Myiasis is the development of larval dipteran flies (bots and warbles) within the tissues or organs of domestic or wild animals or humans. Myiasis that involves the CNS is rather uncommon except for the larval stages of Hypoderma bovis, the cattle heel fly. Its larvae typically burrow between the periosteum and dura mater of the spinal cord in cattle during migration to the subcutaneous tissues of the back. Neurologic signs, varying from a transient, stiff, unsteady gait to paralysis, may occur in cattle administered systemic insecticides when the larvae are present in the spinal canal. (Also see Cattle Grubs.)
The larvae of Oestrus ovis, the nasal bot fly of sheep , are usually found in the nostrils and paranasal sinuses. They rarely penetrate the ethmoid bone and reach the forebrain and it is possible that additional factors facilitate entry of larvae into the brain. The bones of the skull may erode. If the brain is injured, clinical signs, such as a high-stepping gait and incoordination, may mimic infection with Coenurus cerebralis. This condition is often referred to as false gid. Surgical intervention may be useful but can be difficult.
Larval Cuterebra spp, normally found in subcutaneous sites in dogs or cats, have been known to wander into the CNS and localize in the cerebrum or cerebellum. Intracranial migrations by larvae of dipteran flies have been reported in humans (Dermatobia hominis), cattle (Hypoderma bovis), and horses (Hypoderma spp).
Bots and warbles may move rapidly after death of the host and migrate into tissues far from the site of origin.
Treatment of intracranial myiasis is currently experimental. Surgical and medical treatments to alleviate intracranial myiases have been considered. The efficacy of systemic organophosphates against migrating larvae of Hypoderma suggest that organophosphates may effectively eliminate certain dipteran larvae from the nervous system. Parenteral corticosteroids are also recommended to prevent additional inflammatory damage and intracranial pressure throughout the treatment period. Ivermectin (300 mcg/kg on alternate days) used in conjunction with corticosteroids should be considered experimental treatment for intracranial cuterebrosis in cats; it is not approved by the FDA for this use.
