Deerworm Fact Sheet for Sheep and Goat Producers

Courtesy of Cornell Sheep & Goat Program and Cornell Ambulatory Veterinary Services. Modified by Amy Barkley for length.

Funding provided by the Beakman Fund and the Hatch & Smith-Lever Federal Capacity Fund Grant "Application of New Concepts for Control of Internal Parasites in Sheep and Goats".

What is Deer Worm?

Deer worm, aka meningeal worm, (Parelaphostrongylus tenuis (abbreviated P. tenuis)) is a parasitic worm of ruminants. It is very common in white-tailed deer in the Northeast United States (up to 90% of deer harvested during hunting season have been shown to be infected) but does not cause disease in this species. The worm has an indirect life cycle, requiring land snails or slugs as intermediate hosts before being able to infect a sheep or goat from the consumption of slugs/snails and their slime from pastures shared with deer. Sheep and goats are not the ideal host, so this parasite ends up causing serious neurologic disease as it attempts to grow and reproduce in these species.

What are the Signs of Deer Worm Infection?

There are two common types of signs that an infected sheep or goat may show. Many affected animals will have trouble moving around; early signs of deer worm infection include mild stumbling, knuckling and/or dragging the toes of one or both hind limbs, and general weakness of the hind limbs. In more advanced cases the sheep/goat may suffer total paralysis of the hind limbs, leaving it sitting in a dog-sitting posture or down and unable to get up. The front limbs are less commonly affected but can have the same problems as the hind limbs. Even in a case of total limb paralysis, animals may still recover fully given proper treatment and/or time.

Another common sign associated with deer worm infection is excessive itching and rubbing of one area on the side of the body. Migrating larvae can irritate an individual nerve where it merges with the spinal cord, making the animal rub and/or bite incessantly at the area where the nerve runs. This leads to hair loss and occasionally a wound in the skin. Less commonly, infected animals will show signs of brain disease such as a head tilt, walking in circles, rapid eye flickering, and difficulty chewing. Appetite and body temperature typically remain normal in animals affected with the deer worm, which is not often the case with other common diseases of the brain in sheep and goats. It is important to note that not all affected animals will show all these signs. There can be a wide range of manifestations of deer worm infection, so it is important to be vigilant in watching for any problems with your animals.

The diagnosis relies heavily on gazing history combined with symptoms. If you would like to confirm the diagnosis, your vet can submit a sample of brain and spinal fluid to confirm the characteristic microscopic signs of infection.

Cornell's Deer Worm Research Shows a Promising Treatment

A four-year study by the Cornell Sheep & Goat Program and Cornell Ambulatory Veterinary Services investigated the effectiveness of two different treatment protocols for naturally infected sheep and goats. Participating goat and sheep farmers knowledgeable about deer worm contacted the researchers when they observed a suspected case of deer worm infection to determine if the animal qualified for enrollment in the study. Enrolled animals were randomly assigned within herd to a treatment or control group with treatments alternating for succeeding animals from the same farm. The treatment group received 5 days of oral fenbendazole (25 mg/kg), intramuscular dexamethasone (0.2 mg/kg for 3 days followed by 0.1 mg/kg for 2 days) and subcutaneous ivermectin (0.5 mg/kg); the control group received the same 5 day course of fenbendazole and dexamethasone with a similar volume of placebo vehicle subcutaneously. Farmers and study veterinarians were blinded to treatment. Neurologic examinations were conducted on all animals. Animals received a neurological score at the time of enrollment and after treatment that ranged from 1 (unable to stand) to 5 (no detectable neurologic deficits or only an expert would notice), with a score of 4 indicating likely to function in the herd for breeding though noticeably impaired.

Twenty goats and 18 sheep from 10 central NY farms were enrolled in the trial with 9 goats and 9 sheep in the control group and 11 goats and 9 sheep in the treatment group. Animals were classified as recovered if they required no further treatment to potentially remain in the breeding herd after the 5-day treatment period. Six of 9 sheep treated with ivermectin recovered without further treatment but 3 had to be euthanized. Five of 9 sheep treated with the placebo recovered without further treatment; 2 required additional treatment, and 2 had to be euthanized. All 11 of the goats treated with the ivermectin recovered, while six of the nine treated with the placebo recovered without further treatment, and 3 required additional treatment.

Statistical analysis indicated that higher pretreatment neurological scores improved outcome. That is, the less severe the symptoms were before treatment started, the better the chance of recovery. The effect of adding in ivermectin to the treatment protocol showed promise, though not statistically significant. However, all 11 goats treated with ivermectin were categorized as recovered. The better outcomes for goats are probably explained by closer observation of goats (coincidentally all goats were returned to barns at night while all sheep were left in pastures 24/7) which resulted in better pre-treatment neurological scores. Given the importance of pre-treatment score on the odds of recovery, close observation of animals at high risk for P. tenuis infestation is warranted for timely treatment. Studies with larger numbers of animals are needed to definitively state whether including ivermectin in the treatment protocol improves outcome. Inclusion of ivermectin increases the drug withdrawal period for the standard protocol to 96 days or more according to FARAD, the Food Animal Residue Avoidance Databank. If there is no concern about the withdrawal period, we cannot rule out the possib+ility that ivermectin may be beneficial in the treatment of highly valuable animals.

Can Infection be Prevented?

Regular deworming to prevent deer worm is expensive and time consuming, and results in faster dewormer resistance to the other worms of economic importance to sheep and goat producers. Reducing exposure is the most reasonable preventative. Pastures bordering woodlands and low-lying/wet areas are the highest risk of both deer exposure and snail/slug habitat. If these areas need to be part of a regular pasture rotation, consider not grazing them during the wet season or after tree leaves have fallen. Having a livestock guardian regularly patrolling pastures can limit deer visits as well.

While deer worm used to be a disease limited to the eastern side of the state, we are seeing an increased incidence of cases in WNY. Prevention, vigilance, and treatment for this parasite are now necessary. If you are in SWNY and have questions about parasites, feel free to reach out to Amy Barkley at 716-640-0844 or amb544@cornell.edu.

The original article can be found at https://blogs.cornell.edu/smal...