Meningeal worm or brain worm has been transferred from the genus Pneumostrongylus to Parelaphostrongylus, hence is now known as Parelaphostrongylus tenuis.  This species is a slender, delicate roundworm found in the cranial cavity of cervids.  The white-tailed deer is the normal host of P. tenuis.

The adult worms are normally located between the membranes (meninges) covering both the brain and spinal cord, but are found more frequently in the meninges of the brain.  The adult worms deposit eggs either on these membranes or directly into blood vessels.  Those deposited on the membranes, hatch and the larvae enter small blood vessels to be carried to the lungs where they enter the alveoli.  Eggs deposited into blood vessels are carried to the lungs and eventually hatch with larval penetration of the alveoli.  This activity in the lung tissue produces an interstitial pneumonia.  The larvae pass up the respiratory tract from the alveoli, are swallowed and then eliminated in the faeces.  Larvae appear in the faeces about three months after the host becomes infected.

The larvae then penetrate into gastropods (snails and slugs) which act as intermediate hosts.  Development of the larvae in the gastropod to a stage when they are infective to the vertebrate host takes about three weeks.  Cervids become parasitized by ingesting infected gastropods with their food.

In the final host, development of the larva to the adult worm takes place in tissues of the central nervous system, particularly the spinal cord.  Parasites leave the tissues of the spinal cord after about 20-40 days and locate between the spinal membranes where they mature.  Subsequently they tend to accumulate in the cranial region.  The adult worms are about 50 mm in length and may be seen fairly readily when free in the cranial cavity.  From one to 20 worms have been found in the crania of infected deer.  P. tenuis seldom causes damage in white-tailed deer.

In other cervids there is often extensive damage to tissues of the brain and spinal cord.  The resulting neurologic disease is characterized by weakness, fearlessness, lack of coordination of movement, circling, deafness, impaired vision, paralysis and subsequent death.  When in moose this disease is often called “moose sickness” or, “moose disease”.

The life cycle of P. tennis and the relationship between this parasite and "moose sickness" were first elaborated experimentally by Anderson (1963, 1964).  The early experimental work stimulated extensive study of meningeal worm in the natural environment.

In Canada, studies by Anderson (1965, 1971, 1972), Bindemagel and Anderson (1968), Kelsall and Prescott (1971), Lankester and Anderson (1968) and Saunders (1973) have substantiated the importance of this problem in management of big game and have given us some indication of the dynamics of the hostparasite relationships among wild populations.

P. tenuis has a wide distribution in eastern and central North America.  Within Ontario in the last 100 years, changes in the environment have resulted in the northern extension of the deer range.  Both deer and meningeal worm have shared range with moose.  "Moose sickness" has been restricted to areas where deer and moose ranges overlap.  Declines in the moose populations of New Brunswick, Nova Scotia, Maine and Minnesota appear to be associated with this disease.

Failure to consider the importance of the effect of this parasite on cervids other than white-tailed deer resulted in the loss of a small herd of reindeer introduced from Norway to Ontario in 1969.

Selected References:

Anderson, R.C. 1963.  The incidence, development, and experimental transmission of Pneumostrongylus tenuis Dougherty (Metastrongyloidea:Protostrongylidae) of the meninges of the white-tailed deer (Odocoileus virginianus borealis) in Ontario.  Can.  J. Zool. 41:775-792.

Anderson, R.C. 1964.  Neurologic disease in moose infected experimentally with Pneumostrongylus tenuis from white-tailed deer.  Pathol.  Vet. 1:289-322.

Anderson, R.C. 1965.  An examination of wild moose exhibiting neurologic signs, in Ontario.  Can. J. Zool. 43:635-639.

Anderson, R.C. 1971.  Neurologic disease in reindeer (Rangifer tarandus tarandus) introduced into Ontario.  Can.  J. Zool. 49:159-166.

Anderson, R.C. 1972.  The ecological relationships of meningeal worm and native cervids in North America.  J. Wildl.  Dis. 8:304-310.

Bindernagel, J.A. and R.C. Anderson. 1972.  Distribution of the meningeal worm in white-tailed deer in Canada.  J. Wildl.  Manage. 36:1349-1353.

Gilbert, F.F. 1973.  Paretaphostrongylus tenuis (Dougherty) in Maine: I.-The parasite in whitetailed deer (Odocoileus virginianus, Zimmerman).

J. Wildl.  Dis. 9:136-143.

Kelsall, J.P. and W. Prescott. 1971.  Moose and deer behaviour in snow in Fundy National Park, New Brunswick.  Can.  Wildl.  Serv.  Rept.  Series No. 15. 27 pp.

Lankester, M.W. and R.C. Anderson. 1968.  Gastropods as intermediate hosts of Pneumostrongylus tenuis Dougherty of white-tailed deer.  Can.  J. Zool. 46:373-383,

Saunders, B.P. 1973.  Meningeal worm in whitetailed deer in northwestern Ontario and moose population densities.  J. Wildl.  Manage. 37:327-330.