Gastrointestinal nematode infections in cattle in Zimbabwe have been reported by various authors (Bryant & Norval 1985; Pandey 1989; Eysker, Jansen & Pandey 1990; Pandey, Nyanzunda & Chitate 1990a; Pandey et al. 1990b; Duncan & Forbes 1992; Obwolo et al. 1992; Pandey, Chitate & Nyanzunda 1993; Vassilev 1993, 1994, 1999; Obwolo et al. 1995; Moyo et al. 1996; Moyo et al. 1997; Moyo et al. 2003; Magaya et al. 2000; Pfukenyi et al. 2007) and unpublished records from the State Central Veterinary Research Laboratory, Harare, Zimbabwe. Parasitic gastroenteritis (PGE) in cattle is caused by mixed infections of several nematode species (Table 1). Nineteen GI nematode species belonging to seven families have been reported to infect cattle in Zimbawe (Table 1). The most important and widely prevalent nematodes belong to the Family Trichostrongylidae, which is composed of the genera Cooperia, Haemonchus, Trichostrongylus and Ostertagia and to the Family Chabertiidae, which is composed of the genus Oesophagostomum. The genera Bunostomum, Nematodirus, Strongyloides, Toxocara and Trichuris are of lesser frequency (Pandey et al. 1993; Vassilev 1994, 1999; Moyo et al. 1996; Pfukenyi et al. 2007).

The most frequent species is Trichostrongylus axei; there are a few reports of Trichostrongylus colubriformis (Moyo et al. 1996; Moyo 2006). Few reports of T. colubriformis in cattle in South Africa are available (Horak 1981; Horak et al. 2004). Trichostrongylus colubriformis is a parasite of sheep and its presence in cattle is likely to be due to cross-infection (Horak et al. 2004). However, with the exception of T. axei, parasite species belonging to the Trichostrongylus genus are not considered to be important in cattle (Horak et al. 2004).

The survival rate and migration of O. radiatum is lower in comparison to the Cooperia species and H. placei (Reinecke 1960). Its low prevalence is attributed to its relatively long life cycle and low resistance to desiccation during its pre-infective stages (Rivera et al. 1983). Immunity may also play an important role in the differences noted; O. radiatum is extremely effective in eliciting strong, protective immune responses. A primary exposure of previously naïve animals to infection or even to parasite antigens results in a very significant reduction in the number of parasites that can become established after a subsequent infection (Gasbarre & Canals 1989). As a result, these parasites remain largely a problem for only young animals in the herd (Gasbarre & Canals 1989), a finding that is consistent with observations in Zimbabwe (Moyo et al. 1996; Pfukenyi et al. 2007). However, other parasites such as the Cooperia species and H. placei require a longer period of exposure before this level of protective immunity is seen (Gasbarre & Canals 1989).

The survival of B. phlebotomum is extremely low compared to other nematode species and is only present in large numbers when well-distributed summer rainfall provides adequate moisture for larval development and survival (Reinecke 1960). In addition, the possibility of infection, especially under extensive range-type operations with low animal density, is greatly reduced since the larvae do not migrate from the faeces, but penetrate the host’s skin (Reinecke 1960; Rivera et al. 1983). Its occurrence in Zimbabwe is sporadic and tends to be localised, favouring soils with high moisture content; this can be a problem in Kalahari sandveld areas of the country (Jooste 1987).

Toxocara vitulorum is an infrequent parasite under extensive farming conditions (Bianchin & Honer 1987). Its prevalence is low in communal and on commercial farms where cattle are kept under extensive farming conditions (Moyo et al. 1996; Pfukenyi et al. 2007). However, its prevalence in calves on irrigated farms in the highveld region has been found to be high (Pandey et al. 1990b; Moyo et al. 1996). According to Pandey et al. (1990b), under natural conditions T. vitulorum infection is low and the high prevalence that has been observed on irrigated farms is due to the unique system of farm management resulting in persistent moist or wet pastures. However, lactogenic transmission is an important source of infection to calves and the larvae are present in milk for up to three to four weeks after parturition (Pandey et al. 1990b). In cows, resumption of larval development in late pregnancy allows further lactogenic transmission.

Tropical climates do not offer favourable conditions for the transmission and survival of Ostertagia and Nematodirus species (Levine 1963), so both parasites are of little significance in Zimbabwe. However, Nematodirus helvetianus occurs in calves in the moist, temperate climates of the coastal provinces (Eastern Cape, Western Cape and KwaZulu-Natal) of South Africa (Reinecke 1983; Horak et al. 2004). Similarly, O. ostertagi has been reported in cattle on coastal pastures in the Eastern Cape Province of South Africa. However, the parasite species also occurs in cattle in the non-coastal areas of the eastern highveld of Mpumalanga Province (Louw 1999) and the northeastern highveld of the Free State Province (Tsotetsi & Mbati 2003) in South Africa. The presence of these parasites in Zimbabwe could probably be attributed to importation of cattle from South Africa.

Table 2 shows GI nematodes reported in cattle that were also documented in other domestic and wild ruminants in Zimbabwe. The data show that C. pectinata, C. punctata, H. similis, S. papillosus, T. axei, T. globulosa and T. barbertonensis were found in at least two other domestic or wild ruminants, whilst the others were reported in only one other domestic and/or wild ruminant. These findings are important when considering different options for the control of these parasites in cattle.

Thus, during the rainy season (December to March), the rainfall and temperature are favourable for the development, survival and translation of infective larvae on herbage. These conditions result in increased availability of infective larvae on pastures, so the chances of cattle picking up infective stages of the parasites whilst grazing are high, leading to a buildup of high worm burdens in the host. This leads to an increase in FECs with peaks between February and March and April and May. During late autumn to early spring (May to October and November) the decline in FECs is due to lack of rainfall and to some extent lower and higher temperatures; these are conditions that are unfavourable for the development of eggs into infective larvae. Hence, the wet months carry the highest risk of occurrence of parasitic gastroenteritis (PGE) in cattle; therefore control measures should take this factor into consideration. However, it is also important to note that the most pathogenic GI nematode parasites, Haemonchus species, survive the dry season as inhibited early fourth-stage larvae (hypobiosis), whereas the other species from the genera Cooperia and Trichostrongylus survive the dry season mainly as adults (Moyo et al. 1996; Magaya et al. 2000).

The prevalence of GI nematodes as observed through faecal egg counts is higher in animals aged less than 12 months as opposed to adult cattle (Pfukenyi et al. 2007). Hence, adult cattle act as reservoirs of infection and constant sources of infection for the more susceptible young animals. A higher prevalence and egg count of GI nematodes was demonstrated in lactating and pregnant cows when compared to dry cows, bulls and oxen; this confirms the periparturient egg-rise phenomenon (Pfukenyi et al. 2007) and thus serves as a source of pasture contamination.

There are contradictory results on GI nematode infections in cattle on communal land. Earlier studies reported low levels of infection leading to the conclusion that GI nematodes were of no importance and do not contribute significantly to the poor body condition and low productivity recorded in cattle (Bryant & Norval 1985; Pandey et al. 1990a; Obwolo et al. 1992). However, later studies (Pandey et al. 1993; Vassilev 1994; 1999; Moyo et al. 1996; Pfukenyi et al. 2007) showed high levels of GI nematode infections in communal cattle. The levels of infection, particularly during the rainy season, could vary depending on farming systems (Moyo et al. 1996). Faecal egg, pasture larval and worm counts were highest in communally-grazed cattle, intermediate in cattle on irrigated commercial farms and lowest in cattle on conventional commercial farms (Moyo et al. 1996). These differences could be due to different management systems and different microclimatic conditions. Communal cattle are kept under extensive free-range poor communal pastures with very little input of feed supplements. Anthelmintic (AH) treatment of communal cattle is either non-existent or very occasional and irregular and, if given at all, is limited to a few sickly animals (Pandey et al. 1993). In addition, the stocking rate is high, being estimated to be 0.37 livestock units per hectare; this may be increased more than two-fold during the rainy season as 50% – 60% of the land is used for growing crops during that period (Pandey et al. 1993). Thus, a combination of poor nutrition, overgrazing, high stocking rate and lack of AH medication results in heavy worm burdens in communally grazed cattle (Pandey et al. 1993) compared to others. In contrast, cattle on conventional commercial farms are kept under modern extensive or intensive conditions where stocking rates are lower, particularly in the hot and dry lowveld region (Moyo et al. 1996). In addition, commercial farmers tend to treat all animals with an AH at the beginning of the dry season (April to May) and/or the beginning of the rainy season (November to December) (Moyo et al. 1996). Hence, worm burdens in this category of cattle are lower than communally grazed cattle. The obvious reason for higher levels of infection and pasture contamination on irrigated commercial farms when compared to conventional commercial farms is the higher stocking rate that is allowed by the irrigation and furthermore, irrigation implies an increase of the length of infectivity of pasture as shown by higher pasture larval counts (Moyo et al. 1996).

In mixed cattle-game farming systems, cattle and game species, particularly antelopes, are reared together. Mixed farming with cattle and antelope is attributed to bring about the possibility of cross-infection of helminth parasites (Jooste 1987; Knottenbelt 1989; Madzingira et al. 2002a). A high stocking density of antelope and domestic ruminants is known to favour such cross-infection and the random translocation of game may broaden the endemic and host ranges of internal parasites (Horak 1981). Earlier studies in Zimbabwe have shown cross-infections of H. contortus, T. axei, T. colubriformis, Dictyocaulus viviparus, C. pectinata and C. punctata from domestic ruminants, including cattle, to antelopes (Condy 1972; Grobler 1981; Jooste 1984, 1987; Knottenbelt 1989, 1990). Later, Madzingira et al. (2002a) reported evidence of cross-infections of O. radiatum and S. papillosus from cattle to antelope species. No common helminths of antelope were recovered from cattle, implying that antelopes may act as reservoir hosts of infection for cattle. It appears that antelope are better hosts for helminths of cattle than cattle are for those of antelopes (Horak 1980; Boomker, Du Plessis & Boomker 1983; Boomker et al. 1984; Madzingira et al. 2002a). Thus, antelopes are more likely to suffer in a mixed farming system with cattle. However, the degree of cross-infection was low and the parasites involved occurred in low numbers, which may have little effect on the health of the antelopes (Madzingira et al. 2002b).

Gastrointestinal nematodes are considered to be primarily important in first season grazing calves in summer (Charlier et al. 2009). Increased weight gain is the most often observed and best-documented benefit of GI nematode control in first season grazing calves (Charlier et al. 2009). This effect on weight gain was studied in relation to anthelmintic treatment in Western Europe (Shaw et al. 1998; Dimander et al. 2000, Dimander 2003; Larsson et al. 2006). Similar studies were conducted in Zimbabwe (Duncan & Forbes 1992; Vassilev 1993; Magaya et al. 2000; Moyo et al. 2003), which showed a significant effect of anthelmintic treatment on the enhancement of productivity and protection from the effects of infection with nematode parasites in cattle. In the last decade, substantial generated evidence showed that GI nematodes could also have negative impacts on the performance of adult cattle (Charlier et al. 2009). The milk yield response to anthelmintic treatment in pastured dairy cows was observed to be around 1 kg/cow per day (Charlier et al. 2009). In anthelmintic-treated beef cows, increased conception and calving rates, and reductions in calf mortality and calving-to-breeding intervals were shown (Hawkins 1993; Gross, Ryan & Ploeger 1999). Similar studies in dairy cows showed shorter calving-to-conception intervals in treated cows when compared to untreated controls (Walsh, Younis & Morton 1995; Sanchez et al. 2002). However, no such studies have been conducted in Zimbabwe and the impact of GI nematodes on production should be investigated in different categories of cattle.

The diagnosis of PGE can be based on clinical signs confirmed by FECs or by determination of parasite specific serum antibody, gastrin or pepsinogen levels (Berghen et al. 1993). In New Zealand, the FEC strategy achieved a reduction in the frequency of AH use (Leathwick et al. 2006). However, the main drawback is that this is impractical in Zimbabwe due to the limitations of cost and facilities and lack of farmers’ training on diagnosis of cattle PGE based on clinical signs. The measurement of serum pepsinogen and gastrin levels is a specific tool in the diagnosis of ostertagiosis, which is not important in Zimbabwe and other southern Africa countries since the disease is not common in the region. Similarly, an enzyme-linked-immunoabsorbent-assay (ELISA) specific for the temperate C. oncophora has been developed (Poot et al. 1997), but might not be useful for the tropical Cooperia species. However, since the animals that are most susceptible to parasites and/or their effects, and hence require repeated AH treatments, are commonly the same within a season or between seasons (Malan, Van Wyk & Wessels 2001), the therapeutic or targeted (selective) animal treatment remains a beneficial approach. Hence, for the time being, to improve the therapeutic approach in Zimbabwe, simple user-friendly extension material on PGE should be produced and disseminated to farmers and extension staff. For exploitation of the over-dispersion concept, research focused on production of convenient, inexpensive and accurate on-farm methods (e.g., Faffa Malan chart (FAMACHA^©) in small ruminants) for identifying animals for targeted selective treatment should be prioritised.

The threshold for production-based treatment is essentially based on the identification of the so-called ‘sub-clinical’ infections that are characterised by no clinical signs of PGE in untreated animals. However, the level at which GI nematode infection in cattle results in sub-clinical parasitism and any associated production losses has not been defined (Vercruysse & Claerebout 2001). Useful tools to define subclinical parasitism such as pepsinogen and gastrin levels, serum and milk antibody levels and the copro-antigen detection test were evaluated in western Europe for temperate GI nematodes (Hilderson et al. 1992; Kloosterman et al. 1996; Guitian et al. 2000; Agneessens, Vercruysse & Claerebout 2001). However, these tools have not been assessed for tropical GI nematodes.

GI nematode infections should be prevented rather than cured, with the preventive measures being applied at herd level (Vercruysse & Claerebout 2001). Based on the current increased knowledge about the epidemiology of GI nematodes in Zimbabwe, preventive treatment strategies of PGE in cattle can be formulated. GI nematode infections in cattle are high during the rainy season and low during the dry season. A strategic treatment should be carried out when pasture infectivity levels are low during the dry season and a tactical treatment at the peak of pasture infectivity levels during the rainy season (Figure 1). The basis of the strategic treatment is to achieve a long-lasting effect on worm burdens so that re-infection during the dry season will be minimal and additionally the negative impacts of a combination of poor nutrition with nematode infections will be prevented (Moyo et al. 2003). The rationale of the tactical treatment is to prevent clinical disease during the peak period of infection.

FIGURE 1: Seasonal occurrence of gastrointestinal nematode infections in cattle in Zimbabwe based on epidemiological reports and the recommended treatment options.

One of the major constraints with tactical or strategic single-dose treatments is the labour involved in gathering and handling cattle, particularly in communal areas where animal-handling facilities are limited. The use of products with persistent action, particularly the MLs, should be encouraged to reduce the frequency of AH treatments. For ease of use and convenience, pour-on products are now widespread and pour-on ML products are now dominant in the cattle AH market. Furthermore, one of the other developments is the introduction of controlled release devices (boluses), which deliver anthelmintics either continuously or at targeted times. This offers the possibility of a single-dose treatment without re-course to further treatment and should be used in communal areas during dipping sessions.

Despite the acceptable performance of grazing cattle after AH treatment, populations of nematodes that are resistant to the most commonly used AH increasingly threaten this control (Sutherland & Leathwick 2011). Although AH resistance of cattle nematodes has appeared to develop more slowly than those of small ruminants, literature suggests a rapidly escalating problem with 145 documented cases of AH resistance (Sutherland & Leathwick 2011). A review by Sutherland and Leathwick (2011) showed reports of AH resistance of cattle nematodes in Argentina, Australia, Brazil, Belgium, UK, Germany, India, New Zealand and USA. This involves all three of the broad-spectrum AH classes and at least ten species of nematodes. There is no literature about AH resistance of cattle nematodes in Zimbabwe. However, BZ-resistant Cooperia species and H. contortus have been reported in sheep (Boersema & Pandey 1997; Mukaratirwa, Charakupa & Hove 1997). Given the rapid increase of reports in the literature about AH resistance of cattle nematodes, it is important that preventive AH treatments are integrated in the grazing management system to prevent under-treatment or over-treatment (Vercruysse & Claerebout 2001). There is also a pressing requirement to determine the extent of the problem in Zimbabwe and other regional countries, as early detection of AH resistance is an important factor for sustainable chemical control.

Practically no cross-transmission between sheep and cattle occur for Oesophagostomum and Bunostomum species (Porter 1953; Rose 1968; Barger & Southcott 1975; Southcott & Barger 1975). The two genera are important in Zimbabwe and hence, the alteration of cattle and sheep could be an effective method for controlling these parasites in the country. However, species of Cooperia and intestinal Trichostrongylus can reproduce in the alternate host (Porter 1953; Rose 1968). Cross-transmission of T. axei, equally pathogenic to sheep and cattle, is sufficiently effective that alternate grazing of these hosts at intervals of several months has nothing to offer in its control (Kates & Turner 1960). Furthermore, alternate grazing of the most susceptible hosts (young sheep and young cattle) is not reliable to aid control of Haemonchus species in either host (Morley & Donald 1980). Hence, cross-transmission between sheep and cattle appears not to be sufficiently restricted, so that alternate grazing would offer substantial benefits in GI nematode control in Zimbabwe, where the dominant parasites are H. contortus and Trichostrongylus species in sheep, and Cooperia species., H. placei and T. axei in cattle.

Older animals are generally more resistant to GI nematode infections than younger animals. An alternate grazing system through rotational grazing of paddocks by calves ahead of cows or heifers could be employed (Morley & Donald 1980). Two further refinements are the so-called ‘forwards creep grazing’ whereby calves are grazed ahead of their dams, which follow them directly on the same pasture and ‘sideways creep grazing’, in which young animals have access to adjacent pasture that is not accessible to their dams (Van Wyk 1990). All forms can be used to control cattle GI nematode infections in the intensive and extensive commercial farming systems of the highveld and lowveld regions of Zimbabwe. However, the system is managerially complex and therefore relatively difficult to apply (Van Wyk 1990) and is not feasible in the communal grazing areas of the country. In addition, one consequence of mixed or alternate grazing of different age groups of cattle is the exposure of older animals to higher levels of larval intake than they might otherwise experience (Morley & Donald 1980). Except for O. ostertagi where it could lead to the accumulation of very large numbers of arrested larvae in heifers or cows leading to Type II ostertagiosis (Morley & Donald 1980), this may not be important for most genera in Zimbabwe.

Crop aftermaths (stubble lands) can also be used as safe grazing during the dry season in all the cattle farming systems in Zimbabwe and such aftermaths are practical for worm control (Morley & Donald 1980; Van Wyk 1990). They are seldom sufficient to accommodate all the susceptible animals and this also requires careful planning to have pasture available in appropriate quantity and quality (Van Wyk 1990).

Biological control includes the use of predatory fungi to kill a variety of nematode species and substantially reduce the intensity of infection (Larsen 1999). With regard to the effect of nematophagous fungi against cattle parasitic nematodes, it is possible to reduce the number of infective larvae on herbage and subsequently to reduce the number of nematodes establishing in the grazing animals (Wolstrup et al. 1994; Larsen et al. 1995; Nansen et al. 1995). Challenges to fungal control are a requirement for daily administering of fungi to the host and achieving the required fungal density inside the dung. However, a nematode-killing fungus, Duddingtonia flagrans, discovered in New Zealand (Skipp et al. 2002), showed a trapping efficiency rate of 78% and activity for up to 90 days on pasture, providing a viable alternative to reduce animal mortality from nematode infections (Waghorn et al. 2003). Surveys in neighbouring South Africa have shown the presence of local strains of D. flagrans (Durand et al. 2005). Given the limited genetic variation between isolates from different parts of the world (Faedo 2001), a survey of Zimbabwean local strains would address any concerns regarding importing a foreign fungus. When this control approach becomes available for general application, it might be feasible in the small-scale commercial sector, but impractical in the communal set-up.

Plants or crops with natural anti-parasitic properties may play a role in future grazing systems for livestock (Waller & Thamsborg 2004). Several tannin-containing forages, in particular those with condensed tannins (CT), showed AH activity against GI nematodes of sheep and goats (Hoste et al. 2006). Extracts of three tannin-containing plants, Onobrychis viciifolia, Lotus pedunculatus and Lotus corniculatus, were shown to have in vitro AH activity against the cattle nematodes O. ostertagi and C. oncophora, with L. pedunculatus tannins being more active than those of O. viciifolia and L. corniculatus (Novobilsky, Mueller-Harvey & Thamsborg 2011). In Zimbabwe, Acacia karoo was shown to have a negative influence on the establishment and fecundity of H. contortus infection in Boar goats in comparison to goats fed Acacia nilotica and a basal diet (Kahiya, Mukaratirwa & Thamsborg 2003). This was attributed to the high content of CT in A. karoo. Further studies are required to determine the AH effects of tanniniferous plants on cattle GI nematodes and their use in helminth control in order to address the wide problem of AH resistance.

Well-nourished animals are generally more resistant to the effects of parasite infection. Therefore, nutritional supplementation may reduce the requirement for AH control (Knox, Torres-Acosta & Aguilar-Caballero 2006; Torres-Acosta et al. 2004). Additional dietary protein (Torres-Acosta et al. 2004), selenium (Au Yeung et al. 2005; Smith et al. 2005), as well as minerals (Islam et al. 2006; Koski & Scott 2003) may each play a role in countering infections presumably through mechanisms such as enhancing host immunity or maintaining digestive tract integrity.

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