, even endangering vision and putting them at risk of blindness. This is internal ophthalmomyiasis.
So, how do doctors usually diagnose intraocular maggot disease? If the eye is viewed from a microscope, a moving larvae may be found in the anterior chamber, subretinal space, or vitreous; the larva may even leave a distinct trajectory on the retina, as if to tell the observer that it has come.