CASE NOTES
Cattle parasites - what's new?
Dr Peter Rolfe, BVSc, Dip Vet Clin Studies, PhD, District Veterinarian, Yass
Posted Flock and Herd July 2023
The short answer is, not much! There is a wealth of historical data and studies that have examined the epidemiology and control of internal and external parasitism affecting cattle in Australia and overseas. Australia has historically been one of the leading research communities in the world in understanding the basic science of many parasitism affecting cattle. This research has been consolidated into various advisory publications and sources. The ongoing research effort however is largely non-existent and ad hoc, limited by funding and often supported by commercial interests. Of more concern is the lack of trained and capable applied parasitologists that are working in the ruminant space. This decline has been happening over several decades and unlikely to be solved any time soon.
This paper will review contemporary aspects of cattle parasitism that appear relevant.
Chemistry and formulations
The backbone of most parasite control inevitably has been achieved through application of chemicals to the host. The innovation in recent years has been limited and mainly focussed on manipulation of current actives and formulations (see below). The two new anthelmintic classes used in sheep (monepantel and derquantel, each in combination with abamectin) have not been marketed in cattle. The oral monepantel/abamectin combination is registered in cattle but not actively marketed and would present an opportunity for a new class for use in cattle despite available only as an oral formulation.
Injectable and oral formulations for gastrointestinal (GIT) parasites have generally been shown to be more effective, especially in the presence of resistance. The continued use of pour-on and long-acting formulations has arguably exacerbated the development of resistance given the subtherapeutic levels observed in the declining phase of the plasma profiles. The former approach also has the concern around lick off and subtherapeutic or elevated levels of active leading to lack of efficacy or elevated meat residues. It should be noted that the dose limiting parasite for the macrocyclic lactones (MLs) is Cooperia spp. While efficacy of pour-ons may theoretically appear high, the method of calculation of efficacy accepted by registration agencies (geometric means) may mask the variability in residual populations of worms after treatment and lack of high efficacy in individual animals. This also confuses diagnosis of resistance especially in the early stages and using faecal egg counts as a diagnostic tool. The use of any anthelmintic that has a broader spectrum than is required at the time of treatment should be discouraged given the non-target selection for resistance.
Resistance
The ongoing development of resistance is a challenge and with limited opportunity for new chemistry to replace those that are failing. The ongoing trend with animal pharma is to concentrate on the high value companion animal market for new discovery with most innovation coming from manipulation of current generic classes. While this has been extremely useful and has provided medium term solutions there are significant gaps which will only become more serious. Alternate methods of parasite control such as grazing management and biological control have had limited impact despite those options being well documented.
Intestinal nematodes
There is limited recent and robust published data on the extent and prevalence of resistance to intestinal helminths in cattle and that diagnosis can be difficult depending on the species of parasite. Most local contemporary information would seem to be generated from veterinarians in local communities. The development of resistance is continuing for most classes of anthelmintics and importantly to the MLs, the extent dependent on the relevant potency of individual chemistries. Benzimidazoles continue to have surprisingly good efficacy. Levamisole resistance has been reported however care is needed with the interpretation of the results - is a lack of reduction FEC due to resistance per se or an inherent lack of efficacy against specific genera (for example against Ostertagia spp.).
Diagnosis of resistance in GIT parasitism is problematic and a common issue with monitoring burdens by faecal egg count (FEC). Surveys have used FEC to monitor resistance presence and development, and while relevant for fecund parasites, they can be confusing and unreliable in the early stages of resistance development and for some species and genera. The extent of resistance in Ostertagia spp. is unclear given the inherent difficulty in diagnosis as monitored by FEC but less so for genera such as Cooperia spp. and Haemonchus spp. Certainly, severe resistance may be detected by FEC, however, smaller shifts in the population susceptibility are very difficult to interpret. Surveys to detect resistance can be misleading depending on the design (especially randomisation, numbers per group, statistical methods, body weight measurements), detection method (nearly always FEC but there are a variety of techniques, worm reduction is the gold standard but rarely done), formulations used, how well the products are applied/given, and parasite species that are present. The detail is important and often neglected in the summarisation of the data.
The pharma industry has responded by developing combination products containing generic actives, as has occurred in the sheep industry. A number of strategies have been used including oral combinations for use in cattle (originally developed for sheep) and development of new injectable or topical combinations specifically for cattle. These trends are logical and will provide medium term solutions but reflect the ever-increasing challenge.
Ticks
Acaricide resistance in Rhipicephalus microplus is widespread globally and progressing in Australia. The situation in Brazil is severe where all major classes of actives and combinations are failing to some extent including the macrocyclic lactones. This is a challenging situation. Recent resistance data would suggest ongoing development of resistance to fluazuron (reflected in reduction in protection periods), the maintenance of high levels organophosphate and synthetic pyrethroid resistance, limited resistance with amidines and likely no resistance to macrocyclic lactones (ML). Note that the current testing regimes may have the limitations with ML testing which can confuse interpretation of test results.
Liver fluke
Resistance continues to evolve globally in both sheep and cattle and most disturbingly in western Europe where major controls failures have occurred with triclabendazole in cattle but also with the salicylanilides. The halogenated sulphonamides still appear to be effective. The methods to detect resistance are still quite unreliable unless the resistance is at an advanced stage. Certainly, faecal egg counts and faecal antigen results can be over interpreted when examining naturally acquired infections. There are some efforts at new chemistry discovery, however it would appear a low priority within the industry. Generic combinations have been developed as a response to the emergence of resistance and these are largely effective with some demonstration of synergistic effects between the active ingredients.
Grazing management
Grazing management is a largely under-utilised tool for managing infections acquired from pasture and principally for GIT parasites. Several options have been tested and reported, including single- and dual-paddock rotations. Where sheep and cattle are co-grazed, six monthly rotations of pasture were beneficial to parasite control in both species and had agronomic benefits. This information has been in the scientific and other literature for many years but has failed to gain widespread traction. So, the lack of use of such practices is a failure of extension and delivery. To suggest that they are too complicated lacks credibility. Approaches for other parasites such as Rh. microplus and Fasciola hepatica are more straightforward and involve burning (former) and avoidance or treatment of snail habitats (latter).
Breeding for resistance
There has been limited research in this field in cattle, although clearly an option in sheep where the heritability for parasite resistance is ~ 0.2 and breeding values are now available for ram selection. European studies have shown that there is likely to be phenotypic and genotypic immune development in cattle infected with Cooperia spp. This is unsurprising given the acquired immunity that is observed with age, but this study suggests there is innate resistance variability at birth. This resistance however may come at a production cost especially in the early establishment phase as the animal directs resources towards an immune response. This holds the promise that selection for resistant individuals may be possible. This, of course, is difficult and expensive research, and questionable if funding interest would be available.
Parasite vaccines
Despite long term efforts to develop vaccines for a range of parasites, there are limited success stories. Australian research has often been at the forefront of this effort.
Tick vaccine discovery is ongoing in several laboratories across the globe, including Australia, but so far without significant advances. The BM86 antigen, an original Australian discovery, has been used for many years in central and southern America with varying degrees of success. Disappointingly, it was a commercial failure in Australia. There have been attempts to improve the utility of that vaccine by improving efficacy by adjuvants and extending the duration of effect, and so far, not commercially realised.
A vaccine for Neospora in cattle (and dogs) has been researched and developed and available in some countries. The extent of protection appears variable however it is an important tool especially on dairy farms where there is a high incidence of abortions due to this parasite. Historical sero-surveys in Queensland have indicated a significant prevalence in extensively grazed beef, however the significance in terms of reproductive loss is unclear. No vaccine is available in Australia.
Vaccines for liver fluke have been extensively researched but without any of these opportunities being commercialised. The efficacy of the various approaches has varied greatly and failing to meet a threshold for commercial viability. One approach that has not been progressed is a combined vaccine and chemotherapeutic approach which showed early promise in Australian studies.
Gastrointestinal nematode vaccines continue to be the holy grail of ruminant production. The great (and only) success story is the natural H11 vaccine for Haemonchus contortus developed and marketed for sheep in Australia and now globally. This approach has revolutionised the way Haemonchus contortus is managed. This innovation has progressed after a long and expensive failed search for a recombinant approach (and with manufacturing advantages). The natural antigen approach has been both a technical and commercial success despite a degree of complexity in its application. There is some evidence, still to be proven, that the same or similar antigens may be useful in cattle for Haemochus placei and possibly other parasites. This vaccine approach has been only achieved once before with the irradiated larval vaccine for Dictyocaulus viviparus in calves in Europe.
Cestode vaccines have been developed and are highly effective, and again, as a result of Australian research. These have not found a commercial home and are largely used on an experimental basis and in less developed countries where human infections are significant.
Diagnosis of parasitism
Most of these comments will relate to GIT nematodes. FECs have been mentioned several times already and the limitations are clear. They are a useful monitoring tool in young cattle up to one year of age when Cooperia and Haemochus spp. mainly dominate. Beyond this age and for most parasites, the egg count will not reflect the adult burdens and therefore the need to treat. The guides that are available (FEC when treatment is advised) are not well backed by objective data but are the only metrics available. There are a variety of techniques now available at varying costs. As these have become more commercialised some objectivity and expertise has been removed which is unfortunate. Certainly, numerous discrepancies have been seen in reseller-based systems. There is an ongoing need for inter-laboratory quality control to maintain credibility in the results that are generated.
Faecal cultures are always justified, even with low egg counts in cattle, as they provide a guide to the parasite mix, extent of control options that may be required and often a clue if resistance is present (especially post treatment monitoring). Alternate techniques such as quantitative and qualitative PCR to monitor total egg output and species mix would be useful, however that does not appear to have sufficient utility at this time.
Pepsinogen serum levels are still a useful tool for assessing abomasa wall damage, and indirectly parasitism, due to Ostertagia spp. There are well documented limitations at an individual level but it is very useful at a herd level and when repeated over time and especially in the 1-2-year-old animal. Monitoring growth rates is an alternate method of measuring the effectiveness of parasite control and any potential development of resistance. These measurements need to be at regular intervals to investigate anomalies early before there are more serious consequences. Obviously, there are many potential causes of reduced weight gain that can be interrogated at the same time. Technology can assist this process with EID and gated scales associated with water or feed sources.
A gap in the management of parasitism has been tools to measure numbers of larvae on pasture and therefore the safety of that pasture to graze. This is particularly an issue for sheep, however is also relevant for cattle. While research techniques are available, these are impracticable for routine use. Most advisors and producers severely underestimate the time needed for larval numbers to decline to an acceptable level. There is also a paucity of data on what constitutes a safe number of larvae.
Paramphistomes
There is excellent information in the literature around the epidemiology, pathology and chemotherapy of this disease. A subject in itself, it is important to recognise the potential for this infection in areas where there are likely snail habitats such as swamps, irrigation or flood affected areas. The classic presentation is profuse watery diarrhoea characterised by severe hypoproteinaemia. Chemotherapy options are limited to oxyclozanide and historically niclosamide (acute intestinal stage only) and hexachlorophene. A balance needs to be struck with control programs in allowing some infection to establish and animals develop what is a robust immunity to incoming infection (adults may still be present) and preventing production loss and clinical disease.