Veterinary and zoonotic importance, Toxocara canis (犬弓首蛔虫), Toxascaris leonina (狮弓蛔虫) and Toxocara cati (猫弓蛔虫) Veterinary and zoonotic importance, diagnosis and control

Toxocara canis (犬弓首蛔虫) Veterinary Importance Very common in puppies Heavy infections: suboptimal growth-rates or weight-loss pot-belly (swollen abdomen) diarrhoea/ vomiting worms passed Smaller worm burdens in adult dogs

Adults in the small intestine of dogs Distribution: Worldwide, and nationwide in China large white worm up to 10.0 cm long. Can be confused with T. leonina (狮弓蛔线虫). The only useful character for differentiation visible with a hand lens: the presence of a small finger-like process (小的指状突起) on the tail of the male T. canis.

Toxocara canis life-cycle

The egg is dark brown and subglobular, with a thick, pitted shell.

Larvated egg

T. canis has the most complex life cycle in the superfamily Ascaridoid, with four possible modes of infection. 1. The typical ascaridoid form (典型的蛔虫型), similar to that of Ascaris suum with a typical hepatic-tracheal migration. The egg containing the L2 is the infective stage.

After ingestion, and hatching in the small intestine, the L2 travel by the bloodstream via the liver to the lungs, where the second moult occurs, the L3 returning via the trachea to the intestine where the final two moults take place. This form of infection occurs regularly only in dogs of up to three months old.

T. canis: infection of puppies (typical ascaridoid form) Spontaneous expulsion occurs from 6 weeks old

In dogs over three months of age, the hepatic-tracheal migration occurs less frequently, and at six months it has almost ceased. 2. Prenatal infection (先天性感染): This occurs in dog more than three months old. The L2 travel to a wide range of tissues including the liver, lungs, brain, heart and skeletal muscle, and the walls of the alimentary tract.

In the pregnant bitch, larvae becoming mobilized at about three weeks prior to parturition (分娩) and migrating to the lungs of the foetus (胎儿) where they moult to L3 just before birth. In the newborn pup the cycle is completed when the larvae travel to the intestine via the trachea, and the final moults occur.

A bitch, once infected, will usually harbour sufficient larvae to infect all her subsequent litters, even if she never again encounters the infection. A few of these mobilized larvae, instead of going to the uterus, complete the normal migration in the bitch, and the resulting adult worms produce a transient but marked increase in faecal Toxocara egg output in the weeks following parturition.

T. canis: infection of the bitch (prenatal infection) Somatic larvae waiting for: pregnancy

3. Transmammary transmission: The suckling pup may also be infected by ingestion of L3 in the milk during the first three weeks of lactation. There is no migration in the pup following infection by this route.

The subsequent development is confined to the intestinal tract. 4. Infection via paratenic hosts: paratenic hosts such as rodents or birds may ingest the infective eggs, and the L2 travel to their tissues where they remain infective until being eaten by a dog. The subsequent development is confined to the intestinal tract.

their host to be eaten by another animal T. canis: infection of non-canine hosts (infection via paratenic hosts) Somatic larvae Waiting for: their host to be eaten by another animal

Basic epidemiological cycle Prenatal and transmammary transmission Remember! Egg excretion in pups starts between 2 and 3 weeks of age Egg in environment

Patent infections in adult dogs especially nursing bitches

Significant for fox, wolf, dingo etc Paratenic hosts Significant for fox, wolf, dingo etc

Human infection Also a paratenic host!

The minimum prepatent period for direct infection following ingestion of eggs or larvae in a paratentic host is 4-5 weeks. via prenatal infection: 2-3 weeks PATHOGENESIS In moderate infections, both larvae and adults cause no apparent damage to the host.

The adult worms cause a mucoid enteritis (粘性性肠炎). In heavy infections the pulmonary phase of larval migration causes pneumonia (肺炎), which is sometimes accompanied by pulmonary edema(肺水肿). The adult worms cause a mucoid enteritis (粘性性肠炎). In rare cases, perforation (穿孔) with peritonitis (腹膜炎) or in some instances blockage of the bile duct.

CLINICAL SIGNS In mild to moderate infections, there are no clinical signs during the pulmonary phase of larval migration. The adults in the intestine may cause pot-belly (大肚皮), with failure to thrive, and occasional diarrhoea. Entire worms are sometimes vomited or passed in the faeces

In heavy infections the symptoms include coughing, increased respiratory rate, and a frothy nasal discharge (泡沫状鼻排出物). Most fatalities from T. canis infection occur during the pulmonary phase, and pups which have been heavily infected transplacentally (通过胎盘) may die within a few days of birth.

EPIDEMIOLOGY 1. The prevalence of T. canis infection: Surveys of T. canis prevalence in dogs have been carried out in most countries and have shown a wide range of infection rates, from 5% to over 80%. The highest prevalences have been recorded in dogs of less than six months of age, with the fewest worms in adult animals.

2. Factors contribute to the widespread distribution and high intensity of infection with T. canis: A. The extreme fecundity of the females: one female is able to contribute about 700 eggs to each gramme of faeces per day , and egg counts of 15000 EPG (eggs per gram) are not uncommon in pups.

B. The eggs are highly resistant to climatic extremes, and can survive for years on the ground. C. Third, there is a constant reservoir of infection in the somatic tissues of the bitch, and larvae in these sites are insusceptible to most anthelmintics

Toxocara canis public health importance Visceral larva migrans (内脏幼虫移行症, VLM) Ocular larva migrans (眼幼虫移行症, OLM) Occult (隐性) toxocariasis 2.5% seropositive in United Kingdom

The key to the epidemiology of T.canis is .. .. the egg in environment

DIAGNOSIS 1. Only a tentative(=preliminary) diagnosis is possible during the pulmonary phase of heavy infections when the larvae are migrating, and the diagnosis is based on the simultaneous appearance of pneumonic signs in a litter, often within two weeks of birth.

2. Confirmation of diagnosis by the presence of eggs in the faeces: the subglobular and brown eggs with thick pitted shells are species-diagnostic. The egg production of the worms is so high that there is no need to use flotation methods, and the eggs are readily available in simple faecal smears to which a drop of water has been added.

The egg is dark brown and subglobular, with a thick, pitted shell.

Control of Toxocara in dogs Treatment objectives Short term: welfare of pups

Control of Toxocara in dogs Treatment objectives Short term: welfare of pups Longer term: stop eggs getting into environment X X X X X X Reduces accumulation of somatic larvae in bitch Reduces risk of human infection

Chemotherapeutic targets in puppies What worms are we trying to kill? Migrating larvae New adults appear soon after treatment if larvae not killed! Adult worms

Control of T. canis infection: Piperazine (哌嗪) Widely used Cheap, available from supermarkets etc. Efficacy v adults: ~85% Efficacy v larvae: NONE Therefore, frequent doses needed Treat at 2, 4, 6, 8 and 12 weeks of age

Control of T.canis continuing hazard for puppies and humans Compounds that do not kill migrating larvae do not eliminate egg-output (as new adults quickly appear) Therefore, they continuing hazard for puppies and humans

Control of T.canis: Fenbendazole (芬苯哒唑) In pups: 3 x 50 mg/kg Efficacy v adults: ~100% Efficacy v larvae: ~ 95% Treat at 3 and 6 weeks of age

Another chemotherapeutic target in the bitch Somatic larvae X killing these will protect pups and reduce the future output of eggs in the environment X

Prevention of prenatal and transmammary transmission Only licensed treatment in UK is: FENBENDAZOLE 25 mg/kg/day daily from Day 40 of pregnancy to 2 days post-partum

Don’t forget! Hygiene important! Treat nursing bitches for adult worms Note: eggs resistant to disinfectants Treat adult dogs at least 2x per year

Another ascarid worm in dogs - Toxascaris leonina

Toxascaris leonina is like T.canis except: Infects: dogs and cats also bears etc NO prenatal infection NO transmission via milk NO body migration Most important routes of transmission: embryonated eggs eating ‘paratenic’ hosts (mice etc) Zoonotic importance: probably little or none

How to differentiate T. canis and T. leonina? For adults: The only useful character for differentiation visible with a hand lens: the presence of a small finger-like process (小的指状突起) on the tail of the male T. canis; but no such process in T. leonina

For eggs Dark brown and subglobular, with a thick, pitted shell. Slightly ovoid egg, with a smooth thick shell Toxascaris Toxocara Toxocara

Toxocara malaysiensis Ascarid worms in cats: Toxocara cati Toxocara malaysiensis Toxascaris leonina

Toxocara cati is like T. canis except: Infects: cats and other felidae NO prenatal infection Most important routes of transmission: via milk by eating paratenic hosts (mice etc) Zoonotic importance: uncertain

In the cat, differentiation from T In the cat, differentiation from T. leonina is based on the shape of the cervical alae (颈翼). In T. leonina, the cervical alae taper gradually into the body, but are arrow-head shaped in T. cati.

T. cati T. lionina

Identifying ascarids Cat Dog T.canis T.leonina T.cati

Toxocara canis (left), T. cati (right) and T. malaysiensis (middle)

Diagnosis Toxascaris Toxocara Toxocara

Nematode eggs in dog faeces Whipworm Hookworm Toxocara Toxascaris

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