Baylisascaris procyonis
The Raccoon Roundworm

Scott Paulson
Human Biology 103
Parasites and Pestilence
Spring 2002
Prof. D. Scott Smith, MD, MSc., DTM&H

History of Discovery
Clinical Presentation
Incubation Period
Case Study
Prevention Strategies and Public Health
Useful Links



AGENT                                                                                               top of page

Baylisascaris procyonis, from the class Nematoda and the phylum Aschelminthes, results in the infectious disease Baylisascariasis in humans, also known as Raccoon Roundworm.  Additional species of the Baylisascaris genus have been identified, including Baylisascaris columnaris in skunks and badgers, Baylisascaris transfuga in bears, and Baylisascaris laevis in marmots, among others.  Despite intragenus similarities, B. procyonis remains the only one of these species to have demonstrated infectious capacity in humans, and is therefore the most significant Baylisascaris species to be studied in scientific investigation and maintenance of human health.(7)

HISTORY OF DISCOVERY                                                          top of page

The nematode B. procyonis was first identified in 1951 by W. Stefanski and E. Zarnowski as a common resident of the small intestines of raccoons.  Initially classified as Ascaris procyonis, it was later described by J.F. Sprent as Baylisascaris procyonis in 1968.(8)  Its recognition as a helminth infection did not occur until more than a decade later, when, in 1984, it was identified by various physicians (Huff, et. al.) as the source of fatal eosinophilic meningoencephalitis in a 10 month old infant.(9)

INTRODUCTION                                                                            top of page

Since its 1984 discovery, B. procyonis has been identified as the causal agent of disease in ten additional cases throughout the United States, and one additional case reported in Germany.(5,7)  The life cycle of the worm is shown below:


Life cycle of B. procyonis. (from

B. procyonis relies heavily on raccoons for its life cycle.  The adult worms develop from the larval stage in the small intestines of the animals, where they lay numerous eggs that are passed through the feces of the raccoon.  Exposure to the external environment stimulates a 2-4 week incubation period of the eggs, upon termination of which they become infective.  From this point, intermediate hosts ingest the eggs from the contaminated environment.  It is here that the eggs hatch, and the larvae proceed to migrate from the intestinal tract to other tissues, where they subsequently encyst.  The life cycle is completed when raccoons consume the flesh of the infected, paratenic hosts.  Humans are one such host, but are not alone. B. procyonis infections have been discovered in a variety of wildlife, but the selection is relatively restricted to birds and small mammals.(7)

Signs of progressive infection with B. procyonis can become very apparent should the larvae penetrate the gut wall.  Larvae migrate to a wide variety of tissues such as the heart, eyes, brain and liver, causing widespread physiological effects including visceral (VLM), ocular (OLM) and neural larvae migrans (NLM).  These symptoms are characterized by extensive tissue damage and are very difficult to control once infection has entrenched itself.  Established treatment for baylisascariasis is unavailable, although antihelminths are commonly used as stabilizing care.  Progressive disease often produces blindness, devastating neurological effects, and/or death.(1)

CLINICAL PRESENTATION IN HUMANS                             top of page

Presentation of baylisascariasis has a variety of characteristics that are particularly dependent on the directional migration of larvae.  Asymptomatic infection can occur should the ingested B. procyonis eggs never hatch, or should the immune system eliminate the threat prior to migration.  Additionally, if a minimal number of eggs are ingested, it is unlikely that infection will occur (infectious dose is believed to be about 5,000 eggs).(3,5)  However, once a sufficient number of larvae have penetrated the gut wall, rampant migration leads to tissue damage, necrosis, and eosinophilic inflammatory reactions specific to the infected region.  The most commonly affected organ appears to be the brain, which is subject to attack following the development of NLM.  General symptoms of such infection can resemble those of meningoencephalitis, with elevated levels of eosinophils often detected in the cerebrospinal fluid (CSF).  Magnetic resonance imaging (MRI) scans have shown white matter abnormalities and increasing white matter with disease progression (see figure below).  Note that the top row of images were taken only ten days prior to the bottom row, and that white matter prevalence has risen dramatically.

MRI of eleven year old boy infected with B. procyonis.  Top row of scans taken ten
days prior to bottom row.  Note elevated white matter following disease progression.

Behavioral changes are also commonly noted as several case studies have presented increased irritability, lethargy and erratic activity of infected individuals.(4)  Progression of this condition leads to considerable inhibition of brain functioning including seizures, loss of muscle control and, eventually, death, if disease progress is left unchecked.

B. procyonis larvae are also responsible for the OLM condition.  A number of cases have demonstrated only ocular migration without any involvement in the CNS.  Moving worms can be seen on the retinal surface in patients who exhibit OLM, and can also be recognized by the patient as floating objects across the normal field of vision.  Without treatment by laser photocoagulation and worm removal, blindness can result through extensive chorioretinal tissue damage.(6)

Migration of larvae to the viscera induces VLM, which, while far less common than NLM, can be equally devastating.  The larvae tend to show preference in migrating to the cardiac tissue, which can cause death in infected individuals.  One specifc baylisascariasis case involving a 10 year old Massachusettes native clearly demonstrated cardiac pseudotumor induced by the migrating worms, whose numbers where sufficient to cease proper heart functioning and kill the patient.(5)  Generally, youths and infants tend to be at greater risk for severe disease than developed adults, who rarely experience intense neurological or visceral migration, and instead seem to be more prone to ocular infection.(5,4)

RESERVOIRS/VECTORS                                                            top of page

Clearly the most common reservoir of B. procyonis is the common raccoon, Procyon lotor. Dogs have also been shown to act as reservoirs of the disease, but also act as paratenic hosts and succumb to deadly infection.  No vector of B. procyonis exists as transmission is believed to occur strictly through fecal-oral pathways.

TRANSMISSION                                                                             top of page

Transmission of B. procyonis occurs through the ingestion of infective eggs.  Female roundworms that thrive in the intestinal passages of raccoons are capable of producing up to 800,000 eggs per day, and a single raccoon can shed nearly 45,000,000 eggs in its feces on a daily basis.(3)  Eggs tend to become concentrated in one specific area due to raccoon behavior in using a single, communal latrine.(7)  Eggs remain viable in the external environment for months to years, and are therefore easily consumed by a variety of animals, such as mice and birds, that forage in soil.  Human infection typically occurs in infants who explore their environment orally by eating dirt, for example.(3)  All eggs are believed to be introduced to paratenic hosts strictly by this fecal-oral mechanism.

INCUBATION PERIOD                                                                 top of page

Following excretion from their raccoon hosts, B. procyonis eggs require 2-4 weeks of incubation in the external environment, typically in soil, to develop into infective agents.  Eggs ingested prior to proper embryonation of eggs will not develop into larval form in a paratenic host.  However, larval migration from the intestines to systemic circulation, following ingestion of infective eggs, can occur as quickly as 3 days.  A precise incubation period is not yet defined, but following initiation of migration, the larvae grow in size as they wander throughout the body, causing tissue death and necrosis as they move and develop.  Symptoms can then accelerate rapidly over the course of a month as additional larvae embed themselves in various tissues.(3)

Figure: Embryonated (left) and non-infective (right) eggs.

MORPHOLOGY                                                                               top of page

B. procyonis is a relatively large parasite with a larval size of approximately 60mm.  It can be distinguished by a prominent, single lateral ala and large intestinal column.(5)

                                        Cross Section of a single larva from the brain of a sea otter.                 Three encysted B. procyonis larva in another
                                        Note the flared, wing-like alae that protrude laterally from the               cross-section of biopsied brain tissue.
                                        body, and the well-defined intestinal column in the larval center.           (from

B. procyonis eggs are very similar in morphology to the common intestinal nematode Ascaris lumbricoides, but can be distinguished by their larger sizes.  The eggs are oval in shape, with average, approximated dimensions of 80 x 70 microns (see Incubation Period section for photos of eggs).  Adult worms can mature to relatively large sizes, with females reaching 23 cm and males growing to approximately half that size at 12 cm.(8)

Adult worms extracted from the small intestine of a single raccoon.

DIAGNOSIS                                                                                     top of page

Diagnosis is often made through a combination of different clinical, epidemiological, radiologic and laboratory techniques.  Physical examination of actual larvae in tissue sections removed from the infected individual is crucial in producing a conclusive diagnosis.  However, this method requires skill and experience in distinguishing B. procyonis from other larval nematoda, as well as larval cestode infections, and therefore is often inconclusive. Additionally, small numbers of the worm can cause substantial disease, and large numbers of parasites are required to be present in tissue cross-sections for an accurate diagnosis.(5)

Serologic testing is usually performed as diagnostic support, though it can sometimes prove effective in initially alerting investigators to the disease’s presence.  Various neural imaging techniques are also used to determine proliferation of the parasite into the brain.  Prevalence of white matter is a common marker of the disease (see Clinical Presentation).(4)  Diagnosis can be aided by examination of other clinical symptoms, which can include eosinophilic meningoencephalitis, diffuse unilateral subacute neuroetinitis and cardiac pseudotumor.(5)  Epidemiological analysis is important as well, and establishing a clear history of raccoon or contaminated area exposure can be very important in creating an accurate diagnosis.

Raccoon exposure history, positive serology and radiology tests, and appropriate clinical presentation of the disease are the most common methods of diagnosis.  However, because there exists no truly conclusive, commercially available and consistent manner of diagnostic testing, many believe there are significant problems in underrecognition of the disease’s prevalence.(5)

EPIDEMIOLOGY                                                                             top of page

Infection by B. procyonis has been documented in over 90 animal species throughout the United States.  Eleven confirmed cases have been reported since 1984, four of which proved fatal.  A solitary case has been reported in Germany.  Baylisascariasis could exist in a number of other locales, though it is unknown exactly how far the boundaries of the disease extend.(5)   Furthermore, asymptomatic infection is believed present to at least some degree in a number of humans.

Raccoons can shed millions of eggs per day, and several studies have found between sixty and eighty percent of raccoons analyzed to be infected with B. procyonis.(2,4)  The raccoon’s tendency to defecate in concentrated areas, referred to as “latrines,” increases the buildup of eggs and contributes to creating infectious dose levels (approximately 5000 eggs or possibly less) in specific areas (see also Transmission).(7)  The durable nature of these eggs has been well-documented, and they are capable of surviving in soil for up to several years.  Infection also shows a distinct prevalence in youths, particularly infants, believed to be due to their manner of interacting with contaminated environments (i.e. playing in dirt) and their relatively weaker immune systems, which are not as capable as adult systems in eliminating the parasite.(3-5)

MANAGEMENT THERAPY AND TREATMENT                  top of page

No effective therapy is known for the visceral, systemic forms of B. procyonis.  Several methods of managing the disease have been used.  A study in infected mice successfully treated the disease with doses of Albendazole and diethylcarbamezine (DEC) within ten days of the infection.  The drugs appeared to be quite successful in preventing the migration of the larvae to the CNS.(5)   However, administration of antihelminthic agents has proven rather unsuccessful in humans.  Due to the high level of difficulty in producing an accurate and rapid diagnosis, these drugs are often given too late and can only stabilize the state of the disease.  By this point, substantial CNS and/or ocular damage has typically occurred.(4)

It is also believed that many minor infections, many asymptomatic, of B. procyonis are quickly eliminated by the immune system, and therefore do not require any treatment whatsoever.  However, no cases to date have been conclusively proven to exhibit this behavior.(3-5)

In the case of OLM as a result of B. procyonis infection, laser photocoagulation is used to eliminate the worm presence on the retina of the infected individual.  This procedure has proven quite effective in preventing subsequent blindness in OLM patients.(4)

CASE STUDY                                                                                  top of page

Study summarized from Park, et. al. (3)

An 11 month old boy with a clean bill of health arrived in the emergency room in August of 1998 with symptoms of irritability, lethargy and peripheral hypertonia.  Routine examination could not conclude any extraordinary findings, and the patient was dismissed on the presumption of viral infection.  The patient returned several days later with similar complaints but of greater degree.  His interaction with his surroundings had markedly decreased, and his eyes had developed difficulty in focusing.  The patient came from a typical suburban area populated by raccoons, and had been observed interacting orally with stones and dirt in the area surrounding his home.

Funduscopic view of chorioretinal scarring
as a result of ocular larva migrans

Retinal examination showed relatively severe scarring (see above figure).  Magnetic resonance imaging showing increased white matter, negative test results for a variety of viruses, clinical presentation and raccoon exposure history led physicians to suspect an infection of B. procyonis.  Patient began treatment with albendazole and methylprednisolone, and also a number of drugs to counter hypertonia symptoms.  Despite treatment, clinical symptoms progressed rapidly over the course of a month.  Over the next four months, the patient’s condition improved, but central nervous functioning was still significantly impaired.  Eighteen months following infection, the patient was improving, but still suffered from seizures, incomplete motor control, and visual impairment.(3)   CNS effects from similar cases have shown to be irreversible.(3,5)


Crucial avoidance strategies in preventing baylisascariasis infection include exercising high levels of caution in areas frequented and contaminated by a strong raccoon presence.  Education about the disease and the dangers associated with raccoon contact should be sufficient to lower the infective rate of the disease.  Infants should be kept under close watch to ensure that they avoid ingestion of possibly contaminated elements of the environment, such as soil and grass.  Should a possible infection be suspected, treatment with antihelminth agents must be sought immediately to attempt the prevention of NLM and VLM disorders, which can quickly overwhelm an individual and prove fatal.


REFERENCES                                                                                 top of page


2. Kerr CL, Henke SE, Pence DB.  Baylisascariasis in raccoons from southern coastal Texas.  J Wildl Dis 1997 Jul;33(3):653-5.

3. Park S, Glaser C, et. al. Raccoon Roundworm (Baylisascaris procyonis) Encephalitis: Case Report and Field examination.
    PEDIATRICS Vol. 106 No. 4 October 2000, p. e56.

4. Rowley H, Uht R, et. al: Radiologic-Pathologic Findings in Raccoon Roundworm (Baylisascariasis procyonis) Encephalitis.  American
    Journal of Neuroradiology 21:415-420 (2 2000).

5. Sorvello F, Ash L, Berlin OGW, et. al. Baylisascaris procyonis: an emerging helminthic zoonosis.  CDC Vol. 8, No. 4.  April 2002.

6. Markell and Voge.  Medical Parasitology.  New York: WB Saunders Co, 1999.

7. Bowman DD. Baylisascaris procyonis in dogs.  Companion and Exotic Animal Parasitology., March 2000.

8. Valle, T.  The Presence of the Raccoon Roundworm Parasite (Baylisascaris procyonis) in Raccoon Fecal Samples as a Factor in the
    Decline of the Allegheny Woodrat (Neotoma magister): a Continued Assessment.  May 1999.

9. Huff DS, Neafie RC, Binder MJ, De Leon GA, Brown LW, Kazacos KR.  The first fatal Baylisascaris infection in humans: an infant
    with eosinophilic meningoencephalitis.  Pediatr Pathol 1984; 2:345-52.


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