Jae Hoon Lim, MD

Human toxocariasis, or visceral larva migrans, is a zoonotic disease caused by tissue migration of the nematode larva 

, a dog ascaris. The condition involves the liver and lungs producing eosinophilic inflammation.

, live in the small intestine of the wild dog, wolf, dingo, and fox. Eggs produced by these worms are unembryonated when passed in feces, and have a subglubose appearance and a pitted shell. They reach the infective stage in three to four weeks, given favorable moisture and temperature conditions.

Human infection often follows the ingestion of infective- stage eggs in contaminated dog hair, soil, and food. The eggs must contain fully developed larvae for infection to be transmitted. The larvae hatch in the small intestine, penetrate the intestinal wall, flow through the portal vein to reach the liver, migrate within the liver (visceral larva migrans), move to the lungs (hepatopulmonary migration), and distribute throughout the body, including the orbit and central nervous systems, through the general circulation.

 Most larvae remain in the tissues and become encapsulated. They do not grow or develop further, but remain viable and infective indefinitely.

This mode of transmission generally occurs in children who are in frequent contact with dogs or who play on ground contaminated with dog feces.

 This is why toxocariasis used to be considered a disease of children. Nowadays, however, the prevalence of toxocariasis in children is very low, owing to improved hygiene. The incidence of ascaris in pet dogs has also been reduced by the widespread use of antihelminthic drugs.

Adult infection typically follows the ingestion of uncooked animal tissues, such as liver, which contain encapsulated infective larvae (Figure 1). The larvae hatch in the small intestine and, as described above, reach the liver and lungs, where they are encapsulated and remain alive for a certain period.

 Visceral larva migrans of T. canis is one of the most important food-borne parasitic diseases in some Asian countries, including South Korea, Japan, and Taiwan, next to trematodiasis such as clonorchiasis and opisthorchiasis. Visceral larva migrans is rarely reported in Western countries.

 takes place in exactly in the same way, that is, by swallowing embryonated eggs or by ingesting animal tissue containing encapsulated larvae. The liver and lungs are the most frequent site of infection in animals, as they are for humans.

 Infective larvae become encapsulated without developing into adult worms and may remain viable for up to 3.5 years in chicken liver

 and for the life of the host (up to nine years) in small mammals.

 larvae in sexually mature female dogs may be transmitted to offspring though the placenta or through milk feeding. Larvae in the liver or lungs of infected offspring or pups migrate through the trachea to the small intestine. Here they grow into adult worms,

 larvae from the tissues of mammals to another host is a common mode of transmission among carnivorous vertebrates. People who eat raw animal tissue are also at risk of infection via this route. Uncooked liver from cows (Figure 1), pigs, lamb, and chicken, have all been reported as sources of human infection, as has rabbit meat.

 is 0.5 mm in length and 20 µm in diameter.

 It penetrates the portal vein via the intrahepatic portal radicles, and on reaching the parenchyma can cause eosinophilic inflammation, eosinophilic granuloma, or necrotic nodules (Figure 2).

Each lesion is typically oval and measures 1 cm in diameter. It is common to find multiple lesions scattered throughout the liver. They typically consist of central necrosis surrounded by eosinophilic leukocyte infiltration mixed with neutrophils, lymphocytes, and a palisade of epitheloid histiocytes and/or giant cells. Although the larva may be present in the center of a lesion, it will be difficult to identify.

Living larvae move slowly in liver tissue causing visceral larva migrans. The most constant and striking clinical feature is a high, sustained, peripheral eosinophilia.

CT and ultrasound findings reflect the pathology. CT depicts the lesions as multiple small, ovoid, ill-defined nodules with low attenuation.

 The nodules are usually seen best, or seen only, in the portal venous phase (Figure 3). They will be either faint or not seen at all on arterial and equilibrium phase images. These findings seem to be characteristic for eosinophilic infiltration caused by visceral larva migrans of 

Follow-up CT scans reflect the movement of larvae through the liver tissue, and both the shape and location of the lesions may be different (

). Eosinophilic abscesses are present in some cases, appearing as persistent, low-attenuating lesions on arterial, portal venous, and equilibrium phase images.

Ultrasound shows multiple small, focal, hypoechoic lesions in the liver parenchyma. The lesions are usually angulated or trapezoid rather than round, and their margins ill-defined. Lesions sometimes mass together to form a large lesion of mixed echogenicity.

Some larvae in liver tissue enter the hepatic vein and move to the lung parenchyma through the pulmonary circulation, resulting in pulmonary eosinophilic inflammation. Characteristic pulmonary lesions measure approximately 1 cm in diameter and are rather round, comprising a central, tiny, dot-like lesion and a surrounding halo of ground-glass opacity (Figure 5). When the infective larvae move in the lung parenchyma, the lesions move to the adjacent lung on serial chest radiographs or CT images.

Larvae that escape the pulmonary circulation may move to any tissue through the general circulation. The most frequent site of infection is the eye, resulting in endophthalmitis, and the central nervous system, which can cause encephalitis or transverse myelitis (Figure 6).

Articles

Human toxocariasis, or visceral larva migrans, is a zoonotic disease caused by tissue migration of the nematode larva Toxocara canis, a dog ascaris

Imaging proves clinical value in cases of human toxocariasis

Many previously unknown diseases are detected frequently once they have first been recognized. In other words, once seen, easily seen. When the first report of a disease entity overrides similar clinical reports, it becomes evident that the disease must have existed well before it was first described.

Pancreatic intraductal papillary mucinous tumor offers a good example of this phenomenon. First reported in 1987,1 it has subsequently been reported on many occasions in the radiological literature.2-4 Gastroenterologists now know this mucin-causing pancreatic disease well. Before it was first described, however, endoscopists must have drained considerable quantities of mucin from the duodenal papillary orifice during examinations, not realizing that they were being confronted by a novel pathology.

The same is true of intraductal papillary mucin hypersecreting mucinous tumor in the bile duct. This condition is regarded as homologous to pancreatic intraductal papillary mucinous tumor.5 Bile duct mucinous tumor was first reported in 1988,6 and several reports have since been issued concerning this pathology.5,7-11

Bile duct papillary tumors are not rare, accounting for 15% of cholangiocarcinomas.12 The tumors are characterized by the presence of numerous papillary, frondlike foldings. These consist of columnar epithelial cells surrounding slender fibrovascular stalks, which are supported by a connective tissue core from the lamina propria.

The tumors grow slowly and are not aggressive. They often spread along the mucosal surface and may invade the ductal wall and beyond in the late phase.13 Multiple tumors (papillomatosis) involving a fairly long segment along the intra- and extrahepatic bile ducts are common.14-16 Lesions may be classified as intraductal papillary adenocarcinoma, adenoma, or dysplasia, and various histologic patterns often coexist.13,15 Tumors are friable and slough easily, causing intermittent biliary obstructions. The disease may simulate biliary stones and recurrent pyogenic cholangitis.5,9,10

Papillary tumors produce variable amounts of mucin, which is generally retained in the tumor.5,9 Some tumors produce large amounts of mucin that enter the bile duct itself.6-11 Excessive mucin may impede the flow of bile juice intermittently and incompletely, leading to obstructive jaundice. Endoscopy may demonstrate a mucin plug protruding from the patulous orifice of the duodenal papilla (Figure 1).8 Clinical symptoms caused by bile duct blockage, either by mucin or the tumor itself, include recurrent upper abdominal or episodic biliary pain, fever, and jaundice.

The true incidence of bile duct papillary tumor is perhaps underestimated. Patients with this disease are likely to have been misdiagnosed as having a biliary stone or cholangitis. Developments in diagnostic imaging techniques have brought increasing numbers of patients with this disease to clinicians and radiologists, especially in Korea, Japan, and Taiwan.5,11 The incidence of bile duct papillary tumor in Western countries remains unknown.

Ultrasound, CT, and MR cholangiopancreatography demonstrate severe dilatation of the intra- and extrahepatic ducts. Bile ducts proximal and distal to the tumor are dilated because mucin obstructs the papilla of Vater. The tumor may appear on CT as a small, flat mass, either iso- or hypoattenuated compared with adjacent liver parenchyma.9-11

Large tumors can be depicted by ultrasound, CT, and cholangiography. Tumors that spread superficially along the mucosal surface and are relatively small, sessile, and flat are difficult or even impossible to depict on imaging.9,11

On ultrasound, the mucin plug is usually echo-free, like bile juice,11 though it may be weakly echogenic. It is also difficult, but not impossible, to differentiate a papillary tumor from stones. Mucin cannot be depicted by either CT or MR imaging. Its attenuation on CT and its signal intensity on MRI are impossible to distinguish from those of water.9,11 Endoscopic retrograde cholangiopancreatography and percutaneous transhepatic cholangiography show mucin plugs as multiple filling defects in the dilated bile ducts.9,11 The defects can be large or small and linear, elongated, ovoid, or amorphous (Figure 2).

Pancreatic intraductal papillary mucinous tumors are well known as distinct disease entities. The tumors are characterized by excessive mucin production secreted by a papillary tumor of the pancreatic duct.2-4 Clinical symptoms are similar to those of chronic pancreatitis, due to intermittent obstruction of the pancreatic duct from excessive mucin. Intraductal mucin-hypersecreting bile duct papillary tumors have a striking similarity to pancreatic intraductal papillary mucinous tumors in terms of histopathology, amount of mucin produced, pathophysiology, and resulting clinical behavior.5 The shared origin of the biliary tract and pancreas suggests a theoretical homology between these two conditions.17 This is supported by reports describing the same tumors involving the bile ducts and the pancreatic duct.18,19

Further similarities exist between intraductal papillary mucinous tumors of the bile ducts and pancreas. Both tumors grow intraductally in a nodular, polypoid, sessile, or castlike manner, and they do not generally invade the ductal wall. Histopathologically, they are papillary adenoma or adenocarcinoma.

Both produce a large amount of viscous mucin that impedes flow and causes ducts to appear dilated in the absence of obstruction, leading patients to present with recurrent cholangitis or pancreatitis, respectively. Prognosis is excellent in both cases if tumors are resected early enough.

The question remains how patients were managed before the first reported case of bile duct intraductal papillary mucinous tumor in 1987. One publication from 1976 describes 50 autopsied cases of mucin-secreting cholangiocarcinoma.20 On pathologic examination of the tumor, mucin stains are nearly always positive for bile duct carcinoma.5,9,12 Sometimes mucin production is abundant, and it is excreted into the bile ducts. Bile drainage is impaired, due to the viscosity of the mucin, resulting in clinical symptoms of intermittent bile duct obstruction. Without recognizing this disease entity, doctors might have thought they were dealing with recurrent pyogenic cholangitis with passed stones.

Low-tech radiological equipment and doctors' lack of awareness about the condition have probably been responsible for the length of time it has taken to recognize bile duct intraductal papillary mucinous tumor.20 With advances in imaging hardware, the disease can be detected more easily and diagnosed more accurately. Radiologists are less likely to miss characteristic signs or misdiagnose the tumors as recurrent pyogenic cholangitis.

Doctors treat the condition by performing biliary interventions, such as bile duct drainage and stone removal, because patients with recurrent pyogenic cholangitis also present with recurrent abdominal pain, fever, chills, and jaundice. Surgeons at our institution were trying to remove bile duct "filling defects" from such patients just 10 years ago. These "defects" were, in fact, either mucin or friable papillary tumors.

Increasing reports of intraductal papillary mucinous tumor of the bile duct are likely in the future. Advances in biliary tree imaging will make the detection of this minute biliary pathology-and others like it-much easier. Once recognized, the disease is easily seen.

References for this article can be found at www.diagnosticimaging.com.

Prof. Lim is a professor in the department of radiology and the Center for Imaging Science at Samsung Medical Center, Sungkyunkwan University School of Medicine, in Seoul, Korea.

Jae Hoon Lim, MD

Articles

Imaging proves clinical value in cases of human toxocariasis

Imaging helps reveal tumors of the bile duct