Current Advice on Parasite Control:
OVERVIEW OF LIFECYCLE
- Mesocestoides spp. tapeworms have an indirect lifecycle that requires at least two intermediate hosts. The first intermediate host is unknown, but many vertebrates can serve as a final intermediate host harboring larval tapeworms (tetrathyridium) within body cavities, including snakes, birds, lizards, rodents, cats and dogs. These tetrathyridia are able to replicate asexually within the intermediate host. Dogs and cats can be intermediate or definitive hosts for Mesocestoides spp. tapeworms, the latter of which shed egg-laden proglottids in their feces.
- The infectious egg contains a hexacanth embryo.
- Both cephalic and acephalic tetrathyridia (metacestode stage) are reported. The cephalic form has four suckers.
- Adult Mesocestoides spp. are found in the small intestine of an infected dog or cat, and proglottids are shed in the feces.
Two forms of disease are caused by infection of small animals with Mesocestoides spp.
- The intestinal form in which dogs and cats harbor adult Mesocestoides spp. rarely causes clinical disease.
- Peritoneal mesocestodiasis is a rare condition in which tetrathyridium asexually replicate within the peritoneal cavity, causing clinical signs such as ascites, anorexia/weight loss, vomiting, diarrhea, and tachypnea. Scrotal infections have also been documented and scrotal swelling may be clinically visable.
- Peritoneal mesocestodiasis may also be clinically silent and found incidentally upon opening the abdominal cavity for surgical procedures.
- Two forms of disease are caused by infection of small animals with Mesocestoides spp.
- The prevalence of Mesocestoides spp. in dogs and cats is reported to be as low as 0-1%. A number of factors influence the likelihood that a dog or cat will be infected with tapeworms, including the geographic region and the opportunity the animal may have to ingest an infected intermediate host.
- Prevalence data generated by fecal flotation alone underestimates the frequency of infection with Mesocestoides spp. because proglottids (and thus eggs) are focally distributed in fecal material and because eggs are heavy and thus do not readily float; a given fecal sample may be negative for tapeworm proglottids or eggs, even in the presence of an infection.
- Mesocestoides spp. are found throughout North America with higher incidence of reported infections in the western and southeastern United States.
- Peritoneal infections with Mesocestoides spp. have been most commonly reported in California and other western states, and most commonly is reported from dogs.
HOST ASSOCIATIONS AND TRANSMISSION BETWEEN HOSTS
- Both dogs and cats are susceptible to infection with Mesocestoides spp. following ingestion of an infected intermediate host containing tetrathyridia.
- Both dogs and cats have been reported as second intermediate hosts in which tetrahyridia infect the peritoneal space. The route of exposure for development of this form of the disease is not known.
PREPATENT PERIOD AND ENVIRONMENTAL FACTORS
- Dogs and cats may begin shedding proglottids of Mesocestoides spp. as soon as 3 weeks following infection.
SITE OF INFECTION AND PATHOGENESIS
- Intestinal infection: Adult tapeworms are found in the small intestine of dogs and cats. Motile proglottids may be seen in the perianal region as they exit the animal, in the pet’s environment (e.g., on bedding), or in the fecal material itself. Intestinal Mesocestoides spp. infections typically do not cause significant disease in dogs and cats, but because they are aesthetically unpleasant, treatment is warranted.
- Peritoneal infection: The metacestodes (tetrathyridia) are found within the peritoneal cavity of dogs and cats, and have been reported in the scrotum of intact males. Tetrathyridia reproduce asexually, leading to dramatic expansion of the parasite. These infections can remain asymptomatic or can cause irritation within the abdominal cavity leading to ascites and abdominal distension. Other signs that may accompany the peritoneal form of Mesocestoides spp. infection include vomiting, anorexia, depression, diarrhea, tachypnea, and scrotal swelling.
- Diagnosis of intestinal infection with tapeworms is reached by identifying proglottids in the fecal material or by recognizing eggs upon fecal flotation.
- However, because proglottids are not uniformly distributed in the fecal material and eggs do not consistently float, fecal flotation alone is not a reliable means of diagnosing tapeworm infection in dogs and cats.
- Eggs are thin-walled, measure approximately 30-40 microns, and contain a hexocanth embryo.
- Adult Mesocestoides spp. are 30-70cm long and have a scolex with 4 muscular suckers and no rostellum (hooks).
- Mature proglottids are 3-4mm in length and can be identified based on the presence of a parauterine organ, located on the midline of the proglottid, and a genital pore that opens ventrally.
- Diagnosis of pertitoneal Mesocestoides spp. infection is based on clinical signs and diagnostic tools such as laparoscopic exam, cytology, radiographs, ultrasound, and PCR.
- Tetrathyridia are reported in two forms. The cephalic form can be identified by the presence of four suckers. The acephalic form can only be definitively identified by PCR. Both are small (100 microns to 3mm), motile, irregularly shaped organisms.
- Cytology of abdominal fluid may contain tetrathyridia and/or calcareous corpuscles, which are small refractile structures characterized by concentric rings.
- Radiography and ultrasound may be used to support the diagnosis but do not reveal changes pathognomonic for peritoneal cestodosis. Radiographic changes include increased radiopacity and ground-glass appearance, which are consistent with peritoneal effusion. Ultrasound studies have revealed anechoic cystic structures within the abdomen.
- Praziquantel at 5mg/kg can be given orally or subcutaneously for the treatment of Mesocestoides spp. adult tapeworm infections in the intestine of dogs and cats.
- Treatment of adult tapeworms in dogs and cats must be combined with appropriate management, such as prevention of ingestion of prey species; in the absence of these changes, re-infection may occur.
- Treatment of tetrathyridial stages of Mesocestoides spp. requires long-term drug administration and numerous follow up visits.
- Peritoneal lavage should be performed to remove as many tetrathyridia as possible.
- Fenbendazole, (100 mg/kg, twice daily for 28 days) while not labeled for treatment of peritoneal Mesocestoides spp. infection, has been reported as curative in some dogs.
- Long-term monitoring is needed after treatment because reoccurrence of infection is common, which is likely due to asexual reproduction of any remaining tetrathyridia.
- Dogs and cats that become persistently infected with Mesocestoides spp. may be given fenbendazole (50-100mg/kg) daily for the life of the animal.
- The prognosis for the peritoneal form of Mesocestoides spp. infection is guarded.
CONTROL AND PREVENTION
- In dogs that are allowed outside or that are known to have predatory behavior, a heartworm preventive containing praziquantel would be expected to help control intestinal Mesocestoides spp. infections.
- Prevention of predation and scavenging activity by keeping cats indoors and dogs confined to a leash or in a fenced yard will limit the opportunity for dogs and cats to acquire infection with Mesocestoides spp. via ingestion of tetrathyridium in intermediate hosts.
- The life cycle of this parasite is not fully understood, which precludes recommendations regarding specific preventive measures to avoid peritoneal infection with Mesocestoides spp. tapeworms.
PUBLIC HEALTH CONSIDERATIONS
- Dogs and cats that shed gravid proglottids do not pose a direct zoonotic risk as Mesocestoides spp. eggs are not infective to humans.
- Human infection with Mesocestoides spp. is uncommon, and intestinal infections are well tolerated and easily treated. These infections are usually linked to ingestion of improperly prepared food, such as raw viscera or blood that contains tetrathyridia.
- Bonfanti, U., et al. 2004. Clnical, cytological and molecular evidence of Mesocestoides spp. infection in a dog from Italy. J Vet Med A 51:435-438.
- Boyce, W., et al. 2011. Survival analysis of dogs diagnosed with canine peritoneal larval cestodiasis (Mesocestoides spp.). Vet Parasit 180: 256-261.
- Conboy G. 2012. Cestodes of dogs and cats in North America. Vet Clin North Am Small Anim Pract 39:1075-1090.