3.1 Ruminant parasites

In Kenya’s diverse agro-ecological zones, ruminant parasites vary widely, encompassing internal and external species that affect cattle, sheep and goats differently. Understanding the terms used, the types of parasite, and their life cycles and transmission is essential for designing targeted control programmes. Extension officers must be familiar with the parasites found in local herds and flocks and with their economic and health implications.

3.1.1 Definition of terms

Clear terminology allows an extension officer to describe a parasite problem accurately to a farmer or veterinarian. The terms below are used throughout this unit.

• Parasite: An organism that lives on or in another organism (the host), obtains food from it and gives nothing useful in return. A tick feeding on the blood of a dairy cow is an example.
• Host: The animal that carries the parasite and supplies it with food and shelter. Cattle, sheep and goats are the hosts dealt with in this unit.
• Definitive host: The host in which the parasite reaches the adult stage and reproduces sexually. The sheep is the definitive host of the liver fluke.
• Intermediate host: A host in which the parasite passes through a larval stage but does not become adult. Freshwater snails are the intermediate hosts of liver flukes.
• Endoparasite: A parasite that lives inside the host, for example in the gut, liver, lungs or blood.
• Ectoparasite: A parasite that lives on the skin, hair or other external surfaces of the host.
• Infection and infestation: Infection normally describes the presence of internal parasites or microbes in the body, while infestation describes the presence of external parasites such as ticks and lice on the body surface.
• Vector: A living carrier, usually an arthropod, that transmits a disease-causing organism from one animal to another. Tsetse flies are vectors of trypanosomes.
• Life cycle: The series of stages a parasite passes through from egg to adult and back to egg, including any stage outside the host.
• Free-living stage: The part of the life cycle spent on pasture, in water or in soil rather than in or on the host.
• Prepatent period: The time between the animal becoming infected and the parasite first producing eggs that can be found in the faeces.

3.1.2 Types of ruminant parasites

Ruminant parasites are broadly classified into two main groups based on their habitat and mode of infestation within the host animal. Internal parasites, or endoparasites, live inside the animal’s body, primarily in the digestive tract, blood, or other organs. External parasites, or ectoparasites, infest the skin, hair, or external surfaces.

Life cycles and transmission

Knowledge of parasite life cycles is critical for timing control measures effectively, because each stage is exposed to different risks inside the host, on pasture or in the animal house. A life cycle is called direct when the parasite needs only one host, and indirect when it needs an intermediate host or a vector. For example, a worm that passes from a goat’s faeces to the grass and back into another goat has a direct cycle, while a liver fluke that must pass through a snail has an indirect one.

Several factors influence how many infective stages reach the animal:

• Environmental factors: Humidity, temperature, and pasture management influence the survival of free-living parasite stages. For example, prolonged rains on a poorly drained paddock create ideal conditions for snails and liver fluke transmission, while warm, moist grass favours the development of worm larvae.
• Host factors: Young, pregnant, lactating, poorly fed or immunocompromised ruminants are more susceptible to parasitic infections because of weaker immune responses.
• Stocking density: Overcrowded paddocks, bomas and pens increase contamination of the ground with eggs and make contact transmission of lice and mites easier.
• Grazing behaviour: Animals grazing close to the ground, or at the edges of ponds and streams, swallow more infective larvae and cysts.
• Movement of animals: Newly bought or borrowed animals can introduce parasites into a clean herd.

Economic and health impact of ruminant parasites

Parasitic infections in ruminants cause significant economic losses through reduced productivity, treatment costs, and mortality. Extension professionals must quantify these impacts to advocate for investment in parasite control. For example, a dairy farmer who loses part of the daily milk yield from a tick-infested herd also pays for drugs and veterinary visits.

• Reduced weight gain: Parasites compete for nutrients and damage the digestive tract, slowing growth rates in calves and lambs, which delays market readiness.
• Decreased milk production: Infected dairy cows exhibit lower milk yields and quality, affecting household incomes and nutrition.
• Poor reproduction: Parasites can cause infertility, abortions, and weak offspring, undermining herd sustainability.
• Treatment expenses: Costs for anthelmintics, acaricides, and veterinary services strain smallholder farmers’ budgets.
• Mortality: Severe infestations, especially in young animals, lead to high death rates, reducing herd size and future earnings.
• Disease transmission: Ticks and biting flies act as vectors of diseases such as East Coast fever, babesiosis, anaplasmosis and trypanosomiasis, complicating herd health management.

3.1.2.1 Internal parasites

Internal parasites have complex life cycles involving eggs passed in faeces, development in the environment, and ingestion by the host. The animal normally becomes infected by swallowing infective stages with grass, hay or drinking water.

Internal parasites include the following:

• Gastrointestinal nematodes (direct cycle): These roundworms, including Haemonchus, Trichostrongylus, and Ostertagia, inhabit the stomach and intestines, causing damage to the mucosa and leading to poor nutrient absorption. Haemonchus contortus, the barber’s pole worm, feeds on blood and is notorious in sheep and goat flocks for causing anaemia and death. Adult worms lay eggs that leave the animal in the faeces. The eggs hatch into larvae on the pasture, which moult twice to become infective third-stage larvae. These climb onto grass blades and are swallowed during grazing, then develop into adults in the gut, completing the cycle in a few weeks.
• Lungworms (direct cycle): Species such as Dictyocaulus viviparus target the respiratory system, leading to coughing and pneumonia, particularly in young calves. Adult worms live in the airways and produce eggs that hatch there. The larvae are coughed up, swallowed and passed out in the faeces, becoming infective on the pasture. After being swallowed by another animal, the larvae penetrate the intestinal wall and travel through the lymph and blood to the lungs.
• Coccidia (direct cycle): Protozoan parasites such as Eimeria species invade the intestinal lining, causing diarrhoea and dehydration in lambs, kids and calves. Infected animals pass oocysts in their faeces. In warm, damp bedding or soil the oocysts sporulate and become infective, and they are then swallowed by other animals. The parasite multiplies in the cells of the gut lining, which causes the diarrhoea. Crowded, wet calf, lamb or kid pens favour this cycle.
• Liver flukes (indirect cycle): Fasciola hepatica and Fasciola gigantica infect the liver and bile ducts, impairing liver function and causing weight loss. They are associated with wet, poorly drained grazing where snails live, because they need freshwater snails as intermediate hosts, as shown in the steps below.
• Blood parasites (vector-borne transmission): Trypanosomes transmitted by tsetse flies cause trypanosomiasis, a debilitating disease of ruminants in tsetse-infested areas. The fly picks up the parasite when it feeds on an infected animal, the parasite develops in the fly, and it is passed into the next animal through the fly’s saliva when it bites. Other biting flies can also carry blood on their mouthparts from one animal to another.

The life cycle of the liver fluke follows these steps:

1. Adult flukes in the bile ducts of the host lay eggs, which pass down the bile ducts into the gut and out in the faeces.
2. In water or wet mud, and in warm conditions, the eggs hatch into small free-swimming larvae called miracidia.
3. The miracidia find and penetrate a freshwater snail, which is the intermediate host.
4. Inside the snail the parasite multiplies through several larval stages, which greatly increases the number of infective stages produced from one egg.
5. Cercariae leave the snail, swim to grass or water plants and encyst there as metacercariae, which are the infective stage.
6. The grazing animal swallows the metacercariae with wet grass or drinking water.
7. The young flukes leave their cysts in the intestine, burrow through the gut wall and liver tissue, and settle in the bile ducts, where they become adults after about two to three months.

3.1.2.2 External parasites

External parasites spend all or part of their life cycle on the animal. They are transmitted mainly by contact between animals, by contact with infested ground, bedding and housing, or by flying vectors. The speed of the cycle depends strongly on temperature and humidity.

External parasites include the following:

• Ticks: Various species including Rhipicephalus and Amblyomma attach to the skin, feeding on blood and transmitting diseases such as East Coast fever. Ticks are a serious concern for herds grazing in open rangelands. They have eggs, larvae, nymphs and adults. Many species of economic importance are three-host ticks, in which each feeding stage drops off the animal to moult on the ground, as shown in the steps below. Some are one-host ticks, such as the blue tick, which complete all stages on the same animal, and some are two-host ticks, such as the red-legged tick.
• Lice: Chewing and sucking lice infest the coat, causing irritation, weight loss, and secondary infections. Lice spend their whole life on the host. Females glue eggs (nits) to the hairs, nymphs hatch and moult several times, and adults mature on the skin. Lice survive only a short time away from the animal, so they spread mainly by direct contact, especially among crowded and weak animals.
• Mites: Mange-causing mites burrow into or live on the skin, leading to intense itching and skin lesions. They complete the cycle of egg, larva, nymph and adult on the host. Some species tunnel into the skin and lay eggs in the tunnels, while others live on the skin surface. Spread is by close contact between animals and by shared rubbing posts and housing.
• Flies: Biting flies such as tsetse and stable flies cause direct blood loss and act as vectors for disease transmission. Tsetse flies do not lay eggs. The female deposits a fully grown larva in loose, shaded soil, where it pupates, and both male and female adults bite animals for blood. Stable flies breed in rotting vegetation and wet manure, and the adults bite animals during the day.
• Fleas: Less frequent in ruminants but still problematic, fleas cause irritation and may transmit other pathogens. Adult fleas live on the host, but the eggs fall off into bedding and dust. The larvae feed on organic debris, then pupate, and the new adults jump onto an animal. Unclean, dusty bedding in kid or calf pens favours them.

The life cycle of a three-host tick follows these steps:

1. The engorged female drops from the animal, hides in the soil or litter and lays a large batch of eggs, after which she dies.
2. The eggs hatch into six-legged larvae, which climb grass stems and wait for a passing animal.
3. A larva attaches to the first host, feeds on blood, then drops to the ground and moults into a nymph.
4. The nymph climbs vegetation, attaches to a second host and feeds.
5. The engorged nymph drops to the ground and moults into an adult.
6. The adult attaches to a third host, where males and females mate and the female feeds and swells with blood before dropping to lay eggs.

Disease organisms such as the cause of East Coast fever are passed to the animal during feeding by the tick stage that has itself fed on an infected animal earlier.

Chapter Summary

This chapter explored the various types of parasites affecting ruminant animals, distinguishing between internal parasites such as worms and external parasites including ticks and lice. It detailed the common signs and symptoms that indicate parasite infestation in ruminants, emphasizing the importance of early detection for effective management. The chapter then examined different parasite control measures, highlighting chemical, biological, and cultural methods to reduce parasite burdens. It also stressed the necessity of preparing accurate parasite control records to monitor treatment schedules and outcomes. Proper management of parasite control waste was addressed to prevent environmental contamination and ensure safety. Practical procedures for identifying external parasites on ruminants were explained to aid in targeted interventions. Additionally, the chapter covered the implementation of various parasite control techniques, ensuring comprehensive coverage of prevention and treatment. Finally, it reinforced the importance of managing waste generated from parasite control activities to maintain farm hygiene and protect public health.