Sheep nasal bot fly disease is highly harmful; how should it be prevented and treated?
Sheep nasal bot fly disease is a parasitic disease caused by the bot fly parasitizing the nasal cavity and cranial sinuses of sheep. In recent years, among the flocks raised by farmers in our region, because insufficient attention was paid to the prevention and treatment of bot fly disease, the infection rate of sheep nasal bot fly disease rose sharply, affecting the normal growth and development of sheep and seriously affecting the fattening results of sheep. Through in-depth systematic investigation and study of sheep nasal bot fly disease among farmers and farms, and observation of prevention and treatment, the author achieved relatively good results, now reported as follows:
I. Pathogen and its life history
1. Adult: about 25-30 mm in size, with a large head and a markedly protruding frons; the lateral frontal area has many pouch-like depressions, and one hair arises from each depression. The proboscis is degenerated, does not bite animals, and cannot feed. The antennae are short; the base of the arista is very stout, while the distal end is slender and weak; the distance between the two antennae is less than the diameter of the third antennal segment; the arista is hairless. The legs are short; the wings are small and transparent; the fourth longitudinal vein merges with the third longitudinal vein; the first posterior cell is closed; the transverse suture of the mesonotum is complete; thoracic bristles are absent, but long hairs are present. The glandular region is rather thick and large, the abdominal plates are broad, and the abdomen is short. The body surface is gray, but with irregular and iridescent gray spots.
Adult sheep nasal bot flies generally begin to appear from February to April, and are most active especially in summer; they generally stop reproducing in winter, and their lifespan is generally 3-6 days, at most no more than 20 days. After adult emergence, they can mate within 1-3 days; at night and early morning they often crawl in corners of houses or some dark places, and after dawn, especially when the weather is hot at noon, they often fly around sheep flocks and produce larvae around the nostrils of sheep. Sheep are very afraid of this fly; as soon as they hear its buzzing or see it flying, they run wildly, often pressing their noses to the ground or against other sheep to avoid its attack.
2. Larvae: early-stage larvae are about 1-5 mm long, and after growth are about 25-32 mm long; the larvae are stout, with an arched back and a flat belly, and the front end is not very narrow; third-instar larvae have a pair of strong and curved black mouth hooks, and the ventral surface of each segment has numerous small spines, while the dorsal surface has none. The anterior spiracles are inconspicuous or absent, and the posterior end consists of a pair of D-shaped spiracles formed by many small perforated plates.
After the adult fly deposits sheep nasal bot fly larvae around the nostrils of sheep, they gradually crawl into the nostrils, all the way into the frontal cavity of the skull, and in very rare cases also into the trachea, bronchi, eyes, ears, and other tissue and organ ducts. They attach to the mucosa and gradually develop; in the nasal cavity and frontal sinuses they develop to third-stage larvae. After the third-stage larvae mature, they gradually move toward the nostrils again, and when the sheep sneezes they are expelled onto the ground and burrow into the soil. Depending on the climate, after about 1-2 months they again transform from pupae into adult nasal bot flies. In our region, the adult sheep nasal bot fly generally has 1-2 generations per year.
II. Harmfulness of sheep nasal bot flies
1. General harmfulness: during the season of nasal bot fly activity, because sheep fear the flies laying maggots, the sheep become restless, flee in all directions, and cannot graze and feed peacefully in the sun, which affects fattening of the sheep.
When the larvae enter the nasal cavity of sheep, the larval mouth hooks pierce the nasal passage, and the mouth hooks and small spines irritate the nasal cavity and frontal sinus mucosa, causing inflammation of the nasal mucosa. Affected sheep often have nasal discharge, which changes from thin to sticky, and finally to purulent, and they also have difficulty breathing. At the same time, because the larvae have spines on the ventral surface and hooks on the head, when crawling in the nasal cavity they stimulate the affected sheep to sneeze frequently, making the sheep restless in eating and sleeping; in addition, the purulent products in the nasal cavity are absorbed by the sheep, even causing general weakness and malnutrition, and finally making the sheep gradually emaciated; in cases it can cause death.
Sheep nasal bot fly disease is often diagnosed based on clinical symptoms, epidemiology, and necropsy data of sheep (finding larvae in the nasal cavity and frontal sinuses). Over nearly 20 years, the author examined 467 sheep that were slaughtered, died of disease, or died of other causes for sheep nasal bot flies, and the cumulative detection rate reached 87.6%, while the detection rate in sheep was almost 100%, especially from late summer to late winter, when the detection rate was even higher; in the most severe case, 64 larvae were found in the nasal cavity of a single sheep.
2. Special harmfulness: in very rare cases, larvae crawl into the trachea, bronchi, eyes, ears, brain, and other organ ducts of sheep and cause corresponding symptoms. In April 1989, September 2000, and October 2001, the author respectively found one sheep with sudden coughing and restlessness, frequent painful coughing, soon followed by elevated body temperature, and finally death by suffocation. Necropsy revealed the presence of nasal bot fly larvae in the trachea, causing lobar pneumonia in the sheep. In October 2000 and May 2002, respectively, one sheep was found blind in one eye, and after slaughter and necropsy, nasal bot fly larvae were found behind the eyeball, and so on.
III. Prevention and treatment of sheep nasal bot fly disease and summary
1. Eliminating sheep nasal bot flies is relatively difficult, and the policy of "prevention is more important than treatment" must be strictly implemented. In the first, second, and third months of early spring, farmers can be mobilized to dig out maggots and pupae from the corners of walls around sheep pens and destroy them; ordinarily, every morning, if adult sheep nasal bot flies are found in corners of houses and dark places, they should be killed promptly.
2. During the epidemic season of sheep nasal bot fly disease, spraying 3% Lysol solution into the nasal cavity of sheep has a very good effect on eliminating sheep nasal bot fly larvae. In late spring and early summer, in autumn, and in late autumn and early winter, the author respectively sprayed medicinal liquid into the nasal cavity of 836 sheep raised by farmers and farms to eliminate sheep nasal bot fly larvae. The method is: restrain the sheep in lateral recumbency on a downward slope of about 20-30°, with the head down and the tail up (to prevent the medicinal liquid from being sprayed into the trachea and lungs of the sheep, avoiding foreign-body pneumonia and tissue necrosis); use a sprayer connected to a small rubber tube about 5 mm in diameter and about 15 cm long, with the front end closed and with small needle holes made in the front about 5 cm; insert the rubber tube into the nasal cavity of the sheep (the insertion depth depends on the size of the sheep; it is best to compare it on the outside first, and the length from the tip of the sheep's nose to 2-3 cm below the eye socket is suitable); spray 15-20 mL of 3% Lysol solution into each nostril. After spraying the medicinal liquid, the sheep's nasal discharge increases and it tends to sneeze, often expelling the killed nasal bot fly larvae. Five days after deworming, a deworming effect check was carried out, and the deworming rate was about 90%.
According to the activity season of sheep nasal bot flies, deworming once can only achieve one-time effect; afterward, when adult nasal bot flies are active, sheep can still be infected as usual. In practical work, depending on the activity of sheep nasal bot flies, nasal spraying of medicinal liquid for deworming can be carried out multiple times. According to the author's clinical experience, it is advisable to carry out three nasal dewormings against nasal bot flies: during the peak season of nasal bot fly activity, in autumn prevention, and in the late period of nasal bot fly activity.
In addition, some lime can also be spread in the sheep pen so that the sheep's noses contact the ground, prompting the sheep to sneeze and thereby expelling and eliminating the nasal bot fly larvae. Farmers can use this method at any time according to the infection status of sheep nasal bot flies in sheep, but the deworming effect of this method is not high, and generally it can only make the sheep expel a portion of the larvae.
The prevention and treatment of sheep nasal bot flies is still in the exploratory stage, and it is still impossible to control and avoid repeated infection of sheep; some sheep become infected with this disease again shortly after spraying the medicinal liquid. The prevention and treatment of sheep nasal bot flies awaits further research and discussion.