Current Advice on Parasite Control:
Intestinal Parasites - Physaloptera spp. (stomach worm)
- Physaloptera spp. infection should be considered in dogs and cats with chronic vomiting, especially those with a history of consuming insects or vertebrate prey.
- When Physaloptera spp. infection is suspected, eggs can be identified on a direct smear or adults removed at endoscopy; eggs of Physaloptera spp. are often not recovered on fecal flotation.
- Physaloptera spp. infection can be treated by removing adults at endoscopy and with pyrantel (20 mg/kg, orally, every 2 weeks for at least three treatments); courses of anthelmintic treatment should be repeated as necessary until clinical signs resolve.
Canine and feline
- Physaloptera rara
- Physaloptera preputialis
Overview and life cycle
- A variety of wild carnivorous mammals, including coyotes and foxes, are considered to be the normal definitive hosts of the Physaloptera spp. that also infect dogs and cats.
- Dogs and cats are infected with Physaloptera spp. upon ingestion of infected insect intermediate hosts (e.g. crickets, cockroaches, beetles) or ingestion of paratenic hosts that have consumed infected insects.
- Adult nematodes develop in the stomach.
- Larvated eggs are passed in the feces and develop into infective, third-stage larvae in insect intermediate hosts.
- Larvated Physaloptera spp. eggs infective to insect intermediate hosts are passed in the feces of infected definitive hosts. Eggs measure 42-53 by 29-35 microns, have a thicker shell than a hookworm egg, and contain a fully developed first-stage larvae when passed.
- After ingestion from feces or a fecal-contaminated environment, third-stage larvae measuring 2-3 mm develop within insect intermediate hosts, including crickets, cockroaches, and beetles.
- Paratenic hosts may ingest insect intermediate hosts and the nematodes remain as third-stage larvae in the paratenic host tissues. Common paratenic hosts of Physaloptera spp. include amphibians, reptiles, and mammals.
- Adult Physaloptera spp. in the stomachs of dogs and cats are stout, pink to tan in color, 3-6 cm in length, can be recognized by the presence of a cervical collar, and, on male Physaloptera spp., the presence of distinct caudal alae.
- Adult Physaloptera spp. may be found in the vomitus of a dog or cat or identified at endoscopy. Most infected dogs or cats harbor very few (1-3) adult worms although occasionally much higher worm burdens are identified.
- The presence of adult Physaloptera spp. in the stomach of dogs and cats can lead to gastritis that most commonly presents as chronic vomiting.
- Vomiting may persist long-term even when only a few (1-3) adult worms are present.
- In severe, chronic infections or when large numbers of nematodes are present, the gastric mucosa may be thickened, rugose, and edematous and infected animals may become dehydrated or malnourished.
- Some infected animals are clinically unaffected.
- Prevalence of Physaloptera spp. varies widely. Infections appear to be most common in the midwestern United States in dogs and cats with outdoor access and a history of prey consumption.
- Because eggs do not float well in most flotation solutions even with centrifugation, fecal surveys can dramatically underestimate the prevalence of Physaloptera spp. (see Diagnosis).
- Occasionally, heavy infections are reported in individual dogs that ingest large numbers of crickets or other insect intermediate hosts.
Host associations and transmission between hosts
- Dogs and cats become infected with Physaloptera spp. upon ingestion of infected insect intermediate hosts or ingestion of larvae in tissues of paratenic hosts.
- A wide variety of vertebrates can serve as paratenic hosts, including amphibians, reptiles, and mammals.
- Wild carnivores like foxes and coyotes serve as definitive hosts for the Physaloptera spp. that commonly infect dogs and cats in the United States. While these animals serve as a reservoir maintaining a source of infection in nature, they are not directly responsible for infections in dogs and cats.
Prepatent period and environmental factors
- The prepatent period of Physaloptera spp. ranges from 2 – 5 months, depending on how larvae are acquired.
- For transmission to occur, appropriate insect intermediate hosts must be present in the environment or dogs and cats must have access to paratenic host prey species.
Site of infection and pathogenesis
- Adult Physaloptera spp. are found in the stomach and anterior duodenum of dogs and cats.
- The presence of Physaloptera spp. in the stomach irritates the gastric mucosa causing vomiting. Adults may be present in the vomitus although the absence of nematodes in the vomit does not exclude the possibility of an infection.
- Fecal flotation is not considered an effective way to detect infections with Physaloptera spp. Eggs are usually few in number and do not float well in standard specific gravity solutions even when centrifugation is used.
- Direct smear may be more useful for detecting Physaloptera spp. eggs than fecal float although eggs are not consistently found in all infected pets.
- Many Physaloptera spp. infections in dogs and cats are detected by identification of adult nematodes in the vomit or by endoscopic recovery and identification of adult nematodes from the stomach.
- Adults are stout, pink to tan in color, 3-6 cm in length, can be recognized by the presence of a cervical collar, and, on male Physaloptera spp., the presence of distinct caudal alae.
- Effective treatment of Physaloptera spp, can be difficult and often requires repeated courses of anthelmintics to be effective.
- Removal of nematodes by endoscopy is usually curative so long as all Physaloptera spp. are visualized and removed. Because an immature or adult nematode may be overlooked, endoscopic removal should be followed by anthelmintic treatment.
- Pyrantel pamoate is considered the treatment of choice for Physaloptera spp. CAPC recommends 20 mg/kg of pyrantel be given orally, every 2 weeks, for at least three treatments.
- If clinical signs persist, repeated courses of treatment may be necessary.
- Macrocyclic lactones apparently have variable efficacy against Physaloptera spp. Nonetheless, monthly parasite control products with efficacy against intestinal parasites would be expected to limit infections with Physaloptera spp. in pets, reducing the likelihood of clinical signs.
- In patients with severe gastritis, corticosteroids may also be indicated to allow resolution of clinical signs.
Control and prevention
- No products are approved to control Physaloptera spp. infection in dogs or cats. However, monthly parasite control products with efficacy against intestinal parasites would likely be effective at limiting infections and subsequent clinical disease.
- Care should also be taken to limit opportunities for ingestion of insect intermediate hosts, including controlling insect infestations in kennels, catteries, and homes.
- 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 cats and dogs to acquire infection with Physaloptera spp. via ingestion of paratenic hosts.
Public health considerations
- Physaloptera spp. are not known to be zoonotic. Infected dogs and cats do not pose any public health risk.
- Santen DR, Chastain CB, Schmidt DA. 1993. Efficacy of pyrantel pamoate against Physaloptera in a cat. JAAHA 29:53-55.
- Campbell KL, Graham JC. 1999. Physaloptera infection in dogs and cats. Compendium on Continuing Education for the Practicing Veterinarian. 21:299.
- Kazacos KR. 2010. How do you detect Physaloptera species eggs? Veterinary Medicine, October 2010.
- Theisen SK, LeGrange SN, Johnson SE, et al. 1998. Physaloptera infection in 18 dogs with intermittent vomiting. JAAHA 34:74-78.