The beef tapeworm

The life-cycle of T. saginata is typically cyclophyllidean, alternating between the human final host and the bovine intermediate host. Humans are infected by eating undercooked beef. The resulting adult tapeworm can reach 15 m in length. Unlike other cyclophyllideans, the scolex of both the adult tapeworm and the metacestode (which is a cysticercus) has no hooks and is said to be ‘unarmed’. Cattle are infected by ingesting eggs on pasture or from utensils contaminated with human faecal material (e.g. calves drinking from a soiled milk-bucket). Transmission to grassland can be:

1. direct: with eggs reaching pasture by direct deposition of human faeces;
   
2. indirect: by application of inadequately treated sewage sludge as fertiliser;
   
3. by birds: whole T. saginata proglottids can be picked up by sea-gulls, crows etc. feeding at sewage outlets and carried onto pasture-land. If the segment is swallowed, undamaged T. saginata eggs can pass through the bird to be voided in its droppings.

The cysticerci of T. saginata can settle in any bovine muscle (see Figure 5.11) but the greatest densities (i.e. nos. per kg tissue) tend to be in the heart, masseters and triceps. They grow to a final size of about 1 × 0.5 cm and are infective after about 3–4 months. If acquired during calf-hood, they are likely to survive for the lifetime of the animal; if acquired later in life they will eventually be killed by host immune responses. In the latter case, they are gradually replaced by caseous (cheeselike) material which later calcifies. The size and condition of cysticerci in a carcase can be used to provide a rough estimate of when the animal was exposed to infection (which may be helpful in pinpointing the source of contamination). The presence of T. saginata metacestodes in cattle is known as ‘cysticercosis’. The condition is asymptomatic and therefore only diagnosed after slaughter.

The epidemiology of bovine cysticercosis differs from place to place. In the UK, for example, a very low prevalence in the human population coupled with a generally high standard of sanitation results in a low transmission rate to cattle. Consequently, bovine prevalence is also very low (< 5 per 10³) and there is little or no opportunity for herd immunity to develop. The national herd therefore remains vulnerable and any group of cattle that becomes exposed to T. saginata eggs will acquire multiple cysticerci leading to abattoir condemnations. Such sporadic occurrences are known as ‘cysticercosis storms’.

In contrast, in East Africa for example, a relatively high prevalence plus a lack of sanitation in some rural populations results in a high rate of transmission to cattle. Thus, many calves are infected and, although calf-hood infection persists, these animals are immune to reinfection. Thus, the general picture is that of a high prevalence of lightly infected cattle. Meat inspection will not detect all infected carcases and so the potential for new human infection is always present.

Extra information box 5.1

The pork tapeworm

Taenia solium is similar to T. saginata except that the intermediate host is the pig and its scolex has the two rows of hooks typical of the genus. Although the adult tapeworm itself is relatively innocuous, T. solium is nevertheless a very dangerous human parasite as its cysticerci, normally found in porcine muscle, are also capable of developing in human tissues, particularly in the brain and musculature. Human cysticercosis can originate in two ways:

1. by swallowing eggs, e.g. if T. solium eggs are transferred to the mouth on dirty fingers or on salad crops grown on soil contaminated or fertilised with human excrement;
   
2. retroperistalsis, i.e. abnormal intestinal contractions which move gut contents forwards instead of backwards. If T. solium eggs in a person harbouring a pork tapeworm are brought forward to the duodenum, they are stimulated to hatch, releasing oncospheres which invade the body.

T. solium infections are particularly prevalent in communities where pigs are allowed to scavenge freely around human habitation.

Taenia ovis

As can be seen from Table 5.2, the biology of T. ovis is similar to that of T. saginata but with the dog as final host and sheep as intermediate host. The cysticerci are not infective for humans but nevertheless their presence in meat leads to condemnation of the carcase at meat inspection in many countries. It therefore causes significant economic losses in some sheep exporting regions, for example Australasia.

Taenia hydatigena

T. hydatigena is the commonest Taenia species of dogs in Britain and many other countries. Sheep are the usual intermediate host, but the cysticerci can also establish in other livestock species.

The cysticerci grow up to 8 cm in the peritoneal cavity and are often found adhering to the omentum (see Figure 5.12). To reach its predilection site, the oncosphere hatches out of the egg in the small intestine and enters the hepatic portal system to be swept to the liver. Here it transforms into a cysticercus and grows rapidly while migrating through the liver parenchyma. After about a month, it breaks through the liver capsule leaving behind necrotic tracks which become fibrotic. The presence of a single metacestode is unlikely to compromise health but if a sheep swallows a whole proglottid, the simultaneous passage of many thousands of cysticerci will cause massive liver damage and sudden death. This condition, known as ‘cysticercosis hepatica’, is superficially similar to acute fasciolosis (see Section 5.6.2) but is a rare event usually affecting only a single animal in a flock.

Help box 5.2

Taenia pisiformis

T. pisiformis is similar to T. hydatigena except that the cysticerci are pea-sized and are found in the peritoneal cavity of rabbits and hares (see Figure 5.13).

Taenia multiceps

T. multiceps is also known as Multiceps multiceps. It is of widespread distribution, although not reported from the USA or New Zealand. It is the dog Taenia that most commonly produces disease in sheep. After the egg is swallowed, the hatched oncosphere enters the circulation and travels to the central nervous system, usually the brain. Here it migrates across the tissues to settle in the cranial cavity as a slow-growing coenurus. This has multiple scolices arranged in small clusters. It attains a final size of some 5 cm (see Figure 5.14). In so doing, it acts as a space occupying lesion inducing atrophy of adjacent brain and skull tissues. The clinical outcome depends on the site affected. It is often asymptomatic but, if near the cerebellum, can induce walking in circles, a staggering gait, an abnormal head position, blindness and other neurological disturbances. The coenurus may be found in other livestock species and humans on rare occasions.

Taenia serialis

The coenurus of T. serialis forms in intermuscular connective tissues of rabbits. Cases in pet rabbits are often presented as a soft subdermal swelling and probably originate from eggs shed by urban foxes.