Arthropods That Cause CNS Disease
Myiasis is the development of larval dipteran flies (bots and warbles) within the tissues or organs of people and other domestic or wild animals. Myiasis that involves the CNS is rather uncommon except for the larval stages of Hypoderma bovis, the cattle heel fly. Its larvae normally burrow between the periosteum and dura mater of the bovine spinal cord during migration to the subcutaneous tissues of the back. Neurologic signs, varying from a transient, stiff, unsteady gait to paralysis, may be seen in cattle given 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 (see Sheep Nose Bot), are normally found in the nostrils and paranasal sinuses. They rarely penetrate the ethmoid bone and reach the forebrain. However, it is possible that other 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 prove difficult if the larvae are difficult to reach.
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 (see Cuterebra Infestation in Small Animals). Intracranial migrations by larvae of dipteran flies have been reported in people (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 therapies 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 therapy for intracranial cuterebrosis in cats; it is not approved by the FDA for this use.