28 Sept 2015
Andre Baptista
Job Title
The focus of gastrointestinal nematode (GIN) control has shifted from firefighting clinical outbreaks to preventing GIN-associated clinical and subclinical disease at herd level (LeBlanc et al, 2006).
Historically, GIN control has been focused on first grazing season animals, which are considered to be at greatest risk of parasitism. However, studies have now demonstrated the negative impact GIN can have on the productivity of adult dairy cattle. The financial cost of GIN parasites in adult dairy cattle is difficult to quantify, due not only to the direct effect (treatment, extra labour and vet time), but also the indirect effect (poorer performance of the animal and production losses). That said, parasite control forms an essential component of adult dairy cattle management.
A number of GINs can affect adult dairy cattle. Ostertagia ostertagi is the most pathogenic species and is the focus of parasite control in adult dairy cattle. The developments of diagnostic tests using milk (both individual and bulk) are centred on the detection of antibodies against O ostertagi.
The negative effect of GIN infection in first grazing season animals is well recognised and documented (Larsson et al, 2006). However, scientific studies have found a direct link between reduced milk production (Gross et al, 1999) and possibly some fertility/reproductive effects (Sanchez et al, 2002c) and high GIN in adult dairy cattle. One study demonstrated some grazing dairy herds that were treated for GIN saw a milk yield response around 1kg/cow per day (Charlier et al, 2009).
Although there is a strong negative correlation between young animals that have been infested with GIN in the first two years of age and their future milk production (as a direct consequence of reduced weight gains increasing the time to reach breeding weight, consequently involving reproductive and milk yield costs; Zanton and Heinrichs, 2005), exposure to GINs as youngstock is necessary for development of immunity, so appropriate control in rearing heifers that balances GIN exposure with their detrimental effects is necessary.
Work in this area (Ravinet et al, 2014) has defined the concept of time of effective (parasite) contact (TEC) – a measure of parasite exposure that considers the immune status of a herd and “resistance to parasite reinfection” (that is, immunity) at the first calving based on parasite exposure during the rearing period.
Lungworm outbreaks are unpredictable, but tend to be more prevalent in wetter, western areas of Britain. Animals exposed to lungworm usually develop immunity to reinfection, although a lack of exposure may result in clinical signs occurring in older cattle, including milking cows. Animals that were previously immune (either following infection or vaccination) may develop clinical signs if immunity wanes or pasture infectivity is high.
Lungworm can be a problem in young adults if they have not acquired immunity and become exposed. Adults may be carriers, with faecal shedding contributing to pasture contamination. On farms identified as high risk (that is, historical outbreaks) vaccination of youngstock will reduce their risk of infection as adults. Strategic control with anthelmintics is impractical in the adult herd (due to unpredictable challenge and product withholding periods).
Cattle acquire immunity when exposed to roundworms, but it takes roughly one full grazing season for Cooperia oncophora and up to two grazing seasons for O ostertagi.
Consequently, a higher degree of GIN immunity will be present in adult cattle compared to first grazing season animals (assuming appropriate GIN exposure as youngstock); however, that immunity can break down during stressful periods, such as disease, poor nutrition, pregnancy and lactation (Radostits, 2007). There is also variation in individual animals as to how quickly immunity develops and animal exposure will vary depending on pasture parasite burdens.
Therefore, when considering anthelmintic treatment protocols, the risk of exposure in adult cattle and previous grazing history must be considered. Immunity does not completely eliminate infection after exposure, and cattle may tolerate a low worm burden and excrete low numbers of eggs, which contribute to pasture contamination (relevant when developing grazing plans).
GIN burden can be evaluated by several methods using different samples, such as faeces, blood or milk.
Faecal egg counts (FECs) can be used in young animals, but this method should be used cautiously in adults due to its unpredictability in determining a true parasitic burden (animal’s immunity and low egg output; Claerebout and Vercruysse, 2000). FECs have other limitations: being a semi-quantitative method it should generally be interpreted at a group level, rather than in individuals, and cannot detect nematode stages not laying eggs (that is, immature larval stages and hypobiotic larvae) so should be interpreted with caution.
Serum pepsinogen can be used to assess the degree of parasitism of O ostertagi in first season grazing animals, although it should be used more carefully in adult animals (Gross et al, 1999). Unfortunately, the accuracy of the pepsinogen test depends on the life cycle stages of the nematodes present in the host (Berghen et al, 1993). High pepsinogen levels reflect an injured abomasal mucosa, which is mainly caused by O ostertagi larvae.
While FECs reflect the presence of adult GIN, increased pepsinogen indicates destruction of the abomasal mucosa due to Ostertagia species larvae. Pepsinogen concentrations above 1,000mU/ml tyrosine (tyr) are considered the limit between high and low parasite burden (expressed in tyr mU/ml; Mejía et al, 2011). This can be very useful if we want to determine the GIN burden in first season grazing cattle before housing.
The ELISA-based milk O ostertagi (MOO) test can be used at an individual or herd level and will detect the concentration of antibody against O ostertagi in a milk sample. A positive value reflects recent or current exposure to O ostertagi and can be used in adult cows to predict whether anthelmintic treatment is likely to increase productivity at a herd level, as herds with a high MOO antibody value generally respond better to treatment than those with low values (though this response is not seen in every MOO-positive farm; Vanderstichel et al, 2013).
TEC is an interesting concept proposed by Ravinet et al (2014), which takes into consideration the immune status of a herd and reflects its “resistance to reinfection” (that is, immunity) at first calving. The TEC (expressed in months) with GIN larvae before the first calving is calculated (for each herd) as follows:
TEC = duration of grazing season – (duration of persistency of anthelmintic treatments + duration of drought and high supplementation periods)
When heifers grazed two seasons before the first calving, one TEC was calculated for each grazing season and both were added to give the final value. A TEC threshold of eight months was held to predict good immunity in this study and cows from low-TEC herds responded better to treatment than cows from high-TEC herds, suggesting the treatment response correlates with the immune status of the herd. TEC promises to be a useful factor to consider (alongside grazing history/pasture risk and diagnostic test results) when defining parasite control approaches in adult cattle.
Since exposure to infective larvae occurs at pasture, the risk of infection and the establishment of GIN burden is influenced by the length of the grazing period, the pasture parasite burden and TEC/immunity. Consequently, short periods of grazing can pose a risk – for example, in low TEC cows that are housed throughout lactation, but are turned on to infected pastures during the dry period, particularly when permanent pasture is used.
It is assumed adult dairy cows are immune following exposure as youngstock (TEC can help quantify this), but a nutritional cost is still associated with maintaining this immune response (Colditz, 2008; Charlier et al, 2010). In addition, a significant GIN burden can result in reduced appetite, leading to reduced grazing time and forage intake (Forbes et al, 2004) along with a reduced ability to digest forage (Gibb et al, 2005).
Pasture-based dairy herds can face more detrimental GIN effects on milk production and reproduction than semi-confined or totally confined based dairy herds due to its higher level of exposure to GIN. Sanchez et al (2002a; 2002b; 2002c) proved in his varied scientific studies infested pasture-based dairy herds had a marginally significant treatment effect on calving to conception interval, suggesting high GIN burdens had a negative impact on reproductive performance.
On the other hand, semi-confined or totally confined based dairy herds may not require as much attention to the detrimental gastrointestinal (GI) parasitic effect compared to pasture-based dairy herds.
Semi-confined or totally confined based dairy herds do not encounter the same level of exposure to GIN so their parasite burdens are generally low, and monitoring through specific parasitological tests (milk or serum) may be sufficient. Sithole et al (2005; 2006) did not identify any reproductive or milk production benefit in treating dairy cows with low GIN burdens around calving.
In the UK dry period, it is very common to see farmers putting dry cows at pasture and returning them to confinement just before calving, which is a sometimes overlooked risk period in fully/semi-confined herds. Strategically timed deworming or simply monitoring at this stage will help either remove or assess parasite burden (acquired during the dry period) prior to calving. If cows remain in confinement during lactation, no further deworming may be necessary (Sithole et al, 2005; 2006).
Untreated adult cattle may act as carriers at this stage, contaminating pasture that must be considered when planning youngstock grazing due to their higher susceptibility to GIN and lungworms. Pasture management is a very important factor to consider to reduce pasture contamination at critical times during the grazing season.
Several studies have evaluated what time of a dairy cow’s life cycle would be more beneficial to use an anthelmintic treatment, although factors such as TEC, MOO results and GIN burdens were not always considered. Charlier et al (2012) investigated the economic benefits of different treatment protocols, including the difference between treating all cows at calving in comparison to at housing. The results showed daily milk yield response was greater in the group treated around calving. Treating a cow at calving allows it to start its lactation without the pressure of a worm burden, thus helping it to optimise its feed intake in early lactation.
Maximising intakes is critical at the start of lactation as it influences peak production and the subsequent lactation yield. In addition, when treated at calving the resulting milk yield response has the potential to extend over the whole of lactation. However, when treated later in lactation the duration of potential milk yield response is shorter.
In this same study, Charlier et al (2012) proved selective treatment approaches (based on MOO levels, days in milk) can be economically competitive, although they require more “homework” (identifying which animals to treat) and labour effort. Therefore, when devising a treatment plan for a farm, it is important to consider risk of exposure both in terms of period of time (length of grazing season) and pasture management (for example, silage aftermath or permanent pasture). To minimise the risk of unnecessary treatments – which not only has an associated cost, but also increases the risk of anthelmintic resistance – only cows likely to benefit from treatment should be targeted.
Deworming should only be performed when necessary and taking into consideration different factors (such as previous grazing history, TEC, parasitological lab tests, risk of exposure and so on), proper pasture management can reduce the number of parasites ingested by cattle, keeping parasite burdens low.
MOO levels can identify adult animals that could have a production benefit from treatment (especially subclinical cases; Charlier et al, 2012), indicated that, in herds with a low or average level of infection (less than 0.8 bulk tank MOO optical density ratio; ODR) a selective treatment approach will be equally or more economical than whole herd treatment. Furthermore, applying selective treatment instead of whole herd treatment in herds with a bulk tank ODR between 0.3 and 0.8 would reduce the amount of anthelmintics used in these herds by, on average, 52 per cent. To use bulk tank milk and individual serum anti-Ostertagia antibody levels more reliably, grazing history should be taken into account, and perhaps the definition of thresholds should depend on the TEC values.
Only two classes are licensed for use in dairy cattle for GI roundworms (benzimidazoles and macrocyclic lactones). Anthelmintic overuse, underdosing and usage of leftover/out-of-date anthelmintics all represent poor practice, risking inefficacy and potential for selection of resistant parasites through inadequate dosing.
The most commonly used products in lactating dairy cattle contain either eprinomectin or moxidectin. Both have a residual action and continue to have an effect after treatment – up to 28 days for eprinomectin and 35 days after treatment with moxidectin. Benzimidazoles may also be used, but their efficacy against inhibited larvae can be unpredictable. The proportion of fourth stage inhibited larvae can be very high in adult dairy cattle, particularly in autumn (Agneessens et al, 2000). All treatments should be based on an accurate bodyweight weight determination.
New control parasite strategies are emerging that combine laboratory tests, pasture risk assessment and grazing history to deliver integrated, targeted GIN control. Targeting anthelmintic treatment to individual animals is key to improved health and welfare, reduced product use and reduced section pressure for anthelmintic resistance.
Targeting animals for selective treatment against GIN can be based on previous grazing history, TEC and individual and herd-level parasitological indicators, such as FEC, pepsinogen blood levels and MOO levels. To develop future milkers, our goal must be to produce young heifers with a degree of protective immunity and optimise/target anthelmintic use throughout productive life to minimise resistance selection pressure while maintaining health.
