Guinea Pig Ectoparasites Study Guide

Overview and Clinical Importance

Ectoparasites are among the most common dermatological conditions affecting guinea pigs (Cavia porcellus) in clinical practice. These external parasites include mites and lice that can cause significant morbidity and, in severe cases, mortality. The most clinically significant ectoparasite is Trixacarus caviae (sarcoptic mange mite), which causes intense pruritus and can lead to seizures and death if untreated. Understanding the identification, life cycles, clinical presentations, and treatment protocols for these parasites is essential for NAVLE success and clinical practice.

Guinea pigs are susceptible to several ectoparasites including burrowing mites (Trixacarus caviae), fur mites (Chirodiscoides caviae), and chewing lice (Gliricola porcelli and Gyropus ovalis). These parasites are commonly encountered in pet guinea pigs and can have multisystemic effects including dermatological, neurological, and metabolic consequences.

Classification of Guinea Pig Ectoparasites

Guinea pig ectoparasites are classified into two major groups: mites (Class Arachnida) and lice (Class Insecta). Understanding their taxonomic classification helps predict behavior, life cycle, and treatment response.

Trixacarus caviae (Sarcoptic Mange Mite)

Trixacarus caviae is the most clinically significant ectoparasite of guinea pigs. This highly pathogenic sarcoptid mite causes severe pruritus and can be life-threatening if untreated. It is often referred to as the "Sellnick mite" or guinea pig mange mite.

Etiology and Morphology

T. caviae is a burrowing sarcoptid mite belonging to the order Astigmata. Key morphological features include:

• Size: Females measure 135-200 micrometers (smaller than Sarcoptes scabiei which is 200-450 micrometers)
• Round, sac-like body with four pairs of legs
• Prominent sharp dorsal spines (distinguishes from other sarcoptids)
• Long, unsegmented pedicels with caruncles on anterior leg pairs
• Anus position: dorsal in males, ventral in females (key identification feature)

Life Cycle

The life cycle of T. caviae is completed entirely on the host and consists of four stages: egg, larva, nymph, and adult.

Transmission and Epidemiology

Routes of transmission:

• Direct animal-to-animal contact (most common)
• Sow to weanling during nursing
• Contact with contaminated bedding or fomites
• Contact with infested carcasses

Important: Mites can exist subclinically and become active with stressors such as shipping, pregnancy, immunosuppression, or concurrent disease. Off-host survival is limited to approximately 3 weeks, but eggs can survive longer in the environment.

Clinical Signs

Clinical presentation varies from subclinical carrier states to severe, life-threatening disease.

Pathophysiology

The pathogenesis involves both mechanical damage and immunologic response. Female mites burrow into the stratum corneum, creating tunnels where they deposit eggs and feces (scybala). The pruritic response results from a Type IV (delayed) cell-mediated hypersensitivity reaction to mite antigens, eggs, and fecal material. This immune response typically develops 10-30 days post-infestation.

Histopathologic findings include: acanthosis, hyperkeratosis, exfoliative dermatitis, and mites visible in the epidermis. Inflammation extends to the subcutis in severe cases.

Diagnosis

Skin Scraping Technique (Gold Standard):

• Select non-excoriated lesion margins (head, shoulders, inner thighs)
• Apply mineral oil to scalpel blade
• Scrape DEEP - until capillary bleeding occurs (mites burrow deep)
• Transfer material to glass slide with coverslip
• May require 10% KOH digestion to dissolve keratin debris
• Examine under low power microscopy for mites, eggs, and scybala

Treatment

Environmental Management (Critical):

• Discard all bedding and replace with new material
• Clean and disinfect cage with diluted bleach solution
• Treat ALL in-contact guinea pigs (asymptomatic carriers common)
• Continue treatment long enough to eliminate last hatching eggs

Zoonotic Potential

T. caviae is considered transiently zoonotic. Sensitive individuals may develop papular dermatitis and pruritus from contact with infested guinea pigs. However, the mites are species-specific and cannot reproduce on humans - infestations are self-limiting. Advise clients to wash hands thoroughly after handling affected animals and consult their physician if skin symptoms develop.

Chirodiscoides caviae (Fur Mite)

Chirodiscoides caviae is a fur-clasping mite (also called "static lice" by some owners) belonging to the family Listrophoridae. Unlike Trixacarus, this mite does NOT burrow into the skin and is generally considered relatively harmless.

Morphology and Identification

• Size: Males 350-376 micrometers; smaller than Trixacarus
• Body shape: Ovoid and elongated with triangular anterior
• Legs modified for clasping hair shafts
• Paired males and nymphal females often seen together (characteristic feature)
• Eggs (ova) attach to hair shafts, do NOT burrow

Clinical Signs

Most infestations are subclinical or asymptomatic. Heavy infestations may cause mild pruritus and alopecia, particularly over the posterior trunk and hindquarters. The underlying skin is typically unaffected unless there is self-trauma. Mites can be visible as small moving specks in the fur. Coinfections with lice and Trixacarus are common in pet guinea pigs.

Diagnosis and Treatment

Diagnosis:

• Hair pluck examination (mites clasp hair shafts)
• May see paired adults and eggs on hair shafts
• Bumpiness of coat may indicate egg presence

Treatment:

• Ivermectin: 0.3-0.5 mg/kg SC or topical, repeat at 10-14 day intervals
• Selamectin: 15-30 mg/kg topical, repeat at 2-week intervals
• Treatment often unnecessary if asymptomatic

Chewing Lice (Pediculosis)

Guinea pigs are host to two species of chewing (biting) lice belonging to the order Mallophaga: Gliricola porcelli (the slender guinea pig louse) and Gyropus ovalis (the oval guinea pig louse). G. porcelli is the more commonly encountered species.

Morphology and Comparison

Biology and Transmission

• Feeding behavior: Chewing lice abrade the skin with their mouthparts to obtain cutaneous fluids (NOT blood-sucking)
• Host specificity: Species-specific; cannot infest humans, dogs, or cats
• Transmission: Direct contact with infested animals or contaminated bedding
• Off-host survival: Limited; lice migrate away from cooling carcass along hair shafts
• Nits: Eggs attached to hair shafts (white to tan specks)

Clinical Signs

Most infestations are asymptomatic or cause minimal signs. Heavy infestations may produce:

• Mild pruritus (less intense than mite infestations)
• Rough, unkempt hair coat
• Partial alopecia, especially around neck and ears
• Scabs and flaky skin (from scratching and lice feeding)
• Visible lice crawling on hair shafts (near skin surface)
• Nits visible attached to hair shafts

Diagnosis

• Visual inspection with magnifying glass
• Hair pluck examination for nits on hair shafts
• Scotch tape preparation pressed against fur
• Microscopic identification of adult lice or nits

Treatment

Differential Diagnosis for Pruritic Skin Disease

When evaluating a guinea pig with pruritus and/or alopecia, consider the following differentials:

• Trixacarus caviae mange (most common cause of severe pruritus)
• Dermatophytosis (ringworm - Trichophyton mentagrophytes var mentagrophytes)
• Lice infestation (visible lice, less pruritic than mites)
• Barbering (hair chewing by self or cagemates - behavioral)
• Ovarian cysts (bilateral symmetric alopecia in older females)
• Vitamin C deficiency (rough coat, poor wound healing)
• Bacterial pyoderma (often secondary to ectoparasites)

"TRIXIE Makes Guinea Pigs SCRATCH"

T - Trixacarus caviae (the mite) R - Really intense pruritus I - Ivermectin is treatment of choice X - X-treme cases cause seizures I - Inner thighs, shoulders, head affected E - Environmental treatment essential

"LICE vs MITES"

LICE: Large enough to see, Insects (6 legs), Can see Crawling, Easy to treat MITES: Microscopic, In the skin (burrow), Terrible itching, Extreme = seizures, Scraping needed

Key Numbers to Remember

• Trixacarus life cycle: 10-21 days (why we repeat treatment at 7-10 day intervals)
• Ivermectin dose: 0.4-0.5 mg/kg SC for mites; 0.2-0.4 mg/kg for lice
• Selamectin dose: 15 mg/kg (less than 800g) or 30 mg/kg (greater than 800g)
• Treatment repeats: 2-3 times at 7-10 day intervals for mites
• Diazepam for seizures: 1-2 mg/kg IM