Chapter 1: INTRODUCTION

Chapter 10 PARASITE/NUTRITION INTERACTIONS

10.1 PARASITES AS A CAUSE OF PRODUCTION LOSSES

The most striking effect of helmintic infection is the death of the host. Under most pastoral condi tions endoparasitism can result in mortality rates exceeding 50% (Barger 1982). Insidious losses of productivity through red uced feed in take and decreased effi ciency of utilisation of feed are often the largest economic losses.

Cattle and buffalo suffer from the same gastrointestinal parasites but buffalo calves are very much more sensitive to the parasite Toxocara viiulorum. A mortality rate of 25 to 30% for well-managed buffalo calves appears to be tolerated in the humid tropics, and most of the deaths are attributed to T. viiulorum. Moreover, the parasite predisposes buffalo calves to a scour syndrome which may also result in death of the calf.

Information on production losses in small rUllUnants in developing countries is not readily available and although insidious losses are likely to be the greatest economic loss, often only death rates are recorded. It is difficult to determine the other effects of parasites. The death rate due to haemonchosis occasionally reaches catastrophic proportions where conditions are conducive to infestation. For example, in the Koroboya Goat Scheme, Fiji, 50% of the goat population (600 goats) died in the first 3 months of 1984 during an outbreak of haemonchosis (Clarkson 1984).

Insidious losses attributable to parasitism have been recorded in developed countries. For instance, where infected and uninfected groups of cattle grazed on separate areas, the weight gain of the infested cattle was 24 to 40% less than that of the uninfected

Sheep grazing temperate pastures in Australia suffered a very large reduction in weight gain even though they were treated against intestinal parasites (Archer 1980). In the same region a system of strategic drenching based on epidemiological principles has been highly successful. Frequent drenching is important, and lambs can have very heavy worm infestation and suffer severe weight loss when drenched infrequently and non-strategically (Figure 10.1).


Figure 10.1: Effects of anthelmintics and botanical composition of pasture on liveweight gain of Merino lambs. The sheep were drenched at two-week (frequent) 07' six-eight week (infrequent) intervals.

In sheep the productivity of animals that have recovered from severe parasitism appears to be permanently lowered. Reductions of liveweight gain can exceed 50% in animals with mild helmintic infections and this loss of productivity is similar to the permanent loss of productivity observed in animals after they have recovered from severe infestations.

In grazing systems, particularly communal grazing, or where there is overstocking, parasitism becomes a primary constrain t to animal production and even repeated or continllous drenching is not effective if the pasture is heavily contaminated with the parasites. In addition, it is likely that parasites under these conditions rapidly develop resistance to the broad spectrum anthelmintics generally in use in developing countries.

In Bangladesh, calves on pasture that had been grazed previously by a milking herd were free of strongyle worms for only a short time between drenches when treated with Fenbendazole every 4 weeks: Within 3 months of drenching, the number of worm eggs in the faeces of treated calves was similar to that in the faeces of untreated calves (Table 10.1). It is now clear that the approach to ill testinal parasi te control must be to u tilise highly specific and effective drenches at the time that the animal is infected. Major problems of resistance to antehelmintics arise because of indiscriminate use of broad spectrum drenches.

Before discussing the effects of parasites and their interaction with nutrition, some details of the lifecycles of the important groups of parasites are given. It should be stressed that it is the underfed animal or the animal that does not have the correct balance of essential nutrients that suffers the greatest production losses. The most common effect of parasites is to increase the host's need for protein. This is discussed in the next section.

10.2 LIFE CYCLES OF PARASITES

10.2.1 Nematodes

The life-cycles of the gastro-intestinal nematodes of ruminants are relatively simple. The infective larva is usually ingested while grazing at pasture and develops into an adult within the animal. When environmental conditions are favourable, eggs of nematode parasites hatch in the faeces to become first stage larvae. These feed on bacteria in the faeces, grow and moult to second stage larvae, which in turn feed, grow and moult. The larvae migrate from the faeces to pasture in moisture films. Migration to pasture may be delayed if conditions are not suitable (eg. dry and/or cold). Infective larvae can survive on pasture for up to 12 months.

Table 10.1: Effects of anthelmintic treatment on weight gains and strongyle egg counts of young calves.

LWt (kg) Initial Final

LWt gain (g/d)

27 46 123

28.9 47 115

Strongyle

( eggs / g faeces) After treatment After 84d After150d

30 500 690

nil 790 370

Source: Haque and Fraser (1982). 1. Drenched every 28 days.

When ingested by ruminants the infective larvae shed their protective sheath. The third moult takes place within a few days of ingestion and fourth stage larvae, which are attached to or exist within the mucosal surface of the gut, differentiate sexually and grow. On reaching maturity female roundworms start to lay eggs. For most species, including Haemonchus, Ostertagia, Trichostrongylus and Cooperia, the life cycle is completed within 3 weeks. There are some variations from this basic pattern; for instance, freeliving male and female Strongyloides papillosus develop on pasture and reproduce outside the animal. In this species, the parasitic stage consists only of parthenogenetic females and infection is by skin penetration or through the milk of the dam. Bunostomum spp. infect the animals by skin penetration as well as being eaten with the pasture.

The parasitic nematodes of ruminants may be divided broadly into those that ingest blood, ego Haemonchus and Bunostomum, and those that do not (eg. Oste7'iagia and T7'ichostrongylus). Symptoms in animals infested with blood-sucking parasites are directly attributable to anaemia caused by the loss of blood. High levels of infestation with nematodes of the non blood-sucking group produce acute and severe inflammation of the gastro-intestinal mucosa. Massive destruction of the mucosal surface occurs in heavily infested animals and is associated with diarrhoea, a high packed cell volume, decreased blood albumin levels, and high pH of abomasal contents, which allows the incursion of bacteria.

Liver fluke (Fasciola hepatica)

The adult fluke in sheep measures 20 to 30mm in length and the majority are found in bile ducts and the gall bladder, where they produce eggs. Eggs voided in faeces remain dor~nt under cold conditions. When the temperature rises to 25°C the eggs hatch in 9 days but egg mortality is high under conditions of high temperature and low moisture. On hatching the miracidium invades the intermediate snail host, of the species Lymnaea, in which it forms a sporocyst. This in turn gives rise to one or two generations of rediae before the maturation of motile cercariae, which accumulate in the body cavity of the snail. Four to seven weeks after the miracidium invades the snail the cercariae are released, settle on pasture and immediately encyst to become metacercariae, the infective stage for ruminants. The rapid multiplication within the snail can result in 4,000 metacercariae from a single miracidium. High or low temperatures and low humidity kill metacercariae in a few days but under temperate summer conditions and at a relative humidity of 90% or more they survive for at least a year.

Following ingestion by a ruminant, the juvenile flukes enter the peritoneal cavity via the intestinal wall. Within 24 hours of ingestion the flukes enter the liver; over a 4 to 6 week period they cause severe liver disruption by their migratory patterns. At 6 to 8 weeks, young flukes enter the bile duets and grow to three times their size. Eggs are produced 8 to 9 weeks after ingestion of the metacercarial'.

The loss of production in ruminants from fluke infestation is due to the gross disruption of liver tissue during the migration phase and to the loss of blood when the flukes are established in the bile ducts. Anaemia and low levels of plasma proteins are the major symptoms and occur because of the loss of blood proteins into the gut lumen.

Ruminants whose livers are damaged by fluke infestation are prone to ammonia toxicity, since the liver cannot form urea from the absorbed ammonia at a rate sufficient to maintain low concen trations of ammonia in the peripheral blood. Liver dysfunction due to fluke damage may also reduce synthesis of glucose, particularly from propionate. This may be highly important in animals in which availability of propionate is minimal and demand for glucose is high (see Chapters 3 and 4).

10.2.2 Strategic use of drenches

The intermittent and repeated use of broad spectrum drenches have resulted in rapid development of immunity by many intestinal parasites, which may explain the results discussed above from Bangladesh. In A ustralia emphasis is now being placed on the use of highly specific drenches combined with broadspectrum drenches. Based on epidemiological principles, knowledge of the parasites' life cycle and the dynamics of worm populations in relation to their environment, a strategic drenching system has been evolved which has had a major economic impact on sheep production in the temperate grazing areas of Australia (Dash et al. 1984). There is no point in going into details of the drenching programme as any strategy will need to take into account local conditions. The approach, however, is to treat the parasite that is most likely to be infective with highly specific drenches at a time which precedes the build up of infective eggs or larvae and when the environmental conditions are most favourable for them to build up on pasture.

In the summer rainfall grazing regions of South Eastern Australia it is not uncommon to drench sheep at intervals of four weeks with a series of broad spectrum drenches. The new strategy incorporates only three drenches per year which is aimed at producing 'clean' pastures without the need for rotational grazing or "spelling of paddocks" (ie. the parasites are strategically killed at the time of infection and before large numbers of eggs or infective larvae are deposited on pastures).

The approach is to drench with an anthelmintic specific to Haemonchus plus a broad spectrum drench in early August_, ' prior to spring growth of pasture and then again with a broad spectrum plus specific drench in November when, on epidemiological grounds, the infective larval numbers on pasture would be high. A further drench specific for Haemonchus is used again on 1st February when infection should again be high, The results of such a drenching programme on the accumulation and numbers of worm eggs on pasture is shown in Figure 10.2 and illustrates why pastures "clean up" over a period of a few years.

The model "Worm Kill" has been so effective that at least one of the most damaging parasites of sheep ( Haemonchus contortus) has disappeared from some farms in the grazing country in Australia.

This model is one that small farmers in developing countries could not be expected to undertake without government support, but for small farmers dependent on communal grazing for their livestock enterprises the increased productivity could be very large. The importance of knowledge of the life-cyde of parasites to develop a strategic drenching programme is the reason that some of their life cycles are given.

Strategic drenching, combined with strategic supplementation, could have dramatic effects on productivity in the communal grazing areas of third world countries.

10.3 NUTRITION-PARASITE INTERACTIONS

The major effects of parasites on ruminant productivity have been reviewed by Dargie (1980), who stated that:

He also suggested that none of the parasites examined lowered apparent energy digestibility per se by more than 5%, but that their secondary effect was to lower the efficiencies with which apparently digestible and metabolisable nutrients were utilised for fat and protein deposition. In animals infested with T. colubriformis and O. circumcincta, the efficiency of utilisation of metabolisable energy above maintenance was 30 to 40% lower than in pair-fed control animals. If the reasons for the low efficiency of utilisation of metabolisable energy can be determined, methods to overcome the causes could be developed, thus removing a large loss of productivity.

It is apparent that parasitism often depletes body proteins, particularly those circulating in plasma. In addition, anaemia due to blood loss is the most obvious symptom. These effects of parasitism increase the animal's need for absorbed amino acids and thus increase the requirements for protein relative to energy. In productive ruminants (young, growing or lactating) this must be a major constraint since their amino acid requirement is already high relative to their energy needs (Chapter 4). A marked disparity between the requirements of the animal for protein relative to energy and the availability of protein and energy in the end-products of rumen fermentation usually results in decreased feed intake. The greater the disparity the greater the loss of appetite.

In addition, most parasite infestations impair the efficiency of digestion or absorption of nutrients, or both. This in turn allows a greater amount of proteinaceous material to move to the caecum, thus decreasing the availability of amino acids relative to energy (Steel 1978). Any condition that affects the absorption of amino acids from the small intestine is likely to have an effect on feed intake.

10.3.1 Effects of parasites on rumen function

This appears to be a neglected field of study even though there have been reports of large differences in rumen function between animals infested with T, colubriformis and pair-fed, uninfested animals. For instance, the VFA concentration in the rumen of lambs infested with T. colubriformis was similar to that in pair-fed, uninfected animals, but, owing to a higher percentage of dry matter in the rumen contents of parasitised lambs, the pool sizes of both VFAs and ammonia were much lower. Steel (1978) found that acetate and total VFA production rates were reduced by some 30%, whereas the amount of dry matter in the rumen in lambs infested with T. colubriformis was reduced relative to that in control animals.

It thus seems that infestations of T. colubriformis in the small intestine may increase the outflow of organic matter from the rumen. Infestation of sheep with O. circumcincta, on the other hand, increases the non-ammonia nitrogen flow from the abomasum, an effect considered to be due to increased endogenous inputs at the site of infestation (Steel 1978).

A 30% reduction in fermentation in the rumen may be compensated for by increased fermt'ntation in tht' caecum and proximal colon. However, since microbial protein synthesised in the caecum appears to be unavailable 10 tht' animal, the net rt'sult of such a change of si tt' of fermt'n tation would be to decrease the a vailability of amino acids to the animal. This would change the protein-to-energy ratio in the absorbed nutrients markedly, which in turn would significantly reduce the efficiency of utilisation of absorbed nutrients. Feed intake would also fall owing to less amino acids being absorbed from the intestines. It is well t'stablished that an increased uptake of amino acids from tht' intt'stines tt'nds to direct nutrients towards proteinaceous growth rather than fat. As the deposition of protein tissue is associated with body water accumulation, then a primary effect of parasitism may be to direct nutrients into fat synthesis.

10.3.2 Protein digestion in the small intestine / abomasum

Three important facts are apparent:

Although there appears to be an increased input of endogenous nitrogen into the intestines of sheep at the site of infestation there appears to be compensatory digt'stion at other sites in the digestive tract, allowing the apparent digestibility of proteins between the abomasum and ileum to remain constant
In sheep infested with T. colubriformis there is an increase in the water content of all the organs of the tract post-ruminally, whereas the content of water in the rumen is decreased relative to uninfested animals (Roseby 1973)
Most infestations of the intestines result in reductions in the digestibility of protein and amino acid absorption at the site of infestation. Some of the factors that could be involved include:

damage to the wall of the organ by burrowing or migrating larvae, which occurs with O. cir'cumcincta or T. axei infestation

mutilation of the mucosa of the abomasum caused by the biting and piercing mouth parts of parasites such as H. contortus

villous atrophy or flattening and stunting of the microvillae, which occurs in the small intestine of animals infested with T. colubriformis.

These actions of parasites lead to epithelial hyperplasia and inflammation of almost all mucosal surfaces, which in turn apparently leads to leakage of proteinaceous materials, particularly plasma proteins into the lumen of the tract. Additionally, there is excessive mucous production by the epithelial cells and secretions of mast cells, eosinophils and lymphoid cells all of which are high in protein.

Infestations of the abomasum and the small intestine may have similar effects on protein digestion and amino acid absorption. In ruminants the acidity of abomasal fluid is essential in denaturing proteins (including microbial proteins-) leaving the rumen, making them more readily digestible by proteolytic enzymes in the upper tract. Abomasal infestation with O. circumcincta increases the pH of abomasal fluid (Jennings et ai. 1966).

An increase in the pH of abomasal fluid would have two effects. Firstly, bacteria from the rumen would not be killed. Jennings et al. (1966) demonstrated that in calves infested with O. ostertagia there is a 20- to 30-fold increase in the number of aerobic bacteria in the intestinal lumen. Since rumen organisms are anaerobic it is unfortunate that the concentration of viable anaerobic organisms was not measured. Secondly, the increased pH would allow proteins to move to the intestines from the rumen without chemical change. These two effects may help to explain some of the effects of parasites on amino acid retention by ruminants.

Roseby and Leng (1974) demonstrated that synthesis of urea increased in sheep infested wi th T. colubrifOT'mis and attributed this to movement of more protein into the caecum and its fermentation. However, it seems possible that some amino acids are deaminated by bacteria in the small intestine, especially if the impaired gastric digestion ensures that the protein remains in the digestive tract longer and allows bacterial populations to increase. In addition, the lack of acid treatment in the abomasum may also lead to decreased secretion of pancreatic and intestinal enzymes. Decreased proteolysis due to reduced deaturation and secretion of enzymes, combined with lcreased populations of bacteria, suggests that at :ast a proportion of the protein normally degraded ) amino acid and absorbed as such is deaminated by acteria, resulting in a net reduction in the amount f amino acids available for absorption.

To some extent, infestation of the small intestine ith T. colubriformis may have a similar effect to lat of an abomasal infection with Ostertagia spp. T. 'Jlubriformis interferes with the secretion of enzymes I the upper small intestine (eg. dipeptidase activy in the duodenum-jejunum is depressed) (Symons and Jones 1975). In addition, because of damage to Ie epithelial lining, the animal's ability to absorb amino acids from these areas seems to be less. This means that where the duodenum-jejunum is infested, the proteins, despite denaturation by abomasal fluid, will still tend to be digested lower in the small intestine. The movement of extra endogenous protein into istal areas of the gut may overload the digestive enrmes and/or the capacity of the intestines to absorb nino acids, allowing more protein and amino acids ) move to the caecum where it will be fermented to FAs. If there is a large increase in the microflora of Ie small intestine in parasitised animals this may reIlt in increased endogenous nitrogen input into the intestines (Fauconneau and Michel 1970).

10.3 Parasites of the large intestine

Infestations of the large intestine are much more ready described. An organism that damages the wall of the large intestine and increases endogenous inputs of ptoteinaceous materials will have a marked effect on production because the materials will be fermented to VFAs and the microbial protein synthesised will be excreted or degraded in the large intestine. Thus, secretion of endogenous protein into the large intestine presents a drain of amino acids. The absorption of VFAs from the large intestine will decrease the ratio of protein to energy in the nutrients available to the animal. This could markedly reduce appetite and possibly increase heat production, and could, therefore, result in inefficient utilisation of absorbed nutrients.

10.3.4 Effects of site of parasitism on protein nutrition

Irrespective of the N balance in control (uninfested) ruminants, the effect of gastro-intestinal parasites is to decrease the retention of N (see Topps 1983). Parasites in the abomasum and small intestine tend to increase urea excretion in urine, whereas parasites of the large intestine tend to increase faecal N losses (Figure 10.3). The extremely adverse effect on N losses in animals fed low-N diets confirms the general thesis that parasitism is more detrimental to animals on imbalanced, low-N diets.

Figure 10.3: The loss of nitrogen in faeces and urine in ruminants infested with Oesophagostomum radiatum (OR) (calves-large intestine (Ll) parasite), and Ostertagia circumcinta (DC) (sheep-abomasum (A) and Trichostrongylus colubriformis (TC) (sh.eep-small intestine (1)). The data show the dmmatic effect of pamsites in animals on low-N diets. The incr'eased losses of N occur in urine when the infestation is in the abomasum or small intestine, and in the faeces when the infestation is in the largf intestine. (Adapted from Topps 1983).

10.4 METABOLISM IN THE ANIMAL

Probably the most comprehensively studied area of parasitology is the effects of intestinal parasites on erythrokinetics, plasma-protein metabolism and protein turnover in both skeletal muscle and liver. The development of anaemia and hypo-albuminaemia is associated with high fractional rates of removal of red blood cells and albumin, which are attributable to the elevated enteric loss of protein at the site of infestation (Dargie 1980). The net effect of this leakage of protein into the gut is to increase the rates at which haemoglobin and albumin are synthesised to compensate for the losses in order to maintain the levels of these circulating in blood plasma (Dargie 1980; Symons and Steel 1978). An increased rate of protein synthesis would require more amino acids. If there is a significant loss of endogenous amino acids through deamination and/or fermentation in the small or large intestine, amino acid availability will be reduced and requirements wiII not be met. As with all nutritional imbalances, this must result in decreased feed intake.

If some of the proteins that enter the digestive tract are deaminated by bacterial enzymes, a substantial change of the protein-to-energy ratio may result. This would reduce the ratio of protein to energy available to the animal, which would therefore need to dispose of the excess energy in order to balance the nutrient availability. The animal can only do this by increasing fat deposition or by initiating wasteful cycles of metabolism (eg. such as the synthesis and degradation of glucose, synthesis and degradation of fat or other wasteful cycles involving activation and deactivation of particular molecules in which ATP is involved).

Sutherst (1987) has recently reviewed this area and points to four distinct effects of ticks and biting flies which include:

Irritation by biting flies which modify grazing behaviour often leading to decreased grazing time and feed intake
Removal of nutrients (protein and glucose) by the sucking of blood
Injection into the animals of toxins that effect metabolism
Farmers lock up their stock and do not graze infested areas reducing feed intake.

A high infestation of ticks on cattle (say 200 /head) may lead to a blood loss of 120-300ml per day (0.61.5g/tick) (Sutherst and Utah 1981, Sutherst et al. 1983). Under nutritional conditions pertaining to low protein-grass lands, tick infestations would lead to an even lower feed intake (which will always lead to a greater proportion of the feed proteins being degraded in the rumen) and a decreased efficiency of utilisation of feed.

The effects of ticks have been partitioned into feed intake effects and specific effects which are compatible with a decreased efficiency of utilisation of feed (ie. a burning off of nutrients). This is only partially compensated for by increased efficiency of growth after infestation. The increased body weight gain during the apparent compensatory growth period seems to be largely due to water accretion (Springell et al. 1971) (see Table 10.2).

Table 10.2: The effects of ticks on feed intake and litJeweight gain. The cattle were infested during a 77 day trial period (Period A). The liveweight gains were also followed over a post infestation period of variable length (Period B). The inefficient utilization of feed by the tick-infested group compared to their pair fed counterparts is consistent with an increased heat production due to an imbalance of nutl'ients with requirements (see Chapter 4). The cattle wel'e fed pelleted feed (50% lucerne, 30% millet and 20% sorghum).
	Feed intake, kg/d	LW gain, kg/d
	Feed intake, kg/d	Period A	Period B
No ticks	5.7	0.97	0.53
Tick infested ad lib. intake	4.7	0.44	0.63
Pair fed (no ticks)	4.7	0.64	0.89
Source: Seebeck et al. (1971).

The drain of red blood cells and serum proteins by tick infestation would increase the demand for protein relative to energy yielding nutrients. A wide divergence of amino acids to energy available to the demand for these would result in an inefficient utilisation of nutrients. This could explain some of the non-specific effects even though this is believed to be due to toxic substances injected by the tick.

Biting flies appear to cause production losses in cattle not through their effects of removing nutrients by blood sucking but more by a direct effect of the parasite on the "energy expenditure" of the animal which is agitated by the presence of large number of flies. Holroyd et al. (1984) showed positive correlation between individual cattle growth rates and burdens of buffalo fly (H aematobia spp.) on tolerant animals. On the less tolerant animals, however, growth rates and therefore feed conversion efficiency were reduced (Table 10.3). The decreased efficiency was attributed by Sutherst (1987) to the energetically expensive and stressful defensive reactions. However, an alternative that may explain some of this effect encompasses the concept that the balance of nutrients is disturbed.

Table 10.3: The effects of presence or absence of populations of horn fly and stable fly on feed DM conversion (FCR) by young cattle fed "high quality diets ".
Flies / animal	Feed intake, kg/d	LW gain, kg/d	FCR
Horn fly
0	6.1	0.75	8
300	6.3	0.59	11
Stable fly
0	7.7	1.1	7
100	6.9	0.88	8
Source:(Campbell et al.1977; Sutherst 1987).

The stress and annoyance of flies plus increased activity would tend to increase overall glucose utilisation which could lead to a deficiency in metabolism and a burning off of nutrients in order to compensate (see Chapter 4). However, these arguments are difficult to sustain, since, where the effects of flies have been studied, "high quality" diets have been used. For instance, Schwinghaulluer et al. (1986) showed a marked decrease in N balance in response to horn fly infestation, but the animals were fed a high-starch, high protein diet (Table 10.4). If the animals subjected to flies had been in a hot humid environment, the effects of the increased heat production by the animal could have reduced feed intake to a greater extent.

Table 10.4: The influence of stable fly infestation on feed utilisation by beef steers. The steers were fed a diet of 68% cracked corn, 20% cottonseed hulls, 11 % soyabean meal and adequate minerals and vitamins. urine volume increased linearly with the urea excreted.
	Number of flies/head
	0		25	50
Feed intake (kg/d)	5		5	5
N intake (g/d)	99	101		100
N excretion (g/d)
Faeces	31		34	34
Urine	20		29	35
N retained (g/d)	50		40	32
Water intake (kg/d)	17		21	21
Urine output (kg/d)	2.9		3.6	4.8
Source: Schwinghammer et al. (1986).

The effects of intestinal parasite burdens on the efficiency of utilisation of nutrients for tissue synthesis are complex because they are not likely to be the result of a single factor. The effects on protein degradation in the intestine together with resynthesis (increased synthesis) of plasma proteins have associated energy costs: These are small, as are the associated costs of increased urea and glucose synthesis, but together they may account for the decreased efficiency. It appears that there may be an increased loss of amino acids through bacterial deamination in the intestines, which may account for an imbalance in the protein-to-energy ratio in the animal necessitating an increase in fat synthesis or heat production (Leng 1981) .

The overall effect of all parasite infections is to increase the turnover and size of major protein pools, including plasma proteins and red blood cells, increasing the demand for amino acids relative to energy. At the same time there is, in most c.ases, a concomitant reduction in digestion of protein and absorption of amino acids, decreasing the ratio of protein to energy available. The increase in demand for a balance of nutrients with a high protein-to-energy ratio and the decrease in protein-to-energy ratio of absorbed nutrients tends to:

Diets high in bypass protein or which support efficient rumen function would tend to alleviate the effects of parasitism. Conversely, where protein undernutrition is unavoidable then parasite control becomes of paramount importance. With low-protein feeds a low availability of fermentable N would be highly detrimental because of the low ratio of protein relative to energy in the products of fermentative digestion (Chapter 3).

Ticks have clearly two nutritional effects on the animal. The first is a reduced feed intake which might be associated with increased demand for protein relative to energy and which in turn creates an inefficient use of the available feeds.

Biting flies only marginally effect the efficiency of feed utilisation which again could be a result of increasing the demand for critical nutrients (glucose) relative to energy. However, other effects such as injection of toxins are of obvious importance.

Biting flies and ticks have similar depressing effects on the efficiency of feed utilisation (thereby increasing heat production) which could be exaggerated in a hot humid environment where heat load is a major factor affecting feed intake.