Meningeal Worm

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Meningeal worm (Parelaphostrongylus tenuis), also known as deer worm or brain worm, is a parasite of white tail deer that typically causes disease in sheep in the late summer and fall.

In deer, the adult P. tenuis worms live on the membranes covering their brains, the meninges. In most deer there is no adverse effect from the parasites. Female worms lay eggs on the meningeal membranes, they hatch, the larvae migrate into the bloodstream and travel to the lungs. Here larvae are coughed up and swallowed, passing through the intestinal tract and larvae are excreted with the deer manure. These larvae are either ingested or burrow directly into the foot of slugs and snails to infect them when they move over the manure pellets. The larvae grow and develop into 3rd stage larvae in the snail/slug and infect deer when they accidentally ingest a snail/slug or ingest vegetation that is covered in the mucous trails of affected snails/slugs. The ingested larvae migrate out of the deer’s stomach and move through the abdominal cavity into the nerves of the body wall. Deer are the normal host of this parasite so they do not mount a large immune response to this invasion of nervous tissue by parasites. The larvae migrate up these nerves into the spinal cord, where they grow and ultimately migrate up the spinal cord to the brain where they mate and start the whole process over again.

In sheep, when P. tenuis infected snails/slugs/mucous-covered vegetation is ingested, the larvae migrate out of the intestinal tract just like in deer, but when they start invading the nervous tissues, sheep have an immune response to them. This “disorients” the larvae so they are not able to follow their natural life cycle of maturing and then migrating up the spinal cord like they would do in a deer. The disoriented larvae burrow around in the nervous tissue, causing damage and the sheep’s immune response causes swelling. When this process happens in the nerves outside the spine, it causes severe itching and sheep will often scratch themselves raw. When this process happens in the spinal cord (which is enclosed in the spine), nerves are compressed and damaged to the extent that you start seeing abnormal gaits that progress to paralysis, if left untreated. It typically takes 30-60 days after ingestion of the snail to see disease.

What do you see when sheep are infected?

Some sheep only get severe itchiness and do not progress to neurological disease. Lesions are typically on the trunk and neck area and tend to be in a vertical (up and down) line.

If not severely affected, just general wound care should be all that is needed, although animals that show severe itchiness do require the full meningeal worm treatment to stop them from scratching.

The ram lamb in the photo below was severely itchy and required full treatment for meningeal worm as well as topical medication to stop his scratching. You can see he has severe linear scratches on his chest, a raw spot above his ear and is starting to injure himself on his neck as well.

This is a link to a video showing typical early neurological signs in a ewe lamb affected with meningeal worm:
In the resources you can find an additional video showing a more severely affected sheep.

Neurological signs typically start with stumbling, slight knuckling or dragging of a foot and incoordination. From there the disease progresses to weakness and eventually paralysis if left untreated. As long as the worms stay in the spinal cord, the sheep are bright, alert and have a good appetite. If larvae migrate into the brain the disease looks a lot like listeria or polio.

Meningeal worm cannot be diagnosed by doing a fecal egg count. In deer, the larvae are excreted, not eggs. In sheep, the larvae are doing all the damage. The life cycle is not finished, so no eggs are produced.

There is not a meningeal worm test available. The condition is suspected based on signs seen and response to treatment. Your veterinarian can perform a spinal fluid collection and there are specific types of white blood cells (eosinophils) seen in spinal fluid that are very suggestive of meningeal worm infection. But this is a very expensive test in relation to the value of an average meat lamb.

How do I treat meningeal worm?

Cornell did a study on the treatment of meningeal worm and had reasonably good outcomes in sheep and goats (if treatment is started early in the disease) using Fenbendazole (Safeguard) and Dexamethazone. Adding Ivermectin to the protocol might be beneficial but requires a longer drug withdrawal time. You can find the protocol in the “P. tenuis Producer Fact Sheet ” in the resources below.

This treatment comes with a LONG drug withdrawal time as you are using drugs that are not licensed for sheep (Fenbendazole), at much higher doses and for multiple days in a row. The last time I requested withdrawal information from FARAD they gave a 90-day drug withdrawal time before slaughter for just the Safeguard and Dexamethazone protocol. Adding Ivermectin will add to the withdrawal time. Please note that the withdrawal time starts on the last day of treatment. Please check with your veterinarian for the latest withdrawal times for this protocol.

DO NOT USE DEXAMETHAZONE IN PREGNANT EWES; it can cause abortion. Flunixin meglumine (Banamine) can be substituted in these cases. Please check with your veterinarian on dosing for this.

Typically, the more severely that animals are affected before treatment starts, the longer it takes for them to recover. Recovery can take weeks or months. Some severely affected animals might never fully recover.

As a cautionary note: A producer in my area treated a lamb with this protocol and sent it to slaughter with a group of untreated lambs before the withdrawal time was done. He assumed the withdrawal time listed for goats for deworming was sufficient for this treatment protocol. The meat inspector randomly selected the treated lamb carcass to sample for violative residues. The processor he used does not separate and label meat packages by individual lamb and the tests take days for the results to come back. By the time the tests came back the meat packages of all the lambs had been commingled. The packages were not identified by individual lamb, so he was unable to separate the affected meat from the unaffected meat and was unable to sell any meat from the whole group of lambs.


Deer and snails are required for this disease to affect your sheep. If both these are present on your farm, you will probably have some issues unless you confine your sheep off pasture. Deer do not have to be in the pasture at the same time as sheep to transmit P tenuis. The larvae mature in the snails for a few months, so you would have to exclude deer from your pastures at all times (even when the sheep are grazing elsewhere) to prevent transmission. Deer fencing is an option but cost prohibitive for larger acreages.

The snails that transmit meningeal worm are land snails. They do not require standing water, but they do require some moisture. Snails do not respect fences; they can travel short distances into your pastures. Unfortunately, most chemicals used to kill slugs are not safe or cost effective to use on your pastures. Dryer years should produce fewer cases of meningeal worm as there is a smaller slug/snail population.

The good news is that sheep can build up immunity to meningeal worm. Cornell did research that proved that lambs infected with low numbers of infective larvae made antibodies against P. tenuis larvae and showed the same type of immune response that you would expect with a vaccine upon re-exposure. If sheep are exposed to small amounts of infective larvae at first, they can mount an immune response and not show disease if larval loads build up on pasture later in the season.

The earlier in the disease process you can start treatment, the better the outcomes. Doing daily moves to fresh pasture or getting sheep up and moving once a day, to identify sheep moving abnormally, will help you to catch it early.

Ivermectin will kill larvae in the Gi tract and while migrating in tissues before invading the nervous tissue. Farmers who use llamas and alpacas as guard animals treat them monthly with injectable ivermectin to prevent clinical disease. Llamas and alpacas tend to have bad outcomes, even when treated, when they come down with meningeal worm. You could theoretically treat all your sheep monthly with injectable ivermectin to prevent meningeal worm but I would not recommend doing that on a large scale. It will definitely induce ivermectin drug resistance in the GI parasites on your farm. The other concern is that ivermectin excreted in the manure of a large number of animals is detrimental to the dung beetles, other beneficial insects and soil organisms in your pastures. You could consider it for very valuable animals on your farm, if you have severe problems with meningeal worm.

Our farm in Central New York, has lots of deer, lots of snails and heavy clay soils that stay moist, so needless to say we are familiar with the disease. We saw multiple cases (in adults) the first year that we started farming. Over time we have seen fewer cases and we have not had any cases in adults after the first year. The past few years we have only had one or two cases per year with neurological signs, requiring Safeguard treatment, and about five affected with itchiness that resolved without treatment (out of almost 300 lambs). We do select hard for GI parasite resistance, so that might play a role as exciting new research shows that excellent GI parasite resistance does seem to correlate with better immunity in general. We do daily pasture moves which helps a lot to identify affected animals early so we have had good success in treating lambs using the Cornell treatment protocol. I do find that lambs appear to get worse the first 2-3 days of treatment before they start improving.

By: Isabel Richards, EAPK Communications Committee



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