How can the sharp weapons brought by fly larvae hurt the eyes?

Author | chestnut

A 53 year old French gardener felt something in his right eye while working.

Then the eye began to itch, and it didn’t get better after a few hours. The gardener realized that it was impossible to expect the symptoms to disappear by themselves, so he went to the emergency clinic for the help of a doctor.

The doctor checked the horticulturist’s eyesight. The results showed that both left and right eyes were 1.0, and there was no abnormality. But after examining his right eye carefully, the doctor found that he had symptoms of conjunctival congestion. In addition, there are more than a dozen translucent fly larvae “swimming” in the cornea, bulbar conjunctiva, superior conjunctival vault and inferior conjunctival vault of the horticulturist’s right eye.

So where did these young creatures come from? After asking the gardener himself, the doctor learned that his place of work was next to a farm with sheep and horses. The maggot in the gardener’s eyes is the larva of sheep crazy fly.

Life from the nose of a sheep

Many flies like to live in warm, humid places with a rotten smell. The former represents the temperature and moisture suitable for reproduction; The latter may mean that there is enough food to escort reproductive activities.

For example, when animal remains begin to decay, Calliphoridae players can usually fly to the scene quickly and actively breed in some open parts (such as eyes, mouth and nose), followed by Muscidae. Therefore, forensic scholars often speculate the death time of the body according to the breeding cycle of insects.

However, Oestrus ovis is somewhat different from these relatives. It comes from Oestridae, a family of insects that are not known for chasing the smell of death, although they also love warm and humid environments. The ideal breeding ground for sheep crazy flies is often living animals.

This is because the sheep mad fly is an obligate parasite. In other words, part of its life cycle must be completed by parasitism. Most of its life is in the form of larvae, which need a host to continuously provide nutrients for it. It will not leave the host until the larvae are fully developed. Therefore, before giving birth to offspring, mothers should choose the future growth environment for their children.

The nostrils of sheep have become a popular place for sheep crazy flies to give birth. However, the mother does not directly lay eggs, but first hatches the eggs into larvae in her own body, and then gives birth to larvae in the nostril area of the sheep, which is commonly known as maggots. Since then, maggots with a body length of about 1 mm can wriggle upward along the nasal tract and then enter the paranasal sinuses. There, the larvae absorb the nutrition of the host, continue to grow (to about 20mm), fully develop and return to the vicinity of the nostrils. With the sneezing of the sheep, they drill into the soil and begin to pupate; Finally, the pupae break out and mate as adults to start the next reproductive cycle.

In fact, not only sheep, goats and deer are also common hosts of sheep crazy flies. They can carry such life cycle. Compared with these animals, humans may not be a good host of sheep crazy flies, but when humans haunt around those animals, they have a chance to be selected.

If you go deep into the human eye

Each time, there are usually several to more than a dozen larvae produced by sheep crazy fly on the host, and the health status of the host will be affected by the existence of these parasites.

For example, when the larva climbs in the nasal cavity of an animal by using small spines and mouth hooks in the abdomen, it will stimulate the nasal mucosa and make the main stream of the nose runny. At the same time, swelling of nasal mucosa may cause dyspnea. Even if it does not cause dyspnea, it may reduce the amount of food eaten by the host animal due to discomfort or distraction, or even die of malnutrition. A study in 2013 found that after some sheep were parasitized by sheep crazy flies, their weight decreased by 4.5kg, wool production decreased by 500g and goat milk production decreased by 10%.

In humans, the cases of sheep crazy fly infecting the nasal cavity are relatively rare. The typical symptoms are nasal mucosal inflammation and sneezing. Fortunately, maggots can usually be excreted by the body, and it is difficult to leave sequelae. In contrast, eye infections caused by mad sheep flies are more common, as is the unfortunate horticulturist, who suffers from a disease called ophthalmomyiasis.

Although the infection site has changed from the nose to the eyes, the stings and mouth hooks on maggots can also cause damage to the host. As the larva body squirms, these “sharp tools” may make the eyes itchy, red, hot, swollen or shed tears, and also make people feel foreign bodies.

If there is any obvious difference between human eyes and sheep’s nose, it is probably that human eyes are not suitable for the growth of maggots. Sheep’s crazy fly larvae usually die ahead of time before they are fully developed and cannot go through a complete life cycle.

Most of these maggots that cannot escape will not go deep into the internal structure of the eyeball, but live in some external structures. For example, the gardener’s infected sheep crazy fly larvae were moving in the cornea, bulbar conjunctiva, upper and lower conjunctiva when they were found by the doctor.

This kind of condition belongs to external ophthalmomyiasis. A common treatment is to remove foreign bodies manually. The doctor helped the gardener remove the maggots with tweezers. In addition, due to the large number of maggots invaded, doctors are worried that their thorns and mouth hooks will scratch the patient’s cornea, so after removing maggots, they are supplemented with topical antibiotics as a preventive treatment.

The medical team published the gardener’s case report in the New England Journal of medicine. The report said that the patient’s symptoms had been alleviated during a follow-up visit 10 days after the removal of sheep crazy fly larvae.

Of course, not all myiasis patients are as lucky as him. In a few extreme cases, sheep crazy fly larvae also dig holes in their eyeballs and drill into the internal structure. In this way, even if maggots die, the body that is difficult to remove may cause serious inflammation to the eyeball, even endanger vision and bring the risk of blindness. This is internal ophthalmomyiasis.

So, doctor