Ancylostomatoidea - Hookworms
Male and female Ancylostoma caninum recovered from a dog. Adults males are usually 10-12 mm in length, and the females average 14-16 mm in length. Prepatent period varies by route of transmission and can be as short as 2 weeks when larvae are ingested or as long as 4 weeks when larvae are acquired by dermal penetration.
In situ photo of Ancylostoma caninum found on necropsy of a dog. Infected dogs may present with anemia from blood loss due to worm feeding, skin lesions associated with penetration of third-stage larvae, diarrhea associated with fourth-stage larvae damaging the small intestine, and/or hemorrhagic pneumonitis associated with larval migration.
The buccal cavity of the hookworm of cats. Although very similar to the stoma of adult Ancylostoma caninum, the teeth on the ventral margin of Ancylostoma tubaeforme are slightly larger. Infection with this species is the result of skin penetration or ingestion of third-stage larvae, or ingestion of paratenic hosts with larvae encysted in tissues.
A parasite of cattle and zebu, Bunostomum phlebotomum may be found in the small intestine feeding on blood. Penetration of larvae may cause cattle to stamp their feet and lick their legs, and heavy infections with adults may lead to diarrhea, anemia, and hypoproteinemia. In calves this manifests as marked weakness, emaciation, and submandibular edema.
Egg of Uncinaria stenocephala. This hookworm infects dogs (rarely cats) in cooler temperate regions, including the northern US, Canada, and Europe. Eggs resemble those of Ancylostoma spp. in that they are elliptical, thin-shelled, and contain morulae in fresh feces, but Uncinaria eggs are slightly larger (71-92 X 35-58 µm).
Goat lungs showing classic Muellerius capillaris lesions. Adult worms of this genera live in the alveoli and pulmonary parenchyma of sheep and goats. There, they form nodules that turn a greyish color, a result of the degeneration of accumulated leucocytes and necrotic pulmonary tissue. Despite this, hosts generally show no apparent clinical signs.
Neurologic signs of a goat infected with Parelaphostrongylus tenuis. When Parelaphostrongylus tenuis infects a host other than the white-tailed deer, the larvae migrate aberrantly, usually causing damage to the dorsal horns of the grey matter of the spinal cord. Small ruminants are particularly susceptible to neurologic disease due to aberrant migration of Parelaphostrongylus tenuis.
Adult Stephanurus dentatus excised from cysts in the perirenal fat surrounding the ureter of a pig. Adults are usually found in cysts that communicate with the ureter through which they pass their eggs. The eggs leave the host via urine. Host infections may be acquired through ingestion of third-stage larvae, ingestion of a paratenic host, or from dermal penetration by the third-stage larvae. The larvae then migrate extensively through the organs.
Strongyloidea - Large and Small Strongyles
Large buccal cavity of an adult Chabertia ovina. This parasite is found in the colon of sheep, goats, and cattle throughout the world and feeds on the granular layer that it digests with esophageal secretions. Blood feeding is incidental and may be a cause for frank blood in the feces of the host.
Nodules formed by Oesophagostomum columbianum in the colon of a sheep. Oesophagostomum columbianum is a parasite of the colon of sheep, goats, camels, and wild antelope. Their larvae encyst anywhere from the pylorus to the colon to complete their development and become adults that attach in the colon. Frequently, the larval nodules become abcesses that are filled with caseous material.
With the exception of Nematodirus spp., eggs of Trichostrongyloidea species have this typical morphology. They are elliptical, have a thin shell, and are approximately 65-100 X 35-50 µm, depending on the species. In fresh feces eggs contain morulae which develop to first stage larvae within eggs in the environment.
Larva of Ollulanus tricuspis. Third-stage larvae are approximately 500 µm and have a tricuspid tail similar to that of the adult female (2nd and 4th stage larvae also have this type tail). Adults and larvae are found in the stomach of domestic cats and other felids. Diagnosis is based on the identification of larvae or small adults (1 mm) in vomitus using the Baermann test.
Ascaridoidea - Ascarid Roundworms
Egg of Baylisascaris procyonis, commonly known as the raccoon roundworm. Dogs and raccoons acquire the infection through ingestion of a paratenic host. Ingestion of larvated eggs may cause severe central nervous system or ocular disease in dogs, birds, rabbits, rodents, marsupials, and humans
Adult female (top) and male (bottom) Parascaris equorum, a nematode of the small intestine of equids. The males are 15-28 cm in length and the females may reach 50 cm in length. This parasite is most pathogenic to foals 3 to 9 months of age, affecting the respiratory and digestive systems. Adults of the related ascarid Parascaris univalens also infecting equids are considered morphologically identical to P. equorum, but are karotypically distinct.
Adults of Toxocara canis from the small intestine of an infected dog. Transmission of Toxocara canis to canids may occur prenatally, through direct transmission, through a paratenic host, or through nursing. Humans may serve as aberrant or parentetic hosts, resulting in cutaneous larva migrans (CLM) and occular larva migrans (OLM).
A puppy with a heavy load of Toxocara canis parasitizing its small intestine. Note the distended belly and constipation. Other common clinical signs include emaciation, dull hair coat, vomiting, and harsh lung sounds associated with secondary pneumonia. Death frequently occurs two to three weeks after birth when pups are prenatally infected.
Adult male Cruzia americana. This parasite inhabits the cecum and proximal colon of opossums in the United States. In low numbers they are non-pathogenic, but heavy worm burdens may lead to malabsorption and diarrhea. Males measure 11-12 mm in length and females measure 16-18 mm in length.
Fixed and stained microfilaria of Acanthocheilonema reconditum from the blood of an infected dog. Microfilariae of Acanthochilonema reconditum differ from those of Dirofilaria immitis based on size (250-288 µm in length by 4.5-5.5 µm in width), head shape (blunt) and tail shape (curved, or hooked).
Adult Elaeophora schneideri in the common carotid artery of a white-tailed deer. Adults are usually found in breeding pairs in the arteries. The female releases microfilariae which are usually found in capillaries on the forehead and face. An intermediate arthropod host of either the genus Hybomitra or Tabanus will acquire infection from those locations through blood feeding.
Sorehead lesion in a sheep caused by the microfilariae of Elaeophora schneideri. In white-tailed deer, little or no clinical evidence of infection is apparent, but in other hosts, the infections are more pathogenic. In sheep, dermatitis with granulomatous inflammation and pruritus, termed 'sorehead' occurs. In elk, the filarial migration is more aberrant leading to blindness and ischemic necrosis of the brain, muzzle, ears, etc.
Microfilariae of Onchocerca lupi recovered via skin snip from a dog that presented with a subconjunctival granuloma in the eye. Microfilariae of Onchocerca lupiare found in the subcutaneous tissues and are ~100µm in length. O. lupi has been associated with ocular lesions in dogs in the southwestern United States and has also been reported in cats.
Photo by: Guilherme Verocai, Texas A&M University
Lesion caused by Stephanofilaria stilesi on the navel of a brahman cow. Both adults and microfilariae of Stephanofilaria stilesi may be found in the dermis of these dry pruritic lesions. Haematobia irritans (seen on the lesion here) and Stomoxys calcitrans have both been indicated as being competent intermediate hosts for this filarid.
Oxyuroidea - Pinworms
Egg of Syphacia sp. found in a fecal flotation of a dog. Syphacia is a genus of pinworms found in rodents. They may be found in the feces of dogs as pseudoparasites if the dog ingested an infected definitive host or the feces of an infected host. Eggs measure 100-142 μm x 30-40 μm with a smooth, clear shell wall.
Trichinelloidea - Whipworms, Capillarids, and Trichinella spp.
Eucoleus (Capillaria) aerophilus adult found in the trachea of a dog. This parasite can be found in the trachea, bronchi, and bronchioles of dogs, cats and foxes. Infection in dogs and cats are often sub-clinical, but in farmed foxes, infection is an important cause of bronchopneumonia.
Egg of Dispharynx spiralis recovered form the excrement of a pigeon. Dishparynx spiralis is a parasite of birds, occurring in the wall of the proventriculus and esophagus. Eggs are 33-40 μm x 18-25 μm. Infection of the definitive host occurs after ingestion of an infected isopod that serves as an intermediate host.
Adult Dyspharynx spiralis in a peacock proventriculus. Pathogenicity is limited to inflammation and thickening of the proventricular lining in light infections, but in severe infections, deep ulcers and extensive destruction of glands lead to weight loss and anemia in young birds.
Removed from the uterus of female worms, the long, thin tails of first stage Dracunculus insignis larvae are distinctive and measure about 600 μm in length. Intermediate stages are found in copepods (Cyclops) and migrating through the abdomen and thorax of a definitive host (numerous wild carnivore species including raccoons, fisher, mink, muskrat, opossums, skunk, fox, coyote, otter, and occasionally domestic dogs and cats).
Female Dracunculus insignis found in the subcutaneous tissues of a skunk. Dracunculus insignis is a parasite of carnivores in North America. Adult females migrate to the subcutaneous tissues where they form an ulcerative opening in the skin through which they discharge larvae into water.
Egg of Gongylonema spp. Eggs of this genus are passed in the feces of the infected host. In order to complete its life cycle, it is then swallowed by coprophagous beetles or cockroaches where it develops into the infective stage in about 30 days. It may establish infection when the definitive host ingests an infected insect.
Habronema sp. / Draschia megastoma
Larvated eggs of Habronema or Drachia megastoma found in horse feces (the species are morphologically indistinguishable). After larvated eggs are passed in the feces, they are ingested by fly larvae and develop into infective larvae. Flies then deposit the third-stage larvae around the lips of horses. The larvae migrate to the horse's mouth and then to the stomach.
Granulation tissue caused by larvae of Habronema in a non-healing wound on the face of a horse. When flies deposit third-stage larvae of Habronema on open wounds, the larvae prevent the wound from healing. Called, 'summer sores,' these lesions persist during the time that flies are active.
Adult Oxyspirura sp. seen under the nictitating membrane of a Palawan peacock. The adults of this genus measure 10-19 mm long. They deposit their eggs into the nasolacrimal duct which are passed into the digestive tract. As infective larvae are liberated from ingested infected cockroaches, they travel back up the bird digestive tract and nasolacrimal duct to reach the eye.
Eggs of Spirocerca lupi. Though very similar to those of Physaloptera, Spirocerca lupi eggs (30-38 μm x 11-15 μm) are smaller and more elongated than Physaloptera (42-53μm x 29-35μm). These eggs may be found on sedimentation and are found inconsistently on routine fecal flotation exams.
Adult Spirocerca lupi. Adult Spirocerca lupi are found in nodules in the esophagus, stomach, and aorta of canines and wild felines such as bobcats. The eggs are passed in the feces, where they are ingested by a coprophagous beetle intermediate host in which they will become infective. After the beetle is ingested by a definitive host, the larvae extensively migrate through the arteries and aorta until reaching the esophagus.
Third stage larva of Strongyloides stercoralis. This is the infective form which may burrow through the skin of the host and travel in the blood to the lungs. From the lungs, it migrates to the trachea where it is coughed up and swallowed, passing through the stomach and establishing its infection in the small intestines of carnivores.
In fresh feces, Strongyloides spp. larvae rapidly develop to the infective filariform stage, which enters the host via skin or mucosal penetration. Filariform refers to the elongate shape of the esophagus. In dogs and cats, Strongyloides eggs frequently hatch before leaving the body, thus free larvae are most often found in fresh feces.