Unidentified Intra-nasal myasis of Small Ruminants

By: Dr. Kinza Khan

Department of Pathobiology,

BZU, Multan

Myasis

Myasis is the infestation of parasitic larvae of flies to the host, most of the times even the no-parasitic flies when sit on the wound and lay eggs, their larvae become accidently parasitc upon hatching. But all these types of myasis are visible through eyes or can be identified by their apparent symptoms e.g. foul smelling wound, presence of tunnels on wound etc, that are confirmatory for the common myasis. But the myasis caused by the flies of Osteridae family (variously known as bot flies, warble flies, heel flies, gadflies) is usually confused with other bacterial, viral, protozoal or helminth diseases in their symptomology. Larvae of these flies are internal parasites invading the internal organs like intestinal and respiratory tracts.

Myasis by Sheep Bot fly

In Pakistan, common unidentified cases are found of “Oestrus ovis” commonly known as “Sheep Bot fly/Nasal Bot fly”, which invade and damages the nasal sinuses of the host. Sheep are more susceptible, but goats may also be affected if they are kept in herd along with sheep. Animal may show the generalized symptoms of sneezing, circling, inappetitence or striking of head against the wall or with other animals when the larvae move and damage the frontal sinuses but the disease remains undiagnosed and confuses with other diseases of brain. Sometimes the animal remains asymptomatic.

Prevailing Season

These flies prevail in the environment and infect the host in the spring season (september-october and march-april in Pakistan) while the larvae survive in the host during the intense seasons (summer and winter) and 3rd instar larvae are released in the environment in upcoming spring months, pupate in the soil and turn to adult flies which are ready to infect the new host.

Zoonotic Importance

A few cases of zoonosis by nasal bot fly are also reported, when the adult fly carrying the larvae sits/stricks the human eye, it causes ophthalmomyasis which may also be confused with other eye infections but is irresponsive to the common therapy.

Pathogenesis and Clinical signs

The adult flies deposit larvae in the nasal cavity and the larvae migrate to the dorsal turbinates and frontal sinuses. The migrating larvae injure the nasal mucosa with their spiny surface inducing a nasal catarrhal inflammation. Clinical signs include sneezing, snoring respiration and mucopurulent nasal discharge. The affected animals become restless, stamp their feet and shake or press their heads against objects. Secondary bacterial infection may cause suppuration and complicate the clinical picture. At necropsy, the larvae are found in the sinuses.Larval morphology

The first instar larvae isolated in human cases through ocular swabbing were about 1-2mm in length, on microscopic demonstration these larvae were having oral hooks connected to the internal cephalopharyngeal skeleton and multiple spiny projections.

The 3rd instar larvae which come out of the nostrils of sheep and goats are bout 1-2cm in length having the oral suckers with creamy white color of the body, sometimes a blackish line on the dorsal surface originating from the suckers and diminishing in the 2nd half of the length may also be present.

Own observed field case

Life Cycle

Botflies deposit larvae directly on the host, or sometimes indirectly using an intermediate vector such as the common housefly, mosquitoes, and even a species of tick (Dermatobia hominis).

Note: This flow chart is self created not copied

Effects on Host

The larvae do not kill their host animal, thus are true parasites. They complete their life cycle in the host and leave them but if the larvae die inside the host then toxemia develops which is lethal for the host.

Treatment

Once the animal is infested with the bot fly larvae, it is better to care the animal in general way till the 3rd instar larvae are released out and animal recovers. If any anthelmentic is given, the larvae may die inside the host leading to toxemia and animal death.

Common agents used to treat myiasis include ivermectin (0.2 mg/kg), rafoxanide (7.5 mg/kg) and nitroxynil (7.5 mg/kg).

Prevention is better than Cure

Separate the infested animals and care them separately till complete recovery.

Maintain the proper cleanliness and sanitation in animal area.

Use the fly repellents and baits.

Routinely spray of insecticides in the farm premises.

By: Dr. Kinza Khan

Department of Pathobiology,

BZU, Multan.