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Resistance to parasitic diseases

Broadly speaking, resistance to parasitic infections falls into two categories. The first of these, often termed innate resistance, includes species resistance, age resistance and in some cases breed resistance, which, by and large, are not immunological in origin. The second category, acquired immunity, is dependent on antigenic stimulation and subsequent humoral and cellular responses. Although, for reasons explained below, there are few vaccines available against parasitic diseases, natural expression of acquired immunity plays a highly significant role in protecting animals against infections and in modulating the epidemiology of many parasitic diseases.

SPECIES RESISTANCE

For a variety of parasitological, physiological and biochemical reasons, many parasites do not develop at all in other than their natural hosts; this is typified by, for example, the remarkable host specificity of the various species of Eimeria. In many instances however, a limited degree of development occurs, although this is not usually associated with clinical signs; for example, some larvae of the cattle parasite Ostertagia ostertagi undergo development in sheep, but very few reach the adult stage. However, in these unnatural or aberrant hosts, and especially with parasites which undergo tissue migration, there are occasionally serious consequences particularly if the migratory route becomes erratic. An example of this is visceral larva migrans in children due to Toxocara canis, which is associated with hepatomegaly and occasionally ocular and cerebral involvement.

Some parasites, of course, have a very wide host range, Trichinella spiralis, Fasciola hepatica, Cryptosporidium parvum and the asexual stages of Toxoplasma being four examples.

AGE RESISTANCE

Many animals become more resistant to primary infections with some parasites as they reach maturity. For example, ascarid infections of animals are most likely to develop to patency if the hosts are a few months old. If hosts are infected at an older age, the parasites either fail to develop, or are arrested as larval stages in the tissues; likewise, patent Strongyloides infections of ruminants and horses are most commonly seen in very young animals. Sheep of more than 3 months of age are relatively resistant to Nematodirus battus, and in a similar fashion dogs gradually develop resistance to infection with Ancylostoma over their first year of life.

The reasons underlying age resistance are unknown, although it has been suggested that the phenomenon is an indication that the host–parasite relationship has not yet fully evolved. Thus, while the parasite can develop in immature animals, it has not yet completely adapted to the adult.

On the other hand, where age resistance is encountered, most parasitic species seem to have developed an effective counter-mechanism. Thus, Ancylostoma caninum, Toxocara canis, Toxocara mystax, Toxocara vitulorum and Strongyloides spp all survive as larval stages in the tissues of the host, only becoming activated during late pregnancy to infect the young in utero or by the transmammary route. In the case of Nematodirus battus, the critical hatching requirements for the egg, i.e. prolonged chill followed by a temperature in excess of 10°C, ensure the parasites’ survival as a lamb-to-lamb infection from one season to the next.

Oddly enough, with Babesia and Anaplasma infection of cattle, there is generally thought to be an inverse age resistance, in that young animals are more resistant than older naive animals.

BREED RESISTANCE

In recent years, there has been considerable practical interest in the fact that some breeds of domestic ruminants are more resistant to certain parasitic infections than others.

Probably the best example of this is the phenomenon of trypanotolerance displayed by west African humpless cattle, such as the N’dama, which survive in areas of heavy trypanosome challenge. The mechanism whereby these cattle control their parasitaemias is still not fully known, although it is thought that immunological responses may play a role.

In helminth infections, it has been shown that the Red Masai sheep, indigenous to east Africa, is more resistant to Haemonchus contortus infection than some imported breeds studied in that area, whilst in South Africa it has been reported that the Merino is less susceptible to trichostrongylosis than certain other breeds.

Within breeds, haemoglobin genotypes have been shown to reflect differences in susceptibility to Haemonchus contortus infection in that Merino, Scottish Blackface and Finn Dorset sheep which are homozygous for haemoglobin A, develop smaller worm burdens after infection than their haemoglobin B homozygous or heterozygous counterparts. Unfortunately, these genotypic differences in susceptibility often break down under heavy challenge.

Studies within a single breed have shown in Australia that individual Merino lambs may be divided into responders and non-responders on the basis of their immunological response to infection with Trichostrongylus colubriformis and that these differences are genetically transferred to the next generation.

The selection of resistant animals could be of great importance, especially in many developing areas of the world, but in practice would be most easily based on some easily recognisable feature such as ‘coat colour’ rather than be dependent on laboratory tests.

In Australia resistance to ticks, particularly Boophilus, has been shown to be influenced by genetics, being high in the humped, Bos indicus, Zebu breeds and low in the European, Bos taurus, breeds. However, where cattle are 50% Zebu, or greater, in genetic constitution, a high degree of resistance is still possible allowing a limited use of acaricides.

ACQUIRED IMMUNITY TO HELMINTH INFECTIONS

Immune responses to helminths are complex, possibly depending on antigenic stimulation by secretory or excretory products released during the development of the L₃ to the adult. For this reason it has only been possible to develop one or two practical methods of artificial immunisation of which the radiation-attenuated vaccine against Dictyocaulus viviparus is perhaps the best example.

Despite this, there is no doubt that the success of many systems of grazing management depend on the gradual development by cattle and sheep of a naturally acquired degree of immunity to gastrointestinal nematodes. For example, experimental observations have shown that an immune adult sheep may ingest around 50 000 Teladorsagia (Ostertagia) L₃ daily without showing any clinical signs of parasitic gastritis.

THE EFFECT OF THE IMMUNE RESPONSE

Dealing first with gastrointestinal and pulmonary nematodes, the effects of the immune response may be grouped under three headings, the sequence reflecting the usual progression of acquired immunity:

Each of these mechanisms is exemplified in infections of the rat with the trichostrongyloid nematode Nippostrongylus brasiliensis

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