Chapter 51 Gastrointestinal Nematodiasis in Hoofstock
Species of perissodactylid and artiodactylid ungulates evolved to take advantage of the grasses, shrubs, trees, roots, tubers, and fruits on and below the surface of the land. Similarly, nematode parasites evolved to take advantage of the mucosal surfaces of the ungulates’ gastrointestinal (GI) tracts, to maximize the chances of their eggs and larvae surviving outside the host and infecting a new host.
All hoofstock, especially those with access to grass enclosures, should be assumed to carry a range of GI nematodes. The extent of the infestation, the species of nematodes involved, and the clinical significance should be assessed for each host species.6,7,12
In which climatic region is the ungulate collection? In a temperate zone, nematodes would be expected to thrive in the wet summer but would need to survive a harsh winter as resistant stages in the environment and within some of the hosts. In Mediterranean and semiarid regions, however, the cool, wet winters would favor the nematodes, but they would be suppressed and would need to survive during the hot, dry summers.
What ungulate species are held by the collection? Species generally evolved with the nematodes that survive in the same climatic region and would be expected to be relatively resistant to clinical disease, certainly compared with species from different climates. In the temperate climate of the United Kingdom, species of deer such as red deer (Cervus elaphus) show little clinical effects of nematodiasis unless circumstances favor the nematodes (see later discussion). However, antelope such as scimitar-horned oryx (Oryx dammah) and addax (Addax nasomaculatus), from semiarid and arid regions, and musk ox (Ovibos moschatus), from the tundra, are highly susceptible.
What do the clinical and necropsy records reveal? Even when no specific parasite monitoring has been done, the general records for the collection should still have much useful information. How long have species of interest been in a particular enclosure? How has the herd size and stocking density changed over time? Have there been reports of poor body condition, diarrhea, or deaths in the group? Are there fecal egg count results on file?
In many cases, more than one species of nematode may be found, or nematodes and other parasites are found together, and it may be impossible to decide which species is the main pathogen. For example, a mixed infection with a Trichuris sp. and several Eimeria coccidial spp. was thought to be responsible for diarrhea in a 10-day-old red lechwe calf (Kobus leche leche).3
Secondary disease caused by GI nematodiasis is probably more common than primary disease and likely underreported. In these cases, many pathologic processes may be causing loss of body condition, weakness, and death, and the relative importance of each factor may be difficult to determine. Common factors in secondary disease include climate, malnutrition, stress, physiologic state, and concurrent disease.
In temperate regions, animals need to expend more energy to maintain their core body temperature in winter. Species that have evolved to survive winters with thicker coats or insulation will require less energy than species from other regions; even for these animals, however, extended periods of rain may saturate coats and make them less insulating, leading to increased heat loss. In these circumstances, animals may be in negative energy balance for extended periods, and any additional drain on their resources, including parasitism, may be fatal. The increased mortality of deer in winter is recognized as winter death syndrome. Hot and dry summers may be equally stressful to species not regularly exposed to such climate.
Moderate levels of parasitism may be tolerated by hosts if they have plentiful food of good quality. Similarly, the extra energy requirements during cold weather may be met by increased food intake. There are limits, however, and animals may not have sufficient appetite to eat the extra food needed, or GI nematodiasis may depress appetite. Therefore, food of higher energy density must be offered to avoid a negative energy balance at these times. In hoofstock, this can be dangerous because high-energy food may cause acidosis (in ruminants), laminitis (in Perissodactyla), and other metabolic problems.
The intake of vitamins and minerals is important for the general health of any animal, but among hoofstock, copper deficiency may be particularly relevant. At Whipsnade Zoo, London, yak with low blood copper concentrations were found to harbor significant nematode burdens despite regular deworming.4
Besides initiating type II ostertagiasis, as previously mentioned, stress also has a general effect on immune status. What constitutes a stressor will vary between species and individuals and may not always be apparent. Overcrowding is likely to be apparent, but even when the enclosure density is adequate, the makeup and dynamics of the group may be stressful. For example, leaving more than one adult male of certain species with a group of females may or may not result in overt aggression and fighting, but in either case may result in heightened stress in the group, males and females alike. Thus, chronic GI parasitism in Thomson’s gazelles (Gazella thomsonii) at Whipsnade was controlled better by removing all except for one male than by regular anthelmintic treatments.9
Growth, pregnancy, and lactation all require increased energy and protein intake. Therefore, growing animals and pregnant or lactating females are more likely to be affected by heavy parasitism than adult males and nonbreeding females.
It is rare for animals to encounter only one pathogen at a time. If the animals are overcrowded and the enclosure is heavily contaminated, they will be challenged with high doses of viruses, bacteria, and fungi in addition to the nematodes and other parasites. Even in less severe situations, infection with one pathogen may weaken an animal and make it more susceptible to another pathogen. In these cases the GI nematode infestation may predispose the animal to a secondary infection, or nematodes may find it easier to establish in an animal already weakened by a different primary disease.
The level of investigations will depend on the number of species and individuals involved, the severity of the problem, and the resources available. However, consideration should be given to the diagnostic approaches discussed next.
Clinical examination may be more difficult for many large ungulates and those in herds. However, some collections have physical restraint systems that allow close inspection, sample taking, and treatment of deer and antelope. If these are not available, examiners should still be able to observe herds at feeding sites (Figure 51-1) and then select a few individuals to assess while under chemical restraint.