Types of Associations
Nomenclature of Parasitic Organisms
Helminth Life Cycles
Pathology of Helminth Infections
Human Helminth Infections
Reading List
General Parasitolgy - Pathology of Helminth Infections
In terms of human pathology both adult and larval helminths may cause pathology and disease. An important difference between infection with parasitic helminths, and infection with bacterial, viral or protozoan parasites is that, in most cases, the parasites do not increase in numbers within their hosts, (exceptions to this general rule may however be found with larval helminths, or some nematodes such as Strongyloides sp.). That is, each larval helminth that infects the definitive host will give rise to only one adult parasite. Therefore, as pathology due to helminth infection is usually density dependent, (i.e. only with high worm burdens is severe pathology present) this parasite density, and therefore degree of pathology, is governed by the rate at which larval parasites enter the definitive host. This aspect of these diseases has important implications for the control of helminth parasites in that the diseases that they cause may be reduced or even eliminated by control measures that do not completely eliminate the parasite. This is the basis for control of many trichostrongylid nematodes of veterinary importance in Europe, where it is impractical to completely eradicate the parasite, but the disease caused by the parasite may be eliminated by controlled use of drugs at strategic times of the year. This is not the case with parasites that can divide asexually in their hosts such as bacterial, viral or protozoan parasites, where, for example, a single malaria parasite is capable, (at least in theory), of causing a fatal illness.
For adult helminths the schistosomes are probably the most important infectious disease, causing schistosomiasis (or bilharzia). Other important adult helminth diseases include anaemia due to hookworm infections and elephantiasis due to lymphatic filarial infections. In terms of veterinary importance the strongylid nematodes are of greatest economic importance. As has been said above, larval helminth infections in their intermediate hosts may also be important disease organisms, for example hydatid disease in man and domesticated animals (caused by Echinococcus granulosus infection), or hyper infections of Trichinella spiralis L3 larvae in their host's muscle tissues. With viviparous helminths larvae may also cause problems in the definitive host. The most important example of this is with river blindness, due to Onchocerca volvulus infection, with microfilaria (a pre larval L1 stage) migrating through cutaneous tissues (causing skin pathology) and the eye (eventually causing blindness). Pathology with infection of adult helminths may be due to a number of reasons, including:
Immune responses to adult, larval or eggs stages of the lifecycle (for example with the schistosomes).
High densities of adult parasites feeding on host tissues, causing tissue damage (for example many of the digenean flukes), or obstruction of the gut, (as may be the case with Ascaris infections) or lymphatic drainage (as seen with lymphatic filariasis, although in this case this is a gross simplification of what happens).
Depletion of nutrients or other metabolites required by the host (for example vitamin B12 depletion, leading to pernicious anaemia with infection by Diphylobobrium latum)
Parasites feeding on blood, causing anaemia (for example infection with hookworms)
Other reasons
It is generally thought the case that, over time, both partners in the parasitic association evolve together so that the pathological aspects of the association are reduced. Hence infections by facultative parasites which do not require a parasitic lifecycle, such as amoeba of the genus Naegleria, or nematodes such as Micronema delatrix are often associated with rapidly fatal disease.
Other parasitic infections however, where the association is much closer, are generally less pathogenic, extreme pathology generally only being associated with high parasite loads. For example infection with adult cestodes is not associated with much pathology, if at all. That both parasite and host undergo this evolution may be seen with the association between the protozoan parasite Trypanosoma brucei brucei, which in its natural hosts (wild animals such as Zebra and Antelope) do not cause disease. Domesticated animals such as cattle or horses (which have not undergone this co-evolution) when introduced into trypanosome endemic areas are rapidly killed by the same parasites however. The reason for this co-evolution is probably because it is not in the interest of the parasite to kill its host, and the parasite may have evolved ways of reducing any pathogenic effects it may have. This growing association may even go to the extent of the parasites limiting their own densities of infection, (for example the concomitant immunity reported with schistosome infections).
There are however important exceptions to this, particularly for example with infection of intermediate hosts with larval helminth parasites. Here it is often the case that the intermediate host must be eaten by its definitive host to complete its lifecycle. If this is the case, the larval parasites have often evolved to facilitate this, either actively or passively. Examples of parasites actively aiding the predation of their intermediate hosts have been reported from a number of species of helminths that modify the behaviour of these intermediate hosts. For example metacercarial infections of ants by Dicrocoelium dendriticum causes the ants to change their behaviour, by running up, and attaching themselves, by their jaws, to the tops of blades of grass, where they can be accidentally ingested as their herbivorous definitive hosts graze. Other behavioural modifications include the intermediate host not hiding itself from predators, as has been reported in fish infected with larval pseudophyllidean cestodes (e.g. Schistocephalus solidus), or arthropod intermediate hosts infected with larval acanthocephalans. Other more passive means include infection with many larval cyclophyllidean cestodes, where the larval cestode may, as it develops to maturity, cause extreme pathology (for example infection with species of Echinococcus), which will eventually kill the intermediate host. The dead intermediate host is then available for scavenging by the carnivorous definitive hosts, who then become infected, completing the parasites lifecycle.
Larval helminths in accidental or paratenic hosts may cause pathology, two types of condition being important. Firstly Visceral Larval Migrans, as seen with Anisakis and Angiostrongylus infections, and importantly with the larvae of the nematode Toxocara canis, where migrating larvae may causes blindness in infected humans (usually children). Here larvae migrate deep within the paratenic hosts tissues. Secondly Cutaneous Larval Migrans, where the larvae migrate through the skin and subcutaneous tissues. Examples here include dog hookworms of the genus Ancylostoma. Also some cercaria of non-human infecting schistosomes (where the condition is known as cercarial dermatitis) have short lived migrations through the skin before they die. Some larval helminths may cause both conditions in paratenic hosts, such as is the case with plerocercoids of Pseudophyllidean Cestodes such as Spirometra,, where the condition is known as sparganosis.