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https://todaysveterinarypractice.com/table-of-contents-november-december-2021/
Parasitology, Practical Parasitology

Feline Helminths: Recommendations from the Companion Animal Parasite Council

Feline Helminths: Recommendations from the Companion Animal Parasite Council

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Rick Marrinson, DVM, and Susan E. Little, DVM, PhD, Diplomate ACVM (Parasitology)

Helminth infections, both gastrointestinal and extraintestinal, are remarkably common in cats. Transmission, diagnosis, and treatment are discussed.


The mission of the Companion Animal Parasite Council (CAPC) is to foster animal and human health, while preserving the human—animal bond, through recommendations for the diagnosis, treatment, prevention, and control of parasitic infections. For more information, including detailed parasite control recommendations, please visit capcvet.org

Helminth infection in cats is remarkably common. Recent surveys have documented helminths in more than 2/3 of adult cats examined in animal shelters in some areas.1 The most common nematode of cats, Toxocara cati, also has zoonotic potential, making control particularly important.

The high prevalence of infection is often surprising to both veterinarians and parasitologists, in part because fecal flotation may fail to reveal the presence of helminths. An absence of helminth eggs on fecal flotation is especially true for tapeworms; a recent study showed that, in 87% of cats with tapeworms confirmed in the small intestine, eggs were not detected on fecal flotation.1

Dirofilaria immitis (heartworm) also infects and causes disease in cats, although diagnostic test results can be difficult to interpret.2

Broad-spectrum anthelmintic products are widely available to protect and treat cats, but their use and acceptance remain limited. To protect feline health and limit environmental contamination with zoonotic parasites, CAPC recommends use of year-round parasite control products in cats throughout all areas of the United States.3

FELINE GASTROINTESTINAL HELMINTHS

Nematodes

The most common gastrointestinal nematodes among continental North American cats are ascarids (T cati and Toxascaris leonina) and hookworms (Ancylostoma tubaeforme). Other nematodes occasionally seen include Physaloptera species, Strongyloides species, and Ollulanus tricuspis.

Ascarids. Cats acquire infection with ascarids by ingestion of larvated eggs from the environment or ingestion of rodents or birds that have ingested eggs. In some surveys, T cati eggs are identified by fecal flotation in as many as 35% of cats, and necropsy surveys place the prevalence even higher.4,5

Kittens infected with T cati may have a pot-bellied appearance and general ill thrift; adult cats also commonly harbor infection. Migration of adult T cati into the stomach irritates the mucosa, resulting in vomiting, and T cati are commonly found in feline vomitus. The other feline ascarid, T leonina, is less common than T cati and not thought to be associated with clinical disease.3

Hookworms. The hookworm, tubaeforme, is found in the small intestine of cats and has been associated with anemia and weight loss; Ancylostoma braziliense also occurs but is much less common in the United States.

Cats become infected with hookworms upon ingestion of larvae or ingestion of rodent paratenic hosts; unlike with dogs and Ancylostoma caninum, transmammary transmission with tubaeforme is not known to occur.3

Diagnosis. Infection with ascarids and hookworms can often be diagnosed by fecal flotation. To ensure adequate sensitivity to allow detection, CAPC recommends that fecal flotation be performed by centrifugation, using an adequate sample size. However, eggs cannot be detected if only a single sex of worm is present or the nematodes are immature and, thus, prepatent.

Eggs of T cati are spherical and surrounded by a thick, roughly pitted shell (Figure 1), while eggs of A tubaeforme are oval and smooth shelled and contain a developing embryo (Figure 2). The prepatent period for T cati is 4 to 5 weeks, while that of A tubaeforme is 2 to 3 weeks.

Figure 1. Toxocara cati

Figure 1. Egg of Toxocara cati; note the pitted shell and dark appearance.

Figure 2. Ancylostoma

Figure 2. Thin-shelled oval egg of Ancylostoma tubaeforme.

Treatment. Both ascarids and hookworms are readily treated with several different anthelmintics, including pyrantel, emodepside, milbemycin, moxidectin, and selamectin; the topical formulation of some of these compounds facilitates administration to cats.2

Cestodes

Historically, adult tapeworms in the intestinal tract have not been thought to cause significant clinical disease in cats. However:

  • Intestinal impactions with Taenia taeniaeformis necessitating surgical removal have been reported6
  • Spirometra species, a common cestode of cats in some coastal and swampy areas, can cause diarrhea, vomiting, and weight loss.7

Feline intestinal infections with Mesocestoides species are usually asymptomatic, but organisms in extraintestinal stages (tetrathyridia) that multiply asexually in the abdominal cavity can cause severe clinical sequelae, including death, in some cats.

Life Cycle & Transmission. All of these cestodes have an indirect life cycle, requiring 1 or more intermediate hosts:

  • T taeniaeformis and Dipylidium caninum use rodents and fleas, respectively.
  • Spirometra species require a copepod first intermediate host and then use many different vertebrates as second intermediate hosts.
  • The life cycle of Mesocestoides species has not yet been determined, but ingestion of vertebrate intermediate hosts containing tetrathyridia is considered the most likely route of intestinal infection.8

Diagnosis. Diagnosis of cestode infection is challenging. Fecal flotation is recommended, and, when identified, eggs definitively confirm the presence of tapeworms. However, in most cats with cestodes, eggs from the small intestine are not recovered on fecal flotation,1but owner reports of proglottids are helpful in reaching a diagnosis.

Treatment. In cats that commonly ingest prey species, infections are likely, and presumptive treatment may be warranted. Praziquantel and epsiprantel are label-approved as effective against T taeniaeformis and D caninum, and can be used off-label for treatment of intestinal Mesocestoides species.

Effective treatment of Spirometra species is more difficult, requiring administration of an elevated, off-label dose of praziquantel (25 mg/kg PO Q 24 H) for 2 consecutive days.7

Trematodes

Gastrointestinal trematode infection in cats is rare compared with nematode and cestode infection. However, occasional infections with Alaria species or Nanophyetus salmincola are seen in certain geographic areas.

Life Cycle & Transmission. Alaria species infections are acquired when cats prey on intermediate or paratenic hosts, such as frogs and snakes, that harbor the metacercariae. Migration of immature Alaria species through the lungs can lead to pulmonary damage, with focal areas of hemorrhage evident.3

Feline infection with N salmincola occurs when cats ingest metacercariae in salmonid fish. Although trematodes develop in the small intestine, classic “salmon poisoning” disease due to Neorickettsia helminthoeca infection has not been described in cats.3

Diagnosis. Diagnosis of infection with Alaria species, N salmincola, and other trematodes usually requires concentrating eggs by sedimentation, because the eggs do not readily float in the specific gravity of many flotation solutions.

Identity of trematode eggs can usually be confirmed by the presence and shape of the operculum, general egg morphology, and size. For example, eggs of Alaria species are approximately 120 microns × 70 microns, are operculate, and contain an undifferentiated embryo when shed (Figure 3).

Figure 3. Alaria

Figure 3. Large eggs of Alariaspecies recovered from infected cat.

Treatment. Although no products are label-approved for trematode treatment in cats, praziquantel is expected to be effective.3

FELINE EXTRAINTESTINAL HELMINTHS

Nematodes

The 2 most common extraintestinal nematodes infecting cats in North America are heartworms (D immitis) and feline lungworms (Aelurostrongylus abstrusus).3 Both parasites can cause respiratory disease in cats due to pulmonary damage.

The death of developing D immitis may lead to pulmonary, bronchial, and alveolar disease in cats collectively referred to as heartworm-associated respiratory disease; adult heartworms in the pulmonary artery are associated with sudden death.

Life Cycle & Transmission. D immitis is transmitted by mosquitoes, and dogs are considered the main reservoir host. Microfilariae rarely persist in cats long enough for them to serve as a source of infection to mosquitoes.9

Snails and slugs are required intermediate hosts for A abstrusus, but infections are most common in cats allowed to prey on rodent, avian, or lizard paratenic hosts.

Clinical Signs. Vomiting is often present in cats infected with D immitis, while some cats infected with A abstrusus develop chronic cough, dyspnea, and anorexia.

Diagnosis. Diagnosis of heartworm infection in cats is complicated by the low number of worms present and the robust immune response of the feline host.

In cats with respiratory disease or other clinical signs suggestive of heartworm infection, both antigen and antibody tests should be performed (see Feline Heartworm: Antigen Versus Antibody Tests). Heat treating serum prior to antigen testing destroys immune complexes, revealing antigen for detection.2 Radiography and echocardiography can also be helpful in assessing disease status and confirming infection.

Diagnosis of A abstrusus depends on identification of first-stage larvae in fecal samples (Figure 4). Procedures that allow recovery of A abstrusus larvae include direct smear, centrifugal fecal flotation, and Baermann examination.

Figure 4. Aeluro

Figure 4. Larvae of Aelurostrongylus abstrususdetected on fecal examination of infected cat.

Treatment. Adulticide treatment is not recommended for cats with heartworm. However, cats with dyspnea or respiratory distress should receive supportive care, such as corticosteroids, oxygen therapy, or bronchodilators. Preventive use is also recommended to limit additional infections.9

There are no label-approved treatments for A abstrusus infection, but efficacy has been documented with topical moxidectin and topical emodepside.10,11

Feline Heartworm: Antigen Versus Antibody Tests

Both antigen and antibody tests are available to identify feline heartworms. 

Antigen tests are very sensitive and specific, but cats with heartworm often harbor a low number of worms, and antigen levels are correspondingly low.9 In addition, the strong immune response appears to result in antigen—antibody complex formation, preventing detection of antigen on commercial tests.2

Antibody test results can be similarly problematic to interpret. Many cats in endemic areas may be antibody positive, indicating that a past or current infection has likely occurred, but the cat is clinically healthy and infection free.


Cestodes

Mesocestoides species may establish both intestinal infections (described earlier) and, rarely, extraintestinal infections in cats.

Clinical Signs & Transmission. Cats with peritoneal mesocestodiasis develop vomiting, diarrhea, weight loss, and ascites due to asexual replication of tetrathyridia (immature forms) of Mesocestoides species in the peritoneal cavity, but some clinically silent infections are discovered during routine laparotomy and ovariohysterectomy. The route of exposure responsible for the peritoneal form is not known.

Diagnosis. Diagnosis may be suspected at ultrasonography and is confirmed by cytology or laparoscopic examination. Calcareous corpuscles in cytologic fluid help confirm the presence of a cestode.

Treatment. Treatment is difficult, requiring peritoneal lavage followed by long-term administration of fenbendazole. Prognosis for cats with peritoneal mesocestoidiasis is guarded.8

Trematodes

Feline infection with systemic trematodes, including Paragonimus kellicotti in the lungs and Platynosomum fastosum in the liver, is also relatively uncommon. However, because of the severe disease that develops in infected cats, these parasites are important in certain endemic areas.

Life Cycle & Transmission. P kellicotti adults develop in cysts in the lungs of cats; infection with P kellicotti occurs when cats ingest crayfish intermediate hosts from fresh water rivers and streams.

P fastosum is a fluke of the bile ducts of cats in Florida, the coastal southeastern U.S., and Hawaii; infections are also common on several Caribbean islands. The condition is commonly called lizard poisoning because cats usually acquire infection following predation on lizards and other reptile paratenic hosts.

Clinical Signs. P kellicotti infections may be asymptomatic or can lead to respiratory disease characterized by dyspnea, pneumothorax, and hemoptysis.

Cats infected with P fastosum develop hepatic insufficiency and ultimately liver failure, with enlarged bile ducts and biliary epithelial hyperplasia evident; infection also has been associated with metastatic feline cholangiocarcinoma.12

DiagnosisP kellicotti infections often result in large pulmonary cysts that are visible on radiographs, and the characteristic large, operculate eggs may be identified on transtracheal wash or fecal sedimentation (Figure 5).

P fastosum diagnosis can be supported by ultrasonography, which reveals dilated bile ducts and, occasionally, trematodes. Eggs may be detected on fecal sedimentation (Figure 6), but when biliary hyperplasia is severe, liver biopsy may be necessary to achieve a diagnosis because cholestasis prevents egress of eggs.

Figure 5. Paragonimus

Figure 5. Paragonimus kellicottiegg, with distinct operculum, on fecal sedimentation.

Figure 6. Platynosomum

Figure 6. Egg of Platynosomum fastosum recovered on fecal sedimentation from infected cat.

Treatment. For P kellicotti infections, repeated treatment is often necessary, but both fenbendazole and praziquantel have been reported to be effective.13,14

For P fastosum infections, praziquantel is the recommended treatment, although, in severe cases with liver failure, the prognosis is guarded.15

PUBLIC HEALTH CONSIDERATIONS

Many feline helminths are zoonotic, underscoring the importance of their control.

T cati is associated with visceral and ocular larva migrans in humans who ingest larvated eggs from contaminated soil. T cati adults have also been recovered from humans, usually children, but these infections are generally attributed to ingestion of adult worms from feline vomit.16

The zoonotic potential of A tubaeforme is considered low, but cutaneous larva migrans is reported and is commonly seen in humans exposed to A braziliense contaminated soil.3

The literature also contains many reports of D caninum infections in children following ingestion of infected fleas.17

Other feline helminths may occasionally infect humans, but cats are not considered the primary source. For example, human infection with P kellicotti has been reported following ingestion of raw crayfish,18 and Mesocestoides species, Alaria species, Spirometra species, and N salmincola infections have all been described following ingestion of, or contact with, tissues from infected intermediate or paratenic hosts.3,7,19,20

SUMMARY

Although not always appreciated by cat owners or veterinarians, infection with helminths is common in many cats. Cats with outdoor access or those that hunt are at increased risk, but indoor cats are not free of parasites.

Helminths can cause disease in both cats and, potentially, their human families. Reducing parasitism in cats serves both feline and public health. CAPC offers specific strategies to protect cats from parasites year-round through the recommendations published at capcvet.org.

Table. Common Helminths of Domestic Cats in North America
HELMINTH SPECIES INTERMEDIATE HOSTS PARATENIC HOSTS RESERVOIR HOSTS ZOONOTIC
Gastrointestinal
Nematodes Ancylostoma tubaeforme
None None Cat Yes
Toxocara cati
None Rodents, birds Cat Yes
Toxascaris leonina
None None Cat, dog No
Cestodes Dipylidium caninum
Flea, louse None Cat, dog Yes
Mesocestoides species
Unknown; then many vertebrates Unknown Wild carnivores Yes
Spirometra species
Copepod; then many nonfish vertebrates Most vertebrates Cat, dog, wild carnivores Yes
Taenia taeniaeformis
Rodent None Cat No
Trematodes Alaria species
Snail, then tadpole Frogs, reptiles, small mammals, birds Opossums, wild carnivores Yes
Nanophyetus salmincola Snail, then salmonid fish None Many mammals and birds Yes
Extraintestinal
Nematodes Aelurostrongylus abstrusus
Snail, slug Many vertebrates Cat No
Dirofilaria immitis
Mosquito None
Unknown
Many vertebrates
Many vertebrates
Dog Yes
Cestodes Mesocestoides species
Unknown; then many vertebrates Unknown Wild carnivores Yes
Trematodes Paragonimus kellicotti
Snail; then crayfish Many vertebrates Wild carnivores Yes
Platynosomum fastosum Snail; then lizards Many vertebrates Cats No

References

  1. Adolph CA, Downie K, Snider T, et al. High prevalence of covert infection with gastrointestinal helminths in cats. JAAHA 2014; in press.
  2. Little SE, Raymond MR, Thomas JE, et al. Heat treatment prior to testing allows detection of antigen of Dirofilaria immitis in feline serum. Parasit Vectors 2014; 7:1.
  3. Companion Animal Parasite Council. Recommendations for parasite control. capcvet.org, 2014.
  4. Lillis WG. Helminth survey of dogs and cats in New Jersey. J Parasitol 1967; 53:1082-1084.
  5. Amin OM. Helminth and arthropod parasites of some domestic animals in Wisconsin. Wisc Acad Sci Arts Lett 1980; 68:106-110.
  6. Wilcox RS, Bowman DD, Barr SC, et al. Intestinal obstruction caused by Taenia taeniaeformis infection in a cat. JAAHA 2009; 45:93-96.
  7. Little SE, Ambrose DL. Spirometra infection in cats and dogs. Comp Cont Educ Pract Vet 2000; 22:299-305.
  8. Conboy G. Cestodes of dogs and cats in North America. VCNA Small Anim Pract 2012; 39:1075-1090.
  9. Lee AC, Atkins CE. Understanding feline heartworm infection: Disease, diagnosis, and treatment. Top Companion Anim Med 2010; 25:224-230.
  10. Traversa D, Di Cesare A, Milillo P, et al. Efficacy and safety of imidacloprid 10%/moxidectin 1% spot-on formulation in the treatment of feline aelurostrongylosis. Parasitol Res 2009; 105(1):S55-S62.
  11. Traversa D, Milillo P, Di Cesare A, et al. Efficacy and safety of emodepside 2.1%/praziquantel 8.6% spot-on formulation in the treatment of feline aelurostrongylosis. Parasitol Res 2009; 105(1):S83-S89.
  12. Andrade RL, Dantas AF, Pimentel LA, et al. Platynosomum fastosum-induced cholangiocarcinomas in cats. Vet Parasitol 2012; 190:277-280.
  13. Kirkpatrick CE, Shelly EA. Paragonimiasis in a dog: Treatment with praziquantel. JAVMA 1985; 187:75-76.
  14. Peregrine AS, Nykamp SG, Carey H, et al. Paragonimosis in a cat and the temporal progression of pulmonary radiographic lesions following treatment. JAAHA 2014; [epub ahead of print].
  15. Basu AK, Charles RA. A review of the cat liver fluke Platynosomum fastosum Kossack, 1910 (Trematoda: Dicrocoeliidae). Vet Parasitol 2014; 200:1-7.
  16. Lee AC, Schantz PM, Kazacos KR, et al. Epidemiologic and zoonotic aspects of ascarid infections in dogs and cats. Trends Parasitol 2010; 26:155-161.
  17. Molina CP, Ogburn J, Adegboyega P. Infection by Dipylidium caninum in an infant. Arch Pathol Lab Med 2003; 127:e157-e159.
  18. Lane MA, Marcos LA, Onen NF, et al. Paragonimus kellicotti flukes in Missouri, USA. Emerg Infect Dis2012; 18:1263-1267.
  19. Schultz LJ, Roberto RR, Rutherford GW, et al. Mesocestoides infection in a California child. Pediatr Infect Dis J 1992; 11:332-334.
  20. Shoop WL, Corkum KC. Epidemiology of Alaria marcianae mesocercariae in Louisiana. J Parasitol1981; 67:928-931.

C03_bSusan E. Little, DVM, PhD, Diplomate ACVM (Parasitology), is the codirector of the National Center for Veterinary Parasitology at Oklahoma State University’s Center for Veterinary Health Sciences. She received her DVM from Virginia—Maryland Regional College of Veterinary Medicine at Virginia Tech.

C03_cRick Marrinson, DVM, is the owner of Longwood Veterinary Clinic in central Florida and a board member of the Companion Animal Parasite Council. He received his DVM from Oklahoma State University.

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